Use of a receptor competition assay to explore the interaction of the T cell antigen-specific receptor with its ligands.

The observation has previously been made that receptor-bearing cells in culture compete with each other for their ligand. As a result, at a fixed concentration of ligand, the fractional occupancy of the receptor will tend to fall as the number of cells is increased. We have demonstrated that T cells in culture also compete for their ligand, the combination of foreign antigen and the Ia molecule (antigen-Ia), and that this manifests itself as shifts in the antigen dose-response curves as the number of responding T cells is increased. Because of the complexity of T cell activation, modifications to the antigen that affected its stimulatory capacity (i.e., its potency) could come about by altering its interaction with either the T cell receptor or the Ia molecule. We could distinguish between these two possibilities by studying the extent to which the antigen dose-response curves shifted as the T cell number was increased. Amino acid substitutions in the antigen that affected the interaction with the T cell receptor caused changes in the dose-response curve shifts, whereas substitutions that decreased potency by other means did not cause such changes. Finally, two allelic forms of the Ia molecule that differed only slightly in their amino-terminal domain were used to present a single antigen to a T cell clone. Despite a difference in antigenic potency in the presence of these two Ia molecules, no difference was demonstrated in the avidity of the T cell receptor for either antigen-Ia combination. These results suggest that the antigen and the Ia molecule make physical contact during the process of antigen recognition, and that the potency of an antigen can vary as a result of its interaction with either the T cell receptor or the Ia molecule.