B cell intrinsic gain of function mutations in B cell development and function: a story of activated CARD11 and PI3K

Abstract

Gain of function mutations in key signaling molecules downstream of the B cell receptor have been linked to both malignancy and monogeneic primary immunodeficiencies. This study is focused on characterizing the impact of such mutations in Card11 and Pik3cd when restricted to the B cell lineage. Dominant, activating mutations in the adapter protein CARD11 induce inappropriate activation of NF-κB signaling. Somatic mutations in CARD11 are predicted to arise during GC responses in lymphomas, whereas germline CARD11 mutations drive a mild immunodeficiency, B cell expansion with NFκB and T cell anergy (BENTA). Activated PIK3-delta syndrome (APDS) is an immunodeficiency caused by gain-of-function mutations in PIK3CD. Both diseases exhibit complex immune phenotypes including recurrent infection and impaired vaccine responses. Here, we modeled the B cell-intrinsic impact of: 1) the L251P activating mutation in CARD11 (aCARD11) and 2) the common APDS mutation (hPIK3CD-E1021K; referred to as ‘aPIK3CD’). Global B cell aCARD11 expression led to a modest increase in splenic B cells and severe reduction in B1 B cell numbers, respectively. Following T cell-dependent immunization, aCARD11 cells exhibited increased rates of GC formation, resolution and differentiation. In this model, aCARD11 promoted dark zone skewing along with increased cycling, AID levels and class switch recombination. Furthermore, aCard11 GC B cells displayed increased biomass and mTORC1 signaling, suggesting a novel strategy for targeting aCARD11-driven DLBCL. Global B cell expression of aPIK3CD drove expansion of the peripheral innate, B1a and MZ, B cell compartments. aPIK3CD blunted T-independent immune responses and reduced class-switched antibodies following T-dependent immunization. Thus, aPIK3CD and aCARD11 differentially alter B cell development and function post immunization, providing insight into B cell intrinsic contributions of these mutations to APDS and BENTA, as well as post GC malignancy.