Immunopharmacology and Immunotoxicology, 2009; 31(4): 524–535

R E S E A R C H A R T I C L E

The structure, genetic polymorphisms, expression and biological functions of complement receptor type 1 (CR1/CD35)

Dong Liu1,2, and Zhong-Xiang Niu1

1College of Animal Science & Veterinary Medicine, Shandong Agriculture University, Tai’an, People’s Republic of China, and 2Jinan Animal Husbandry and Veterinary Bureau, Ji’nan, People’s Republic of China

Address for Correspondence: Zhong-Xiang Niu, Daizong Road No. 61, Taian, Shondong, China. 271018. E-mail:zxniu@cdau.edu.cn

(Received 15 February 2009; accepted 24 February 2009 )

Introduction

Complement receptor type 1 (CR1), also called CD35, is a type I membrane-bound glycoprotein that belongs to the regulators of complement activity (RCA) family.(1) The complement receptor 1 (CR1, CD35) is best charac-terized as a receptor for the activated form of the com-plement protein C3, C3b.(2,3) Binding with lower affinity was also demonstrated for the degraded form of C3b, C3bi, and for the complement proteins C4b, C1q and mannan-binding lectin.(4–6)

CR1 has emerged as a molecule of immense interest in gaining insight to the susceptibility, pathophysiology, diagnosis, prognosis and therapy of such diseases. Such as the extensive research on CR1 has brought insight to the pathophysiology of a galaxy of diseases including the autoimmune and inflammatory disorders, current research on different aspects of CR1 formed the basis of exploring the importance of CR1 in its functions and implications. This review attempts at giving a briefly

view of CR1 with special emphasis on the structure, genetic polymorphisms, expression and biological func-tions of this protein.

The importance of the complement system

Complement, as a vital part of the body’s immune sys-tem, provides a highly effective means for the destruc-tion of invading microorganisms and for immune complex elimination.(7–9) Complement has also been shown to participate in the generation of normal humoral immune responses to foreign antigens.

Complement is a key component of innate immunity (10)

The complement system is comprised of a number of serum and membrane-bound proteins that play an important role in the elimination of foreign micro-organisms while protecting the host organism from

ISSN 0892-3973 print/ISSN 1532-2513 online © 2009 Informa UK LtdDOI: 10.3109/08923970902845768

AbstractThe complement system is comprised of soluble and cell surface associated proteins that recog-nize exogenous, altered, or potentially harmful endogenous ligands. In recent years, the complement system—particularly component C3 and its receptors—have been demonstrated to be a key link between innate and adaptive immunity. Complement receptor type 1 (CR1), the receptor for C3b/C4bcomplement peptides, has emerged as a molecule of immense interest in gaining insight to the susceptibility, patho-physiology, diagnosis, prognosis and therapy of such diseases. In this review, we wish to briefly bring forth the structure, genetic polymorphisms, expression and biological functions of CR1.

Keywords: Complement receptor type 1; molecular structure; genetic polymorphisms; expression; biological functions

http://www.informahealthcare.com/ipi

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 525

complement-related damage. It has three critical physi-ologic activities:

defending against microbial infections by triggering •the generation of a membranolytic complex (C5b9 complex) at the surface of the pathogen and C frag-ments (named opsonins, i.e., C1q, C3b and iC3b) which interact with C cell surface receptors (CR1, CR3 and CR4) to promote phagocytosis. Soluble C anaphylatoxins (C4a, C3a and C5a) greatly control the local pro-inflammatory response through the chemotaxis and activation of leukocytesbridging innate and adaptive immunity (essen-•tially through C receptor type 2, CR2, expressed by B cells)disposing of immune complexes and the products •of the inflammatory injury (i.e., other danger signals such as toxic cell debris and apoptotic corpses) to ensure the protection and healing of the host.

The regulatory mechanisms of C are finely balanced so that, on the one hand, the deposition of C is focused on the surface of invading microorganisms and, on the other hand, the deposition of C on normal cells is lim-ited by several key C inhibitors (e.g., CD46, CD55 and CD59).(11)

C can be activated essentially by three distinct routes, the classical, alternative and lectin pathways (CP, AP, and LP, respectively) (Figure 1). The initiation of the AP (involving C3, factor B (fB), factor D (fD) and properdin (P)) does not depend upon the presence of immune complexes but is initiated following interactions with carbohydrate-rich particles lacking sialic acid for review.(12)

The AP is for instance activated by a diverse set of “natural” substances, including yeast walls, bacterial cell walls and cobra venom factor. This will lead to the depo-sition of C3 fragments on the target cells. The CP (involv-ing C1q, C1r, C1s, C4, C2 and C3 components) is acti-vated primarily by the interaction of C1q with immune complexes (antibody-antigen) but activation can also be achieved after interaction of the C1q with non-immune molecules such as polyanions (bacterial lip polysaccha-rides, DNA and RNA), certain small polysaccharides, viral membranes, C reactive protein (CRP), serum amy-loid P component (SAP), and, more importantly, some bacterial, fungal and virus membrane components, yet to be fully characterized.(13)

CRP in mammals is an acute phase protein which can interact directly with microorganisms in a calcium-dependent manner to bring about C activation through the CP but which, allegedly, does not initiate an efficient terminal pathway with the formation of C5a, and the membranolytic complex C5b9.(14)

Along with SAP, CRP, is a member of the pentraxin family of proteins that are unrelated to other known pro-teins, but are themselves stably conserved in vertebrate and invertebrates (e.g., Limulus polyphemus, horseshoe crab).(15–17) In vertebrates, CRP binds to a plethora of microbial polysaccharides while SAP interacts with car-bohydrate moieties. Pentraxins are also know to bind to cell components (e.g., fibronectin, chromatin) in dam-aged tissues to aid in their removal through interactions with the opsonic C system as well as stimulating mac-rophages through direct binding to Fc receptors (types I and II).(18)

Mannose binding lectin (MBL) is a member of the family of calcium-dependent lectins, the collectins

Classicalpathway (CP)

Immune complexesnonimmune activators

Carbohydrates(e.g. mannose/pathogen)

Lectinpathway (LP)

Activating surfaces

Alternativepathway (AP)

C1INH

C4bpfI

S proteinClusterin

C5b-9(pore formingprotein)

CD59

C3b

fHfI

(+)

(+)

(+)

C1q + (C1r, C1s)

MBL + (MASP1, MASP2)

+C3

+C4

(+C5)

+C2+C3

+fD+fB

C3a

C3a

C5a

C5b

C3bBbC3b

C4b2a3b

+C6 +C7 +C8 +C9DAF, CR1MCP

Figure 1. The three activation pathways of the complement system. The C system is tightly regulated by a number of soluble (in bold italics) and membrane associated proteins (in bold, boxed).

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

526 Liu and Niu

(collagenous lectins), and is homologous in structure to C1q. MBL is probably the most remarkable PRM of the innate immune system owing to its selective bind-ing to arrays of terminal mannose groups on a variety of bacteria.(19,20) MBL activates C by interacting with two serine proteases called MASP1 and MASP2 (MBL-associated serine protease). MASP2 cleaves and acti-vates C4 and C2 and MASP1 may cleave C3 directly. The lectin pathway is initiated by the binding of MBL or serum ficolins to repeating carbohydrate moieties found primarily on the surface of microbial pathogens.(21–23)

In addition, though, the protein cytokeratin, when it is exposed on ischemic endothelial cells(24) also activates this pathway, as can antibodies bearing a specific form of agalactosyl carbohydrate designated G0.(25) Of inter-est, although the best evidence for the mechanism by which MBL binding leads to C3 activation is that this pathway proceeds through the initial cleavage of C4 and C2, several lines of research suggest that C3 may directly activated by the lectin pathway without utilizing C4 or C2.(21,26) These components of the C system are part of the lectin pathway.(27–29) (Figure 1).

Each of these activating pathways converges at the step of C3 and lead to the covalent attachment of C3b to the activating substance. C3b fragments generated from C3 cleavage are covalently attached to the activating substrate and serve as ligands, along with C4b-bound antigens, for complement receptor type 1 (CR1/CD35). Inactivation of C3b to iC3b and C3d generates the specific ligands for complement receptor type 2 (CR2/CD21), and serves as a means of targeting antigen or immune complexes to cells expressing CR2, linking complement activation to B cell biology and tolerance.(30)

The family of complement receptors(10)

Four complement receptors have been described: CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) of which CR1 and CR2 are mainly expressed on B cells and follicular dendritic cells, while CR3 and CR4 are integrins expressed on macrophages and dendritic cells.(31)

CR1 is a principal regulator of the activation of the complement system of plasma proteins. It is a membrane- bound single-chain glycoprotein, ranging in size from 210 to >300 kDa(32–35) and expressed on eryth-rocytes, monocytes, neutrophils, B cells, some T cells, follicular dendritic cells and glomerular podocytes.(36) The structure of CR1 consists of a linear series of struc-turally related modules designated short consensus repeats (SCR) followed by a transmembrane and short intracytoplasmic domain, which places it in the family of proteins designated the Regulators of Complement Activation. It is capable of binding C4b and C3b frag-ments of complement,(37) functioning on phagocytic

cells to facilitate the ingestion by these cells of particles that have activated complement.(38)

CR2 is a receptor for the C3d/C3d,g and iC3b forms of C3 as well as the Epstein-Barr virus (EBV)(2) and soluble or membrane-bound CD23. Like CR1, CR2 is composed of a series of repeating short consensus repeats and is a member of the RCA family. CR2 is expressed on mature B lymphocytes, FDC, a small subset of peripheral T cells, early thymocytes, basophils, mast cells, kerati-nocytes, and many types of epithelial cells (nasopha-ryngeal, oropharyngeal, cervical, lacrimal, and ocular surface.)(48)

As to CR2, the complement system through CR2 plays immunoregulatory roles such as enhancing humoral immunity to T-dependent and T-independent foreign antigens(39–42) and in regulating T cell immunity to self and non-self antigens.(43–45) CR2 demonstrates a number of biologic roles. On B cells CR2 interacts with C3d/iC3b-bound antigens and promotes B cell activation and differentiation through associations with CD19 and TAPA-1 (CD81)(141,142) Major roles of this CR2/CD19/CD81 complex are to lower thresholds of B cell activa-tion to foreign(42,143,144) and likely self-antigens(145–147) as well as promote the retention of the B cell receptor (BCR) in lipid rafts, resulting in more effective activation and internalization for Ag presentation.(148,149)

In addition, by binding CD23 CR2 may enhance B cell class switching to IgE and antigen presentation by CR2 expressing B cells to CD23-positive T cells.(150) Recently, an additional complement receptor designated CRIg for circulating C3b-bound antigens has been found on Kuppfer cells, which likely plays the major role in clear-ance of pathogens from the blood system.(46)

CR3 and CR4 are members of the integrin family and share common chains of the 2 form(48,151) CR3 and CR4 are expressed on professional phagocytic cells such as neutrophils, monocytes and macrophages as well as FDC, a subset of lymphocytes and eosinophils and B-1 cells. CR3 is the primary receptor for iC3b and interacts less well with C3b and C3d, while CR4 is a receptor for iC3b and C3b. CR3 is a multifunctional molecule and also demonstrates non-complement-dependent func-tions in cellular adhesion by interacting with a variety of ligands, including ICAM-1, factor X, and fibrinogen.(48,152) Because of these additional activities, CR3 may mediate adhesive interactions of neutrophils with endothelium in both a C3 ligand-dependent and C3-independent manners. An additional important interaction of CR3 may be with CD23(153) CR3 and CR4 can also augment T cell activation. CR3 and CR4 can mediate phagocytosis of iC3b-opsonized antigens on antigen presenting cells, and thus, may augment antigen presentation.(47)

In addition to immunoregulatory functions, other studies have focused attention on the roles that the complement system plays in recognition and effector

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 527

functions during self tissue injury(48,49) and in shaping the development of the natural antibody repertoire by influencing the development of reactivity with certain self antigens.(50–55)

The C3b interacts with CR1, which promotes its pro-teolytic processing eventually to C3dg that is the ligand for CR2.(56,57) Therefore, we will subsequently focus our attention on the available data of CR1 with special emphasis on the structure, genetic polymorphism, expression and biological functions of this protein, as it has been shown to be the main receptor of C4b and C3b.

The structure of complement receptor type 1 (CR1)

CR1 is a multifunctional polymorphic glycoprotein which is variably present on different cells, plasma and urine, including the plasma membrane of erythrocytes, eosinophils, monocytes, macrophages, B-lymphocytes, a subpopulation of CD4+ T cells, dendritic cells, Langerhan cells in the skin and glomerular podocytes.(58–60)

Non-membrane bound soluble form of CR1 (sCR1) found in plasma is released from leukocytes especially polymorphonuclear leukocytes into the circulation by cleavage of the surface form of CR1.(61) Urinary CR1 (uCR1) is found in urine in association with the mem-brane vesicles.(62) derived from the glomerular podo-cytes. CR1 is an −200 kDa, single-chain glycoprotein. The extramembranous portion of its most common size allotype is composed of 30 complement-control-protein repeats (CCPs) or short consensus repeats (Figure 2).

The 28 N-terminal CCPs can be organized, based on a degree of homology, into four long homologous repeats (LHRs) A–D, each composed of seven CCPs. Each CCP is 59–72 amino acids long and is characterized by the presence of four invariant cysteines and one invariant tryptophanIn addition, 13 (mostly hydrophobic) amino acids are conserved in >40% of CCPs. Because of this conservation, CCPs show an overall structural similar-ity, featuring two disulfide bonds located at the opposite ends of an elongated structure, several -sheets, and connecting loops and turns.

The structure of complement receptor type 1 (CR1)

CR1 belongs to the regulators of complement activity (RCA) family and it is a type I membranebound glyco-protein. Its extracellular region is formed from 30 short complement regulator (SCR) domains, also known as short consensus repeat, Sushi or complement control protein domains.(63)

Other polymorphic forms of CR1 (not studied here) contain 23, 37 or 44 SCR domains instead of the 30 SCR domains found in the most common major allelic form. CR1 is the largest member of the RCA family, where factor H (FH) and complement receptor type 2 (CR2) contain 20 and 15 or 16 SCR domains, respectively. Characteristically each SCR domain is comprised of a 61-residue structure with two conserved disulphide bridges and a buried conserved Trp residue.(1,64)

Within the CR1 sequence, the 30 SCR domains are arranged as four long homologous repeat (LHR-A to LHR-D) regions with seven SCR domains in each, fol-lowed by two C-terminal SCR domains(65) (Figure 2). Site

LHR A

1 2

Site 1Binds C4b

Decay-acceleratingactivity

Site 2Binds C3b and C4b

Cofactor activity

Site 2Binds C3b and C4b

Cofactor activity

Site 3

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 TM Cyt

LHR B LHR C LHR D

NH2

Figure 2. The most common size variant of complement receptor type 1 (CR1). Each box represents a complement-controlprotein-repeat mod-ule (CCP) or short complement regulator (SCR) domains. Sited 1, 2 and 3 are indicated. Site 1 spans CCPs 1–3 in LHR A, and binds C4b and, weakly, C3b; Site 2 spans CCPs 8–10 in LHR B and the nearly identical CCPs 15–17 of LHR C, and show binding to C3b, C4b PfEMP1, Site 3 (SCR 25) is the site for Swain–Langley (Sl) and McCoy (McC) Knops blood group polymorphism. Within these sites, the same coloring of boxes reflects (near) identity of CCPs (e.g., CCPs 3, 10 and 17 differ by only one to three amino acids). CCP 25, which carries blood group antigens frequent in Africans but rare in Caucasians, is also highlighted, in red. Abbreviations: Cyt, cytoplasmic tail; LHR, long homologous repeat; NH2, amino terminus; TM, transmembrane domain. The Complement Receptor 1 is composed of 30 short consensus repeats (~60 amino acids each). The first 28 are organized into four long homologous repeats (LHRs) each with seven consecutive SCRs. The functional domains of the CR1 pro-tein are also specifically distributed among the different LHRs. LHR-A principally binds with C4b. LHR-B and LHR-C both bind with C3b/C4b and PfEMP1 and have the cofactor activity for factor I The LHR-D has binding sites for Mannan-binding lectin (MBL) and C1q. The SCR 25 in LHR-D has the binding sites of Swain–Langley (Sl) and McCoy (McC) Knops blood group antigens. The structure of sCR1 is similar to that of sur-face CR1 except that it lacks a cytoplasmic tail and is generated by proteolytic cleavage in the C-terminal region of the transmembrane domain of the surface CR1 molecule.

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

528 Liu and Niu

1 in SCR-1/3 in LHR-A binds C4b and is the active site for the decay acceleration activity of the classical and alternative pathway C3 convertases; site 2 in SCR-8/10 in LHR-B and site 3 in SCR-15/17 in LHR-C binds C3b and C4b and are the main sites for its cofactor activity, i.e. the factor I-mediated cleavage of C3b and C4b.(1,66)

In humans, there are only minor variations in the size of CR1 when expressed by different cell types, resulting from variations in N-glycosylation. By contrast, in non-human primates, major size differences exist between CR1 expressed on erythrocytes and on white blood cells; CR1 on white blood cells is similar in size (and >95% identical) to the −200 kDa human CR1, but the CR1-like protein expressed on erythrocytes is much smaller

For example, in chimpanzees and baboons, the two species that have been most thoroughly investigated, the CR1-like proteins have masses of 75 and 65 kDa for erythrocytes, respectively. They are generated by a par-tial duplication of the CR1 gene. These small forms of CR1-like proteins are composed of five or six CCPs, cor-responding to the initial CCPs of CR1. They are followed by one or two other CCPs and GPI-anchor. The small CR1-equivalent on erythrocytes is derived from a partial duplication of the gene encoding CR1. These primate erythrocyte forms do not contain a site 2. However, this is compensated for by modifications of site 1 that allow binding of both C3b and C4b.(66)

X-ray scattering analyses of sCR1(1)

The extra-cellular component of CR1 is comprised of 30 short complement regulator (SCR) domains,(63) whereas complement receptor type 2 (CR2) has 15 SCR domains and factor H (FH) has 20 SCR domains. A recent report by Furtado et al. (2008) showed that the domain arrange-ment of a soluble form of CR1 (sCR1) was studied by X-ray scattering and analytical ultracentrifugation. The radius of gyration RG of sCR1 of 13.4(±1.1) nm is not much greater than those for CR2 and FH, and its RG/R0 anisotropy ratio is 3.76, compared to ratios of 3.67 for FH and 4.1 for CR2.

Unlike CR2, but similar to FH, two cross-sectional RG ranges were identified that gave RXS values of 4.7(±0.2) nm and 1.2(±0.7) nm, respectively, showing that the SCR domains adopt a range of conformations including folded- back ones. The distance distribution function P(r) showed that the most commonly occurring dis-tance in sCR1 is at 11.5 nm. Its maximum length of 55 nm is less than double those for CR2 or FH, even though sCR1 has twice the number of SCR domains compared to CR2 sedimentation equilibrium experiments gave a mean molecular weight of 235 kDa for sCR1. This is consistent with the value of 245 kDa calculated from its composition including 14 N-linked oligosaccharide sites, and confirmed that sCR1 is a monomer in solution.

Sedimentation velocity experiments gave a sedimenta-tion coefficient of 5.8 S. From this, the frictional ratio (f/f0) of sCR1 was calculated to be 2.29, which is greater than those of 1.96 for CR2 and 1.77 for FH.

The constrained scattering modeling of the sCR1 solu-tion structure starting from homologous SCR domain structures generated 5000 trial conformationally ran-domized models, 43 of which gave good scattering fits to show that sCR1 has a partly folded-back structure. Furtado et al. conclude that the inter-SCR linkers show structural features in common with those in FH, but dif-fer from those in CR2, and the SCR arrangement in CR1 will permit C3b or C4b to access all three ligand sites.(1)

The genetic polymorphisms of complement receptor type 1 (CR1)

The complement receptor 1 gene is located on the Chromosome 1 at the locus 1q32.(63) It has been mapped to the region of regulators of complement activation (RCA) gene cluster.(64) Various gene polymorphisms have been studied for CR1. Three types of genetic polymor-phisms of human CR1have been identified.(65,66) The gene variance may range from grossly molecular weight alter-ing insertion-deletion polymorphisms to the minimally protein altering intronic or exonic single nucleotide polymorphisms that affect the density of CR1 molecules on the cell surface. In addition to the above two, there is another set of polymorphisms, which are generated by single nucleotide polymorphisms and alter the CR1 pro-tein to generate a separate group of blood group antigen variants, the Knops blood group antigens.

The first discovered was a size variation, created by LHR duplications and deletions.(66) The CR1 protein has four known allotypic variants varying in size from 160 kDa to 250 kDa.(32–34) These are not post-translational modifications as their unglycosylated primary transcript possesses the same variation(67)

The genomic difference between each allotype ranges from 1.3 to 1.5 kb, equivalent to a single LHR.(68,69) The insertion-deletion mechanisms due to unequal crossing over of chromosomes have been con-sidered responsible for such a variation.(69) The CR1-C (160 kDa), CR1-A (190 kDa), CR1-B (220 kDa) and CR1-D (250 kDa) are the four variants as analyzed on SDS-PAGE under non-reducing conditions.(70) The LHR repeats give an additional C3b binding site to the larger variants (CR1-B and CR1-D) while the small CR1-C has only one such site, the functional implication, however, is not clear.

The most frequent alleles are the CR1-A (F allotype) and the CR1-B (S allotype) followed by the CR1-C. CR1-D allele is very rare.(33,71) The gene frequencies of CR1-A and CR1-B are 0.87 and 0.11 in Caucasians, 0.82 and

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 529

0.11 in African Americans, 0.89 and 0.11 in Mexicans(71) and 0.916 and 0.084 in Asian Indians(72) AA and AB are the most prevalent genotypes in all the populations studied.

In addition, the Q981H polymorphism(73) is charac-terized by an exonic SNP leading to the replacement of a single guanine with thymine (G3093T). The poly-morphism is in the binding sites of ECR1 ligands(74) and has drawn special attention in relation to the severity of Plasmodium falciparum malaria.(75)

The second type of variation is a Hind III restriction-fragment-length polymorphism that correlates with CR1 copy number on erythrocytes. Homozygotes for the L (low expression) allele usually express fewer than 200 copies of CR1, homozygotes for the H (high expression) allele express several times this number, and heterozy-gotes are intermediate.(66)

Various polymorphisms have been studied for their ability to alter the density of erythrocyte CR1 on the cell membranes. There might be a 10-fold difference between the ECR1 in different individuals owing to their specific genotypic composition.(76) The levels of CR1 on the leu-cocytes do not show variability like ECR1.(77) The Hind III restriction fragment length polymorphism (Hind III RFLP) corresponds to a single nucleotide polymorphism in the intron 27(with a single base substitution, T520C) of the CR1 gene.(78) The Hind III 7.4 kb genomic fragment, associated with high expression allele (H allele), and the 6.9 kb Hind III genomic fragment, associated with low expression allele (L allele).

Presence of several exonic single nucleotide poly-morphisms (SNPs)(74) (Table 1), in linkage disequilib-rium with the Hind III RFLP polymorphism is suggested as a probable mechanism for alteration of ECR1 by the intronic Hind III RFLP. The H allele is now considered to contain G3093, A3650, and C5507 (encoding Gln981, His1167, and Pro1786); and the L allele is considered to contain T3093, G3650, and G5507 (encoding His981, Arg1167, and Arg1786). These variances in the amino acid composition are suggested to influence the stability of CR1 protein.

The ECR1 levels show a trimodal distribution, high expression phenotype (–1000 molecules per cell) in association with the homozygous H, intermediate expression with heterozygous HL and low expression (~100molecules per cell) with homozygous L genotype in Caucasians(74) and Indians(72) but not in Africans.(79)

The third polymorphism is represented by the Knops blood-group system.(66) The various antigens studied under this grouping include the allelic pairs Kna/Knb (Knops), McCa/McCb (McCoy), Sla/Vil (Swain-Langley/Villien) and Yka (York)(80,81) These antigens were identi-fied by the occurrence of high avidity non-complement fixing and non-hemolyzing antibodies in the circulation. Subsequently, it was identified that the corresponding antigens were present on the CR1 molecule(82,83) and the genes coding them were located in the LHR-D region of the CR1 gene.(84,85)

Recently Sla has been sub-classified into various conformational variants.(86) The various Knops blood group genotypes are generated by single nucleotide polymorphism in exon 29 (SCR-25).(87) The identifica-tion of Knops blood group antigens as CR1 phenotypes was indicated by the discovery that the serologically null phenotype, the ‘Helgeson phenotype’ of the Knops blood group showed reduced CR1 levels.(88) The asso-ciation of Knops blood groups with malaria and various inflammatory disorders is a topic of interest for many present day researches.

The expression of complement receptor type 1 (CR1)

Fragments of complement component C3 generated upon activation of the cascade play an important role in the induction and regulation of immune responses. Receptors interacting with various fragments of this ver-satile complement protein are expressed on a wide vari-ety of cell types including lymphocytes, macrophages, erythrocytes, granulocytes, dendritic cells, follicular dendritic cells and certain type of mast cells.

My previously published review(10) brings forth a composite view of the current understanding on the expression of human, sheep and murine CR2. In mice, both receptors CR1 and CR2 (mCR1/2) are encoded by the same locus and the two different products are gener-ated by alternative splicing of the mRNA.(89) Receptors produced in this way are different only in the number of short consensus repeats they contain. In humans CR1 and CR2 are coded by different genes on different loci, consequently these receptors may exert different functions.

Additionally, expression of CR2 on follicular den-dritic cells (DC) is required for B cell survival within the germinal center, affinity maturation, and the

Table 1. Genetic polymorphisms in the coding sequence of CR1.(74)

Nucleotide Exon Amino acid SCR

A207G 2 Silent in Glu19 1

T981C 6 Silent in Pro277 5

A1356G 9 Silent in Gly402 7

A1360G 9 T404A 7

T2078C 13 I1643T 10/11

T2367C 14 Silent in Tyr739 12

G3093T 19 Q981H 16

A3650G 22 H1167R 19

A4870G 29 I1574V 25

C5507G 33 P1786R 28

C5654T 34 T1835I 29

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

530 Liu and Niu

establishment of B cell memory.(47,90–92) In addition, CR1 (CD35), a type I integral membrane protein that binds C3b, C4b, and C1q, and MBL, also plays a role in estab-lishment of B cell responses.(5,93,94) This glycoprotein is expressed on all peripheral blood cells in humans with the exception of platelets, natural killer cells and most T cells.(93,95)

In primates, CR1 expression on erythrocytes con-tributes to immune complex clearance and transfer of C3b-opsonized antigens to splenic and hepatic macrophages.(96,97) In mice, CR1 is expressed as an alter-native splice product of the Cr2 gene and is restricted to B cells and follicular dendritic cells.(47,98–100) Profound defects in humoral immunity have been observed in CR1/CR2-/- mice(39,42,101,102) with little effect on T cell activity.(103,104) CR1/CR2-mediated antigen trapping on follicular dendritic cells enhances antigen presentation to B cells, and is required for both primary and second-ary humoral responses.(47,58,105)

Other factors on CR1 expression

With increasing realization that there is a close relation-ship between the disease pathology and the level of CR1 expression, it is desired that the factors that contribute to the maintenance of the normal CR1 levels and its modulation in the disease states be unraveled. The effect of several cytokines in this context is being investigated. Depending on the type of the cell, variable effects have been observed.(106)

It is observed that TNF-, TNF- and IL-4 increased CR1 levels on monocytes. The chemotactic peptides were shown to upregulate CR1 in different cells tested.(107) In a recent investigation it was found that IFN- increased the levels of CR1 transcript in the neutrophils of healthy individuals and patients with SLE(108) IL-4 and IL-10 sup-pressed the effects of IFN- on CR1 expression.

While elucidating the factors responsible for the decline of CR1 transcript in SLE, it was found that serum opsonized immune complexes reduced the lev-els of CR1 transcript in the neutrophils from the healthy individuals as well those from the patients.(108) TNF- however, had no effect on NCR1 expression. Immune complexes, if not opsonized, had no direct effect on CR1 expression, but it suppressed the IFN--induced CR1 expression. With these findings, immune com-plexes have emerged as the major factor responsible for decline of NCR1 in SLE.(108) Studies on different cell lines have shown retinoic acid to up-regulate CR1gene expression.(109)

An interesting relationship between CR1 Hind III pol-ymorphism and the modulation of NCR1 levels by IFN- and immune complexes have been observed(110). It was observed that the HL genotype was more responsive to either the up-regulation or down-regulation of NCR1

levels by IFN- or immune complexes, respectively. This gives some clue to why in Asian Indians L allele appeared protective against SLE.

The biological functions of complement receptor type 1 (CR1)

CR1 acts as a complement inhibitor by reversibly bind-ing to C3b and C4b and inactivating the C3 and C5 con-vertases, multi-protein complexes that include C3b and C4b, and by promoting the dissociation of the catalytic subunits C2a or Bb (decay accelerating activity). CR1 also serves as a necessary cofactor in the proteolytic cleavage of C3b and C4b by factor I (cofactor activity).(66) Most CR1 is found at low levels on erythrocytes where it binds to complement-fixed immune complexes, namely those with covalently bound C3b and C4b, to facilitate their clearance from the circulation.

The interaction of leukocyte CR1 with C3b and C4b deposited on antigen, bacteria, viruses, or other foreign surfaces leads to the proteolysis of C3b and C4b by fac-tor I to iC3b and iC4b, and subsequently to C3d and C4d, which are ligands for other complement recep-tors including CR2, CR3 and CR4. The overall effect of the decay accelerating and factor I cofactor activities of CR1 is a downregulation of the classical, alternative and lectin complement pathways and a channeling of the immune response to foreign antigen to appropriate cell types bearing other complement receptors.(1)

The biological function of CR1 varies with the cell type on which it is expressed(1) CR1 on erythrocytes is the major cellular attachment site for large circulating immune complexes that have fixed complement, fol-lowing which this cell transports complexes to the liver and spleen for further processing and disposal. CR1 on monocytes, macrophages, and neutrophils promotes the phagocytosis of C3-bound targets. CR1 on FDC is believed to play a role in the trapping of immune com-plexes within germinal centers in lymphoid organs, and CR1 on B cells regulates B cell activation in addi-tion to facilitating antigen binding and presentation to T cells.(48)

Biological functions of the cell surface complement receptor type 1

The Complement receptor type 1 has multiple actions that define its role as a complement regulatory pro-tein. The CR1 molecule acts as a receptor for C3b and C4b thereby destabilizing and enhancing the decay of Classical Pathway C3 (C4b2a) and C5 (C4b2a3b) covertases and Alternate Pathway C3 (C3bCBb) and C5 (C3bBb3b) covertases.(111) Another prominent func-tion is its activity as the cofactor for Factor I-mediated

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 531

inactivation of C3b(112) and C4b(113) acting as another step in the regulation of the alternate pathway. The role of CR1 as a MBL receptor(5) may be involved in regulating the MBL pathway of complement activation.

The erythrocyte CR1 (ECR1) is able to bind to C3b/C4b opsonized immune complexes(114) and localize them on the erythrocytic membrane. The immune complexes thus trapped are transferred to the macrophages in the liver and spleen.(115,116) The Kupffer cells and other large phagocytes of the liver and spleen engulf and metabo-lize the immune complexes.(117,118)

Various immune complexes and particles coated with polymeric C3b can be recognized by the CR1 molecules on the polymer-phonuclear (PMN) cells and mono-cytes particularly those in the clathrin coated pits.(119,120) The Fc-gamma receptors and CR1 operate in synergism to promote uptake of particles opsonized by immu-noglobulins and complement proteins. The particles are internalized and destroyed in the lysosomes.(121,122) The C3b receptor molecules are present as cytoplas-mic secretory vesicles (demonstrated in neutrophils) which are translocated to the plasma membrane on cell activation.(123)

Although the presence of CR1 on CD4+ and CD8+ T lymphocytes has been proved(59) the functional signifi-cance of the same is not clear. Increased expression of CR1 on poly clonally activated T cells had been demon-strated. This suggested a possible role of CR1 in T-cell mediated immune regulation.(126)

Biological functions of soluble CR1

The sCR1 appears to have highly efficacious comple-ment regulatory and anti-inflammatory activities.(127) The physiological plasma levels of sCR1, however, are too low to have any significant functional role.(128) Soluble CR1 had been isolated from the bronchoal-veolar lavage of the patients with inflammatory lung disorders(129) and from the plasma of the patients suffer-ing from hepatic and renal failure and lymphomas and leukemias.(128) Increased levels of sCR1 in plasma had also been reported in SLE and glomerulonephritis.(130) It is hypothesized that sCR1 is a locally active molecule. sCR1 is envisaged as a molecule of choice for various novel CR1-based therapeutic strategies.

Conclusion

In recent years, foundational work on the relationship of complement C3, CD19, CR2/CD21 and CR1/CD35 has resulted in a growing appreciation of the importance of complement for the induction and recall of antibody responses to foreign antigens.(131) Complement recep-tor type 1 (CR1), the receptor for C3b/C4bcomplement

peptides,(36,38) has drawn substantial attention from sci-entists because of its gene polymorphisms, structural variances, diversity of functions and disease associa-tions. Complement receptor 1 genetics and its relation-ship with disease processes open up a fascinating area of research. Extensive research has brought insight to the potential of CR1 as a diagnostic and prognostic marker is being increasingly realized.

In addition, more than its capability of binding C4b and C3b fragments of complement, functioning on phagocytic cells to facilitate the ingestion by these cells of particles that have activated complement. On eryth-rocytes, CR1 binds C3b or C4b that has been fixed on immune complexes, and these are then carried to the liver and spleen where macrophages bind, internalize and destroy the complexes.(132–135) The rate of clearance from the circulation is directly related to the number of CR1 per erythrocyte.(38,136–139) Therefore, the several aspects of CR1 (such as the structure, genetic polymor-phisms, expression and biological functions of CR1) as assembled in this review may intensify further investiga-tions on CR1 in achieving better insight into its structural variances, diversity of functions, disease associations and therapeutic implications

Although much progress has been made in the char-acterization of the structure/function relationship of individual C components, there are many more funda-mental questions to address: importantly, we will have to decipher when, where and with what associated molecules do C molecules engage to promote either an anti-inflammatory or rather a full pro-inflammatory response.

Moreover, we will have to decipher the ill-character-ized cross talks between the C and other innate immune signaling pathways (e.g., TLR and scavenger receptors) to promote the safe clearance of the intruders. The answers to these questions will greatly expand our understand-ing of the complex interactions between pathogens and the host immune response.(10,11)

It is expected that future studies will reveal more and more complement-mediated links between innate and adaptive immunity.(140) Future challenges include understanding how complement and complement receptors contribute to the prevention or, conversely, exacerbation of autoimmunity in particular contexts. In addition, the molecular interplay of the BCR, CD19, CR2/CD21 and CR1/CD35 remains to be fully resolved. Much work about the functions of these three sorts of CR1/2 needs to be conducted to determine their struc-tural differences and relation to complement biology. A better understanding of complement function and the antibody response holds great promise for the develop-ment of complement (e.g. C3d)-based vaccines as well as modalities for the treatment of B cell-dependent autoimmune disease.(10)

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

532 Liu and Niu

Acknowledgements

This work was supported by the Shandong Natural Science Foundation (grant number: Y2007D47) and the Independent Innovation Fund of Jinan (grant number: 200807071).

Declaration of interest: The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

References

1. Furtado, P.B., Huang, C.Y., Ihyembe, D., et al. The partly folded back solution structure arrangement of the 30 scr domains in human complement receptor type 1 (cr1) permits access to its C3b and C4b ligands. J. Mol. Biol. 2008, 375, 102–118.

2. Ahearn, J.M., Fearon, D.T. Structure and function of the com-plement receptor CR1 (CD35) and CR2 (CD21). Adv. Immunol. 1989, 46, 183–219.

3. Delibrias, C., Fischer, E., Kazatchkine, M.D. Receptors for human C3 fragments. In: Rother, K., Till, G.O., Hansch, G.M. (Eds.), The Complement System. Springer, Berlin: Springer, 1997; 211–220.

4. Wagner, C., Ochmann, C., Schoels, M., et al. The complement receptor 1, CR1 (CD35), mediates inhibitory signals in human T-lymphocytes. Molec. Immunol. 2006, 43, 643–651.

5. Ghiran, I., Barbashov, S.F., Klickstein, L.B., et al. Complement receptor 1/CD35 is a receptor for mannan-binding lectin. J. Exp. Med. 2000, 192, 1797–1808.

6. Tas, S., Klickstein, L.B., Barbashov, S.F., Nicholson-Weller, A. C1q and C4b bind simultaneously to CR1 and additively support erythrocyte adhesion. J. Immunol. 1999, 136, 5056–5063.

7. Walport, M. J. Complement. Second of two parts. N. Engl. J. Med. 2001, 344, 1140–1144.

8. Walport, M. J. Complement. First of two parts. N. Engl. J. Med. 2001, 344, 1058–1066.

9. Michael Kirschfink, M., Mollnes, T.E. Modern Complement Analysis. Clin.Diag. Lab. Immunol. 2003, 10(6), 982–989.

10. Liu, D., Zhu, J.Y., Niu, Z.X. Molecular structure and expression of anthropic, ovine, and murine forms of complement receptor type 2. Clin. Vacc. Immunol. 2008, 15(6), 901–910.

11. Gasque, P. Complement: A unique innate immune sensor for danger signals. Molec. Immunol. 2004, 41, 1089–1098.

12. Farries, T.C., Lachmann, P.J., Harrison, R.A. Analysis of the inter-actions between properdin, the third component of comple-ment (C3), and its physiological activation products. Biochem. J. 1988, 252, 47–54.

13. Gewurz, H., Ying, S.C., Jiang, H., Lint, T.F. Nonimmune activa-tion of the classical complement pathway. Behring Inst. Mitt. 1993, 93, 138–147.

14. Kilpatrick, J.M.,Volanakis, J.E. Molecular genetics, structure, and function of C-reactive protein. Immunol. Res. 1991, 10, 43–53.

15. Armstrong, P.B., Armstrong, M.T., Quigley, J.P. Involvement of alpha 2-macroglobulin and C-reactive protein in a complement-like hemolytic system in the arthropod, Limulus polyphemus. Mol. Immunol. 1993, 30, 929–934.

16. Liu, T.Y., Minetti, C.A., Fortes-Dias, C.L., et al. C-reactive pro-teins, limunectin, lipopolysaccharide-binding protein, and coagulin, molecules with lectin and agglutinin activities from Limulus polyphemus. Ann. N.Y. Acad. Sci. 1994, 712, 146–154.

17. Tharia, H.A., Shrive, A.K., Mills, J.D., et al. Complete cDNA sequence of SAP-like pentraxin from Limulus polyphemus: Implications for pentraxin evolution. J. Mol. Biol. 2002, 316, 583–597.

18. Bharadwaj, D., Stein, M.P., Volzer, M., et al. The major recep-tor for C-reactive protein on leukocytes is fcgamma receptor II. J. Exp. Med. 1999, 190, 585–590.

19. Jack, D.L., Klein, N.J., Turner, M.W. Mannose-binding lec-tin: Targeting the microbial world for complement attack and opsonophagocytosis. Immunol. Rev. 2001,180, 86–99.

20. Petersen, S.V., Thiel, S., Jensenius, J.C. The mannan-binding lectin pathway of complement activation: Biology and disease association. Mol. Immunol. 2001,38, 133–149.

21. Fujita, T., Matsushita, M., Endo, Y. The lectin-complement pathway—Its role in innate immunity and evolution. Immunol. Rev. 2004, 198, 185–202.

22. Matsushita, M., Endo, Y., and Fujita, T. Complement-activating complex of ficolin and mannose-binding lectin-associated ser-ine protease. J. Immunol. 2000, 164, 2281–2284.

23. Reid, K.B.M., Turner, M.W. Mammalian lectins in activation and clearance mechanisms involving the complement system. Springer Semin. Immunopathol. 1994, 15, 307–325.

24. Calender, A., Billaud, M., Aubry, J.P., et al.. Epstein-Barr virus EBV induces expression of B-cell activation markers on in vitro infection of EBV-negative B-lymphoma cells. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 8060–8064.

25. Malhotra, R., Wormald, M.R., Rudd, P.M., et al. Glycosylation changes of IgG associated with rheumatoid arthritis can activate complement via the mannose-binding protein. Nat. Med. 1995, 1, 237–243.

26. Selander, B., Martensson, U., Weintraub, A., et al. Mannanbinding lectin activates C3 and the alternative complement pathway without involvement of C2. J. Clin. Invest. 2006, 116, 1423–1434.

27. Dahl, M.R., Thiel, S., Matsushita, M., et al. MASP-3 and its association with distinct complexes of the mannan-binding lectin complement activation pathway. Immunity 2001, 15, 127–135.

28. Rossi, V., Cseh, S., Bally, I., et al. Substrate specificities of recom-binant mannan-binding lectin-associated serine proteases-1 and -2. J. Biol. Chem. 2001, 276, 40880–40887.

29. Stover, C., Endo, Y., Takahashi, M., et al. The human gene for mannanbinding lectin-associated serine protease-2 (MASP-2), the effector component of the lectin route of complement acti-vation, is part of a tightly linked gene cluster on chromosome 1p36.2-3. Genes Immun. 2001, 2, 119–127.

30. Boackle, S.A. Complement and autoimmunity. Biomed.Pharmacother. 2003, 57, 269–273.

31. Petty, H.R., Worth, R.G., Todd, R.R. Interactions of integrins with their partner proteins in leukocyte membranes. Immunol. Res. 2002, 25, 75– 95.

32. Dykman, T.R., Cole, J.L., Iida, K., Atkinson, J.P. Polymorphism of human erythrocyte C3b/C4b receptor, Proc. Natl. Acad. Sci. U.S.A. 1983, 80,1698–1702.

33. Dykman, T.R., Hatch, J.A., Aqua, M.S., Atkinson, J.P. Polymorphism of the C3b/C4b (CR1) receptor: Characterization of a fourth allele, J. Immunol. 1985, 134,1787.

34. Dykman, T.R., Hatch, J.A., Atkinson, J.P. Polymorphism of the human C3b/C4b receptor. Identification of a third allele and analysis of receptor phenotypes in families and patients with systemic lupus erythematosus, J. Exp. Med. 1984, 159, 691–703.

35. Wong, W.W., Wilson, J.G., Fearon, D.T. Genetic regulation of a structural polymorphism of human C3b receptor, J. Clin. Invest. 1983, 72, 685–693.

36. Fearon, D.T., Ahearn, JM. Complement receptor type 1 (C3b/C4b receptor; Cd35) and complement receptor type 2 (C3d/Epstein-Barr virus receptor; CD21), Curr. Top. Microbiol. Immunol. 1989, 153,83–98.

37. Fearon, D.T., Wong, W.W. Complement ligand-receptor interac-tions that mediate biological responses. Annu. Rev. Immunol. 1983, 1, 243–271.

38. Funkhouser, T., Vik, D.P. Promoter activity of the 5’ flanking region of the complement receptor type 1 (CR1) gene: Basal and induced transcription. Biochim. Biophys. Acta 2000, 1490, 99–105.

39. Ahearn, J.M., Fischer, M.B., Croix, D.A., et al. Disruption of the Cr2 locus results in a reduction in B-1a cells and in an impaired B cell response to T-dependent antigen. Immunity 1996, 4, 251–262.

40. Carroll, M.C. The role of complement in B cell activation and tolerance. Adv. Immunol. 2000, 74, 61–88.

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 533

41. Haas, K.M., Hasegawa, M., Steeber, D.A., et al. Complement receptors CD21/35 link innate and protective immunity during Streptococcus pneumoniae infection by regulating IgG3 anti-body responses. Immunity 2002, 17, 713–723.

42. Molina, H., Holers, V.M., Li, B., et al. Markedly impaired humoral immune response in mice deficient in complement receptors 1 and 2. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 3357–3361.

43. Fairweather, D., Frisancho-Kiss, S., Njoku, D.B., et al. Complement receptor 1 and 2 deficiency increases coxsackievi-rus B3- induced myocarditis, dilated cardiomyopathy, and heart failure by increasing macrophages, IL-1beta, and immune com-plex deposition in the heart. J. Immunol. 2006, 176, 3516–3524.

44. Kaya, Z., Afanasyeva, M., Wang, Y., et al. Contribution of the innate immune system to autoimmune myocarditis: A role for complement. Nat. Immunol. 2001, 2, 739–745.

45. Pratt, J.R., Abe, K., Miyazaki, M., et al. In situ localization of C3 synthesis in experimental acute renal allograft rejection. Am. J. Pathol. 2000, 157, 825–831.

46. Helmy, K.Y., Katschke, K.J., Gorgani, N.N., et al. CRIg: A mac-rophage complement receptor required for phagocytosis and circulating pathogens. Cell 2006, 124, 915–927.

47. Avirutnan, P., Mehlhop, E., Diamond, M.S. Complement and its role in protection and pathogenesis of flavivirus infections. Vaccine 2008, 26S 1100–1107.

48. Holers, V.M. The complement system as a therapeutic target in autoimmunity. Clin. Immunol. 2003, 107, 140–151.

49. Thurman, J.M., Holers, V.M. The central role of the alternative complement pathway in human disease. J. Immunol. 2006, 176, 1305–1310.

50. Fingeroth, J.D., Weis, J.J., Tedder, T.F., et al. Epstein–Barr virus receptor of human B lymphocytes is the C3d receptor CR2. Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 4510–4514.

51. Holers, V.M. Complement receptors and the shaping of the nat-ural antibody repertoire. Springer Sem. Immunopathol. 2005, 26, 405–423.

52. Holers, V.M., Carroll, M.C., Holers, V.M. Innate Autoimmun. Adv. Immunol. 2005, 86, 137–157.

53. Reid, R.R., Woodstock, S., Shimabukuro-Vornhagen, A., et al. Functional activity of natural antibody is altered in Cr2-deficient mice. J. Immunol. 2002, 169, 5433–5440.

54. Zhang, M., Alicot, E.M., Chiu, I., et al. Identification of the tar-get self antigens in reperfusion injury. J. Exp. Med. 2006, 203, 141–152.

55. Zhang, M., Austen Jr., W.G., Chiu, I., et al. Identification of a specific self-reactive IgM antibody that initiates intestinal ischemia/reperfusion injury. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 3886– 3891.

56. Erdei, A., Prechl, J., Isaak, A., Molnar, E. Regulation of B-cell activation by complement receptors CD21 and CD35. Curr. Pharm. Des. 2003, 9, 1849– 1860.

57. Fearon, D.T. The complement system and adaptive immunity. Immunology 1998, 10, 355–361.

58. Fang, Y., Xu, C., Fu, Y.X., et al. Expression of complement recep-tors 1 and 2 on follicular dendritic cells is necessary for the gen-eration of a strong antigen-specific IgG response. J. Immunol. 1998, 160, 5273–5279.

59. Rodgaard, A., Christensen, L.D., Thomsen, B.S., et al. Complement receptor type 1 (CR1, CD35) expression on periph-eral T lymphocytes: Both CD4- and CD8-positive cells express CR1. Complement. Inflamm. 1991, 8, 303–309.

60. Weiss, L., Fischer, E., Haeffner-Cavaillon, N., et al. The human C3b receptor (CR1). Adv. Nephrol. Necker Hosp. 1989, 18, 249–269.

61. Danielsson, C., Pascual, M., French, L., et al. Soluble comple-ment receptor type 1 (CD35) is released from leukocytes by sur-face cleavage. Eur. J. Immunol. 1994, 24, 2725–2731.

62. Pascual, M., Steiger, G., Sadallah, S., et al. Identification of membrane-bound CR1 (CD35) in human urine: Evidence for its release by glomerular podocytes. J. Exp. Med. 1994, 179, 889–899.

63. Weis, J.H., Morton, C.C., Bruns, G.A.P., et al. A complement receptor locus: Genes encoding C3b/C4b receptor and Cad/Epstein-Barr virus receptor map to 1g32. J. Immunol. 1987, 138, 312.

64. Rodriguez de Cordoba, S., Rubinstein, P. Quantitative variations of the C3b/C4b receptor (CR1) in human erythrocytes are con-trolled by genes within the regulator of complement activation (RCA) gene cluster. J. Exp. Med. 1986, 164, 1274–1283.

65. Krych-Goldberg, M., Atkinson, J.P. Structure-function relation-ships of complement receptor type 1. Immunol. Rev. 2001, 180, 112–122

66. Krych-Goldberg, M., Moulds, J.M., Atkinson, J.P. Human com-plement receptor type 1 (CR1) binds to a major malarial adhesin. TRENDS in Molec. Med. 2002, 8 (11), 31–37.

67. Lublin, D.M., Griffith, R.C., Atkinson, J.P. Influence of glycosyla-tion on allelic and cell-specific Mr variation, receptor process-ing, and ligand binding of the human complement C3b/C4b receptor. J. Biol. Chem. 1986, 261, 5736–5744.

68. Wong, W.W., Kennedy, C.A., Bonaccio, E.T., et al. Analysis of multiple restriction fragment length polymorphisms of the gene for the human complement receptor type I. Duplication of genomic sequences occurs in association with a high molecular mass receptor allotype. J. Exp. Med. 1986, 164, 1531–1546.

69. Holers, V.M., Chaplin, D.D., Leykam, J.F., et al. Human comple-ment C3b/C4b receptor (CR1) mRNA polymorphism that corre-lates with the CR1 allelic molecular weight polymorphism. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 2459–2463.

70. Van Dyne, S., Holers, V.M., Lublin, D.M., et al. The polymor-phism of the C3b/C4b receptor in the normal population and in patients with systemic lupus erythematosus. Clin. Exp. Immunol. 1987, 68, 570–579.

71. Moulds, J.M., Reveille, J.D., Arnett, F.C. Structural polymor-phisms of complement receptor 1 (CR1) in systemic lupus ery-thematosus (SLE) patients and normal controls of three ethnic groups. Clin. Exp. Immunol. 1996, 105, 302–305.

72. Katyal, M., Sivasankar, B., Ayub, S., Das, N. Genetic and struc-tural polymorphism of complement receptor 1 in normal Indian subjects. Immunol. Lett. 2003, 89, 93–98.

73. Birmingham, D.J., Chen, W., Liang, G., et al. A CR1 polymor-phism associated with constitutive erythrocyte CR1 levels affects binding to C4b but not C3b. Immunology 2003, 108, 531–538.

74. Xiang, L., Rundles, J.R., Hamilton, D.R., Wilson, J.G. Quantitative alleles of CR1: Coding sequence analysis and comparison of haplotypes in two ethnic groups. J. Immunol. 1999, 163, 4939–4945.

75. Thomas, B.N., Donvito, B., Cockburn, I., et al. A complement receptor-1 polymorphism with high frequency in malaria endemic regions of Asia but not Africa. Genes Immun. 2005, 6, 31–36.

76. Wilson, J.G., Wong, W.W., Schur, P.H., Fearon, D.T. Mode of inheritance of decreased C3b receptors on erythrocytes of patients with systemic lupus erythematosus. N. Engl. J. Med. 1982, 307, 981–986.

77. Wilson, J.G., Ratnoff, W.D., Schur, P.H., Fearon, D.T. Decreased expression of the C3b/C4b receptor (CR1) and the C3d receptor (CR2) on B lymphocytes and of CR1 on neutrophils of patients with systemic lupus erythematosus. Arthritis Rheum. 1986, 29, 739–747.

78. Wilson, J.G., Wong, W.W., Murphy III, E.E., et al. Deficiency of the C3b/C4b receptor (CR1) of erythrocytes in systemic lupus erythematosus: Analysis of the stability of the defect and of a restriction fragment length polymorphism of the CR1 gene. J. Immunol. 1987, 138, 2708–2710.

79. Rowe, J.A., Raza, A., Diallo, D.A., et al. Erythrocyte CR1 expres-sion level does not correlate with a HindIII restriction fragment length polymorphism in Africans: Implications for studies on malaria susceptibility. Genes Immun. 2002, 3, 497–500.

80. Moulds, M.K. Serological investigation and clinical signifi-cance of high-titer, low-avidity (HTLA) antibodies. Am. J. Med. Technol. 1981, 47, 789–795.

81. Daniels, G.L., Anstee, D.J., Cartron, J.P., et al. Blood group ter-minology 1995. ISBT working party on terminology for red cell surface antigens. Vox Sang 1995, 69, 265–279.

82. Moulds, J.M., Nickells, M.W., Moulds, J.J., et al. The C3b/C4b receptor is recognized by the Knops, McCoy, Swain-Langley, and York blood group antisera. J. Exp. Med. 1991, 173, 1159–1163.

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

534 Liu and Niu

83. Rao, N., Ferguson, D.J., Lee, S.F., Telen, M.J. Identification of human erythrocyte blood group antigens on the C3b/C4b recep-tor. J. Immunol. 1991,146, 3502–3507.

84. Moulds, J.M., Zimmerman, P.A., Doumbo, O.K., et al. Molecular identification of Knops blood group polymorphisms found in long homologous region D of complement receptor 1. Blood 2001, 97, 2879–2885.

85. Tamasauskas, D., Powell, V., Schawalder, A., Yazdanbakhsh, K. Localization of Knops system antigens in the long homolo-gous repeats of complement receptor 1. Transfusion 2001, 41, 1397–1404.

86. Moulds, J.M. Understanding the Knops blood group and its role in malaria. Vox Sang 2002, 83 (Suppl. 1), 185–188.

87. Moulds, J.M., Thomas, B.J., Doumbo, O., et al. Identification of the Kna/Knb polymorphism and a method for Knops genotyp-ing. Transfusion 2004, 44, 164–169.

88. Moulds, J.M., Moulds, J.J., Brown, M., Atkinson, J.P. Antiglobulin testing for CR1-related (Knops/McCoy/Swain–Langley/York) blood group antigens: Negative and weak reac-tions are caused by variable expression of CR1. Vox Sang 1992, 62, 230–235.

89. Andrásfalvy, M., Prechl, J., Hardy, T., et al. Mucosal type mast cells express complement receptor type 2 (CD21). Immunol. Lett. 2002, 82, 29–34.

90. Barrington, R.A., Pozdnyakova, O., Zafari, M.R., et al. Blymphocytememory: Role of stromal cell complement and FcgammaRIIB receptors. J. Exp. Med. 2002, 196(9), 1189–99.

91. Fischer, M.B., Goerg, S., Shen, L., et al. Dependence of germinal center B cells on expression of CD21/CD35 for survival. Science 1998, 280(5363), 582–5.

92. Wu, X., Jiang, N., Fang, Y.F., et al. Impaired affinity maturation in Cr2−/− mice is rescued by adjuvants without improvement in germinal center development. J. Immunol. 2000, 165(6), 3119–27.

93. Fearon, D.T. Identification of the membrane glycoprotein that is the C3b receptor of the human erythrocyte, polymorphonu-clear leukocyte, B lymphocyte, and monocyte. J. Exp. Med. 1980, 152(1), 20–30.

94. Klickstein, L.B., Barbashov, S.F., Liu, T., et al. Complement receptor type 1 (CR1 CD35) is a receptor for C1q. Immunity 1997, 7(3), 345–55.

95. Tedder, T.F., Fearon, D.T., Gartland, G.L., Cooper, M.D. Expression of C3b receptors on human be cells and myelo-monocytic cells but not natural killer cells. J. Immunol. 1983, 130(4), 1668–73.

96. Bogers, W.M., Stad, R.K., Van Es, L.A., Daha, M.R. Both Kupffer cells and liver endothelial cells play an important role in the clearance of IgA and IgG immune complexes. Res. Immunol. 1992, 143(2), 219–24.

97. Craig, M.L., Bankovich, A.J., McElhenny, J.L., Taylor, R.P. Clearance of antidouble-stranded DNA antibodies: The natural immune complex clearance mechanism. Arthritis Rheum. 2000, 43(10), 2265–75.

98. Kinoshita, T., Takeda, J., Hong, K., et al. Monoclonal antibodies to mousecomplement receptor type 1 (CR1). Their use in a dis-tribution study showing that mouse erythrocytes and platelets are CR1-negative. J. Immunol. 1988, 140(9), 3066–72.

99. Kurtz, C.B., O’Toole, E., Christensen, S.M., Weis, J.H. The murine complement receptor gene family. IV. Alternative splic-ing of Cr2 gene transcripts predicts two distinct gene products that share homologous domains with both human CR2 and CR1. J. Immunol 1990, 144(9), 3581–91.

100. Molina, H., Kinoshita, T., Inoue, K., et al. A molecular and immunochemical characterization of mouse CR2. Evidence for a single gene model of mouse complement receptors 1 and 2. J. Immunol. 1990, 145(9), 2974–83.

101. Chen, Z., Koralov, S.B., Gendelman, M., et al. Humoral immune responses in Cr2−/− mice: Enhanced affinity maturation but impaired antibody persistence. J. Immunol. 2000, 164(9), 4522–32.

102. Croix, DA, Ahearn, JM, Rosengard, A.M., et al. Antibody response to a T-dependent antigen requires B cell expression of complement receptors. J. Exp. Med. 1996, 183(4), 1857–64.

103. Kopf, M., Abel, B., Gallimore, A., et al. Complement component C3 promotes T-cell priming and lung migration to control acute influenza virus infection. Nat. Med. 2002, 8(4), 373–8.

104. Suresh, M., Molina, H., Salvato, M.S., et al. Complement com-ponent 3 is required for optimal expansion of CD8 T cells during a systemic viral infection. J. Immunol. 2003, 170(2), 788–94.

105. Qin, D., Wu, J., Carroll, M.C., et al. Evidence for an important interaction between a complement-derived CD21 ligand on fol-licular dendritic cells andCD21 on B cells in the initiation of IgG responses. J. Immunol. 1998, 161(9), 4549–54.

106. Limb, G.A., Hamblin, A.S., Wolstencroft, R.A., Dumonde, D.C. Selective upregulation of human granulocyte integrins and complement receptor 1 by cytokines. Immunology 1991, 74, 696–702.

107. Doi, T., Takemura, S., Ueda, M., et al. Suppressed increase of C3 receptors on polymorphonuclear leukocytes by stimulation with C5a in diabetes mellitus. Arerugi 1995, 44, 1223–1228.

108. Arora, V., Mondal, A.M., Grover, R., et al. Modulation of CR1 transcript in systemic lupus erythematosus (SLE) by IFN-gamma and immune complex. Mol. Immunol. 2007, 44, 1722–1728.

109. Mucida, D., Park, Y., Kim, G., et al. Reciprocal TH17 and regu-latory T cell differentiation mediated by retinoic acid. Science 2007, 317, 256–260.

110. Raju, KR., Sivasankar, B., Anand, V., et al. Use of complement receptor 1 (CD35) assay in the diagnosis and prognosis of immune complex mediated glomerulopathies [J]. Asian Pac J Allergy Immunol, 2001, 19(1), 23–27.

111. Iida, K., Nussenzweig, V. Complement receptor is an inhibitor of the complement cascade. J. Exp. Med. 1981, 153, 1138–1150.

112. Ross, G.D., Lambris, J.D., Cain, J.A., Newman, S.L. Generation of three different fragments of bound C3 with purified factor I or serum. I. Requirements for factor H vs. CR1 cofactor activity. J. Immunol. 1982, 129, 2051–2060.

113. Medof, M.E., Nussenzweig, V. Control of the function of sub-strate-bound C4b-C3b by the complement receptor Cr1. J. Exp. Med. 1984, 159, 1669–1685.

114. Yoshida, K., Yukiyama, Y., Miyamoto, T. Interaction between immune complexes and C3b receptors on erythrocytes. Clin. Immunol. Immunopathol. 1986, 39, 213–221.

115. Cosio, F.G., Shen, X.P., Birmingham, D.J., et al. Evaluation of the mechanisms responsible for the reduction in erythrocyte com-plement receptors when immune complexes form in vivo in pri-mates. J. Immunol. 1990, 145, 4198–4206.

116. Craig, M.L., Bankovich, A.J., Taylor, R.P. Visualization of the transfer reaction: Tracking immune complexes from erythro-cyte complement receptor 1 to macrophages. Clin. Immunol. 2002, 105, 36–47.

117. van Es, L.A., Daha, M.R. Factors influencing the endocytosis of immune complexes. Adv. Nephrol. Necker Hosp. 1984, 13, 341–367.

118. Skogh, T., Blomhoff, R., Eskild, W., Berg, T. Hepatic uptake of circulating IgG immune complexes. Immunology 1985, 55, 585–594.

119. Fearon, D.T., Kaneko, I., Thomson, G.G. Membrane distribution and adsorptive endocytosis by C3b receptors on human poly-morphonuclear leukocytes. J. Exp. Med. 1981, 153, 1615–1628.

120. Abrahamson, D.R., Fearon, D.T. Endocytosis of the C3b recep-tor of complement within coated pits in human polymor-phonuclear leukocytes and monocytes. Lab. Invest. 1983, 48, 162–168.

121. Ehlenberger, A.G., Nussenzweig, V. The role of membrane receptors for C3b and C3d in phagocytosis. J. Exp. Med. 1977, 145, 357–371.

122. Schorlemmer, H.U., Hofstaetter, T., Seiler, F.R. Phagocytosis of immune complexes by human neutrophils and monocytes: Relative importance of Fc and C3b receptors. Behring Inst. Mitt. 1984, 88–97.

123. Sengelov, H., Kjeldsen, L., Kroeze, W., et al. Secretory vesicles are the intracellular reservoir of complement receptor 1 in human neutrophils. J. Immunol. 1994, 153, 804–810.

124. Fingeroth, J.D., Heath, M.E., Ambrosino, D.M. Proliferation of resting B cells is modulated by CR2 and CR1. Immunol. Lett. 1989, 21, 291–301.

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.

Complement receptor type 1 535

125. Jozsi, M., Prechl, J., Bajtay, Z., Erdei, A. Complement receptor type 1 (CD35) mediates inhibitory signals in human B lym-phocytes. J. Immunol. 2002, 168, 2782–2788.

126. Rodgaard, A., Thomsen, B.S., Bendixen, G., Bendtzen, K. Increased expression of complement receptor type 1 (CR1, CD35) on human peripheral blood T lymphocytes after polyclo-nal activation in vitro. Immunol. Res. 1995, 14, 69–76.

127. Hamer, I., Paccaud, J.P., Belin, D., et al. Soluble form of comple-ment C3b/C4b receptor (CR1) results from a proteolytic cleav-age in the C-terminal region of CR1 transmembrane domain. Biochem. J. 1998, 329 (Pt 1), 183–190.

128. Pascual, M., Duchosal, M.A., Steiger, G., et al. Circulating soluble CR1 (CD35). Serum levels in diseases and evidence for its release by human leukocytes. J. Immunol. 1993, 151, 1702–1711.

129. Hamacher, J., Sadallah, S., Schifferli, J.A., et al. Soluble com-plement receptor type 1 (CD35) in bronchoalveolar lavage of inflammatory lung diseases. Eur. Respir. 1998, J. 11, 112–119.

130. Das, N., Sivasankar, B., Tiwari, S.C., et al. Modulation of CR1 expression in glomerulonephritis. Int. Immunopharmacol. 2002, 2 (9), 1386.

131. Rickert R. C. Regulation of B lymphocyte activation by com-plement C3 and the B cell coreceptor complex. Curr.Opinion Immunol. 2005, 17, 237–243.

132. Medof, G. M., Oger Prince, J. Kinetics of interaction of immune complexes with complement receptor on human blood cells: Modification of complexes during interaction with red cells, Clin. Exp. Immunol. 1982, 48, 715–725.

133. Cornacoff, J.B., Hebert, L.A., Smead, W.L., et al. Primate erythrocyte-immune complex clearing mechanism. J. Clin. Invest. 1983, 71, 236–247.

134. Schifferli, J., Ng, Y., Estreicher, J., Walport, M. The clearance of tetanus toxoid/anti-tetanus toxoid immune complexes from the circulation of humans. Complement- and erythrocyte comple-ment receptor 1 dependent mechanisms. J. Immunol. 1988, 140, 899–904.

135. Davies, K., Hird, V., Stewart, S., et al. A study of in vivo immune complex formation and clearance in man. J. Immunol. 1990, 144, 4613–4620.

136. Tausk, F.A., McCutchan, J.A., Spechko, P., et al. Altered erythro-cyte C3b receptor expression, immune complexes, and comple-ment activation in homosexual men in varying risk groups for acquired immune deficiency syndrome. J. Clin. Invest. 1986, 78, 977–982.

137. Schifferli, J.A., Ng, Y.C., Paccaud, J.P., Walport, M.J. The role of hypocomplementaemia and low erythrocyte complement receptor type 1 numbers in determining abnormal immune complex clearance in humans. Clin. Exp. Immunol. 1989, 75, 329–335.

138. Davies, K.A., Peters, A.M., Beynon, H.L.C., Walprot, M.J. Immune complex processing in patients with systemic lupus

erythematosus. In vivo imaging and clearance studies. J. Clin. Invest. 1992, 90, 2075–2083.

139. Gibson, N.C., Waxman, F.J. Relationship between immune com-plex binding and release and the quantitative expression of the complement receptor type 1 (CR1, CD35) on human erythro-cytes. Clin. Immunol. Immunopathol. 1994, 70, 104–113.

140. Prechl, J., Erdei, A. Immunomodulatory functions of murine CR1 /2. Immunopharmacology 2000, 49, 117– 124.

141. Fearon, D.T., Carter, R.H. The CD19/CR2/TAPA-1 complex of B-lymphocytes: Linking natural to acquired immunity. Annu. Rev. Immunol. 1995, 13, 127–149.

142. Tedder, T.F., Zhou, L.-J., Engel, P. The CD19/CD21 signal trans-duction complex of B lymphocytes. Immunol. Today 1994, 15, 437–442.

143. Ahearn, J.M., Fischer, M.B., Croix, D.A., et al. Disruption of the Cr2 locus results in a reduction in B-1a cells and in an impaired B cell response to T-dependent antigen. Immunity 1996, 4, 251–262.

144. Carter, R.H., Fearon, D.T. CD19: Lowering the threshold for anti-gen receptor stimulation of B-lymphocytes. Science 1992, 256, 105–107.

145. Boackle, S.A., Holers, V.M., Chen, X., et al. Cr2, a candidate gene in the murine Sle1c lupus susceptibility locus, encodes a dys-functional protein. Immunity 2001, 15, 775–785.

146. Prodeus, A., Goerg, S., Shen, L.-M., et al. A critical role for com-plement in maintenance of self-tolerance. Immunity 1998, 9, 721–731.

147. Wu, X., Jiang, N., Deppong, C., et al. A role for the Cr2 gene in modifying autoantibody production in systemic lupus ery-thematosus. J. Immunol. 2002, 169, 1587–1592.

148. Cherukuru, A., Cheng, P.C., Sohn, H.W., Pierce, S.K. The CD19/CD21 complex functions to prolong B cell antigen receptor sign-aling from lipid rafts. Immunity 2001, 14, 169–179.

149. Cherukuru, A., Pierce, S.K. The role of the CD19/CD21 complex in B cell processing and presentation of complement-tagged antigens. J. Immunol. 2001, 167, 172.

150. Aubry, J.P., Pochon, S., Graber, P., Jansen, K.U., Bonnefoy, J.Y. CD21 is a ligand for CD23 and regulates IgE production. Nature 1992, 358, 505–507.

151. Kishimoto, T.K., O’Connor, K., Lee, A., Roberts, T.M., Springer, T.A. Cloning of the B subunit of the leukocyte adhesion proteins: Homology to an extracellular matrix receptor defines a novel supergene family. Cell 1987, 48, 681–690.

152. Brown, E.J. Complement receptors and phagocytosis. Curr. Opin. Immunol. 1991, 3, 76–82.

153. Rezonnico, R., Chicheportiche, R., Imbert, V., Dayer, J.M. Engagement of CD11b and CD11c beta2 integrin by antibod-ies or soluble CD23 induces IL-1beta production on primary human monocytes through mitogen-activated protein kinase-dependent pathways. Blood 2000, 95, 3877.

Imm

unop

harm

acol

ogy

and

Imm

unot

oxic

olog

y 20

09.3

1:52

4-53

5.D

ownl

oade

d fr

om in

form

ahea

lthca

re.c

om b

y R

yers

on U

nive

rsity

on

05/0

3/13

. For

per

sona

l use

onl

y.