T-cell activation in cAMP isomer– and PDE4 LMWI–treated cells. (A) Proliferative response and (B) IL-2 production of purified mature CD3⁺ T cells in the presence of the inhibitors as specified. The PKA activators (SP-8-Br-cAMP, 250 μM; PDE4 LMWI, 5 nM) reduce the proliferation and the IL-2 production significantly (P < .05). Cells were left unstimulated or were stimulated with anti-CD3 (precoated at a concentration of 10 μg mL^–1) plus soluble anti-CD28 (1 μg mL^–1), as indicated. Note the significant (P < .001 in the proliferation assay; P = .015 in the IL-2 assay) rescue of the PDE4 LMWI effect by the Rp-8-Br-cAMP (the PKA inhibitor used at 1000 μM). Results shown are the mean ± SD of at least 3 independent experiments.