Winslow, Gary; Levack, Russell (2020)
      CD11c+ T-bet+ B cells serve crucial roles in both protective immunity and autoimmunity.However, the ontogeny of these cells remains unclear, and strategies to target them in vivo have yet to be identified. Here, we demonstrate that developing CD11c+ T-bet+ B cells received help in the form of IL-21, IFN-γ, and CD40L from a population ofT follicular helper 1(TFH1)cells outside of formal germinal centers (GC). These TFH1cells provided help to developing CD11c+ T-bet+ B cells in two distinct phases: IFN-gwas provided early following infection, and CD40L was provided later. Unlike the TFH1cells, CD11c+ T-bet+ B cells required the GC-associated transcription factor Bcl-6 for their development, but not T-bet. While the CD11c+ B cells that arose in the absence of T-bet appeared nearly identical to their T-bet-competent counterparts,they did not switch to IgG2c. These data support a model where, in the absence of formal GCs, TFH1cells provide GC-like help to developing CD11c+ T-bet+ B cells and while T-bet is not required for the development of these T-bet+ B cells,it is required for appropriate class-switch recombination (CSR). Our work also demonstrates that mature CD11c+ T-bet+ B cells, which arise in both immunity and autoimmunity,wereeliminated following treatment with the adenosine 2a receptor (A2aR) agonist CGS-21680. Depletion of these CD11c+ T-bet+ B cells occurred in a B cell-intrinsic manner and was corelated with improved disease outcome in a mouse model of lupus. Preliminary data indicated that human CD11c+ B cells expressed the A2aR,and these cells were depleted following CGS-21680 treatment in vitro, suggesting that A2aR-agonistadministrationmay also be effective in the treatment of human autoimmune diseaseswhere CD11c+ Bcell play a role. Overall, this work provides novel insight into the development of T-bet+ B cells and identifies the first pharmacological approach to target these cells in vivo.

      Papillion, Amber (2017)
      IgM memory cells are recognized as an important component of B cell memory, based on several studies both in mice and humans. Our studies of B cells elicited in response to ehrlichial infection identified a population of CD11c/T-bet-positive IgM memory cells and an IgM T-bet-positive bone marrow antibody-secreting cell population (ASCs). The origin of these populations was unknown, although an early T-independent spleen CD11c-and T-bet-positive IgM plasmablast population precedes both, suggesting a linear relationship. The majority of IgM memory cells detected after day 30 post-infection had undergone somatic hypermutation, indicating that they expressed activation-induced cytidine deaminase (AID). Therefore, to identify early AID-expressing precursor cells, we infected an AID-regulated tamoxifen-inducible Cre-recombinase-EYFP reporter strain. Tamoxifen administration led to labeling of both the IgM memory cells and bone marrow ASCs on day 30 and later post-infection. High frequencies of labeled cells were identified on day 30 post-infection,following tamoxifen administration on day 10 post-infection. Both IgM memory cells and IgM bone marrow ASCs were labeled when tamoxifen was administered as early as day 4 post-infection. We also identified mechanisms involved in maintenance of the IgM bone marrow ASCs and IgM+ memory cells, namely proliferation and FcγRIIb respectively. BrdU studies revealed that the bone marrow IgM ASCs were maintained by proliferation, unlike the IgM memory cells. RNAseq analysis revealed a 2-fold higher expression of inhibitory Fc receptor, FcγRIIb. Because FcγRIIb inhibits B cell activation, we hypothesized that FcγRIIb negatively regulates IgM+ memory B cells by binding immune complexes present during low-level chronic infection. E. murisinfection of FcγRIIb-deficientmice revealed a 3-fold expansion of the IgM+ memory 30 days post-infection. We further demonstrated that the expansion of the IgM+ memory cells was not due to increased proliferation, but a decrease of apoptosis, due to a lack of Fas expression in FcγRIIb-deficient mice. This result was mimicked in AID-deficient mice, which lack the ability to class switch to IgG and make immune complexes, revealing a role for immune complexes in regulating IgM+ memory. Altogether, these studies demonstrate a novel germinal center-independent pathway for the generation of two distinct long-term IgM-positive B cell populations.