Bruton’s Tyrosine Kinase Synergizes with Notch2 To Govern Marginal Zone B Cells in Nonobese Diabetic Mice

Abstract

Expansion of autoimmune-prone marginal zone (MZ) B cells has been implicated in type 1 diabetes (T1D). To test disease contributions of MZ B cells in NOD mice, Notch2 haploinsufficiency (Notch2+/−) was introduced, but failed to eliminate the MZ, as it does in C57BL/6 mice. Notch2+/−/NOD have MZ B cell numbers similar to WT C57BL/6, yet still develop diabetes. To test whether BCR-signaling supports Notch2+/−/NOD MZ B cells, Bruton's tyrosine kinase (Btk)-deficiency was introduced. Surprisingly, MZ B cells failed to develop in Btk-deficient Notch2+/−/NOD mice. Expression of Notch2 and its transcriptional target, Hes5, were increased in NOD MZ B cells compared with C57BL/6 MZ B cells. Btk-deficiency reduced Notch2+/− signaling exclusively in NOD B cells, suggesting that BCR-signaling enhances Notch2 signaling in this autoimmune model. The role of BCR-signaling was further investigated using an anti-insulin transgenic BCR (125Tg). Anti-insulin B cells in 125Tg/Notch2+/−/NOD mice populate an enlarged MZ, suggesting that low level BCR signaling overcomes reliance on Notch2. Tracking clonotypes of anti-insulin B cells in H chain only VH125Tg/NOD mice showed that BTK-dependent selection into the MZ depends on strength of antigenic binding, while Notch2-mediated selection does not. Importantly, anti-insulin B cell numbers were reduced by Btk-deficiency, but not Notch2-haploinsufficiency. These studies show that: 1) Notch2-haploinsufficiency limits NOD MZ B cell expansion without preventing T1D, 2) BTK supports the Notch2 pathway in NOD MZ B cells, and 3) autoreactive NOD B cell survival relies on BTK more than Notch2, regardless of MZ location, which may have important implications for disease-intervention strategies.

Anti-Insulin B Cells Are Poised for Antigen Presentation in Type 1 Diabetes

“…This observation highlights the importance of physiologic models of tolerance compared with Tg expression of foreign proteins (13,(55)(56)(57). Previous work by our group shows that different insulin-binding populations are characterized by two predominant L chains (5,17,40). In V H 125 SD .NOD mice, distinct insulin-binding populations can also be visualized.…”

“…Significantly increased frequencies of T2 and MZ B cells were observed in the insulin-binding B cells compared with non-insulin-binding B cells (p , 0.001 for each subset). Although anti-insulin B cells can enter all peripheral B cell compartments, they are enriched in the T2 and MZ subsets, which are recognized for their heightened responsiveness to innate signals and enhanced capacity for Ag presentation (23,(38)(39)(40). Conversely, the frequencies of FO B cells were significantly decreased in the insulin-binding population compared with noninsulin-binding B cells (p , 0.001).…”

The role of Bruton’s tyrosine kinase in autoimmunity and implications for therapy

“…Anti-insulin B cells and autoantibodies were severely reduced, while total B cell numbers and total IgG levels were not (26, 37). We also showed that Btk supports naturally occurring autoreactive-prone anergic (An1) and autoreactive marginal zone (MZ) NOD B cells (26, 37, 91), which may also have implications for arthritis, as this subset was recently shown to be a primary source of anti-collagen B cells in collagen-induced arthritis (92). …”

Bruton's Tyrosine Kinase Deficiency Inhibits Autoimmune Arthritis in Mice but Fails to Block Immune Complex–Mediated Inflammatory Arthritis

“…Equally striking is the profound reduction of the marginal zone compartment, which began even at the pre-marginal zone stage. Although in most models the marginal zone develops independently of BTK, we recently reported that NOD mouse marginal zone B cells rely in part on BTK signals (44). Even in NOD mice, however, loss of BTK causes a block at the premarginal zone stage, with an increase in numbers, and only partial reduction of marginal zone B cells (4).…”