ICAM-1 and B7-1 provide similar but distinct costimulation for CD8(+) T cells, while CD4(+) T cells are poorly costimulated by ICAM-1

The function of purified ICAM-1 in costimulating CD4(+) and CD8(+) T cell r esponses has been directly compared to that of B7-1 in a model system that minimizes contributions of other receptor-ligand interactions. While B7-1 c ostimulates both subsets of T cells, ICAM-1 is much more effective in the c ostimulation of CD8(+) cells. ICAM-1 also synergizes with B7-1 for the indu ction of IL-2 production in CD8(+) but not CD4(+) T cells. These difference s are not explained by differences in LFA-1 receptor expression on the two subsets of T cells. The CD8(+) T cell response to ICAM-1 costimulation is a ssociated with increased proliferation and IL-2 production at levels simila r to those seen with B7-1 costimulation, but clonal expansion in response t o ICAM-1 is not as great due to decreased cell survival. ICAM-1-mediated co stimulation is effective for both naive and memory CT8(+) T cells, is indep endent of CD28 engagement, and does not appear to be due solely to effects on adhesion. These results suggest that ICAM-l-dependent, B7-independent co stimulation may be important in initiating a CTL response to class I antige n presented by cells that are not professional APC.