title: Evidence for a physical association of CD4 and the CD3:alpha:beta T-cell receptor.
abstract: CD4 is a molecule expressed on the surface of T lymphocytes which recognize foreign protein antigens in the context of class II major histocompatibility complex (MHC) molecules. Recognition of antigen:class II MHC complexes by CD4+ T cells can be inhibited by anti-CD4 (ref. 3). Nevertheless, specific recognition of the antigen:Ia complex is clearly a function of the T-cell receptor, which is composed of CD3 and the variable polypeptides alpha and beta. Thus, it has been proposed that CD4 serves an accessory function in the interaction of CD4+ T cells and Ia-bearing antigen-presenting cells by binding to non-polymorphic portions of class II MHC molecules and stabilizing the cell interaction. Based on our observation that anti-CD4 could inhibit activation of a cloned line of CD4+ T cells by antibodies directed at a particular epitope on the variable region of the T-cell receptor, we have recently proposed that CD4 is actually part of the T-cell antigen recognition complex, physically associated with CD3:alpha:beta. But numerous studies showing that CD3 and CD4 are not stably associated on the T-cell surface would appear to contradict this model. Here we show that anti-T-cell-receptor antibodies can co-modulate expression of the T-cell receptor and CD4, and that the monovalent Fab fragment of such an anti-T-cell-receptor antibody can, in conjunction with bivalent anti-CD4 antibody, generate an activating signal for the T cell. These findings provide further evidence for a physical association of the T-cell receptor complex and CD4.
mesh_terms: D000911:Antibodies, Monoclonal; D000945:Antigens, Differentiation, T-Lymphocyte; D000954:Antigens, Surface / Q000502:physiology*; D002462:Cell Membrane; D004705:Endocytosis; D005434:Flow Cytometry; D006684:HLA-DR Antigens / Q000276:immunology; D006801:Humans; D008213:Lymphocyte Activation; D046911:Macromolecular Substances; D011948:Receptors, Antigen, T-Cell / Q000502:physiology*; D013601:T-Lymphocytes / Q000276:immunology / Q000648:ultrastructure*
chemical_list: D000911:Antibodies, Monoclonal; D000945:Antigens, Differentiation, T-Lymphocyte; D000954:Antigens, Surface; D006684:HLA-DR Antigens; D046911:Macromolecular Substances; D011948:Receptors, Antigen, T-Cell