New infectious mammary tumor virus superantigen with Vβ‐specificity identical to staphylococcal enterotoxin B (SEB)

Only few infectious mouse mammary tumor viruses (MMTV) have been characterized which induce a potent superantigen response in vivo. Here we describe the characterization of an MMTV which was isolated from milk of the highly mammary tumor‐prone SHN mouse strain. Exposure of newborn mice to milk‐borne MMTV (SHN) results in a very slow deletion of Vβ7, 8.1, 8.2 and 8.3 expressing peripheral T cells. Subcutaneous injection of adult mice with this virus induces a rapid and strong stimulation of all four affected Vβ‐subsets in vivo. Besides the strong T cell effect we observed an early proliferation and activation of the local B cell pool leading to the initial secretion of IgM followed by preferential secretion of IgG2a, by day 6. Sequence comparison of the polymorphic C terminus with known open reading frames revealed high homology to the endogenous provirus Mtv‐RCS. This is the first report of a virus having a complete overlap in Vβ‐specificity with a bacterial superantigen stimulating as many as 35 % of the whole CD4+ T cell repertoire including Vβ8.2.