The co-receptor function of CD4.
CD4 is a critical component of the T cell receptor complex that recognizes peptides bound to MHC class II molecules. This can be observed at all stages of T cell development, activation, and function. CD4 has been termed a co-receptor to indicate that its most important activity is to bind the same peptide: self class II MHC complex as the T cell receptor and to transduce positive activating signals in conjunction with the T cell receptor. This behavior has been shown by several independent experimental systems: direct cross-linking of the T cell receptor to CD4, the inhibition of T cell activation by anti-CD4, the transfection of CD4 into CD4- T cells, and by the phenomenon of epitope interference, as described in this review. All of these approaches suggest that the participation of CD4 as a co-receptor in antigen: self class II MHC recognition potentiates activation by 100-fold. Given the complex nature of the ligand recognized by the T cell receptor, the co-receptor function of CD4 virtually eliminates the possibility of CD4 T cells recognizing peptides presented by class I MHC molecules, in keeping with many in vivo observations.
Janeway
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CA
Section of Immunobiology, Yale University School of Medicine, New Haven, CT 06510.
D000818:Animals; D015703:Antigens, CD / Q000502:physiology; D015704:CD4 Antigens / Q000502:physiology*; D000939:Epitopes; D006649:Histocompatibility Antigens / Q000502:physiology; D000949:Histocompatibility Antigens Class II / Q000502:physiology; D006801:Humans; D017493:Leukocyte Common Antigens; D008213:Lymphocyte Activation; D011948:Receptors, Antigen, T-Cell / Q000502:physiology*; D013601:T-Lymphocytes / Q000276:immunology
D015703:Antigens, CD; D015704:CD4 Antigens; D000939:Epitopes; D006649:Histocompatibility Antigens; D000949:Histocompatibility Antigens Class II; D011948:Receptors, Antigen, T-Cell; D017493:Leukocyte Common Antigens