Lecture 8

1. Lecture 8 • Antigen presentation • MHC restriction • MHC genetics • Cytotoxic CD8 T cell killing • Subversion by viruses Reading Chapter 3, and 6.13-6.14

2. 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 T cells see “presented” antigen Antigen presenting cell CD8 T cells 20 16 13 14 15 17 19 18 4 5 3 2 9 6 8 7 1 11 12 Note: the naïve T cell is not in killing mode the ability to kill comes after antigenic stimulus Note: for both T and B cells antigen is a growth factor

3. Review of the Zinkernagel-Doherty findings • Killer T cells isolated from a previously infected mouse were used to kill different target cells in tissue culture. • Say the mouse from which the killer cells came had MHC Class I molecules of type "k" and was infected with LCMV virus. • The killer cells kill LCMV-infected cells from strains of mice carrying type "k" molecules • The killer cells do not kill LCVM infected cells from mice that have type "d" MHC I molecules. • Note that "k" and "d" are different natural variants of the same gene(s).

4. MHC proteins are polymorphic Different variants of the gene occur in the population so that most of use are heterozygous, carrying two different allelic variants. For example, there are over 200 different variants of the human MHC class I gene HLA-A

5. Killer cells are specific for both the virus peptide and the host MHC class I Virus peptide H-2Kd H-2Kk Mouse MHC class I on target cells Host type Different type

6. Figure A-38 Chromium release assay for T cell killing From Janeway book

7. T cell recognition of a peptide-MHC complex

8. MHC class I and class II molecules are encoded by several genes Mouse Class I H-2K,D, L Class II H-2I-A,E Human Class I HLA-A, B, C Class II HLA-DR,DP, DQ

9. Figure 3-28

10. Each T cell recognizes peptide associated with a single self MHC molecule

11. T cell recognition of a peptide-MHC complex

12. Figure 3-29 MHC molecules have binding preference for peptides carrying certain sequences. (Residues in green point down, into MHC pockets)

13. Surprising, but true • If a microbe had proteins such that none of its peptides can bind to MHC, it will be invisible to T cells! • If a microbe mutates to avoid MHC binding of its peptides it would be a disaster! • These considerations probably explain the polymorphism in MHC (large number of alleles within the populations) • In each person there are also several different class I and class II genes (isotypes). • This specificity of MHC binding also explains the link between antigen non-responsiveness to simple or small proteins and the MHC.

14. The human MHC class I genes

15. Figure 3-24 part 2 of 2 The human MHC class II genes

16. Figure 3-23

17. Because the polymorphic MHC molecules map near to each other on human chromosome 6, one often talks about "haplotypes" which refers to the collection of linked gene versions. Typically, you inherit one haplotype from your mother and one from your father. Less frequently, a new haplotype is formed by a crossover in germ cells of your mother or father.

18. Figure 3-27 Other genes related to antigen presentation map to the MHC

19. Figure 3-31

20. Figure 3-32 part 1 of 2

21. Figure 3-32 part 2 of 2

22. Figure 3-33 part 1 of 2 Over relatively short evolutionary time, rare mutations are continually generating new variants of MHC molecules that may be selected for.

23. Figure 3-33 part 2 of 2

24. Figure 3-34 Heterozygotes at all polymorphic MHC loci Homozygous at one locus Homozygous at 2 or 3

25. MHC molecules are essential Bare Lymphocyte Syndrome--congenital deficiency of either class I or class II molecules. Patients lack killer cell or helper cell responses and are prone to many types of opportunistic infections.

26. Microbes often try to suppress antigen presentation Herpes Virus ICP-47 blocks TAP function Adenovirus E1A inhibits transcription of MHC class I genes E3 binds and retains class I molecules in the ER Cytomegalovirus US3 protein sequesters class I molecules in the ER US2 & US11 proteins “dislocates” class I molecules from the ER to cytoplasm HIV Vpu and Nef inhibit class I expression

27. CD8 T cell mediated killing Killing involves the generation of pores in the plasma membrane of target cells and also the initiation of programmed cell death by different pathways Perforin is a pore forming protein found in the granules of activated, but not naïve, CD8 killer cells. (It is related to complement component C9.)

28. Figure 6-28

29. Figure 6-29

30. Figure 6-30

31. Figure 6-31

32. Concepts • MHC molecules are polymorphic and polygenic. • MHC restriction occurs because T cells see a complex of foreign peptide and self-MHC. • MHC molecules are semi-specific for peptide binding, and require "anchor residues." • MHC allelic forms often have different peptidfe binding specificities and different T cell contact residues. • MHC molecules are under intense evolutionary selection by microbes, favoring diversity. • Microbes try other ways to suppress MHC mediated recognition. • Activated CD8 T cells lyse target cells with the appropriate MHC class I /peptide complex. • Killer cells induce programmed cell death (apoptosis) of targets.