TLR9 ligand sequestration by chemokine CXCL4 negatively affects central B cell tolerance.

Central B cell tolerance is believed to be regulated by B cell receptor signaling induced by the recognition of self-antigens in immature B cells. Using humanized mice with defective MyD88, TLR7, or TLR9 expression, we demonstrate that TLR9/MYD88 are required for central B cell tolerance and the removal of developing autoreactive clones. We also show that CXCL4, a chemokine involved in systemic sclerosis (SSc), abrogates TLR9 function in B cells by sequestering TLR9 ligands away from the endosomal compartments where this receptor resides. The in vivo production of CXCL4 thereby impedes both TLR9 responses in B cells and the establishment of central B cell tolerance. We conclude that TLR9 plays an essential early tolerogenic function required for the establishment of central B cell tolerance and that correcting defective TLR9 function in B cells from SSc patients may represent a novel therapeutic strategy to restore B cell tolerance.

Positive and negative selection shape the human naive B cell repertoire.

Although negative selection of developing B cells in the periphery is well described, yet poorly understood, evidence of naive B cell positive selection remains elusive. Using 2 humanized mouse models, we demonstrate that there was strong skewing of the expressed immunoglobulin repertoire upon transit into the peripheral naive B cell pool. This positive selection of expanded naive B cells in humanized mice resembled that observed in healthy human donors and was independent of autologous thymic tissue. In contrast, negative selection of autoreactive B cells required thymus-derived Tregs and MHC class II-restricted self-antigen presentation by B cells. Indeed, both defective MHC class II expression on B cells of patients with rare bare lymphocyte syndrome and prevention of self-antigen presentation via HLA-DM inhibition in humanized mice resulted in the production of autoreactive naive B cells. These latter observations suggest that Tregs repressed autoreactive naive B cells continuously produced by the bone marrow. Thus, a model emerged, in which both positive and negative selection shaped the human naive B cell repertoire and that each process was mediated by fundamentally different molecular and cellular mechanisms.