Inhibition of the FGL2:FcγRIIB/RIII Immunosuppressive Pathway Enhances Antiviral T cell and B Cell Responses Leading to Clearance of LCMV cl-13.

Persistent viruses evade immune detection by interfering with virus-specific innate and adaptive antiviral immune responses. Fibrinogen-like protein-2 (FGL2) is a potent effector molecule of CD4(+)CD25(+)FoxP3(+) regulatory T cells and exerts its immunosuppressive activity following ligation to its cognate receptor, FcRIIB/RIII. The role of FGL2 in the pathogenesis of chronic viral infection caused by lymphocytic choriomeningitis virus clone-13 (LCMV cl-13) was assessed in this study. Chronically infected fgl2(+/+) mice had increased plasma levels of FGL2, with reduced expression of the maturation markers, CD80, CD86 and MHC-II on macrophages and dendritic cells and impaired production of neutralizing antibody. In contrast, fgl2(-/-) mice or fgl2(+/+) mice that had been pre-treated with antibodies to FGL2 and FcRIIB/RIII and then infected with LCMV cl-13 developed a robust CD4(+) and CD8(+) antiviral T-cell response, produced high titred neutralizing antibody to LCMV and cleared LCMV. Treatment of mice with established chronic infection with antibodies to FGL2 and FcRIIB/RIII was shown to rescue the number and functionality of virus-specific CD4(+) and CD8(+) T cells with reduced total and virus-specific T-cell expression of programmed cell death protein 1 leading to viral clearance. These results demonstrate an important role for FGL2 in viral immune evasion and provide a rationale to target FGL2 to treat patients with chronic viral infection.