Within the last decade there have been several severe combined immunodeficient (SCID) pig models discovered or genetically engineered. The animals have mutations in ARTEMIS, IL2RG, or RAG1/2 genes, or combinations thereof, providing SCID pigs with NK cells, but deficient in T and B cells, or deficient in NK, T, and B cells for research studies. Biocontainment facilities and positive pressure isolators are developed to limit pathogen exposure and prolong the life of SCID pigs. Raising SCID pigs in such facilities allows for completion of long-term studies such as xenotransplantation of human cells. Ectopically injected human cancer cell lines develop into tumors in SCID pigs, thus providing a human-sized in vivo model for evaluating imaging methods to improve cancer detection and therapeutic research and development. Immunocompromised pigs have the potential to be immunologically humanized by xenotransplantation with human hematopoietic stem cells, peripheral blood leukocytes, or fetal tissue. These cells can be introduced through various routes including injection into fetal liver or the intraperitoneal (IP) space, or into piglets by intravenous, IP, and intraosseous administration. The development and maintenance of transplanted human immune cells would be initially (at least) dependent on immune signaling from swine cells. Compared to mice, swine share higher homology in immune related genes with humans. We hypothesize that the SCID pig may be able to support improved engraftment and differentiation of a wide range of human immune cells as compared to equivalent mouse models. Humanization of SCID pigs would thus provide a valuable model system for researchers to study interactions between human tumor and human immune cells. Additionally, as the SCID pig model is further developed, it may be possible to develop patient-derived xenograft models for individualized therapy and drug testing. We thus theorize that the individualized therapeutic approach would be significantly improved with a humanized SCID pig due to similarities in size, metabolism, and physiology. In all, porcine SCID models have significant potential as an excellent preclinical animal model for therapeutic testing.

The use of HLA-identical hematopoietic stem cell transplantation (HSCT) demonstrates overall survival rates greater than 75 % for T-B-NK+ severe combined immunodeficiency secondary to pathogenic mutation of recombinase activating genes 1 and 2 (RAG1/2). Limited data exist regarding the use of HSCT in patients with hypomorphic RAG variants marked by greater preservation of RAG activity and associated phenotypes such as granulomatous disease in combination with autoimmunity. We describe a 17-year-old with combined immunodeficiency and immune dysregulation characterized by granulomatous lung disease and autoimmunity secondary to compound heterozygous RAG mutations. A myeloablative reduced toxicity HSCT was completed using an unrelated bone marrow donor. With the increasing cases of immune dysregulation being discovered with hypomorphic RAG variants, the use of HSCT may advance to the forefront of treatment. This case serves to discuss indications of HSCT, approaches to preparative therapy, and the potential complications in this growing cohort of patients with immune dysregulation and RAG deficiency.

The V(D)J recombination is a DNA rearrangement process that generates the diversity of T and B lymphocyte immune repertoire. It proceeds through the generation of a DNA double-strand break (DNA-DSB) by the Rag1/2 lymphoid-specific factors, which is repaired by the non-homologous end joining (NHEJ) DNA repair pathway. V(D)J recombination also constitutes a checkpoint in the lymphoid development. V(D)J recombination defect results in severe combined immune deficiency (SCID) with a lack of T and B lymphocytes. The V(D)J recombination represents one of the few programmed molecular events leading to DNA-DSBs that strictly relies on NHEJ. Two NHEJ factors, Artemis and XLF/Cernunnos, were identified through the molecular studies of SCID patients. Mutations in PRKDC and DNA Ligase IV genes also result in SCID. Studies in mice have demonstrated that XLF/Cernunnos is dispensable for V(D)J recombination in lymphoid cells but not for the repair of genotoxic-induced DNA-DSBs, which raises the question of the implication of Rag1/2 factors in the DNA repair phase of V(D)J recombination. New factors of NHEJ, such as PAXX, are being identified. Patients with NHEJ deficiency (XRCC4) without immune deficiency were recently reported. We, therefore, may not have yet the complete picture of DNA-DSB repair in the context of V(D)J recombination.