Immune Control of Hepatitis B Virus infection by innate lymphocytes.
PhD thesis, National University of Ireland Maynooth.
Hepatitis B virus (HBV) infects over 300 million people worldwide and despite the availability of a vaccine, it remains the second biggest carcinogen in the world and a serious global health problem. The majority of adult infections are resolved while 90% of neonates fail to clear infection. Persistently-infected individuals harbour a risk of developing liver disease and hepatocellular carcinoma (HCC). Treatment of HBV is met with several problems including poor tolerance and antiviral resistance and therefore, novel well-tolerated and effective therapies are required.
Innate lymphocytes include NK, NT, NKT and T cells and comprise a group of MHC-unrestricted lymphocytes that can elicit potent cytokine production and cytotoxicity at a very early stage of virus infection. We proposed that a better understanding of the role of innate lymphocytes in the control of HBV infection might eventually facilitate the development of new cell-based immunotherapies.
We have quantified the frequencies and cytokine profiles of natural killer (NK), natural T (NT), natural killer T (NKT) and T cells in a group of HBV patients with relatively low viral load and little evidence of liver disease in order to ascertain the role of these cells in controlled HBV infection. We found expansions in the frequencies of circulating NK and NT cells and the V2 and V1 subsets of T cells in these patients, compared to uninfected control subjects. Furthermore, expansions of interferon- (IFN-)-producing NT, V2 T and conventional T cells were significantly higher in this group as were the frequencies of total IFN--producing lymphocytes. Higher levels of IFN- expression in HBV were confirmed using qRT-PCR and we, therefore, propose that such expansions are indicative of an active antiviral immune response and that IFN- plays an important role in the control of HBV replication. Higher frequencies of IL-10-producing NK cells observed in the HBV patients may indicate a possible regulatory role of these cells in asymptomatic HBV infection. IL-17-producing NT and T cells exhibit enhanced responses to in vitro stimulation in HBV suggesting that IL-17 plays a supporting role in the control of HBV infection and that such Th17-biased cells can expand rapidly following sufficient stimulation. Since IL-10 and IL-17 can elicit inhibitory effects on IFN- production, we propose that the increased potential of IL-10 and IL-17 production in these patients might represent a regulatory mechanism that limits the antiviral immune response to prevent liver injury. We have also found reduced expression of the cytotoxicity-associated receptor NKG2D by V2 T cells
which may be another mechanism by which immune-mediated damage is limited in these HBV patients. Furthermore, we have found that HBV proteins have negative effects on IFN--production by V2 T cells suggesting that HBV can inhibit the effector mechanisms of these cells.
HCC is a serious endpoint of chronic HBV infection for which there are few curative options. We performed a preliminary study to survey the potential of HMBPP/IL-2 expanded T cells as an immunotherapy against HCC by investigating the interactions between Vδ2 T cells and the HCC-derived Hep3B cells. We found that the expression of NKG2D by V2 T cells and its ligand MICA/B on Hep3B cells was downregulated following co-culture. We propose that the shedding of MICA/B by the carcinoma-derived cells might be the mechanism by which NKG2D is downregulated and the effector functions of V2 T cells are subverted. Quantification of cytokine expression in Hep3B cell / Vδ2 T cell co-culture supernatants revealed higher IL-10 expression and reduced IFN- and TGF-β1 expression. From our analysis of these findings, we suggest that Hep3B cell-derived IL-10 might inhibit IFN- production by Vδ2 T cells as an immune evasion strategy while the Vδ2 T cells may suppress TGF-β1 expression by the epithelial cells to avoid the immunosuppressive effects of the growth factor.
From this study, we propose that we have identified a new model of the non-cytolytical control of HBV replication in which NT and V2 T cells minimise viral load without causing significant liver pathology via mechanisms that involve enhanced but regulated IFN- production. These results place HBV infection as a candidate disease that might benefit from cellular therapies involving innate T cells.
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