Signal transduction by the major histocompatibility complex class I molecule

Ligation of cell surface major histocompatibility class I (MHC-I) proteins by antibodies, or by their native counter receptor, the CD8 molecule, media tes transduction of signals into the cells. MHC-I-mediated signaling can le ad to both increased and decreased activity of the MHC-I-expressing cell de pending on the fine specificity of the anti-MHC-I antibodies, the context o f CD8 ligation, the nature and cell cycle state of the MHC-I-expressing cel l and the presence or absence of additional cellular or humoral stimulation . This paper reviews the biochemical, physiological and cellular events imm ediately after and at later intervals following MHC-I ligation. It is hypot hesized that MHC-I expression, both ontogenically and in evolution, is driv en by a cell-mediated selection pressure advantageous to the MHC-I-expressi ng cell. Accordingly, in addition to their role in T-cell selection and fun ctioning, MHC-I molecules might be of importance for the maintenance of cel lular homeostasis not only within the immune system, but also in the interp lay between the immune system and other organ systems.