Oestrogen-mediated protection of experimental autoimmune encephalomyelitis in the absence of Foxp3+ regulatory T cells implicates compensatory pathways including regulatory B cells.

Oestrogen (17β-oestradiol, E₂) is a highly effective treatment for experimental autoimmune encephalomyelitis (EAE) that may potentiate Foxp3+ regulatory T (Treg) cells, which in turn limit the expansion of encephalitogenic T-cell specificities. To determine if Treg cells constitute the major non-redundant protective pathway for E₂, we evaluated E₂ protection of EAE after targeted deletion of Foxp3 expression in Foxp3-DTR mice. Unexpectedly, E₂-treated Foxp3-deficient mice were completely protected against clinical and histological myelin oligodendrocyte glycoprotein (MOG)-35-55 peptide-induced EAE before succumbing to diphtheria toxin-induced mortality. This finding indicated the presence of alternative E₂-dependent EAE-protective pathways that could compensate for the lack of Treg cells. Further investigation revealed that E₂ treatment inhibited proliferation and expression of CCL2 and CXCL2, but enhanced secretion of interleukin-10 (IL-10) and IL-13 by MOG-35-55-specific spleen cells. These changes occurred concomitantly with increased expression of several chemokines and receptors, including CXCL13 and CXCR5, and the negative co-activation molecules, PD-L1 and B7.2, by B cells and dendritic cells. Furthermore, E₂ treatment resulted in higher percentages of spleen and lymph node T cells expressing IL-17, interferon-γ and tumour necrosis factor-α, but with lower expression of CCR6, suggesting sequestration of MOG-35-55 peptide-specific T cells in peripheral immune organs. Taken together, these data suggest that E₂-induced mechanisms that provide protection against EAE in the absence of Foxp3+ Treg cells include induction of regulatory B cells and peripheral sequestration of encephalitogenic T cells.