Feline leukemia virus envelope gp70 of subgroups B and C defined by monoclonal antibodies with cytotoxic and neutralizing functions.

Nine murine monoclonal antibodies (MAb) to the envelope proteins of feline leukemia virus (FeLV) are described. Eight MAb are directed to epitopes of the same molecular species of gp70 and the other MAb is directed to the p15E moiety. Six of the gp70 epitopes are discrete; two are closely associated or overlapping. Four anti-gp70 MAb (2 of IgG2A and 2 of IgG2B subclasses) were directly cytotoxic for FeLV-producer lymphoma cells with cat or with rabbit complement (C). Another MAb (IgG2B), which was not cytotoxic alone, specifically and synergistically increased the cytotoxic effects of both IgG2A MAb. Cytotoxic anti-gp70 MAb also had virus-neutralizing capacity; one MAb recognized a determinant common to all FeLV subgroups (A, B, and C), the others recognized gp70 epitopes not present on subgroup A but common to both B and C subgroups. Competitive inhibition of MAb binding was employed to map spatial distributions of the epitopes, and the results fitted a molecule shaped as an incomplete loop. According to the model, epitopes involved with cytotoxic and virus neutralizing antibody functions were closely associated; the region involved is approximately in the center of the molecule, and it contains epitopes that are variably expressed among individual isolates of FeLV derived from different cat lymphoma cell lines.