Cnp Promoter-Driven Sustained ERK1/2 Activation Increases B-Cell Activation and Suppresses Experimental Autoimmune Encephalomyelitis

The ERK1/2 signaling pathway promotes myelin wrapping during development and remyelination, and sustained ERK1/2 activation in the oligodendrocyte (OL) lineage results in hypermyelination of the CNS. We therefore hypothesized that increased ERK1/2 signaling in the OL lineage would 1) protect against immune-mediated demyelination due to increased baseline myelin thickness and/or 2) promote enhanced remyelination and thus functional recovery after experimental autoimmune encephalomyelitis (EAE) induction. Cnp-Cre;Mek1DD-eGFP/+ mice that express a constitutively active form of MEK1 (the upstream activator of ERK1/2) in the OL lineage, exhibited a significant decrease in EAE clinical severity compared to controls. However, experiments using tamoxifen-inducible Plp-Cre(ERT);Mek1DD-eGFP/+ or Pdgfrα-Cre(ERT);Mek1DD-eGFP mice revealed this was not solely due to a protective or reparative effect resulting from MEK1DD expression specifically in the OL lineage. Because EAE is an immune-mediated disease, we examined Cnp-Cre;Mek1DD-eGFP/+ splenic immune cells for recombination. Surprisingly, GFP(+) recombined CD19(+) B-cells, CD11b(+) monocytes, and CD3(+) T-cells were noted when Cre expression was driven by the Cnp promoter. While ERK1/2 signaling in monocytes and T-cells is associated with proinflammatory activation, fewer studies have examined ERK1/2 signaling in B-cell populations. After in vitro stimulation, MEK1DD-expressing B-cells exhibited a 3-fold increase in CD138(+) plasmablasts and a 5-fold increase in CD5(+)CD1d(hi) B-cells compared to controls. Stimulated MEK1DD-expressing B-cells also exhibited an upregulation of IL-10, known to suppress the initiation of EAE when produced by CD5(+)CD1d(hi) regulatory B-cells. Taken together, our data support the conclusion that sustained ERK1/2 activation in B-cells suppresses immune-mediated demyelination via increasing activation of regulatory B10 cells.