p38 MAP kinase signaling in microglia plays a sex-specific protective role in CNS autoimmunity and regulates microglial transcriptional states

Abstract Multiple sclerosis (MS) is an autoimmune demyelinating disease of the central nervous system. MS is three times more common in women, yet more severe in men. The mechanisms underlying these sex differences remain largely unknown. MS is initiated by autoreactive T cells, but CNS-infiltrating myeloid cells are key effector cells driving pathology. The role of CNS-resident microglial cells is less clear. We have previously shown that genetic ablation of p38α MAP kinase broadly in the myeloid lineage is protective in the autoimmune model of MS, experimental autoimmune encephalomyelitis (EAE), but only in females, and not males. Here, we sought to define the mechanisms responsible, using genetic approaches and bone marrow chimeras to ablate p38α in microglial cells, peripheral myeloid cells, or both. Deletion of p38α in both cell types recapitulated the previous sex difference, with reduced EAE severity in females. Unexpectedly, deletion of p38α in the periphery was protective in both sexes. In contrast, deletion of p38α in microglia exacerbated EAE in males only, revealing opposing roles of p38α in microglia vs. periphery. Single-cell RNAseq revealed a diversity of microglial states, connected by distinct convergent transcriptional trajectories. Microglial p38α deficiency resulted in enhanced transition from homeostatic to disease-associated microglia, with downregulation of regulatory genes such as Atf3, Rgs1, and Btg2, and upregulation of inflammatory genes such as Cd74, Trem2, and MHC class I and II. These results reveal a p38α-dependent male-specific molecular pathway in microglia that is protective in CNS autoimmunity, suggesting that autoimmunity in males and females may be driven by distinct cellular and molecular pathways.