PI3K plays multiple roles throughout the life of a B cell. As such, its signaling is tightly regulated. The importance of this is illustrated by the fact that both loss-and gain-of-function mutations in PI3K can cause immunodeficiency in humans. PIK3IP1, also known as TrIP, is a transmembrane protein that has been shown to inhibit PI3K in T cells. Results from the ImmGen Consortium indicate that PIK3IP1 expression fluctuates throughout B cell development in a manner inversely correlated with PI3K activity; however, its role in B cells is poorly understood. In this study, we define the consequences of B cell-specific deletion of PIK3IP1. B cell development, basal Ig levels, and T-independent responses were unaffected by loss of PIK3IP1. However, there was a significant delay in the production of IgG during T-dependent responses, and secondary responses were impaired. This is likely due to a role for PIK3IP1 in the extrafollicular response because germinal center formation and affinity maturation were normal, and PIK3IP1 is not appreciably expressed in germinal center B cells. Consistent with a role early in the response, PIK3IP1 was downregulated at late time points after B cell activation, in a manner dependent on PI3K. Increased activation of the PI3K pathway was observed in PIK3IP1-deficient B cells in response to engagement of both the BCR and CD40 or strong cross-linking of CD40 alone. Taken together, these observations suggest that PIK3IP1 promotes extrafollicular responses by limiting PI3K signaling during initial interactions between B and T cells.

Systemic Lupus Erythematosus (SLE) is characterized by pathogenic autoantibodies against nucleic acid-containing antigens. Understanding which B-cell subsets give rise to these autoantibodies may reveal therapeutic approaches for SLE that spare protective responses. Mice lacking the tyrosine kinase Lyn, which limits B and myeloid cell activation, develop lupus-like autoimmune diseases characterized by increased autoreactive plasma cells (PCs). We used a fate-mapping strategy to determine the contribution of T-bet + B cells, a subset thought to be pathogenic in lupus, to the accumulation of PCs and autoantibodies in Lyn -/mice. Approximately, 50% of splenic PCs in Lyn -/mice originated from T-bet + cells, a significant increase compared to WT mice. In vitro, splenic PCs derived from T-bet + B cells secreted both IgM and IgG anti-dsDNA antibodies. To determine the role of these cells in autoantibody production in vivo, we prevented T-bet + B cells from differentiating into PCs or class switching in Lyn -/mice. This resulted in a partial reduction in splenic PCs and anti-dsDNA IgM and complete abrogation of anti-dsDNA IgG. Thus, T-bet + B cells make an important contribution to the autoreactive PC pool in Lyn -/mice.

Systemic Lupus Erythematosus (SLE) is characterized by autoantibodies (autoAbs) against nucleic acid containing antigens. These antibodies form immune complexes that promote inflammation and cause organ damage. Defining B cell subsets that give rise to these autoAbs may reveal therapeutic approaches for SLE that spare responses to foreign antigens. Mice lacking the tyrosine kinase Lyn, which limits B and myeloid cell activation, develop lupus-like autoimmune disease characterized by a large increase in autoreactive plasma cells (PCs). We used several strategies to identify the origin of these PCs. We used fate mapping to assess the contribution of T-bet+ age associated B cells (ABCs), a subset that is increased and thought to be pathogenic in both murine and human lupus. 40% of splenic PCs in Lyn−/− mice were derived from T-bet expressing cells, a significant increase compared to wild type mice that was not seen in the bone marrow PC population. Marginal zone (MZ) B cells, also implicated in some lupus models, are enriched in autoreactivity, prone to PC differentiation, and reduced in Lyn−/− mice. We hypothesized that this reduction might be due to their unchecked transit to the PC stage. However, impairing MZ B cell development in Lyn−/− mice did not reduce autoAbs. Nor did preventing PC differentiation restore MZ B cell numbers. Instead, Lyn−/− mice accumulated B1a cells when PC differentiation was impaired. B1a cells tend to be polyreactive or weakly autoreactive and are primed for terminal differentiation. Our results suggest that both ABCs and B1a cells, but not MZ B cells, contribute to the autoreactive PC pool in Lyn−/− mice. Future studies will assess the role of these subsets in the development of autoimmune disease. Supported by grants from the NIH (R21 AI161307, R21 AI137746)