Expanding the PRAAS spectrum: De novo mutations of immunoproteasome subunit ß-type 10 in six infants with SCID-Omenn syndrome.

Mutations in proteasome ß-subunits or their chaperone and regulatory proteins are associated with proteasome-associated autoinflammatory disorders (PRAAS). We studied six unrelated infants with three de novo heterozygous missense variants in PSMB10, encoding the proteasome ß2i-subunit. Individuals presented with T-B-NK± severe combined immunodeficiency (SCID) and clinical features suggestive of Omenn syndrome, including diarrhea, alopecia, and desquamating erythematous rash. Remaining T cells had limited T cell receptor repertoires, a skewed memory phenotype, and an elevated CD4/CD8 ratio. Bone marrow examination indicated severely impaired B cell maturation with limited V(D)J recombination. All infants received an allogeneic stem cell transplant and exhibited a variety of severe inflammatory complications thereafter, with 2 peri-transplant and 2 delayed deaths. The single long-term transplant survivor showed evidence for genetic rescue through revertant mosaicism overlapping the affected PSMB10 locus. The identified variants (c.166G>C [p.Asp56His] and c.601G>A/c.601G>C [p.Gly201Arg]) were predicted in silico to profoundly disrupt 20S immunoproteasome structure through impaired ß-ring/ß-ring interaction. Our identification of PSMB10 mutations as a cause of SCID-Omenn syndrome reinforces the connection between PRAAS-related diseases and SCID.

Causative mechanisms and clinical impact of immunoglobulin deficiencies in ataxia telangiectasia.

BACKGROUND: Ataxia telangiectasia (AT) is characterized by cerebellar ataxia, telangiectasia, immunodeficiency, and increased cancer susceptibility and is caused by mutations in the ataxia telangiectasia mutated (ATM) gene. The immunodeficiency comprises predominantly immunoglobulin deficiency, mainly IgA and IgG2, with a variable severity. So far, the exact mechanisms underlying the immunoglobulin deficiency, especially the variable severity, remain unelucidated. OBJECTIVE: We characterized the clinical impact of immunoglobulin deficiencies in AT and elucidated their mechanisms in AT. METHODS: We analyzed long-term immunoglobulin levels, immunophenotyping, and survival time in our cohort (n = 87, median age 16 years; maximum 64 years). Somatic hypermutation and class-switch junctions in B cells were analyzed by next-generation sequencing. Furthermore, an in vitro class-switching induction assay was performed, followed by RNA sequencing, to assess the effect of ATM inhibition. RESULTS: Only the hyper-IgM AT phenotype significantly worsened survival time, while IgA or IgG2 deficiencies did not. The immunoglobulin levels showed predominantly decreased IgG2 and IgA. Moreover, flow cytometric analysis demonstrated reduced naive B and T lymphocytes and a deficiency of class-switched IgG2 and IgA memory B cells. Somatic hypermutation frequencies were lowered in IgA- and IgG2-deficient patients, indicating hampered germinal center reaction. In addition, the microhomology of switch junctions was elongated, suggesting alternative end joining during class-switch DNA repair. The in vitro class switching and proliferation were negatively affected by ATM inhibition. RNA sequencing analysis showed that ATM inhibitor influenced expression of germinal center reaction genes. CONCLUSION: Immunoglobulin deficiency in AT is caused by disturbed development of class-switched memory B cells. ATM deficiency affects both germinal center reaction and choice of DNA-repair pathway in class switching.

A large single-center cohort of bare lymphocyte syndrome: Immunological and genetic features in Turkey.

Major histocompatibility complex class II (MHC-II) deficiency or bare lymphocyte syndrome (BLS) is a rare, early-onset, autosomal recessive, and life-threatening inborn error of immunity. We aimed to assess the demographic, clinical, laboratory, follow-up, and treatment characteristics of patients with MHC-II deficiency, together with their survival. We retrospectively investigated 21 patients with MHC-II deficiency. Female/male ratio was 1.63. The median age at diagnosis was 16.3 months (5 months-9.7 years). Nineteen patients (90.5%) had parental consanguinity. Pulmonary diseases (pneumonia, chronic lung disease) (81%), diarrhoea (47.6%), and candidiasis (28.6%) were common. Four (19%) had autoimmunity, two developed septic arthritis, and three (14%) developed bronchiectasis in the follow-up. Three patients (14%) had CMV viraemia, one with bilateral CMV retinitis. Eight (38.1%) had lymphocytopenia, and four (19%) had neutropenia. Serum IgM, IgA, and IgG levels were low in 18 (85.7%), 15 (71.4%), and 11 (52.4%) patients, respectively. CD4+ lymphocytopenia, a reversed CD4+/CD8+ ratio, and absent/low HLA-DR expressions were detected in 93.3%, 86.7%, and 100% of the patients, respectively. Haematopoietic stem cell transplantation (HSCT) was performed on nine patients, and four died of septicaemia and ARDS after HSCT. The present median age of patients survived is 14 years (1-31 years). Genetic analysis was performed in 10 patients. RFX5 homozygous gene defect was found in three patients (P1, P4 and P8), and RFXANK (P2 and P14) and RFXAP (P18 and P19) heterozygous gene defects were found in each two patients, respectively. This large cohort showed that BLS patients have severe combined immunodeficiency (SCID)-like clinical findings. Flow cytometric MHC-II expression study is crucial for the diagnosis, differential diagnosis with SCID, early haematopoietic stem cell transplantation (HSCT), and post-HSCT follow-up. Genetic studies are required first for matched family donor evaluation before HSCT and then for genetic counselling.

Case report: Persistent hypogammaglobulinemia and mixed chimerism after HLA class-II disparate-hematopoietic stem cell transplant.

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for various hematological, immunological and metabolic diseases, replacing the patient's hematopoietic system with donor-derived healthy hematopoietic stem cells. HSCT can be complicated by early and late events related to impaired immunological recovery such as prolonged hypogammaglobulinemia post-HSCT. We present a 16-year-old female patient with sickle-cell disease who underwent HSCT with stem cells from a human leukocyte antigen (HLA) class-II mismatched family donor. While cellular recovery was good post-HSCT, the patient developed mixed chimerism and suffered from cervical lymphadenopathy, recurrent airway infections and cutaneous SLE. She presented with hypogammaglobulinemia and was started on immunoglobulin substitution therapy and antibiotic prophylaxis. B-cell phenotyping showed that she had increased transitional and naïve mature B cells, reduced memory B cells, and diminished marginal zone/natural effector cells. In-depth immunophenotyping and B-cell receptor repertoire sequencing ruled out an intrinsic B-cell defect by expression of activation-induced cytidine deaminase (AID), presence of somatic hypermutations and differentiation into IgG- and IgA-producing plasma cells in vitro. Immunohistochemistry and flow cytometry of lymph node tissue showed a clear block in terminal B-cell differentiation. Chimerism analysis of sorted lymph node populations showed that exclusively patient-derived B cells populated germinal centers, while only a minor fraction of follicular helper T cells was patient-derived. Given this discrepancy, we deduced that the HLA class-II disparity between patient and donor likely hinders terminal B-cell differentiation in the lymph node. This case highlights that studying disturbed cognate T-B interactions in the secondary lymphoid organs can provide unique insights when deciphering prolonged hypogammaglobulinemia post-HSCT.

NUDCD3 deficiency disrupts V(D)J recombination to cause SCID and Omenn syndrome.

Inborn errors of T cell development present a pediatric emergency in which timely curative therapy is informed by molecular diagnosis. In 11 affected patients across four consanguineous kindreds, we detected homozygosity for a single deleterious missense variant in the gene NudC domain-containing 3 (NUDCD3). Two infants had severe combined immunodeficiency with the complete absence of T and B cells (T -B- SCID), whereas nine showed classical features of Omenn syndrome (OS). Restricted antigen receptor gene usage by residual T lymphocytes suggested impaired V(D)J recombination. Patient cells showed reduced expression of NUDCD3 protein and diminished ability to support RAG-mediated recombination in vitro, which was associated with pathologic sequestration of RAG1 in the nucleoli. Although impaired V(D)J recombination in a mouse model bearing the homologous variant led to milder immunologic abnormalities, NUDCD3 is absolutely required for healthy T and B cell development in humans.