Cytotoxic cells are a key factor governing the outcome of acute hepatitis C in humans and chimpanzees, and there is a consensus view that this response may define chronic infection vs. spontaneous resolution.
Although cytotoxic cells use multiple mechanisms to exert their antiviral activity, the release of granzymes into infected target cells is considered a dominant player in the cellular immune response against viruses and other intracellular pathogens. Interestingly, the recent findings that viruses can block granzyme activities, and that granzymes can directly target viral proteins suggest that the efficient clearance of viruses may be an integrated function, affected by viral and host factors at the granzyme-virus interface. These factors include (1) susceptibility of viral proteins to cleavage by specific granzymes, (2) presence of granzyme inhibitors, and (3) expression and access of distinct granzymes to infected target cells.
There is a great need for further understanding of the direct role of granzymes in the control of human viruses through the identification of viral substrates and inhibitors for these proteases. In addition, since different populations of cytotoxic lymphocytes express distinct subsets of granzymes, this information may guide the identification of cytotoxic cells with potential granzyme combinations that may naturally inactivate hepatitis C virus (HCV). Thus, the proposed studies are designed to identify novel antiviral pathways used by human cytotoxic lymphocytes to target HCV. We will dissect whether HCV encodes inhibitors against human granzymes, identify whether HCV-encoded proteins are cleaved and inactivated by granzymes, and define how different protease specificities of the various granzymes might converge to restrain HCV infection.
To search for these granzyme-HCV interactions, we will apply our significant expertise in defining viral inhibitors and substrates for all five human granzymes in HCV-expressing target cells using infectious and replicon systems from different strains of HCV. Defining whether and how granzymes interact with HCV, and the consequences of these interactions (viral resistance vs. viral clearance) is potentially critical to the understanding of mechanistic pathways in HCV pathogenesis.