Hypomorphic Rag1 mutations alter the preimmune repertoire at early stages of lymphoid development

Abstract

Hypomorphic <i>RAG1</i> mutations allowing residual T- and B-cell development have been found in patients presenting with delayed-onset combined immune deficiency with granulomas and/or autoimmunity (CID-G/AI) and abnormalities of the peripheral T- and B-cell repertoire. To examine how hypomorphic <i>Rag1</i> mutations affect the earliest stages of lymphocyte development, we used CRISPR/Cas9 to generate mouse models with mutations equivalent to those found in patients with CID-G/AI. Immunological characterization showed partial development of T and B lymphocytes, with persistence of naïve cells and preserved serum immunoglobulin but impaired antibody responses and presence of autoantibodies, thereby recapitulating the phenotype seen in patients with CID-G/AI. By using high-throughput sequencing, we identified marked skewing of <i>Igh V</i> and <i>Trb V</i> gene usage in early progenitors, with a bias for productive <i>Igh</i> and <i>Trb</i> rearrangements after selection occurred and increased apoptosis of B-cell progenitors. Rearrangement at the <i>Igk</i> locus was impaired, and polyreactive immunoglobulin M antibodies were detected. This study provides novel insights into how hypomorphic <i>Rag1</i> mutations alter the primary repertoire of T and B cells, setting the stage for immune dysregulation frequently seen in patients.