Regulatory B cells suppress GVHD and prolong survival of GVHD via membrane-bound IL-35 and secretion of exosomes with membrane-bound IL-35

Interleukin 35-producing B-cells (i35-Bregs) suppress autoimmune diseases and IL-35 is immunosuppressive member of IL-12 cytokine family. It is comprised of p35 and Ebi3 subunits which are assumed to be independently secreted proteins that associate in-vivo to form non-covalently linked heterodimeric IL-35. In view of recent reports that p35 and Ebi3 are not secreted as a heterodimer but act as independent anti-inflammatory cytokines, we investigated whether i35-Bregs can suppress GVHD by secreting p35, Ebi3 or heterodimeric IL-35. Surprisingly, i35-Bregs ameliorate and prolong survival of Graft-Versus-Host-Disease (GVHD) via membrane-bound IL-35 and secrete exosomes with membrane-bound IL-35 (i35-exosomes) that upregulate checkpoint inhibitors (PD-1/LAG-3) and induce T-cell exhaustion, thereby rendering allogeneic T-cells less active, with diminished proinflammatory effects. i35-exosomes also propagate infectious tolerance mechanism by binding bystander B-lymphocytes and converting them to IL-35 producers. Adoptive transfer of i35-Bregs or i35-exosomes suppress GVHD through these mechanisms, indicating that i35-Bregs/i35-exosomes combination-immunotherapy maybe effective therapy for GVHD.