Coexpression of PD-1, 2B4, CD160 and KLRG1 on exhausted HCV-specific CD8+ T cells is linked to antigen recognition and T cell differentiation

CD8+ T-cell differentiation in chronic HCV infection

HCV has the tendency to persist in the majority of infected patients despite the presence of virus-specific CD8+ T-cell responses. The HCV-specific CD8+ T-cell pool consists of heterogenous subsets that can be elegantly distinguished by CD127 expression. Indeed, HCV-specific CD8+ T cells with high levels of CD127 expression are characterized by features consistent with memory differentiation, while HCV-specific CD8+ T cells with low levels of CD127 expression display an exhausted phenotype characterized by the coexpression of PD-1 and other inhibitory receptors. In this article, the current knowledge about the phenotypes and differentiation profiles of HCV-specific CD8+ T cells is summarized. Clearly, the differentiation of HCV-specific CD8+ T cells in chronic HCV infection results from complex virus–host interactions, and both viral and immunological factors shape the HCV-specific CD8+ T-cell pool.

Adaptive immunity to the hepatitis C virus

The hepatitis C virus (HCV) is a global public health problem affecting approximately 2% of the human population. The majority of HCV infections (more than 70%) result in life-long persistence of the virus that substantially increases the risk of serious liver diseases, including cirrhosis and hepatocellular carcinoma. The remainder (less than 30%) resolves spontaneously, often resulting in long-lived protection from persistence upon reexposure to the virus. To persist, the virus must replicate and this requires effective evasion of adaptive immune responses. In this review, the role of humoral and cellular immunity in preventing HCV persistence, and the mechanisms used by the virus to subvert protective host responses, are considered.

Induction and function of virus-specific CD4+ T cell responses

CD4+ T cells - often referred to as T-helper cells - play a central role in immune defense and pathogenesis. Virus infections and vaccines stimulate and expand populations of antigen-specific CD4+ T cells in mice and in man. These virus-specific CD4+ T cells are extremely important in antiviral protection: deficiencies in CD4+ T cells are associated with virus reactivation, generalized susceptibility to opportunistic infections, and poor vaccine efficacy. As described below, CD4+ T cells influence effector and memory CD8+ T cell responses, humoral immunity, and the antimicrobial activity of macrophages and are involved in recruiting cells to sites of infection. This review summarizes a few key points about the dynamics of the CD4+ T cell response to virus infection, the positive role of pro-inflammatory cytokines in the differentiation of virus-specific CD4+ T cells, and new areas of investigation to improve vaccines against virus infection.

T-cell dysfunction and inhibitory receptors in hepatitis C virus infection

Dysfunction of the virus-specific T cells is a cardinal feature in chronic persistent viral infections such as one caused by hepatitis C virus (HCV). In chronic HCV infection, virus-specific dysfunctional CD8 T cells often overexpress various inhibitory receptors. Programmed cell death 1 (PD-1) was the first among these inhibitory receptors that were identified to be overexpressed in functionally impaired T cells. The roles of other inhibitory receptors such as cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) and T cell immunoglobulin and mucin domain-containing molecule 3 (Tim-3) have also been demonstrated in T-cell dysfunctions that occur in chronic HCV patients. Blocking these inhibitory receptors in vitro restores the functions of HCV-specific CD8 T cells and allows enhanced proliferation, cytolytic activity and cytokine production. Therefore, the blockade of the inhibitory receptors is considered as a novel strategy for the treatment of chronic HCV infection.