Type 4 phosphodiesterases (PDE4) are critical regulators in TCR signaling by attenuating the negative constraint of cAMP. In this study, we show that anti-CD3/CD28 stimulation of human primary CD4(+) T cells increases the expression of the PDE4 subtypes PDE4A, PDE4B, and PDE4D in a specific and time-dependent manner. PDE4A and PDE4D mRNAs as well as enzyme activities were up-regulated within 5 days, PDE4B showed a transient up-regulation with highest levels after 24 h. The induction was shown to be independent of different stimulation conditions and was similar in naive and memory T cell subpopulations. To elucidate the functional impact of individual PDE4 subtypes on T cell function, we used PDE4 subtype-specific short-interfering RNAs (siRNAs). Knockdown of either PDE4B or PDE4D inhibited IL-2 release 24 h after stimulation (time point of maximal IL-2 concentrations) to an extent similar to that observed with the panPDE4 inhibitor RP73401 (piclamilast). Substantial amounts of IFN-gamma or IL-5 were measured only at later time points. siRNA targeting PDE4D showed a predominant inhibitory effect on these cytokines measured after 72 h. However, the inhibition of all cytokines was most effective when PDE4 siRNAs were applied in combination. Although the effect of PDE4 inhibition on T cell proliferation is small, the PDE4D-targeting siRNA alone was as effective as the panPDE4 inhibitor, whereas PDE4A or PDE4B siRNAs had hardly an effect. In summary, individual PDE4 subtypes have overall nonredundant, but complementary, time-dependent roles in propagating various T cell functions and PDE4D is the form likely playing a predominant role.