Macrophage CD40-signaling drives experimental autoimmune encephalomyelitis.

The co-stimulatory CD40L-CD40 dyad plays a major role in multiple sclerosis (MS). CD40 is highly expressed on MHCII+ B cells, dendritic cells and macrophages in human MS lesions. Here we investigated the role of the CD40 downstream signaling intermediates TRAF2 and TRAF6 in MHCII+ cells in experimental autoimmune encephalomyelitis (EAE). Both MHCII-CD40-Traf2-/- and MHCII-CD40-Traf6-/- mice showed a reduction in clinical signs of EAE and prevented demyelination. However, only MHCII-CD40-Traf6-/- mice displayed a decrease in myeloid and lymphoid cell infiltration into the central nervous system (CNS) that was accompanied by reduced levels of TNF-α, IL-6, and IFN-γ. As CD40-TRAF6 interactions predominantly occur in macrophages, we subjected CD40flfl LysMcre mice to EAE. This myeloid specific deletion of CD40 resulted in a significant reduction in EAE severity, reduced CNS inflammation and demyelination. In conclusion, the CD40-TRAF6 signaling pathway in MHCII+ cells plays a key role in neuro-inflammation and demyelination during EAE. Concomitant with the fact that CD40-TRAF6 interactions are predominant in macrophages, depletion of myeloid CD40 also reduces neuro-inflammation. CD40-TRAF6 interactions thus represent a promising therapeutic target for multiple sclerosis.