Isotype Distribution and Specificity of the Antibody Response to Primary Moloney Murine Sarcoma Virus Infection in BALB/c Mice

The development and isotype distribution of Moloney murine leukemia virus (M-MuLV)-specific serum antibodies following primary inoculation with Moloney murine sarcoma/leukemia virus (M-MuSV/M-MuLV) in adult BALB/c mice have been investigated using an enzyme-linked immunosorbant assay (ELISA). The primary antibody responses to M-MuSV/M-MuLV consisted of the IgM, IgG2a, IgG2b, and IgG3 isotypes; no M-MuLV-specific serum IgG1 or IgA antibodies were detected. The detectable antibody response was biphasic, with an early peak of virus-specific titers seen between 10 and 15 days after inoculation and a second peak seen in regressor sera. Pooled regressor sera contained IgM, IgG2a, and IgG2b antibodies which bound to M-MuLV-expressing lymphoma cells. Immunoelectron microscopy with regressor sera showed IgG bound both to infected cell surfaces and to mature viral particles, while IgM bound only to infected cell surfaces. These findings were supported by immunoprecipitation analyses which demonstrated binding of the M-MuLV-specific antibodies to both virion-associated and cell-associated antigens encoded by the gag and env genes.