Synthesis of Th2 cytokines by clonal T cells requires signaling through CD28 and CD2 molecules. Purified CD3⁻CD4⁺ cells from hypereosinophilic patients were cocultured with mature allogeneic irradiated DC in the absence (patient 1) or in the presence (patient 2) of 150 U/mL rIL-2 at a DC:T-cell ratio of 1:30. In successive experiments, CTLA4-Ig (10 μg/mL) or blocking MoAbs against B7-1 (CD80), B7-2 (CD86), CD2 (all used at 10 μg/mL), CD40 (100 ng/mL), or isotypic controls at the same concentrations were added to cocultures. After 5 days, supernatants were harvested for measurement of IL-5 concentrations by ELISA. Solid bars represent IL-5 synthesis during cocultures in the presence of the above-mentioned blocking molecules, and diamonds represent IL-5 production in the presence of corresponding isotypic controls. Results are expressed as percentage of IL-5 levels reached in the presence of blocking MoAbs compared with T-DC cocultures prepared in the absence of these MoAbs. Data from one of two experiments, which yielded similar results for each coculture condition, are shown.