TCR-independent activation of human CD4+ 45RO- T cells by anti-CD28 plus IL-2: Induction of clonal expansion and priming for a Th2 phenotype.

In this study we show that uncommitted human CD4+ CD45RA+ RO- CD25- CD71- HLA-DR- T cells can be primed for a Th2 phenotype before they encounter TCR signals and before they are exposed to IL-4. We found that >99% of uncommitted T cells proliferated upon costimulation by immobilized anti-CD3 plus anti-CD28 mAbs and differentiated into pure Th1 cells. In contrast, uncommitted T cells did not respond to stimulation by either anti-CD3 or anti-CD28, or by IL-2 alone. Interestingly, 5% of uncommitted T cells proliferated efficiently in response to stimulation by immobilized anti-CD28 plus IL-2 (in the absence of TCR/CD3 signals) and differentiated into pure Th2 "precursor" cells. Like murine CD4+ NK1.1+ T cells, human Th2 precursors promptly expressed mRNA for Th2 cytokines upon stimulation via the TCR/CD3 complex by anti-CD3 mAb or staphylococcal enterotoxin B, and secreted up to 50 ng of IL-4, IL-5, and IL-13 per 10(6) cells. Th2 "precursors" developed only in the complete absence of IL-4, as addition of 0.1 U (5 pg) of exogenous IL-4 suppressed their clonal expansion by >90%, whereas addition of neutralizing anti-IL-4 mAb had no effect. Together these results suggest that, in vivo, a significant fraction of uncommitted T cells may be primed for a Th2 phenotype independent of Ag and IL-4 if they are exposed to Th1 cell-derived IL-2 and simultaneously interact with accessory cells bearing the natural CD28 ligands B7-1 and B7-2. When stimulated by specific Ag, such primed Th2 precursor cells may provide a source of IL-4 to promote Th2 immunity.