Blocking of cell-mediated immunity to Moloney murine sarcoma virus-transformed cells by lactate dehydrogenase virus-antibody complex

The enhancement of Moloney murine sarcoma virus (M-MuSV) tumor growth in BALB/c mice previously infected with lactate dehydrogenase virus (LDV) was shown to depend on the timing of the inoculations. Mice infected with LDV either 3 or 4 days prior to the M-MuSV inoculations demonstrated enhanced tumor growth, whereas mice infected simultaneously with LDV and M-MuSV or with LDV 30 days prior to M-MuSV challenge did not demonstrate M-MuSV tumor enhancement. Circulating immune complexes (CIC) were isolated from serum pools of LDV-infected mice. IgM-containing CIC were isolated only between 3 and 9 days post LDV infection. CIC containing IgG₁ were isolated only between days 6 and 13, whereas IgG₂-containing CIC were not detected until 18 days post LDV infection. Coincubation of isolated immune complexes (IC) with M-MuSV-sensitized cytotoxic lymphocyte-transformed and M-MuSV-transformed target cells produced a 56% reduction in M-MuSV-specific cytotoxicity, with LDV-antibody IC isolated from the sera of mice 11 days post LDV infection. CIC isolated from the sera of mice 5 days and 25 days after infection with LDV demonstrated 11 and 9% reductions in M-MuSV cytotoxicity, respectively. LDV-antibody IC composed of IgG₁ antibody may act as blocking factors to M-MuSV-sensitized lymphocytes in vitro and may be responsible for the transient M-MuSV tumor enhancement observed in dually infected mice.