PI3Kδ in B cell Homeostasis and Transformation

Abstract

The lipid kinase phosphoinositide 3-kinase δ (PI3Kδ) is vital for lymphocyte function and development. Hyperactivation of PI3Kδ caused by mutations in PIK3CD drives Activated PI3Kδ Syndrome (APDS), a primary immunodeficiency with increased risk of pulmonary infection, autoimmunity, and B cell lymphoma development. This thesis explores the role of hyperactive PI3Kδ in B cell homeostasis, maintenance, and malignant transformation. The Okkenhaug lab has previously identified a novel mouse model for Diffuse Large B Cell Lymphoma which is driven by the cooperation of hyperactive PI3Kδ with B Cell Lymphoma 6 (BCL6), a master regulator of the Germinal Center reaction. High dimensional phenotyping allowed for the identification of a pre-malignant pre-Plasmablast subset characterised by high expression of CD73. Adoptive cell transfer of CD73hi B cells into immunocompromised mice lead to malignancy after ~100 days, validating the transformation potential of these cells. To evaluate the contribution of T and B cell-intrinsic PI3Kδ hyperactivity on B cell lymphoma development, transgenic mouse models with lymphocyte-specific PI3Kδ hyperactivity and deregulated BCL6 have been generated. PI3Kδ hyperactivity in either B or T cells drove malignancy reminiscent of B cell lymphoma in an immunocompromised background. This emphasises that PI3Kδ and BCL6 can cooperates as oncogenes in different cells to drive malignancy. Hyperactive PI3Kδ drives expansion of a novel B regulatory cell population characterised by the lack of B220 expression. Bulk RNA-seq revealed upregulation of CYP11A1, the rate limiting enzyme for steroidogenesis within B220- B cells. CYP11A1 expression was further validated by flow cytometry, and B cell specific CYP11A1 knockout (KO) mice were generated. KO of CYP11A1 alone did not influence immune cell populations in young mice but led to splenomegaly in old mice. Furthermore, mass spectrometry was performed to assess the steroid secreting potential of B cells in response to PI3Kδ. In summary, this thesis highlights PI3Kδ-driven changes in B cell populations in the context of health and disease. This knowledge will be valuable for understanding the role of B cells in promoting or controlling pathogenesis in B cell lymphoma, APDS or other autoimmune conditions.