p38α Mitogen-activated protein kinase is activated by CD28-mediated signaling and is required for IL-4 production by human CD4⁺CD45RO⁺ T cells and Th2 effector cells

T cell proliferation and cytokine production usually require stimulation via both the TCR/CD3 complex and the CD28 costimulatory receptor. Using purified human CD4⁺ peripheral blood T cells, we show that CD28 stimulation alone activates p38α mitogen-activated protein kinase (p38α). Cell proliferation induced by CD28 stimulation alone, a response attributed to CD4⁺CD45RO⁺ memory T cells, was blocked by the highly specific p38 inhibitors SB 203580 (IC₅₀ = 10-80 nM) and RWJ 67657 (IC₅₀ = 0.5-4 nM). In contrast, proliferation induced by anti-CD3 plus anti-CD28 mAbs was not blocked. Inhibitors of p38 also blocked CD4⁺ T cell production of IL-4 (SB 203580 IC₅₀ = 20-100 nM), but not IL-2, in response to CD3 and CD28 stimulation. IL-5, TNF-α, and IFN-γ production were also inhibited, but to a lesser degree than IL-4. IL-4 production was attributed to CD4⁺CD45RO⁺ T cells, and its induction was suppressed by p38 inhibitors at the mRNA level. In polarized Th1 and Th2 cell lines, SB 203580 strongly inhibited IL-4 production by Th2 cells (IC₅₀ = 10-80 nM), but only partially inhibited IFN-γ and IL-2 production by Th1 cells (<50% inhibition at 1 μM). In both Th1 and Th2 cells, CD28 signaling activated p38α and was required for cytokine production. These results show that p38α plays an important role in some, but not all, CD28-dependent cellular responses. Its preferential involvement in IL-4 production by CD4⁺CD45RO⁺ T cells and Th2 effector cells suggests that p38α may be important in the generation of Th2-type responses in humans.