p38γ and p38δ modulate innate immune response by regulating MEF2D activation

Evidence implicating p38{gamma} and p38{delta} (p38{gamma}/p38{delta}) in inflammation are mainly based on experiments using p38{gamma}/p38{delta} deficient (p38{gamma}/{delta}-/-) mice, which show low levels of TPL2, the kinase upstream of MKK1-ERK1/2 in myeloid cells. This could obscure p38{gamma}/p38{delta} roles, since TPL2 is essential for regulating inflammation. Here we generated a p38{gamma}D171A/D171A/p38{delta}-/- (p38{gamma}/{delta}KIKO) mouse, expressing kinase-inactive p38{gamma} and lacking p38{delta}. This mouse exhibited normal TPL2 levels, making it an excellent tool to elucidate specific p38{gamma}/p38{delta} functions. p38{gamma}/{delta}KIKO mice showed a reduced inflammatory response and less susceptibility to LPS-induced septic shock and Candida albicans infection than wild-type mice. Gene expression analyses in LPS-activated WT and p38{gamma}/{delta}KIKO macrophages revealed that p38{gamma}/p38{delta} regulated numerous genes implicated in innate immune response. Additionally, phospho-proteomic analyses and in vitro kinase assays showed that the transcription factor myocyte enhancer factor-2D (MEF2D) was phosphorylated at Ser444 via p38{gamma}/p38{delta}. Mutation of MEF2D Ser444 to the non-phosphorylatable residue Ala increased its transcriptional activity and the expression of iNOS and IL-1{beta} mRNA. These results suggest that p38{gamma}/p38{delta} govern innate immune responses by regulating MEF2D phosphorylation and transcriptional activity.