Characterization of two infectious mouse mammary tumour viruses: Superantigenicity and tumorigenicity

Mouse mammary tumour virus (MMTV) is a type B retrovirus that causes mammar y rumours in susceptible mice. MMTV encodes a superantigen (SAg) that has t he property of stimulating T-cell populations expressing a particular varia ble region of the T-cell receptor (TCR) beta chain (VP) and needs to be pre sented in the context of major histocompatibility complex (MHC) class II mo lecules. Previously, we described two exogenous MMTV, MMTV BALB14, which en codes a superantigen that induces the deletion of V beta 14+ T cells, and M MTV BALB2, which encodes a SAg that induces the deletion of V beta 2+ T cel ls. We now describe their biological activity: the deletions involve both C D4(+) and CD8(+) populations, are progressive and can be detected in blood, lymph nodes and spleen. Such deletions reflect, at least in part, those oc curring during intrathymic development. Both BALB2 and BALB14 viral variant s an capable of inducing a strong increase of VP-specific T cells in BALB/c mice (I-AS, I-E+). However, when injected into the footpad, their initial stimulatory capacity differs in that the presence of MHC I-E molecules is e ssential only for the stimulation of V beta 2+ T cells. Both viral variants are able to induce deletion even in the absence of the I-E molecule in whi ch case, however, deletion appears later and is less pronounced. Both exoge nous MMTVs induce, at the end of a year, 30-35% of pregnancy-dependent mamm ary adenocarcinomas.