T cell proliferation in response to stimulation with Staphylococcus enterotoxin A (SEA) requires accessory cells that express class II major histocompatibility complex (MHC) molecules. Murine fibroblasts transfected with genes encoding the alpha and beta subunits of HLA-DR, DQ, or DP were used to show that the proliferative response of purified human T cells to SEA is dependent on class II molecules but is not restricted by the haplotype of the responder. Binding of fluoresceinated SEA to class II transfectants and precipitation of class II heterodimers with SEA-Sepharose show that the proliferative response is a result of SEA binding to class II molecules. The binding is specific for class II molecules and is independent of class II allotype or isotype. The ability of SEA to bind class II molecules may be a general characteristic of this class of antigens, now called "superantigens".
Class I-b genes constitute the majority of MHC class I loci. These monomorphic or oligomorphic molecules have been described in many organisms; they are best characterized in the mouse, which contains a substantial number of potentially intact genes. Two main characteristics differentiate class I-b from class I-a molecules: limited polymorphism and lower cell surface expression. These distinguishing features suggest possible generalizations regarding the evolution and function of this class. Additionally, class I-b proteins tend to have shorter cytoplasmic domains or in some cases may be secreted or may substitute a lipid anchor for the transmembrane domain. Some are also expressed in a limited distribution of cells or tissues. At least six mouse MHC class I-b molecules have been shown to present antigens to alpha beta or gamma delta T cells. Recent advances have provided insight into the physiological function of H-2M3a and have defined the natural peptide-binding motif of Qa-2. In addition, significant progress has been made toward better understanding of other class I-b molecules, including Qa-1, TL, HLA-E, HLA-G, and the MHC-unlinked class I molecule CD1. We begin this review, however, by arguing that the dichotomous categorization of MHC genes as class I-a and I-b is conceptually misleading, despite its historical basis and practical usefulness. With these reservations in mind, we then discuss antigen presentation by MHC class I-b molecules with particular attention to their structure, polymorphism, requirements for peptide antigen binding and tissue expression.
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