aasld abstract, off of embargo.......
Publishing ID: 237 Abstractid ID: 102152
DIFFERENTIAL REGULATION OF CD4+ AND CD8+ MEMORY T CELLS DURING HCV RECHALLENGE OF CHIMPANZEES
Michelina Nascimbeni, Eishiro Mizukoshi, Markus Bosmann, National Institutes of Health, Bethesda, MD; Marian Major, Kathy Mihalik, CBER, FDA, Bethesda, MD; Charles Rice, Rockefeller University, New York, NY; Stephen Feinstone, CBER, FDA, Bethesda, MD; Barbara Rehermann, National Institutes of Health, Bethesda, MD
The search for an HCV vaccine is linked to the question whether protective immunity exists and how it is regulated. Intriguingly, spontaneously recovered individuals maintain HCV-specific cellular rather than humoral immune responses. To determine components and kinetics of HCV-specific memory immune responses during reexposure to the virus, we rechallenged three HCV recovered, HCV envelope antibody negative chimpanzees with increasing doses of homologous HCV (1a) and one animal subsequently with heterologous HCV (1b). Frequency and effector function of HCV-specific CD4+ and CD8+ T cells in the blood and liver were analyzed by tetramer staining, cytokine enzyme linked immunospot assays, proliferation and cytotoxicity assays and related to the clinical and virological outcome. CD4+ memory T cells represented the most sensitive components of the HCV-specific immune response, because proliferative and IFNã+ CD4+ T cell recall responses were observed even after challenge with low titer inocula that caused neither detectable viremia nor CD8+ T cell responses. When viremia occurred after rechallenge with higher dose inocula, all chimpanzees controlled viral load at 2-4 log lower levels than in the previous infection. Viral clearance was achieved within 2-5 weeks without significant liver enzyme elevation. During this phase, HCV-specific CD4+ T cells responses preceded CD8+ T cell responses. Specifically, CD4+ T cells proliferated and produced IFN-g immediately at the onset of viremia. In contrast, HCV-specific, tetramer+ CD8+ T cells required a functional switch from proliferating CCR7+ central memory cells to cytokine producing CCR7- effector memory cells and started to produce IFN-ã only after the virus was cleared. HCV-specific CD4 + T cells were also the predominant T cell population expanded from the liver during rechallenge and were targeted against all viral nonstructural proteins. Finally, HCV-specific CD4+ T cell responses needed to be maintained for several months to completely eradicate the virus. In one animal, IFN-ã effector function in the absence of significant expansion of antigen-specific CD4+ and CD8+ T cells was only able to control, but not able to ultimately eradicate the virus. After a PCR-negative period of 4 months, disappearance of IFN-ã+ CD4+ T cell response was followed by viral recrudescence. Recrudescent viremia triggered weaker cellular immune responses which controlled but did not completely eradicate the virus. In summary, HCV-specific memory T cells of recovered chimpanzees responded sensitively to HCV challenge resulting in reduced viremia, earlier viral clearance and significantly less liver disease than in primary HCV infection. While viral control was associated with IFN-ã producing CD4 T cells and a switch of CD8 T cells from central memory to effector memory cells, ultimate eradication of the virus required proliferation and maintenance of HCV specific CD4+ T cells for several months. These data may guide the development and evaluation of candidate vaccines to induce protective immune responses. |
