Flow Cytometric Detection of MMTV Superantigens

Mouse mammary tumor viral superantigens (vSAgs) are produced by germline-encoded proviruses and infectious viruses (for reviews see refs. 1,2 . Like other superantigens, they interact with class II major histocompatiblity complex (MHC) proteins and trigger T-cell proliferation via recognition of particular variable elements (V_β ) of the T-cell receptor (TCR). Prior to the discovery of their viral origin, the provirus-encoded vSAgs were referred to as Mls (minor lymphocyte stimulatory) antigens (3). Unlike the bacterial superantigens, which are produced as small soluble proteins, the vSAgs are produced as membrane glycoproteins that transit the exocytic pathway and undergo partial proteolytic processing by cellular endoproteases (4). Proteolytic processing appears to be necessary for vSAgs to activate T cells (5-7), and available data suggest that a soluble carboxy terminal processing product may be sufficient for vSAg function (8 ). The vSAgs require class II MHC proteins to be presented to T cells (9 , although not necessarily for surface expression (8 ).