Role of Accessory Molecules in the Superantigen-Induced Activation of Peripheral Blood T Cells

Among the antigens presented by the major histocompatibility complex (MHC) class II molecules, the superantigens (SAgs) constitute a particular family of ligands that are able to signal via the MHC class II molecules (1 ). The SAg are characterized by their ability to bind to both the MHC class II molecules (2 ,3 ) and to the Vβ region of the TCR (4 ). The formation of this trimolecular complex results in the activation of all T cells bearing the appropriate T-cell receptor (TCR) V_β family (5 ). This T-cell activation by SAg has been described as being MHC class II molecule-dependent (although unrestricted). However, the expression of MHC class II molecules on some antigen-presenting cells (APCs) is not always sufficient to induce a T-cell response to the SAg as was observed in studies of MHC class II expressing astrocytes (6 ), keratinocytes (7 ) and a class II-transfected L line (8 ). These results could be interpreted as suggesting that co-stimulatory molecules are necessary, in addition to MHC class II molecules, to lead to a full response to SAg.