Lecture 10 CTL evasion HIV interferes with steps of initiating a CTL response.

1. Lecture 10 CTL evasion HIV interferes with steps of initiating a CTL response. Epitopes bound by MHC class I alleles Viral escape from CTL by epitope mutation. Data from case studies of HIV-infected patients. -epitopes that are easily changed -epitopes that are difficult to change -epitopes that are impossible to change Viral escape by down-modulating surface expression of MHC -HIV nef protein interferes with MHC expression -poliovirus inhibits protein transport

2. Lecture 10: Viral evasion from cytotoxic T lymphocytes Immunobiology. Janeway et al. The CTL response keeps HIV under control for many years. When CTL declines, opportunistic infections and disease start.

3. Dendritic cells bring antigen into the lymph node where it can activate specific T helper cells. HIV can hitchhike on outside of DC to get taken to lymph node. HIV can be taken up as antigen for expression on MHC class II, and HIV can also infect dendritic cells. Macrophages at the site of entry, or responding T helper cells are the first infected.

4. Dendritic cells are potent activators of naïve cytotoxic T cells. Infected dendritic cells are presenting antigens on MHC class I. The CD8 cell with signal 1-MHC class I presenting peptide to TCR, and signal 2-the co-stimulation, can make its own IL-2. These are effector CTL and can kill infected DC, and TH cells right in lymph node.

5. The activation of CTL requires T cell help HIV-infected individuals have 80%of their CTL remaining immature. Perhaps because they are not getting enough IL-2 from TH cells.

6. MHC class I presents peptides from intracellular proteins Nascent proteins are degraded and peptides presented on MHC class I Peptides are linear parts of the intracellular protein that fits right in the cleft of MHC class I.

7. Humans co-dominantly express three MHC class I (HLA) genes A1 B27 C3 A3 B57 C7

8. The structure of peptide antigens bound by an MHC class I allele An epitope bound by MHC class I is a linear 9 amino acid sequence from within the protein. Viral peptides can be presented if they fit. Each peptide varies except at anchor residues. Peptides bound by HLA-A3 most important amino acids

9. HIV/AIDS. HLA leaves its footprints on HIV. HIV is evolving to eliminate CTL epitopes (especially for common HLA alleles). Science. 2002 May 24;296(5572):1410-1. HLA molecules present specific epitopes that are recognized by CTL. (center) Effective CTL exert strong selective pressure on the virus. Viral variant epitopes that escape these CTLs by failing to bind to HLA or to interact with T cell receptors have an advantage in vivo. (Left) Other responses are less effective and are not associated with escape. (Right) If an effective CTL response is directed against an epitope region that constrained functionally or structurally, there may be no selection or slow selection because a mosaic of mutations is required for escape to occur.

10. Epitopes that are easy to change Case study: HIV infected patient with CTL escape. • Patient was identified early and serum collected over time. Determined that the primary immune response was an immunodominant epitope in the env gene (encodes gp120 and gp41 proteins) which was presented by HLA-B44 allele. • Immunodominant HLA-B44 epitope AENLWVTVY • Mutation of this sequence to AXNLWVTVY allowed escape for the virus. Immunodominant epitopes: CTL immune response tends to focus on a few epitopes originally seen. Sometimes called ‘original antigenic sin”.

11. Epitopes that are easy to change Case studies: attempts to treat HIV-infected individuals with their own CTL. • CTL from patients were expanded in vitro and re-infused in high numbers • HLA-A3 bound epitope of Nef protein QVPLRPMTYK Mutant HIV deleted part encoding peptide in 30% of HIV isolated • HLA-B8 individuals had dominant epitope of Nef protein FLKEKGGL. • Patients with mutants in this region included deletion and FLKEXGGL

12. Epitopes that are difficult to change Case studies: HIV-infected individuals that progressed slowly to disease. • All individuals were HLA-B27 (normally progress slowly to disease) • Immunodominant epitope KRWILGLNK • Mutations arose late in infection, and required complementary mutations to gag capsid • Virus in four patients had KKWIMGLNK showing escape is slow, but happens. J Immunol. 2011 Jan 1;186(1):479-88. Epub 2010 Nov 29. HIV immune escape at an immunodominant epitope in HLA-B*27-positive individuals predicts viral load outcome.

13. Epitopes that are impossible to change Case studies: HIV-infected individuals that have been infected for decades without progressing to disease (Amsterdam cohort). MHC class I genotype 11/13 long-term HIV nonprogressors have the HLA-B57 allele. All individuals have normal CD4 counts, low viral concentration and no symptoms of AIDS Migueles et al., 2000 PNAS 97, 2709 The HLA-B57 MHC class I gene presents a peptide in the capsid protein that the virus cannot mutate.

14. Viral escape by down-modulating surface expression of MHC Nef targets HLA-A and HLA-B for degradation NK HLA-B HLA-A HLA-C Nef targets HLA-A and HLA-B to endosome for degradation. HLA-A and HLA-B have target sequences in tail, HLA-C does not. HLA-C is left on surface of cell to prevent lysis by NK cells.

15. Viral escape by down-modulating surface expression of MHC: Poliovirus interferes with protein transport in the cell to make vacuoles. Vesicles budding from ER never make it to the membrane. No new surface proteins get to the cell surface. Old MHC, with self peptides bound remain on surface, even though the cell is infected. Figure 2 Appendix Virology

16. Problems. How would you make an effective HIV vaccine? Why did the HIV subunit (gp120) vaccine fail? Would you take a live attenuated HIV vaccine? What type of vaccine would improve the CTL response? What factors in the human population must be considered?