TCR reserve: a novel principle of CD4 T cell activation by weak ligands.

Some ligand-receptor systems have a receptor reserve where a maximal response can be achieved by occupation of a fraction of available receptors. An implication of a receptor reserve is the expansion of the number of ligands for response. To determine whether T cells follow receptor reserve, we have characterized the effect of reducing TCR levels on CD4 T cell responses elicited by altered peptide ligands that vary in potency. Agonist peptide is unaffected by a 90% reduction in TCR level while proliferation to weak agonists is significantly inhibited when TCR expression is reduced by 40%. Thymocyte-negative selection similarly demonstrates a differential requirement of TCR for response to agonist, weak agonist, and partial agonist. Therefore, our data demonstrate receptor reserve as a novel principle of T cell activation in which excess TCRs expand the antigenic repertoire to include less potent ligands.