Virus-specific CD8+ T cells play an essential role for the elimination of HCV infection;
however, the exact mechanisms contributing to CD8+ T cell failure in persistent infection
are not completely understood. In chronically infected patients, HCV-specific CD8+
T cells are rarely detectable ex vivo by conventional methods. Thus, it is still an
open question whether the absence of these cell populations is due to a lack of priming
early in infection or caused by deletion during persistent infection. To address this
important question, we performed a novel tetramer enrichment protocol by using HLA-A*02-restricted
NS31073- and NS31406-tetramers and a large panel of surface marker antibodies to detect
and phenotypically characterize HCV-specific CD8+ T cells ex vivo in chronically infected
patients (genotype 1a). By using this sensitive method, we were able to detect HCV-specific
CD8+ T cells in all tested patients. Interestingly, these cells displayed two different
differentiation states: in the majority of patients HCV-specific CD8+ T cells displayed
an exhausted effector-memory phenotype characterized by the expression of PD-1. Surprisingly,
however, 40% of HCV-specific CD8+ T cell populations displayed a naïve phenotype,
suggesting that these cells have never been primed even despite ongoing viral replication.
In order to analyze potential mechanisms that may explain the presence of naïve CD8+
T cells, we stimulated HCV-specific CD8+ T cells in vitro by using monocyte-derived
autologous dendritic cells pulsed with respective peptides. Importantly, the majority
of naïve HCV-specific CD8+ T cells could be primed and expanded in culture, suggesting
the absence of intrinsic T cell defects. Next, we analyzed the corresponding viral
sequence in respective patients. Consensus sequences were present in half of patients
harboring naïve HCV-specific CD8+ T cells clearly suggesting that the presence of
naïve HCV-specific CD8+ T cells cannot be explained by the absence of matching antigen.
Taken together, our results indicate that HCV-specific CD8+ T cells are present in
all chronically HCV-infected patients, that the majority of these cell populations
display an exhausted effector-memory phenotype but that also a substantial fraction
displays a naïve phenotype. This suggests that a lack of priming contributes to the
absence of detectable HCV-specific CD8+ T cells in many chronically infected patients.
Our data also suggest that the lack of priming is not due to intrinsic T cell defects
or viral sequence variations. Thus, failure of HCV-specific CD8+ T cells is not limited
to viral escape or T cell exhaustion but may also be explained by impaired T cell
priming. Since naïve HCV-specific CD8+ T cells in chronically infected patients can
sufficiently respond to antigen-specific stimulation, these data offer new insights
for therapeutic immunotherapy.
