Hyperactivated PI3K\u3b4 promotes self and commensal reactivity at the expense of optimal humoral immunity

Gain-of-function mutations in the gene encoding the phosphatidylinositol-3-OH kinase catalytic subunit p110\u3b4 (PI3K\u3b4) result in a human primary immunodeficiency characterized by lymphoproliferation, respiratory infections and inefficient responses to vaccines. However, what promotes these immunological disturbances at the cellular and molecular level remains unknown. We generated a mouse model that recapitulated major features of this disease and used this model and patient samples to probe how hyperactive PI3K\u3b4 fosters aberrant humoral immunity. We found that mutant PI3K\u3b4 led to co-stimulatory receptor ICOS\u2013independent increases in the abundance of follicular helper T cells (TFH cells) and germinal-center (GC) B cells, disorganized GCs and poor class-switched antigen-specific responses to immunization, associated with altered regulation of the transcription factor FOXO1 and pro-apoptotic and anti-apoptotic members of the BCL-2 family. Notably, aberrant responses were accompanied by increased reactivity to gut bacteria and a broad increase in autoantibodies that were dependent on stimulation by commensal microbes. Our findings suggest that proper regulation of PI3K\u3b4 is critical for ensuring optimal host-protective humoral immunity despite tonic stimulation from the commensal microbiome