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Characterization of Two Infectious Mouse Mammary Tumour Viruses: Superantigenicity and Tumorigenicity

Mouse mammary tumour virus (MMTV) is a type B retrovirus that causes mammary tumours in susceptible mice. MMTV encodes a superantigen (SAg) that has the property of stimulating T‐cell populations expressing a particular variable region of the T‐cell receptor (TCR) β chain (Vβ) and needs to be presented in the context of major histocompatibility complex (MHC) class II molecules. Previously, we described two exogenous MMTV, MMTV BALB14, which encodes a superantigen that induces the deletion of Vβ14+ T cells, and MMTV BALB2, which encodes a SAg that induces the deletion of Vβ2+ T cells. We now describe their biological activity: the deletions involve both CD4+ and CD8+ populations, are progressive and can be detected in blood, lymph nodes and spleen. Such deletions reflect, at least in part, those occurring during intrathymic development. Both BALB2 and BALB14 viral variants are capable of inducing a strong increase of Vβ‐specific T cells in BALB/c mice (I–A+, I–E+). However, when injected into the footpad, their initial stimulatory capacity differs in that the presence of MHC I–E molecules is essential only for the stimulation of Vβ2+ T cells. Both viral variants are able to induce deletion even in the absence of the I–E molecule in which case, however, deletion appears later and is less pronounced. Both exogenous MMTVs induce, at the end of a year, 30–35% of pregnancy‐dependent mammary adenocarcinomas.