Impaired V(D)J Recombination and Lymphocyte Development in Core RAG1-expressing Mice

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Abstract
RAG1 and RAG2 are the lymphocyte-specific components of the V(D)J recombinase. In vitro analyses of RAG function have relied on soluble, highly truncated “core” RAG proteins. To identify potential functions for noncore regions and assess functionality of core RAG1 in vivo, we generated core RAG1 knockin (RAG1c/c) mice. Significant B and T cell numbers are generated in RAG1c/c mice, showing that core RAG1, despite missing ∼40% of the RAG1 sequence, retains significant in vivo function. However, lymphocyte development and the overall level of V(D)J recombination are impaired at the progenitor stage in RAG1c/c mice. Correspondingly, there are reduced numbers of peripheral RAG1c/c B and T lymphocytes. Whereas normal B lymphocytes undergo rearrangement of both JH loci, substantial levels of germline JH loci persist in mature B cells of RAG1c/c mice, demonstrating that DJH rearrangement on both IgH alleles is not required for developmental progression to the stage of VH to DJH recombination. Whereas VH to DJH rearrangements occur, albeit at reduced levels, on the nonselected alleles of RAG1c/c B cells that have undergone D to JH rearrangements, we do not detect VH to DH rearrangements in RAG1c/c B cells that retain germline JH alleles. We discuss the potential implications of these findings for noncore RAG1 functions and for the ordered assembly of VH, DH, and JH segments.