The role of CD28/B7 and LFA-1/ICAM costimulation in proliferation and Th1/Th2 differentiation of naive CD4+ T cells was addressed using T cells from DO11. 10 TCR transgenic mice stimulated by dendritic cells. The blockade of either CD28/B7 or LFA-1/ICAM interactions partially inhibited T cell proliferation. By comparison, blocking CD28/B7 costimulation inhibited IL-4 and IL-5 (Th2 cytokine) production, whereas blocking LFA-1/ICAM-1 or LFA-1/ICAM-2 led to a significant increase (15-to 40-fold) of Th2 cytokines. The combination of anti-ICAM-1 and anti-ICAM-2 mAbs had a synergistic effect with a 100-to 1000-fold increase of Th2 cytokine production. Thus, these two costimulatory pathways have opposing roles in the regulation of Th2 development.