DNA repair consists of several cellular pathways which recognize and repair damaged DNA . The classical nonhomologous DNA end‐joining ( NHEJ ) pathway repairs double‐strand breaks in DNA . It is required for maturation of both B and T lymphocytes by supporting V(D)J recombination as well as B‐cell differentiation during class switch recombination (CSR). Inactivation of NHEJ factors Ku70, Ku80, XRCC 4, DNA ligase 4, DNA ‐ PK cs, and Artemis impairs V(D)J recombination and blocks lymphocyte development. Paralogue of XRCC 4 and XLF ( PAXX ) is an accessory NHEJ factor that has a significant impact on the repair of DNA lesions induced by ionizing radiation in human, murine, and chicken cells. However, the role of PAXX during development is poorly understood. To determine the physiological role of PAXX , we deleted part of the Paxx promoter and the first two exons in mice. Further, we compared Paxx ‐knockout mice with wild‐type (WT) and NHEJ ‐deficient controls including Ku80‐ and Dna‐pkcs ‐null and severe combined immunodeficiency mice. Surprisingly, Paxx ‐deficient mice were not distinguishable from the WT littermates; they were the same weight and size, fertility status, had normal spleen, thymus, and bone marrow. Paxx ‐deficient mice had the same number of chromosomal and chromatid breaks as WT mice. Moreover, Paxx ‐deficient primary B lymphocytes had the same level of CSR as lymphocytes isolated from WT mice. We concluded that PAXX is dispensable for normal mouse development.

Autoantibodies to transglutaminase 2 (TG2) are hallmarks of celiac disease. To address B cell tolerance and autoantibody formation to TG2, we generated immunoglobulin knock-in (Ig KI) mice that express a prototypical celiac patient–derived anti-TG2 B cell receptor equally reactive to human and mouse TG2. We studied B cell development in the presence/absence of autoantigen by crossing the Ig KI mice to Tgm2−/− mice. Autoreactive B cells in Tgm2+/+ mice were indistinguishable from their naive counterparts in Tgm2−/− mice with no signs of clonal deletion, receptor editing, or B cell anergy. The autoreactive B cells appeared ignorant to their antigen, and they produced autoantibodies when provided T cell help. The findings lend credence to a model of celiac disease where gluten-reactive T cells provide help to autoreactive TG2-specific B cells by involvement of gluten–TG2 complexes, and they outline a general mechanism of autoimmunity with autoantibodies being produced by ignorant B cells on provision of T cell help.

To ensure genome stability, mammalian cells employ several DNA repair pathways. Nonhomologous DNA end joining ( NHEJ ) is the DNA repair process that fixes double‐strand breaks throughout the cell cycle. NHEJ is involved in the development of B and T lymphocytes through its function in V(D)J recombination and class switch recombination ( CSR ). NHEJ consists of several core and accessory factors, including Ku70, Ku80, XRCC 4, DNA ligase 4, DNA ‐ PK cs, Artemis, and XLF . Paralog of XRCC 4 and XLF ( PAXX ) is the recently described accessory NHEJ factor that structurally resembles XRCC 4 and XLF and interacts with Ku70/Ku80. To determine the physiological role of PAXX in mammalian cells, we purchased and characterized a set of custom‐generated and commercially available NHEJ ‐deficient human haploid HAP 1 cells, PAXX Δ , XRCC 4 Δ , and XLF Δ . In our studies, HAP 1 PAXX Δ cells demonstrated modest sensitivity to DNA damage, which was comparable to wild‐type controls. By contrast, XRCC 4 Δ and XLF Δ HAP 1 cells possessed significant DNA repair defects measured as sensitivity to double‐strand break inducing agents and chromosomal breaks. To investigate the role of PAXX in CSR , we generated and characterized Paxx −/− and Aid −/− murine lymphoid CH 12F3 cells. CSR to IgA was nearly at wild‐type levels in the Paxx −/− cells and completely ablated in the absence of activation‐induced cytidine deaminase ( AID ). In addition, Paxx −/− CH 12F3 cells were hypersensitive to zeocin when compared to wild‐type controls. We concluded that Paxx ‐deficient mammalian cells maintain robust NHEJ and CSR .

Antibodies specific for peptides bound to human leukocyte antigen (HLA) molecules are valuable tools for studies of antigen presentation and may have therapeutic potential. Here, we generated human T cell receptor (TCR)-like antibodies toward the immunodominant signature gluten epitope DQ2.5-glia-2 in celiac disease (CeD). Phage display selection combined with secondary targeted engineering was used to obtain highly specific antibodies with picomolar affinity. The crystal structure of a Fab fragment of the lead antibody 3.C11 in complex with HLA-DQ2.5:DQ2.5-glia-2 revealed a binding geometry and interaction mode highly similar to prototypic TCRs specific for the same complex. Assessment of CeD biopsy material confirmed disease specificity and reinforced the notion that abundant plasma cells present antigen in the inflamed CeD gut. Furthermore, 3.C11 specifically inhibited activation and proliferation of gluten-specific CD4 + T cells in vitro and in HLA-DQ2.5 humanized mice, suggesting a potential for targeted intervention without compromising systemic immunity.