Immunology of HIV

1. Immunology of HIV Rupert Kaul

2. “The immunology of HIV” • Review of HIV-1, life cycle, transmission • How does HIV infect a person? • Mucosal immune events • How does HIV cause disease? • Direct vs bystander, gut events • How does the host fight back? • Implications for vaccines, therapeutics

3. HIV structure

4. HIV - virus, genetics • HIV is a lentivirus - an RNA virus from the class of retroviruses • 2 HIV species (1 and 2) - 40-50% homologous • Several HIV clades - A,B,C,D,A/E,O (others) - 70-80% homologous • Within a clade - 85-90% homologous • Within an individual - “quasispecies” >95% homologous • About 109 viruses produced per day, error-prone reverse transcriptase (q 10-4-10-5)

5. HIV-1 life cycle (1) HIV-1 attachment; (2) Fusion; (3) Cell entry; (4) Reverse transcription, formation of the pre-integration complex (PIC); (5) Nuclear transport; (6) Chromosomal integration of DNA provirus; (7) Transcription of viral RNA; (8) Nuclear export of RNA; (9) Translation and processing; (10) Membrane transport; (11) Virion assembly; (12) Budding; (13) Maturation.

6. HIV - clinical progression

7. Two contrasting facts: HIV has spread widely and rapidly…

8. …and yet HIV is relatively difficult to transmit UK PEP Guidelines, 2006.

9. Blood viral load correlates with transmission. Quinn, T. 2000.

10. Blood HIV levels predict amount of virus in “genital fluids” Sheth P; J Immunol, 2005. Kovacs A; Lancet, 2001.

11. Genital/mucosal protective factors • Genital tract repels >99% of HIV exposures • Combination of factors: • Intact epithelium • Mucus, pH, SLPI, lactoferrin, Trappin-2, etc • ?Adaptive mucosal immunity • Lack of co-infections also important

12. What are the major genital HIV targets? Hladik F. Immunity, 2007. Haase A. Nat Imm Rev, 2005.

13. Penile HIV target cells

14. Mucosal immune protection vs HIV… • 3 large RCTs in SSA showed clear benefit • Very consistent results in Uganda, Kenya, SA • Efficacy: ITT ~55%, OTA ~63% • In Kenya: incidence 2.1% vs 4.2% • No short term behavioural disinhibition • is being followed prospectively Viewpoint. Coates T, et al. Lancet, 2007.

15. Mucosal immunology and coinfections Freeman E, AIDS, 2006;

16. Cervical target cells in HIV(-) women • These associations were seen in HSV-2 infected women in the absence of HSV-2 DNA shedding or clinically apparent ulceration

17. How does HIV cause disease? • Not direct depletion of CD4+ T cells • See a number of immune effects that contribute: • Increased immune activation • ? Via switched on innate immunity, ? damage to gut mucosa • Leads to skewed T cell function, apoptosis • Loss/dysfunction of many cell types: • pDCs, other dendritic cell subsets • CD4 and CD8 T cells • NK cells, NKT cells, GD cells, etc etc

18. HIV: immune effects on the gut Mehandru et al. PLoS Med, 2006. Brenchley et al. JEM, 2004

19. 47 and HIV infection Mora J. Nature, 2003. Johnson P. NEJM, 2008.

20. Gut events and HIV pathogenesis HYPOTHESIS: • GI mucosal immune defects  bacterial translocation  systemic immune activation  CD4 depletion. Brenchley J. Nat Med, 2006.

21. Bacterial translocation andinflammation • Systemic inflammation correlates closely with both: • Bacterial translocation • Rate of CD4 depletion Silvestri G. AIDS Rev, 2008.

22. Non-pathogenic SIV models:Sooties and AGMs Silvestri G. Blood, 2008; Immunity, 2003

23. Lessons from non-pathogenic models* • Do not see enhanced cellular immunity • Do see reduced inflammation - initial “blip”, rapidly downregulated • Do see CD4+ depletion in the gut, but transient and then recovers • Target “shielding”?? • SM - reduced CCR5 expression if activated • AGM - “CD4(-)” T helpers not depleted

24. Host defenses: antibodies

25. HIV: antibody responses • IgG response is ubiquitous - basis of diagnosis • Most people do make neutralizing Abs against their own virus • BUT only work against the virus that was there a few months ago - not the one that is there today • Failure of infused “cocktail” to impact infection for more than a few days

26. HIV antibody responses (2) • Conformational masking - entropy • Lack of broad neutralization • Shielding of highly-conserved coreceptor binding regions by hypervariable loops • “Irrelevant" antibodies vs gp120 monomers, or non-critical regions of the gp120-trimer (debris) • Surface glycosylation: focused changes in glycan packing prevent neutralizing Ab binding but not receptor binding

27. Wei X. Nature, 2003.

28. HIV antibody responses (3) • BUT: some are specific for conserved regions, do neutralize primary virus, synergize • F105, b12 - CD4 binding site of gp120 • 2G12 - complex gp120 epitope • 2F5, 4E10, Z13 - gp41 • OTHERS just described • **Passive infusion of cocktail = ONLY model of sterilizing immunity (MCH, PEP trials) • ?Pre-formed Ab applicable via microbicides

30. Host defenses: CTL

31. Sewell A 2001

32. CTL responses: any good? • In primate models, vaccine-induced CTL can slow progression, improve viral control • Timing of CTL and control • CD8+ depletion experiments • CTL (CD8+) impose major immune pressure on virus (SIV, HIV) • HIV-specific CD4+, CD8+ responses found in exposed, uninfected populations

33. Immune time course post infection

34. Kiepela et al. Nature, 2004

35. CTL: not good enough… • Proviral latency - no antigen expressed • Downregulation of HLA class I (nef, vpu) • Upregulation of Fas ligand • Mutation: • epitope mutation prevents HLA binding, TLR binding • flanking mutations prevent processing • BUT do see benefits from a “less fit” virus • Impaired CD8+ function

36. Escape from CTL control Mutation: Other:

38. Cellular immune “exhaustion” Ahmed R, et al. J Exp Med, 2006.

39. HIV superinfection can occur • Despite strong CTL, can be infected by a second strain of HIV-1 • But may be less common than initial infection • ?? Half as likely to happen (very unclear)

40. Real life HIV protection? exposed uninfected individuals • People who “should be infected but aren’t” • sex workers, discordant couples, etc • Several correlates: • Lack of CCR5 • HIV specific cellular immunity: lysis, IFNg, proliferation (generally low level) • HIV neutralizing IgA • Dampened immune activation • ? Actually mediating protection vs. paraphenomenon

41. Immune correlates of HIV protection: long-term nonprogressors • People who “should be sick but aren’t” • Infected for >10 years, normal immune system, low VL • Also “elite controllers” - low/undetectable VL • Several correlates: • Certain class I HLA types: B5701/03, B27, etc • HIV specific cellular immunity: breadth? Function? • No good humoral associations

42. Polyfunctionality and survival Progressors LTNP Betts, M Blood, 2006

43. Vaccine-induced CTL: are they useful? • Macaque models - several show that inducing SIV/SHIV-specific CD8+ T cells can lower viral load, slow/prevent progression • Generally don’t prevent infection - but maybe could protect against “real” challenge? • Hard to induce using candidate vaccines • Case of human infection post vaccine despite strong CD8+ responses against dominant epitope

44. STEP TRIAL • Merck HIV vaccine • Adenovirus (Ad5) based, sole goal was to induce cellular immunity • Did so fairly well, BUT… • No protection against infection • No impact on post-infection VL • Increased HIV rates if prior adeno infection

45. Summary • Resistance to acquisition is the norm • Gut events / immune activation and disease • Cellular responses are primarily responsible for (inadequate) control post-infection • Antibody responses against specific epitopes may provide passive protection • Circumcision is an effective mucosal intervention