Hepatitis C virus clearance correlates with HLA-DR expression on proliferating CD8+ T cells in immune-primed chimpanzees.

Worldwide >150 million individuals are chronically infected with hepatitis C virus (HCV) and ~3.2 million are infected in the USA alone (1). Studies show that 60–85% of acute infections become chronic and it has been determined that ~15–20% of all individuals that become infected with HCV progress to serious end-stage liver disease, such as cirrhosis or hepatocellular carcinoma (2). With ~18,000 new HCV infections occurring in the U.S. annually (3) the development of a vaccine against this virus is imperative. Although HCV persists in the majority of cases, acute infections are spontaneously resolved in ~20–25% of humans (4) and ~40% of chimpanzees (5). Chimpanzees represent the only animal model available to study pathogenesis of this viral disease and replicate the features of HCV infection in humans, such as kinetics of viremia and persistent infection even in the presence of immune responses (6). Spontaneous clearance following primary infection with HCV has been associated with effective T-cell responses, the major characteristics of which have been shown to be strong proliferating capacity of HCV-specific T-cells (7, 8), higher frequencies of CD8+ T-cells expressing perforin in the liver (9) and early and multispecific IFNγ-production by CD4+ and CD8+ T-cells (10, 11). Due to this association, several chimpanzee prophylactic vaccine studies have focused on T-cell induction targeting non-structural proteins for prevention of chronic HCV infection. Although these T-cell-based studies confirm that vaccine-induced T-cell responses can lead to memory cells and early control of viral replication (5) there is no greater incidence of viral clearance in vaccinees compared to naive animals when non-structural components are included in vaccines (5). In addition, we have previously shown that an ineffective T-cell vaccine can create greater pressure for viral mutation and therefore immune escape, which may lead to persistence of HCV (12). The failure of most T-cell vaccines to result in clearance of HCV is at odds with the knowledge that following spontaneous clearance and re-infection, viremia and liver disease are significantly reduced in both intensity and duration in chimpanzees (13, 14) and humans (4) and that this rapid clearance appears to be associated with faster T-cell activation (7, 14). We wished to assess if vaccine-induced T-cell responses differ qualitatively from those induced by natural infection and if memory T-cells with specific phenotypes in immune-primed animals can be associated with rapid and successful clearance of HCV. The initiation of the immune response starts with activation of T-cell receptors which leads to a signal cascade, up-regulation of activation markers and cellular proliferation. Analyses of hepatic gene expression in chimpanzees have suggested that efficient T-cell activation or recruitment is critical for HCV clearance during primary infections (15). The surface markers CD38 and/or HLA-DR have been used as measures of T-cell activation in many studies of acute and chronic infections, including HCV (16, 17), and during studies of yellow fever and smallpox vaccinees (18). These two markers in combination with Ki-67 and Bcl2 have been suggested as a means to identify CD8+ effector T-cell responses after immunization or infection (18). Ki-67 is a marker of proliferation in T-cells (19) while reduced Bcl2 expression indicates susceptibility to apoptosis; resting T-cells express high levels of the protein (20). We analyzed the frequency and kinetics of these markers of activation and proliferation on CD4+ and CD8+ T-cells in chimpanzees that were previously infected with HCV and had spontaneously cleared infection, vaccinated and control chimpanzees before and after infection with HCV and correlated this data with rapid control of the virus or progression to persistent infection.