A maternally inherited superantigen encoded by a mammary tumour virus

A collection of superantigens, molecules which in combination with class II major histocompatibility complex (MHC) engage T cells bearing particular V beta chains as part of their alpha beta receptors, have recently been described. The mouse self superantigen, Mls-1a, for example, in conjunction with many MHC class II proteins, engages mouse T cells bearing V beta 6, 7, 8.1 and 9, almost regardless of the sequences of the other variable components of the receptors on the T cells. Two types of superantigen have been identified so far: first, superantigens encoded in the mouse genome, such as Mls-1a; second, superantigens produced by bacteria, such as the staphylococcal enterotoxins. Although the latter type of superantigens are in many cases known to be proteins of about 220 amino acids, nothing is known about the structures of any of the superantigens encoded in mouse. Here we describe the properties of a new mouse superantigen. The antigen is maternally transmitted in milk and is probably encoded by a mammary tumour virus (MTV). Given the known genetic linkage between at least one of the mouse genomic superantigens and endogenous MTV integration sites, it is tempting to speculate that the superantigen described here and some of the endogenous mouse superantigens are encoded by MTVs.