Recombination Activating Genes 1 and 2 (RAG1 and RAG2) play a critical role in T and B cell development by initiating the recombination process that controls expression of T cell receptor (TCR) and immunoglobulin genes. Mutations in the RAG1 and RAG2 genes in humans cause a broad spectrum of phenotypes, including severe combined immune deficiency (SCID) with lack of T and B cells, Omenn syndrome, leaky SCID, and combined immune deficiency with granulomas or autoimmunity (CID-G/AI). Using next generation sequencing, we analyzed the T and B cell receptor (TCR, BCR) repertoire in 12 patients with RAG mutations presenting with Omenn syndrome (n=5), leaky SCID (n=3), or CID-G/AI (n=4). Restriction of repertoire diversity skewed usage of Variable (V), Diversity (D), and Joining (J) segment genes, and abnormalities of CDR3 length distribution were progressively more prominent in patients with a more severe phenotype. Skewed usage of V,D and J segment genes was present also within unique sequences, indicating a primary restriction of repertoire. Patients with Omenn syndrome had a high proportion of class-switched immunoglobulin heavy chain transcripts and increased somatic hypermutation rate, suggesting in vivo activation of these B cells. These data provide a framework for better understanding the phenotypic heterogeneity of RAG deficiency.

Background Patients with severe combined immunodeficiency disease (SCID) who have matched sibling donors (MSD) can proceed to hematopoietic cell transplantation (HCT) without conditioning chemotherapy. Objective To determine whether the results of HCT without chemotherapy-based conditioning from matched unrelated donors (URD), either from volunteer adults or umbilical cord blood, are comparable to those from matched sibling donors (MSD). Methods A multicenter survey of SCID transplant centers in North America, Europe, and Australia to compile retrospective data on patients who have undergone unconditioned HCT, from either URDs (n = 37) or MSDs (n = 66). Results Most patients undergoing URD HCT (92%) achieved donor T-cell engraftment, compared to 97% for MSDs, however, estimated 5-year overall and event-free survival were worse for URD recipients (71% and 60%, respectively), compared to MSD recipients (92% and 89%, respectively; P <0.01 for both). URD recipients who received pre-HCT serotherapy had similar 5-year OS (100%) to MSD recipients. The incidences of Grade II-IV acute and chronic GVHD were higher in URD (50% and 39%, respectively), compared to MSD recipients (22% and 5%, respectively; P <0.01 for both). In the surviving patients, there was no difference in T-cell reconstitution at last follow-up between the URD recipients and MSD recipients, however MSD recipients were more likely to achieve B-cell reconstitution (72% vs. 17%; P <0.001). Conclusion Unconditioned URD HCT achieves excellent rates of donor T-cell engraftment similar to MSD recipients, and reconstitution rates are adequate. However, only a minority will develop myeloid and B cell reconstitution and attention must be paid to GVHD prophylaxis. This approach may be safer in children ineligible for intense regimens to spare potential complications of chemotherapy.

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Mutations of the recombinase-activating genes 1 and 2 (RAG1 and RAG2) in humans are associated with a broad range of phenotypes. For patients with severe clinical presentation, hematopoietic stem cell transplantation (HSCT) represents the only curative treatment; however, high rates of graft failure and incomplete immune reconstitution have been observed, especially after unconditioned haploidentical transplantation. Studies in mice have shown that Rag−/− natural killer (NK) cells have a mature phenotype, reduced fitness, and increased cytotoxicity. We aimed to analyze NK cell phenotype and function in patients with mutations in RAG and in non-homologous end joining (NHEJ) genes. Here, we provide evidence that NK cells from these patients have an immature phenotype, with significant expansion of CD56bright CD16−/int CD57− cells, yet increased degranulation and high perforin content. Correlation was observed between in vitro recombinase activity of the mutant proteins, NK cell abnormalities, and in vivo clinical phenotype. Addition of serotherapy in the conditioning regimen, with the aim of depleting the autologous NK cell compartment, may be important to facilitate engraftment and immune reconstitution in patients with RAG and NHEJ defects treated by HSCT.