Inhibition of the responses of a cloned CD4 T cell line to different class II major histocompatibility complex ligands by anti-CD4 and by anti-receptor Fab fragments are directly related

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Abstract
The responses of a single cloned T cell line to three different class II major histocompatibility complex (MHC) ligands have been compared for avidity, determined by inhibition with anti-T cell receptor Fab fragments directed at two different receptor epitopes, and for ease of inhibition with anti-CD4 antibody. It has, thus, been directly demonstrated that ease of inhibition of the response of a T cell to a class II MHC ligand by anti-CD4 is inversely related to the avidity of the T cell receptor for that ligand. The difficulties in inferring from this finding that CD4 acts primarily by increasing receptor avidity for its class II MHC ligand are discussed in the light of evidence suggesting that CD4 is an active signaling component of the T cell receptor for class II MHC ligands.