THE MINIMAL NUMBER OF ANTIGEN-MAJOR HISTOCOMPATIBILITY COMPLEX CLASS-II COMPLEXES REQUIRED FOR ACTIVATION OF NAIVE AND PRIMED T-CELLS

Although previous studies have shown that 50-200 antigen-major histoco mpatibility complex complexes (Ag-MHC) are sufficient to stimulate sig nificant secretion of interleukin (IL)-2 from MHC class II-restricted T cell hybridomas, there have been no studies of this nature on more p hysiologically relevant T cell populations. In this study we have anal yzed the ligand requirements for stimulation of responses from naive a nd previously primed T cells derived from T cell receptor (TCR)-transg enic animals whose TCR is specific for the pigeon cytochrome c (PCC) 8 8-104 peptide presented by I-E-k. Primed T cells were as sensitive as the previously reported T cell hybridomas, requiring about 100 Ag-MHC complexes to synthesize readily detectable quantities of IL-2, whereas naive T cells required 15 times more ligand to produce equivalent qua ntities of IL-2. Similarly, primed T cells required about 40 Ag-MHC co mplexes to produce a significant proliferative response, whereas naive T cells required about 400 complexes. In contrast to these results, n aive and primed T cells showed similar ligand requirements when early events in the T cell activation pathway were analyzed; i.e. TCR down-m odulation, CD69 and CD25 expression, and blast transformation. A furth er analysis of IL-2 and IL-2R expression indicated: 1) The first synth esis of IL-2 was detected at the same ligand concentration in both pri med and naive T cells, but primed T cells made much more IL-2 as the l igand concentrations increased; 2) primed T cells expressed about five fold more IL-2 receptor (R) than naive T cells, despite the fact that the antigen dose-response curves with respect to the percentage of cel ls expressing IL-2R were identical. These results suggest that naive a nd primed T cells have the same threshold with respect to the number o f Ag-MHC complexes required to initiate T cell activation, but that du e to the inefficient expression of IL-2 and IL-2R, engagement of more complexes is needed to enable naive T cells to synthesize the necessar y amounts of these two molecules to allow T cells to go through a comp lete cycle of replication.