Induction of cell surface expression of HLA antigens by human IFN-γ encoded by recombinant vaccinia virus

A recombinant vaccinia virus (VV) encoding human IFN-γ (VV-huIFN-γ) was constructed and its effects on MHC Ag expression in human and murine cells in vitro analyzed by flow cytometry. At high multiplicities of infection (5 pfu/cell) the IFN-γ expressed by vaccinia was not able to overcome the profound decrease of MHC concentrations, normally associated with VV infection, in any of the cells tested. However, at successively decreasing multiplicities of infection, a gradual increase in MHC class I concentration above control levels was observed in human 143B cells but not in murine L929 cells, thus indicating that the species specificity of IFN-γ is preserved in VV-huIFN-γ-infected cells. We infer from these data that the IFN-γ secreted by infected 143B cells is able to exert an MHC up-regulating effect on uninfected cells in the vicinity. Antiviral activity of the IFN-γ expressed by the virus was also assessed. Pretreatment for 24 to 48 h of human 143B cells with IFN-γ containing supernatants had a significant antiviral effect comparable to rhuIFN-γ. However, when added 1 h after virus infection, antiviral activity was much less evident. Also, the IFN-γ secreted by infected 143B cells in monolayers infected at low multiplicity did not efficiently inhibit spread of infection to other cells in the vicinity.