Induction of competence to respond to IL-4 by CD4+ T helper type 1 cells requires costimulation.

Rested murine CD4+ Th1 clones do not produce IL-4, but have previously been shown to be capable of responding to IL-4 if they are first activated with Ag and APC. In this study, we have examined the activation requirements for induction of competence to respond to IL-4 in these clones. TCR occupancy alone (given either as chemically fixed APC and Ag, anti-CD3, Con A, or ionomycin and PMA) was inadequate, but the addition of a source of costimulation to any of these stimuli resulted in complete induction of competence to respond to IL-4. Pretreatment of the Th1 clones with TCR occupancy alone induced an anergic state from which subsequent full stimulation with Ag and APC failed to give IL-4 responsiveness. Pretreatment of the cells with IL-2 alone was an inadequate signal to induce IL-4 responsiveness and only a partial response was obtained when TCR occupancy was combined with IL-2. Addition of anti-IL-2 and anti-IL-2R antibodies during full activation with APC and Ag gave a 50% inhibition of competence induction. These results demonstrate that costimulation, in addition to its role in IL-2 production, is an important second signal for inducing T cells to become competent to respond to IL-4.