Chapter 3: Antigen Recognition by T lymphocytes

TCRTH1 - T Helper cell type 1TH2 - T Helper cell type 2TC or TCL - cytotoxic T cell

Mechanism of Self-tolerance

Positive selection = bind to selfDouble selection = bind to strongly to self

• Two Types1) TCR and TCR

T cellsTCR and TCR

T cells

• Antigen Binding site- V and V

• Similar to Fab fragment

T-cell Receptor

• T cells- Recognize MHC:peptide complex- Diverse Functions

A) Stimulate other immune cellsB) Cytotoxic - kill infected host cells

- Cell:cell interactions

• T cells- Dominant T-cell in epithelial tissue

(only 1-5% in circulation)- Recognizes more than MHC:peptide- Not well characterized

Functions and Properties of T Cells

• Antigen-Recognition site = Peptide:MHC Recognition site

• Single V region -CDR1-3 for each chain

• All TCRs on a single T cell are the same

• Different T cells express different TCRs

• Diversity mechanisms like BCRs

Figure 3-3

No Ds in Vgene

DJ first then VDJ in gene

rearrangement

Occurs in the Thymus

Figure 3-8• -chain locus is within -chain locus

• Fewer V segments then and

• Two D segments can be incorporated

Functional T-cell Receptor Complex

• Core complex

• CD3 complex: ,

• (zeta) chain

• Function of CD3 and :TransportSignal Transduction

Invariant Chains

Avidity

PROPERTY B CELLS T CELLSInitial Development Bone Marrow Thymus

Pre-antigen Diversity

YES YES

Post antigen Diversity

YES NO

Single antigen specificity

YES YES

Antigen recognized Variety Peptide:MHC

Secreted form of Receptor

Yes No

Invariant signaling subunits

Yes Yes

Comparison of B and T cells

• SCID - severe combined immunodeficiency disease- Many causes but a rare disease- Classified according to lymphocyte profile (T B NK)- Bone Marrow transplant can cure

• Omenn syndrome- RAG proteins have reduced activity- Patient is: T+ B- NK+

• CD3 and CD3 deficiency diseases- Mutations in some CD3 genes- Patient is: T+/TCR- B+ NK+

or T- B+ NK+

Immunodeficiency diseases

How do T cells recognize antigens?

Antigen ProcessingAntigen PresentationAntigen Presenting Cell (APC)Professional APC

MHC class I communicates with Tc cells

MHC class II communicates with TH cells

IR to Extracellular Pathogens(CD4-MHC II)

1. Antibodies needed

2. Pathogen recognition/internalization by professional APCs

a. B cellsb. Macrophagesc. Dendritic cells

3. Phagolysosome degrades proteins to peptides

4. Peptides:MHC II complex transported to surface

5. Professional APC contacts CD4 T cells

6. CD4 TH cells secrete cytokines to signal B cell maturation

IR to Intracellular Pathogens(CD8-MHC I)

1. Antibodies ineffective

2. Pathogen replicates in the cell and proteins are degraded in the cytoplasm of the cell

3. Peptides are transported into ER and bind MHC I and transported to the surface

4. MHC I expressing cells present to CD8 T cells

5. CD8 T cells (cytotoxic T cell, CTL) kills host cell

Figure 3-11

Structures involved in MHC Presentation

TCR

CD4 and CD8

MHC1 and MHCII

T cell Co-Receptors

Binds MHC I

Binds MHC IITH

TC

Figure 3-13 part 1 of 2

Variable

Invariant

CD8-MHC I CD4-MHC II

CD8-3CD4-2

Figure 3-15Closed Open

8-10 amino acids 13-25 amino acids

Degenerate binding specificity

Figure 3-16

Proteosome=Shredder

Transporter associated with antigen processing

Peptide Degradation and Transport

Chaperones

CalnexinCalreticulinTapasin

Bare lymphocyte syndrome-TAP deficiency-Viral infections

Figure 3-19

Acidic process

Golgi Transport

Vacuolar Transport

CLIP - class II-associated invariant-chain peptide

Prevention of Peptide:MHC II formation in ER

Invariant Chain (li) - Prevent peptide binding in ER - Deliver to vesicles

HLA-DM - Release of CLIP, peptide loading

Figure 3-21 part 2 of 3

TCR binds MHC and peptide

CDR3 binds peptideCDR2 binds MHCCDR1a binds N-terminalCDR1b bind C-terminal

• Cytokines can regulate expression

• Interferon induces MHC II expression

T-cell Diversity- Recombination- Structure of TCR and associated molecules- Immunodeficiency diseases

Antigen Processing and Presentation- Intracellular vs Extracellular- MHC structure

INTRA

Proteasome

TAP

MHC I

Golgi

Calnexin

Calreticulin

Tapasin

CD8 TC

EXTRA

Li

MHC II

Golgi

Vesicle

CLIP

HLA-DM

CD4 TH

Summary

Alleles: different forms of one geneAllotypes: different forms of one protein (isoforms)

Polymorphic: alternative forms of one gene = Many allelesOligomorphic: a few forms of one gene = Few allelesMonomorphic: no polymorphism

Homozygous: same allele on both inherited chromosomesHeterozygous: different allele on both inherited chromosomes

MHC in humans is called HLA (human leukocyte antigen complex)

Figure 3-13 part 1 of 2

Variable

Invariant

No rearrangements or somatic changesDiversity is derived from 1) Gene families

2) Genetic polymorphism

HLA-A,B,C-present peptide antigens to CD8Tcells and interactwith NK-cells

HLA-E,G-interactwith NK-cells

HLA-F-?

HLA-DP,DQ,DR- present peptide antigens to CD4Tcells

HLA-DM,DO-regulate peptide loading of DP,DQ,DR

Human leukocyte antigen complexAbs used to ID MHC molecules react with leukocytes not RBCs

Figure 3-24 part 1 of 2

Heavy Chain

Heavy Chains

2-microglobulin on chr15• and chain = GeneA and GeneB• Haplotype - combination of alleles inherited from Chr6• 2% meiotic recombination rate generates population diversity•Crossover: Haplotypes, normally, are inherited intact and hence antigens encoded by different loci are inherited together (e.g., A2; B27; Cw2; DPw6; DQw9; DRw2). However, on occasions, there is crossing over between two parental chromosomes resulting in new recombinant haplotypes. Thus, any one specificity encoded by one locus may combine with specificities from other loci. This results in vast heterogeneity in the MHC make-up in a given

population.

Chr6

Chromosome Organization of HLA complex

• Cytokines (Interferons) coordinately regulate the group of genes - class I heavy chain and other associated genes

• TAP transporter, Tapasin, Proteasome subunits - LMP2 and LMP7 ---> all proteins involved in Antigen Processing

Interferon , , and ----> Class I , 2M, TAP, LMP2, LMP7

Interferon ----> CIITA transcription factor --->

MHC class II transactivator (CIITA) - deficiency leads to bare lymphocyte syndrome

HLA II genes, li chain

MHC I (single peptide binding chain ): 3 genes to present antigen

HLA-A, HLA-B, HLA-C

MHC II (two chains, and ): 3 genesb to present antigen

HLA-DQ, HLA-DP, HLA-DR

Each MHC II locus encodes a gene for the chain and a gene for the chain:

e.g. HLA-DQA, HLA-DQB => MHC II isoformsHLA-DPA, HLA-DPB => MHC II isoformsHLA-DRA, HLA-DRB => MHC II isoforms

Maternal: 3 MHC I genes HLA-AM, HLA-BM, HLA-CM

Paternal: 3 MHC I genes HLA-AP, HLA-BP, HLA-CP

Maternal: 3 MHC II genes HLA-DPAM, HLA-DPBM

HLA-DQAM, HLA-DQBM

HLA-DRAM, HLA-DRBM

Paternal: 3 MHC II genes HLA-DPAP, HLA-DPBP

HLA-DQAP, HLA-DQBP

HLA-DRAP, HLA-DRBP

6 different MHC I proteins on all cells

6 different MHC II proteins on all cells(some individuals have 8 due to two HLA-DRB genes)

Heterozygous

Homozygous = one DR type

Heterozygous = up to four DR combinations, but only 3 types can be made by one person

HLA-DRAP, HLA-DRB1P

HLA-DRAP, HLA-DRB3P

HLA-DRAP, HLA-DRB4P

Figure 3-34

Red – heterozygous for all the highly polymorphic HLA I & IIYellow - Homozygous for one locusBlue - Homozygous for two or three loci

Correlation is mainly with HLA class I -

consistent with killing of virus infected cells

Seroconversion - when antibodies have first been detected

Figure 3-28 part 1 of 2

MHC

One MHC isoform can bind multiple peptidesContact residues - the MHC amino acid residues that interact with TCR or the bound peptide

Figure 3-28 part 2 of 2

Error in the textbook on HLA-DR (oligomorphic vs invariant)

chain

chain

HLA-DR

Figure 3-29

Peptide binding motif - combination of anchor residues

Anchor residues (green) - peptide amino acids interacting with MHC

Figure 3-30

Co-

MHC restriction - TCR recognizes the complex of both the peptide and MHC by interacting with exposed amino acid residues

Figure 3-31Large circles- total # antigenicpeptides that can be presentedvia MHCI & MHCII

small circles- total # antigenicpeptides that can be presentedvia an individual MHCI & MHCII haplotype

Balancing SelectionFavors multiple alleles

Figure 3-32 part 1 of 2

Advantages for heterozygous for the MHC

Figure 3-32 part 2 of 2

Different mechanism from recombination - DNA is exchanged between alleles and copied in one direction to generation new MHC allele

HLA B*5301-Found in African populations and associated with resistance to severe malaria

Figure 3-33 part 2 of 2

HLA B*4601- Found in southeast Asian populations andassociated with susceptibility to nasopharyngeal carcinoma.

Recombination betweenalleles of a differentgene

Generation of newMHC alleles

MHC selection by Infectious Disease

• Pathogens adapt to avoid MHC - recent MHC isoform may provide a survival advantage (hence higher frequency level)

• Epidemic diseases place survival advantages on those who can best present pathogenic peptides

• Only a minority of HLA alleles are common to all humans- most are recent and specific to ethnic groups

HLA Type and Disease Susceptibility

Ankylosing spondylitis B27IDDM DR4/DR3Multiple Sclerosis DR2Narcolepsy DR2Rheumatoid arthritis DR4Lupus (SLE) DR3AIDS (rapid) HLA-A29, HLA-B22

HLA-C16, HLA-DR11AIDS (slow) HLA-B14, B27, B57

HLA-C8, C14

MHC polymorphism and Organ Transplants

• Developing T cells that recognize complexes of peptide and MHC molecules on HEALTHY tissue (self-peptides presented by self MHC) are DESTROYED

• This results in the preservation of T cells that recognize non-self MHC (allogeneic MHC). These are alloreactive T cells and are 1-10% of total T-cell repertoire

• Immune system is primed for rejection of foreign organs that express allogeneic MHC

• THIS IS WHY YOU WANT TO MATCH HLA TYPE