Thymic and T-cell intrinsic critical roles associated with Severe Combined Immunodeficiency and Omenn syndrome due to a heterozygous variant (G201R) in PSMB10.

Background: Heterozygous immunoproteasome subunit beta-type 10 (PSMB10) mutations can cause severe combined immunodeficiency (SCID) and Omenn syndrome (OS). Hematopoietic stem cell transplantation in these patients is associated with severe complications and poor immune reconstitution, often resulting in death.

Objective: To perform immunological and molecular characterization of an infant with a PSMB10 heterozygous variant.

Methods: A heterozygous variant in PSMB10 (p.G201R) was identified in the index but not her parents. Detailed immunophenotyping and functional studies, including flow cytometry, immunoblotting, and T-cell development in artificial thymic organoids (ATO), were performed.

Results: The patient presented with severe B-, NK-, as well as T-cell lymphopenia, with a progressive increase in memory CD4+ and loss of CD8+ T-cells, diminished Vbeta family diversity, and abnormal IL-7 signaling. Immunoproteasome protein expression (PSMB10 and 9) was markedly reduced in the patient's cells, including PBMCs, EBV-transformed B cells, and fibroblasts, the mutation likely acting in a dominant negative fashion. Patient CD34+ cells showed a normal early T-cell development but slightly impaired generation of CD3+TCRαβ+ cells in ATO, and human thymus single cell RNA sequencing demonstrated that PSMB10 is expressed in different subsets of cortical and medullary thymic epithelial cells. Collectively, these data indicate that PSMB10 mutations affect positive selection of CD8 T-cells, generation of a diverse T-cell repertoire, and negative selection of autoreactive T-cells.

Conclusion: The PSMB10 G201R variant is associated with reduced immunoproteasome expression levels that appear to play vital roles in hematopoietic and extra-hematopoietic immune system development and function. PSMB10-associated thymoproteasome dysfunction leads to impaired thymopoiesis and the development of SCID and OS, suggesting possible benefit from thymus implantation.