Visualizing T cell competition for peptide/MHC complexes: A specific mechanism to minimize the effect of precursor frequency

In vivo antigenic competition of naive CD4(+) TCR transgenic T cells was vi sualized by tracking cell division. Competition reduced both recruitment in to cell division and burst size per recruited precursor cell, minimizing th e effect of differences in precursor frequency while maintaining the dose-r esponse relationship with antigen. Competition was restricted to T cells of the same specificity, indicating that cells were competing for access to A S-MHC complexes rather than for Ag nonspecific factors. Moreover, the quali tative distinction between the responses to i.v. peptide and s.c. peptide/C FA was unaffected by precursor frequency. These data explain the paradoxica l ability of the immune system to tailor responses to the type and dose of Ag even in individuals with large differences in initial precursor frequenc y.