Effects of CD28 co-stimulation on Th1 and Th2 cytokine production by ATRA-treated T cells. IL-5 and IFN-(proteins were quantitated by ELISA in supernatants of purified T cells cultured (1 × 106/ml) with immobilized anti-CD3 mAb (200 ng/ml) ± 1 ug/ml anti-CD28 mAb and treated with ETOH (□) or ATRA 10-7 M (■) for 48 h. The results are expressed as pg/ml (+/- SD) for IFN-(and IL-5. A paired T test was performed on the values derived from three separate experiments as described in the Methods. The P values listed with the brackets compared the significant changes in IFN-γ expression between anti-CD3 mAb +/- CD28 mAb in the presence or absence of ATRA. *P < 0.05 indicates significant differences between anti-CD3 mAb + control IgG1 treated samples and the other experimental groups.