Background/Purpose: Age-associated B cells (ABCs) are a novel B cell subset, which expresses CD11c and preferentially accumulates with age in female mice. Increased expansion of ABCs has also been observed in some autoimmune-prone mice. ABC differentiation is promoted by T cell help and TLR7 stimulation. Although the generation of ABCs is known to rely on the transcription factor T-bet, the molecular mechanisms regulating this novel B cell subset are poorly understood. Our laboratory previously identified a protein termed DEF6, which shows significant homology to only one other protein, SWAP-70. Notably, the human DEF6 locus has recently been identified as a new SLE risk variant and mice deficient in both DEF6 and SWAP-70 (DKO mice) spontaneously develop a lupus-like disease on a C57BL/6 background. Similarly to human SLE, the lupus-like disorder in DKO mice preferentially affects the female sex. Autoimmunity in DKO mice is characterized by a number of T cell abnormalities including accumulation of T_FH cells and increased IL-21 production. Here we have investigated whether DEF6 and SWAP-70 regulate the ABC subset.

Methods: Presence of CD11c⁺CD11b⁺B220⁺CD19⁺ B cells (ABCs) was evaluated by FACS in WT and DKO mice as well as in DKO mice concomitantly lacking either IL-21 or SAP. ABCs were FACS-sorted from spleens and stimulated in vitro with TLR7 ligands or generated in vitro by culturing CD23⁺ follicular B cells with aIgM, aCD40, and IL-21±TLR7 ligands. ABC markers and gene expression were analyzed by FACS, qPCR and western blot.

Results: We have found that DKO mice exhibit a marked accumulation of ABCs in the spleen but not in the draining lymph nodes or bone marrow. ABC expansion was higher in DKO female mice as compared to DKO male mice. ABC expansion in DKO mice was dependent on both DEF6 and SWAP-70 since mice lacking only DEF6 or only SWAP-70 did not show accumulation of ABCs. Expansion of ABCs in DKO mice was dependent on both IL-21 and T-B cell interactions as demonstrated by a significant reduction of ABCs in DKO mice concomitantly lacking either IL-21 or SAP. In vitro stimulation of CD23⁺ B cells with aBCR, aCD40, +/- IL-21, furthermore, demonstrated increased frequencies of CD11c⁺CD11b⁺Tbet⁺ ABCs from DKO B cells stimulated with IL-21. FACS-sorted ABCs from DKO mice stimulated in vitro with TLR7 ligands produced high levels of IgG2a/c anti-dsDNA autoantibodies compared to follicular B cells, confirming that ABCs are capable of producing autoantibodies.

Conclusion: Our study demonstrate that the absence of DEF6, a genetic risk factor for lupus, leads to the aberrant expansion of ABCs, a novel B cell subset previously associated with the development of autoimmunity. We also show that accumulation of ABCs in murine lupus is dependent on T-B cell interactions and IL-21. Finally, we demonstrate that ABCs may contribute to SLE pathogenesis by producing high levels of anti-dsDNA IgG2a/c antibodies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/expansion-of-age-associated-b-cells-in-murine-lupus-is-regulated-by-def6-and-its-homologue-swap-70/