Genome-Wide CRISPRi Screening Identifies XPO5 as a Regulator of B Cell Mutation and Fitness

The B cell receptor (BCR) is the defining factor of B lymphocyte identity and function, allowing for a robust adaptive immune response through antigen recognition. Strict regulation of BCR surface density dictates proper B cell signaling, immune regulation, and the prevention of malignancy, yet the factors regulating this density remain undefined. Here, we performed a genome-wide CRISPR interference (CRISPRi) screen in Ramos B cells, which undergo constitutive somatic hypermutation (SHM) and identified Exportin-5 (XPO5) as a central regulator of BCR surface expression. XPO5 depleted cells exhibited an accelerated loss of surface BCR with no change in transcript levels, suggesting a potential post-transcriptional regulatory mechanism. Further analysis revealed XPO5 depletion led to an accumulation of non-functional BCR light chain sequences driven by an increase in AID signature mutations, implicating XPO5 in balancing mutagenesis and repair during somatic hypermutation (SHM). Transcriptomic and small RNA sequencing revealed a global reduction in miRNA levels and enrichment of target gene sets indicative of cell cycle arrest and increased DNA damage response. These data suggest that XPO5 plays a multi-faceted regulatory role in B cells via a miRNA-mediated control, supporting both proliferation and regulating DNA repair thresholds to maintain B cell receptor expression and functionality.