Chronic Infection
In an experimental model of fulminant hepatic failure (FHF) caused by the coronavirus mouse hepatitis virus strain 3 (MHV-3), increased plasma levels of FGL2 as well as increased frequencies of Treg pre- and post- viral infection are predictive of susceptibility and severity of disease. Inhibition of FGL2 by antibody or an siRNA to exon-1 of the mouse fgl2 gene which included the translation initiation site AUG fully protected susceptible animals from the lethality of MHV-3 whereas adoptive transfer of wild-type Treg into resistant fgl2-deficient (fgl2-/-) animals accelerated their mortality. These studies provide strong evidence for the role of FGL2 in the pathogenesis of FHF (Published in Shalev et al., Hepatology, 2009 and Zhu C et al., Hum. Gene. Ther., 2006). Our laboratory has examined the contribution of fgl2 in both a model of acute viral hepatitis caused by LCMV WE and chronic hepatitis caused by LCMV Cl 13 Following LCMV WE infection, plasma levels of FGL2 in littermate control (fgl2+/+) mice were markedly increased at all time points post-infection (pi) compared to uninfected mice and remained elevated until day 40 pi. Targeted deletion of fgl2 led to increased expression of maturation markers of DC as well as increased numbers of LCMV-specific CD8+ T cells in both the peripheral and the intrahepatic compartments of fgl2-/- mice compared to fgl2+/+ mice pi. LCMVinfected fgl2-/- mice had increased numbers of CD138+CD45lowCD19low plasma cells and increased titers of total LCMV specific and neutralizing antibodies. Enhanced anti-viral immunity in fgl2-/- mice was associated with increased hepatic necrosis and inflammation following LCMV WE infection (Abstract published in The Liver Meeting AASLD 2011; Manuscript in preparation- will be provided upon request). These findings are consistent with the immunosuppressive activity of FGL2 observed previously in the mouse model of FHF caused by MHV-3 infection and provide important new information on the importance of FGL2 on adaptive T and B cell immunity in a relevant model of acute viral infection caused by LCMV WE. We have now initiated studies to define the role of FGL2 in a mouse model of chronic hepatitis caused by LCMV Cl 13. C57BL/6J wild-type mice infected with LCMV Cl 13 develop as reported by others a persistent chronic hepatitis as indicated by persistently elevated levels of ALT, a marker of liver injury; marked periportal and portal lymphocytic infiltration and a lobular hepatitis on day 28 pi which persisted for greater than 120 days; together with bridging fibrosis detected by trichrome staining. By plaque assay and immunohistochemistry, virus was localized primarily in the cytoplasm and on the membrane of hepatocytes although virus was also seen in RE cells. Levels of FGL2 were markedly elevated in LCMV Cl 13 infected mice and at all time points were higher than levels of FGL2 in LCMV WE-infected mice. While in LCVM WE-infected mice FGL2 levels returned to normal coincident with resolution of acute infection, in Cl 13-infecetd mice levels of FGL2 remained persistently elevated. In a preliminary set of studies with Cl 13, we observed enhanced T cell responses as well as accelerated clearance of virus in fgl2-/- mice compared to fgl2+/+ mice postinfection. Furthermore, at day 28 post-infection fgl2-/- mice were shown to be free of liver disease as indicated by normal levels of serum ALT, whereas wild-type mice displayed elevated levels of ALT indicating the on-going chronic disease in these mice (Unpublished data- will be provided upon request ). A recent report from Wherry and Ahmed has shown that mice chronically infected with LCMV Cl 13 have increased expression of mRNA for fgl2 and FcγRIIB in functionally impaired virus-specific CD8+ T cells supporting the key role of the FGL2-FcγRIIB inhibitory pathway in inhibition of anti-viral T cell responses, which results in chronic infection and disease (Published in Wherry et al., Immunity, 2007)