Identification of genes involved in class switch recombination by genome-wide CRISPR/Cas9 knock-out screen

This screen aims to identify genes involved in class switch recombination (CSR), a process that allows B cells to change the isotype they express from IgM/IgD to IgG, IgA or IgE. For that purpose, the murine B cell line CH12 stably expressing Cas9 was used. This cell line can be induced to undergo CSR from IgM to IgA very efficiently in vitro when cultured with TGF-b, IL-4 and an anti-CD40 antibody. CH12Cas9 cells were transduced with the second-generation mouse Brie (mBrie) retroviral gRNA library (pMX-mBrie) and selected with puromycin for two weeks to allow for the Cas9-mediated generation of knockouts. Cells were then stimulated to undergo CSR for 72h. Those which failed to undergo CSR (which remained IgM+) or that succeed (which became IgA+) were sorted using magnetic beads. Genes, whose loss-of-function affect the efficiency of CSR, were identified by looking at gRNA enrichment in the IgM+ versus IgA+ populations through deep sequencing. Comparison of gRNAs enrichment between IgM+ and IgA+ populations by high-throughput sequencing