The RAG1 Ubiquitin Ligase Domain Enhances Assembly and Selection of T Cell Receptor Genes to Restrain the Autoimmune Hazard of Generating T Cell Receptor Diversity

RAG1/RAG2 (RAG) endonuclease-mediated assembly of diverse lymphocyte antigen receptor genes by V(D)J recombination is critical for the development and immune function of T and B cells. However, this process creates highly self-reactive cells that must be negatively selected to suppress autoimmunity. The RAG1 protein contains a ubiquitin ligase domain that stabilizes RAG1 and stimulates RAG endonuclease activity in vitro. We report that mice with a mutation that inactivates the RAG1 ubiquitin ligase in vitro exhibit decreased assembly of T cell receptor (TCR) b and a genes in thymocytes and impaired thymocyte developmental transitions that require assembly of these genes and signaling by their proteins. The mice also exhibit a combined phenotype of: 1) decreased expression of TCR signaling proteins in thymocytes, 2) less efficient positive selection of conventional ab T cells and iNKT cells, 3) impaired negative selection of highly self-reactive thymocytes, and 4) CD4+ ab T cells with elevated autoimmune potential. Our findings demonstrate that the RAG1 ubiquitin ligase domain functions in vivo to stimulate efficiencies of TCRb and TCR a gene assembly and ab TCR selection, thereby establishing replete ab TCR diversity and ab T cell lineages while restraining the inherent autoimmune hazard of creating diverse antigen specificities.