Regulatory B cell CCL3 competency promotes disease resolution and oligodendrogenesis in experimental autoimmune encephalomyelitis.

Regulatory B cells (Bregs) contribute to immune homeostasis via IL-10-dependent and independent pathways. To dissect additional mediators, we investigated splenic derived GM-CSF/IL-15 fusokine (GIFT15)-induced Bregs in experimental autoimmune encephalomyelitis (EAE) using transcriptomics and functional validation. Bulk RNA-seq of splenic Bregs revealed upregulation of Ccl3, GzmB, and Il27 subunits compared to resting B cells. Functional studies showed that CCL3-deficient Bregs failed to suppress disease, whereas GzmB-deficient Bregs retained efficacy, and IL-27 receptor signaling in recipients was dispensable. Flow cytometry demonstrated that CCL3 expression correlated with FoxP3⁺ Treg and Tr1 expansion, along with CD206⁺ macrophage polarization. In the CNS, transient, tissue-dependent increases in oligodendroglial markers O4 and GalC were detected. Collectively, these findings identify CCL3 as a non-redundant effector of Breg-mediated protection, acting primarily through peripheral T-cell and myeloid remodeling, with secondary CNS impacts. These results highlight the translational potential of CCL3-competent, spleen-derived GIFT15 Bregs for therapeutic modulation of autoimmune demyelination.