MHC Class II Deficiency: Clinical, Immunological, and Genetic Insights in a Large Multicenter Cohort

Background: Major histocompatibility complex class II deficiency, a combined immunodeficiency, results from loss of HLA class II expression on antigen-presenting cells. Currently, hematopoietic stem cell transplantation stands as the sole curative approach, although factors influencing patient outcomes remain insufficiently explored. Objectives: To elucidate the clinical, immunologic, and genetic profiles associated with MHC-II deficiency and identify prognostic indicators that affect survival rates. Methods: In this multicenter retrospective analysis, we gathered data from 35 patients with a diagnosis of MHC-II deficiency across 12 centers in Turkey. We recorded infection histories, gene mutations, immune cell subsets, and surface MHC-II expression on blood cells. We conducted survival analyses to evaluate the impact of various factors on patient outcomes. Results: Predominant symptoms observed were pneumonia (n = 29; 82.9%), persistent diarrhea (n = 26; 74.3%), and severe infections (n = 26; 74.3%). The RFXANK gene mutation (n = 9) was the most frequent, followed by mutations in RFX5 (n = 8), CIITA (n = 4), and RFXAP (n = 2) genes. Patients with RFXANK mutations presented with later onset and diagnosis compared with those with RFX5 mutations (P =.0008 and .0006, respectively), alongside a more significant diagnostic delay (P = .020). A notable founder effect was observed in five patients with a specific RFX5 mutation (c.616G>C). The overall survival rate for patients was 28.6% (n = 10), showing a significantly higher proportion in individuals with hematopoietic stem cell transplantation (n = 8; 80%). Early death and higher CD8⁺ T-cell counts were observed in patients with the RFX5 mutations compared with RFXANK-mutant patients (P = .006 and .009, respectively). Conclusions: This study delineates the genetic and clinical panorama of MHC-II deficiency, emphasizing the prevalence of specific gene mutations such as RFXANK and RFX5. These insights facilitate early diagnosis and prognosis refinement, significantly contributing to the management of MHC-II deficiency.