Determination of the relationship between T cell responsiveness and the number of MHC-peptide complexes using specific monoclonal antibodies

We describe the generation of three mAbs that recognize the complex of the class II MHC molecule IEk bound to a peptide derived From the carboxyl term inus of moth cytochrome c (residues 95-103), Reactivities of these mAbs are sensitive to single alterations in the sequence of both helices of the MHC molecule and to the bound peptide, The epitopes of these reagents are dist inct but overlap substantially. One of these mAbs specifically blocks lymph okine release by T cells responsive to this complex but not others. We have used another to examine how the number of complexes on an APC is related t o its ability to stimulate T cells, We find that 200-400 complexes per cell are necessary and sufficient to induce a degree of stimulation, whereas ma ximum stimulation is achieved only if more than 5000 complexes are present. The analysis indicates that T cell activation Is a stochastic process.