The minimal number of antigen‐major histocompatibility complex class II complexes required for activation of naive and primed T cells

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Abstract
Although previous studies have shown that 50–200 antigen‐major histocompatibility complex complexes (Ag‐MHC) are sufficient to stimulate significant secretion of interleukin (IL)‐2 from MHC class II‐restricted T cell hybridomas, there have been no studies of this nature on more physiologically relevant T cell populations. In this study we have analyzed the ligand requirements for stimulation of responses from naive and previously primed T cells derived from T cell receptor (TCR)‐transgenic animals whose TCR is specific for the pigeon cytochrome c (PCC) 88–104 peptide presented by I‐Ek. 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 quantities of IL‐2. Similarly, primed T cells required about 40 Ag‐MHC complexes to produce a significant proliferative response, whereas naive T cells required about 400 complexes. In contrast to these results, naive and primed T cells showed similar ligand requirements when early events in the T cell activation pathway were analyzed; i.e. TCR down‐modulation, CD69 and CD25 expression, and blast transformation. A further analysis of IL‐2 and IL‐2R expression indicated: 1) The first synthesis of IL‐2 was detected at the same ligand concentration in both primed and naive T cells, but primed T cells made much more IL‐2 as the ligand concentrations increased; 2) primed T cells expressed about fivefold more IL‐2 receptor (R) than naive T cells, despite the fact that the antigen doseresponse curves with respect to the percentage of cells expressing IL‐2R were identical. These results suggest that naive and primed T cells have the same threshold with respect to the number of Ag‐MHC complexes required to initiate T cell activation, but that due to the inefficient expression of IL‐2 and IL‐2R, engagement of more complexes is needed to enable naive T cells to synthesize the necessary amounts of these two molecules to allow T cells to go through a complete cycle of replication.