New Developments

Autoimmunity: Present-Day Concepts and Future Prospects

Richard Hong, MD

Introduction The purpose of this brief review is to present new information on the subject of autoimmunity. Although most readers may not be practicing immunologists, the impact pf these newer concepts impinge in a very pervasive way on much of the average practitioner’s clinical experience. Consequently, he or she must incor- porate these principles into an intellectual framework in order to use the most up-to-date diagnostic approaches and help patients make important decisions about newer therapeutic options. I direct most of my attention to what I believe will be the effects of this information on the future practice of medicine.

I confine this review to the inflammatory response generated by the T cells, the role of the major histocompatibility complex (MHC), and potentials for novel therapy.

Inflammation Secondary to Immune System Activation The reaction of the immune system with its antigen results in some sort of bind- ing, either of a T lymphocyte or an immunoglobulin molecule, to the immunogen. This binding alone usually does not result in sufficient neutralization of any po- tential severe or life-threatening consequences. For example, if the antigen is an infectious agent, it must be eliminated or effectively contained or else it will inflict harm on the host. Effective removal of such agents requires an inflammatory re- sponse. For B-cell immunoglobulin responses, the complement system is used. In T-cell reactions, the inflammatory process is mediated by cytokines. Cyto- kines are secreted by the various cells of the immune system as well as resident cells of the tissue involved. They include the interleukins, colony-stimulating fac- tors, tumor necrosis factors (TNF) alpha and beta, interferons, and other mole- cules

An inflammatory response may be initiated by the secretion of interleukin-1 (IL-l). The functions of IL-l are many and varied (see review by Durum and col- leagues’). One of the major effects of IL-l in the inflammatory response is to in- crease the expression of adhesion molecules, leading to the attachment of lym- phocytes and granulocytes to the walls of vessels at the site of inflammation. TNF, a product of macrophages, is also important in this activity. Together, IL-1 and TNF-alpha cause vessels to express increased amounts of endothelial cell leukocyte adhesion molecule-l (ELAM-I), which binds to neutrophils, as well as vascular cell adhesion molecule-l (VCAM-1) and intercellular adhesion mole- cule-l (ICAM-I), which bind T cells via their ligands, very late antigen (VLA-4) and leukocyte function antigen-l (LFA-1). 2, 3 These relationships are summa- rized in Table 1.

Attachment encourages the extravascular migration of cells and is further enhanced by their deformability, properties controlled by proteins known as in- tegrins. The extravasated cells are directed in their movements by a concentra-

Richard Hong, MD, is Professor of Pediatrics, University of Wisconsin Center for Health Sci- ences, Madison.

Current Problems in Pediatrics / July 1991 253

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TABLE 1”

Cell Endothelial Receptor Cells-t Ligand ..-_ -.. -.. CD1 la PMN, MO, T

CDllb PMN, MO, T CDllc PMN, MO, T Unknown PMN, MO, T VLA-4 MO, T Unknown PMN, MO, T

Homin,g receptor: PMN, MO, T MEL-14, LAM-l, hLHRc, Leu-8

ICAM-I (?ICAM-2)

ICAM-I Unknown El-AM-1 VCAM-1 CD 62

(GMP-140, PADGEM)

Unknown

*Adapted from Carlos TM, Harlan JM: lmmunol Rev 1990; 1145-28. t;Cells which bear receptors. PMN = polymorphonuclear leukocyte; MO = monocyte; T = T lym- phocyte; ELAM-1 = endothelial cell leukocyte adhesion mole- cule-l; GMP-140 = granule membrane protein-140; hLHRc = hu- man lymph node ‘homing receptor core protein; ICAM- 5 intercel- lular adhesion molecule-l; LAM-I = leukocyte adhesion mole- cule-l ; MEL-1 4 = mouse lymph node homing receptor; PADGEM = platelet activation-dependent granule-external membrane protein; VCAM-1 = vascular cell adhesion molecule-I

tion gradient of IL-8 and mononuclear cell-activating factor (MCAF), products of monocytes that have infiltrated the lesions4

The elucidation of cytokine processes will offer many different approaches to therapy. (See section on therapy below.)

Role of the Major Histocompatibility Complex The transplantatron antigens are products of genes located on chromosome 6, in the MHC region. Their function is not to prevent organ transplantation, but to play a ,major role in the acquisition of T-cell immunity. T lymphocytes only recog- nize antigen when it is presented to their receptors as a peptide held in a cleft of the major transplantation antigens HLA-A, -B, -C, or -D. The HLA antigens are formed by two chains, an alpha and beta (HLA-0) or an alpha chain with beta, microglobulin (HLA-A, 3, or -C). In their three-dimensional configuration, these chains are arranged as two parallel alpha helices on top of a beta-pleated sheet floor. The cleft so formed contains a peptide that is recognized as antigen by the T-cell receptor.5 Extensive study of the amino acid sequences of the HLA chains suggests that amino acids at certain locations can profoundly influence peptide binding. ’ Since the amino acid sequence determines the serologic identity of the particular HLA group (i.e., whether it is HLA-Dw4 or HLA-Dw2, for example), the reason for the well-known association of autoimmune disease with the MHC becomes apparent. A serologic reactivity implies that a certain amino acid sequence is present. If that sequence can influence peptide binding, au- toreactive peptides could be preferentially bound or presented in a particularly immunogenic configuration. Such individuals would be predisposed to the au- toimmune disease. With other amino acids, similar peptides would not be bound, and that serologic type would have a negative association with a partic- ular disease.

Studies of the susceptibility TV insulin-dependent diabetes mellitus provide a striking example of the influence of individual amino acids. The presence of as- panic acid at position 57 in the beta chain of the DQ allele is protective. The

New Developments

absence of aspartic acid at that site in association with an arginine on the alpha chain at position 52 confers susceptibility.7 These two positions are at opposite ends of the alpha helices which form the sides of the antigenic peptide groove. It is easy to envision that at those sites, they could influence the mode of pep- tide presentation to break natural tolerance and contribute to the development of autoimmune diabetes.

No single factor entirely accounts for susceptibility to autoimmune disease. A consideration of the many factors involved has been presented recently.*

From the above it can be seen that in the future, it will be possible to ascer- tain those individuals who are at high risk for various autoimmune disorders. Al- though it is unlikely that we can alter their genes, more careful follow-up and avoidance of aggravating factors can be utilized in their health management. Routine’screening of the general population is frequently not cost-effective be- cause of low prevalence of a given disease or health abnormality. Delimiting the various population groups and using relevant tests will make future preventive medicine much more efficent and effective.

Therapy In recent years, much more emphasis has been placed on T cells as major of- fenders in autoimmune reactions. With the increased knowledge of the mecha- nisms of immune responses, many novel approaches to the treatment of autoim- munity are now possible. These approaches may be divided into two major cat- egories, those designed to control the inflammation cascade and those based on T-lymphocyte manipulation. Extensive preclinical, and in a few cases, clinical testing has begun.

Control of lnflamma tion For the moment, we must still rely primarily on prednisone for control of the in- flammatory response. From the previous discussion, one can see that interrup- tion of cytokine activity offers many possibilities for amelioration of symptoms. Unfortunately, inhibitors, either as pharmacologic agents or as antibodies, are not yet available in suitable quantities for investigation, but it should only be a matter of time before that problem is alleviated.

A monoclonal antibody directed against the adhesion molecule, LFA-1, has been employed in Europe to prevent rejection of bone marrow transplants. There is a high rate (73.9%) of rejection when T cell-depleted marrows are used. Giving anti-LFA-1, the Europeans decreased the numbers of failures to 28%. Infection was a major complication.g Long-term chronic administration of anti-LFA-I is probably not desirable for treatment of autoimmunity, because in addition to infection susceptibility, patients would develop antibodies against the mouse antibody.

Development of human-mouse chimeric immunoglobulin molecules could delay the emergence of antibodies by the patient, however. It is only desirable to have a mouse (or other animal) antibody because there are essentially no re- strictions concerning the antigens employed and the type of immunization scheme, and finally, the spleen can be removed as the source of the B cells for the monoclonal antibody production. These options would not be available if hu- mans were to serve as the source of B cells. Yet the mouse antibody, being a foreign protein, stimulates an immune response so rapidly that the therapeutic potential is compromised. Human-mouse chimeric antibodies are constructed so that most of the antibody is of human origin, making the major part of the

Current Problems in Pediatrics / July 1991 255

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immunoglobulin the same as the rest of our own IgG pool. Only the small part (no more than 25%) of the molecule necessary for binding to the antigen, the variable region, comes from the mouse. Fortunately, the immunoglobulin mole- cule is made from several genes, so the genes that code for different regions of the molecule can be isolated. Thus, the desired genes are isolated from human or mouse sources, spliced to code for the entire molecule, and subsequently transfected into an appropriate cell line for production of large quantities. Such antibodies are being used in clinical trials at this time. (See the article by Morri- son and Schlom” for review.)

An attractive alternative would be the use of soluble cytokine receptors. Cy- tokines activate their target cells via surface receptors. These receptors, in- jected in a soluble form, could bind to the cytokines and thus block their reac- tions with the target lymphocytes.

Manipulation of the T Lymphocytes (Rogue Lymphocytes) Antibodies against T lymphocytes have been widely used in the treatment of aplastic anemia, organ graft rejection, and graft versus host disease. The lack of specificity, resulting in profound lymphopenia (and sometimes neutropenia and thrombocytopenia as well), limits their prolonged and repeated use in chronic disease. A monocional antibody with activity only against the rogue lymphocytes with its more restricted target would avoid some of those problems. Such an an- tibody could be developed if the T-cell receptors of the pathogenetic clones were used as the antigen.

For this approach to have clinical utility, however, it is necessary that the re- sponses in autoimmune disease be oligoclonal; that is, only a few clones are involved and these clones are the same from patient to patient. If each patient had his or her own set of unique clones causing the disease, any treatment based on those clones would be applicable only for that patient. There is only limited information on the heterogeneity of T-cell receptor response in autoim- munity, but studies in both mouse and humans suggest limited T-cell receptor usage.

The T-cell receptors involved in causing the neurologic damage of experi- mental allergic encephalomyelitis (EAE) show remarkable homogeneity.” For example, a comparison of residues in the T-cell receptor alpha chain showed virtual identity over a stretch of 11 amino acids. This was true despite the fact that two different strains of mice and a rat strain were compared, and despite th’e fact that the encephalitogenic antigens are different among the various spe- c’ies. It thus appears that only a few T-cell receptors are capable of causing e.n- cephalomyelitis. Use of monoclonal antibodies specific for these T-cell recep- tors Iprevented EAE and rewe’rsed established disease (reviewed by Howell and colleagues”).

Iln humans, the ‘evidence is less compelling, but T-cell receptors in patients w’i’th myasthenia gravis, ‘mu’ltiple sclerosis, or rheumatoid arth’ritis were com- posed of certain alspha and beta chains in percentages far higher than the inci- de:nce in the general popu’lation (reviewed by Wraith and associates’*) The pref- erential us’e of ce#rtain alpha and beta chains provides suggestive, but in no way co’nc’lusive, evidence for restricted clonal’ity. The assumption is that if a poly- clonal response were seen, there ,would be equal usage of the various alpha and ‘beta chains avai’l’a’ble. ‘In that case, analysis of T-ce’ll receptor chains would rePlect tihe distributions seen in the general population and no preference or bi- ased usage cou.ld be discerned.

256 Current Problems in Pediatrics / July 1991

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Knowledge of the T-cell receptor sequence in the EAE mouse model has been exploited by two groups. Vandenbark and associates13 and Howell and colleagues” injected mice with synthetic peptides representing a portion of the encephalitogenic T-cell receptor. They obtained complete protection from sub- sequent attempts to induce EAE. The precise mechanisms of these immuniza- tion procedures in the prevention or amelioration of disease is unclear. Jane- way14 provided an insightful analysis. He proposed that the peptide-HLA com- plex from the antigen-presenting cell as well as the reacting T-cell receptor is internalized by the CD4+ responder cell and reexpressed on its surface, in the form of three new stimulatory peptides. The peptides would be derived from (1) the original antigen, (2) the HLA, and (3) the T-cell receptor, and would be bound in the cleft of the HLA displayed on the surface. These peptides, acting as neoantigens, would react with CD8+ T cells, stimulating regulatory (suppres- sor) clones. Those clones reacting against the T-cell receptor peptide would de- stroy or otherwise inhibit clones capable of reacting with the encephalitogenic antigen, thus making the animals unresponsive.

Even without the detailed knowledge of the receptor, access to the clones would be of potential therapeutic value. A few years ago, Lider and coworkersi showed, again in the mouse EAE model, that injection of T-cell clones reactive against myelin basic protein could protect the recipients from subsequent at- tempts to induce the disease by the injection of myelin basic protein. The T cells were effective, whether or not exposed to such treatments as irradiation or chemical crosslinking. The injections appeared to stimulate the development of suppressor T-cell clones which prevented the emergence of the pathogenetic T lymphocytes following immunization.

With increased knowledge of factors involved in peptide binding, synthesis of nonimmunogenic peptides that can compete with the immunogenic peptide offers yet another approach. The strategy here is to “immunize” the susceptible or affected individual with peptides that can fit1 the HLA cleft. They could bind with greater affinity than would the pathogenic peptide, but on the basis of in vitro experiments and knowledge of the structural requirements for immunoge- nicity, would be nonimmunogenic.

Summary Autoimmune disease has been controlled somewhat in recent years, primarily by nonspecific antiphlogistic or cytotoxic regimens. The results have varied Erom excellent to no response. With increased un,derstanding of the various aspects of the pathogenesis of these disorders, particular@ in the areas of genetics, cy- tokines, and molecular immunology, we are now in a position to us,e precise ap- proaches. Hopefully, the days of blunderbuss therapy, with unacceptable costs of infection and other complications are numbered.

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munol Rev 1990; 114:5-28. 4. Matsushima K, Oppenheim JJ: lnterleukin 8 and MCAF: Novel inflammatory cytokines, inducibl:e

by IL-1 afld TNF. Cyfokine 1989; 1’:2-73. 5. Bjorkman PJ, Saper MA, Samraoui B, et al: The foreign antigen binding site and T cell. recogni-

tion regions of class I histocompatibifity antigens. Nafure 1987; 329:XZ518.

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6. Grey HM, Sette A, Buus S: How T cells see antigen. Sci Am 1989; 261:56-64. 7. Khaki I, d’Auriol L, Gobet M, et al: A combination of HLA-DQf3 asp57negative and HLA DQu

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9. Fischer A, Friedrich W, Fasth A, et al: Reduction of graft failure by a monoclonal antibody (anti- LFA-1 CDlla) after HLA nonidentical bone marrow transplantation in children with immunodefi- ciencies, osteopetrosis, and Fanconi’s anemia: A European Group for ImmunodeficiencyiEuro- pean Group for Bone Marrow Transplantation report. Blood 1990; 77:249%256.

10. Morrison SL, Schlom J: Recombinant chimeric monoclonal antibodies, in DeVita VT Jr, Hellman S, Rosenberg SA (eds): important Advances in Oncology 1990. Philadelphia, JP Lrpprncott, 1990, pp 3-18.

11. Howell MD, Winters ST, Olee T, et al: Vaccination against experimental allergic encephalomyeli- tis with T cell receptor peptides. Science 1989; 246:668-670.

12. Wraith DC, Smilek DE, Mitchell DJ, et al: Antigen recognition in autoimmune encephalomyelitis and the potential for peptide-mediated immunotherapy. Cell 1989; 59:247-255.

13. Vandenbark AA, Hashim G, Offner H: Immunization with a synthetic T-cell receptor V-region peptide protects against experimental autoimmune encephalomyelitis. Nature 1989; 341:541-544.

14. Janeway CA. Immunotherapy by peptides? Nature 1989; 341:482-483. 15. Lider 0, Reshef T, Beraud E, et al: Anti-idiotypic network induced by T cell vaccination against

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