Murine models of hypomorphic defects of v(d)j recombination

V(D)J recombination is the process by which the subgenic elements of T and B cell receptors are assembled. The Rag1 and Rag2 proteins initiate this process by cleaving the DNA, whereas the Non Homologous End-Joining pathway (that includes Ku70/80, XRCC4, LigIV, DNA-PKcs and Cernunnos)mediates DNA joining. Rag1 and Rag2 null mutations cause SCID with complete lack of T and B lymphocytes in humans. Hypomorphic mutations that reduce, but do not abrogate, V(D)J recombination activity, may lead to Omenn syndrome (OS) or to “leaky SCID”, both of which are characterized by activated, anergic and autologous T cells, whereas severe erythroderma and cholitis are typical of OS only. Hypomorphic LigIV mutations in humans are associated with increased cellular radiosensitivity, microcephaly, facial dysmorphisms, growth retardation, developmental delay, and a variable degree of immunodeficiency. These disorders define the LIGIV syndrome. The work presented in this thesis focused on the characterization of two murine models with hypomorphic mutations in Rag1 (rag1S723C/S723C) and LigIV (ligIVR278H/R278H) that recapitulate leaky SCID. A minority of rag1S723C/S723C mice (but none of ligIVR278H/R278H mice) showed features of OS without phenotypic features of OS. Both mouse models show a severe, but incomplete, block in T and B cell development, with residual generation of single-positive thymocytes. However, peripheral T cells are present and show an activated and anergic phenotype, with a restricted repertoire, reminiscent of human leaky SCID. The impaired in vitro proliferation in response to anti-CD3 is mostly due to increased apoptosis, and can be only partially rescued by addition of costimulatory factors such as anti-CD28 or by exogenous IL-2. Despite a severe block at the pro to pre-B stage of B cell differentiation and of profound B cell lymphopenia, IgG, IgM, IgA and IgE serum levels are maintained, and a high proportion of immunoglobulin-secreting cells was observed in the spleen of both rag1S723C/S723C and ligIVR278H/R278H mice. Antibody responses to T-dependent and T-independent antigens are impaired, however both hypomorphic mouse models spontaneously produce high amounts of low-affinity antibodies that include self-reactive specificities. The Rag1 and LigIV hypomorphic mouse models show attempts to organize the thymic medulla and are able, to various degrees, to generate nTregs. However, the expression of aire and of tissue-rectricted antigens, required for the maintenance of the central tolerance, is reduced. Taken together, these data indicate that the stochastic generation of an autoreactive B cell repertoire, associated with defects in central and peripheral T cell tolerance, are an important component of the immunopathology of immunodeficiencies associated with hypomorphic mutation in the V(D)J recombination process. Therefore using these mouse models, it is now possible to elucidate the mechanisms underlying the phenotypic variability between OS and leaky SCID in humans.