A patient with RFX5 variant causing an expression defect in both HLA ABC and HLA DR.

The major histocompatibility complex (MHC) encompasses a group of genes critical for immune system regulation. In humans, these molecules are referred to as human leukocyte antigens (HLA) due to their initial discovery in human leukocytes. Class I molecules present antigens to CD8 + T cells, while Class II molecules present to CD4 + T cells. Here we report a patient who had a background of parental consanguinity and a family history suggestive of immunodeficiency. He presented with clinical symptoms including fever, septic arthritis, recurrent moniliasis. Preliminary diagnostic tests revealed hypogammaglobulinemia and CD4 lymphopenia. Further immunological assessment indicated extremely low expression levels of HLA molecules: HLA ABC at 5% and HLA DR at 0%. Genetic analysis showed a mutation in the regulatory factor X5 (RFX5) gene, leading to a combined immunodeficiency diagnosis. Consequently, hematopoietic stem cell transplantation (HSCT) was planned. Regulatory factor X5plays a pivotal role in immune function by transactivating genes critical for the expression of MHC Class I and Class II molecules, as well as beta- 2-microglobulin (B2M). MHC Class I transcription is controlled indirectly by RFX5, and the RFX5 gene mutation in the patient likely caused the markedly reduced expression of HLA ABC in addition to HLA DR. Combined HLA-ABC and HLA-DR expression analyses via flow cytometry may serve as a valuable diagnostic tool for identifying RFX5-related immunodeficiency at an early stage, facilitating timely genetic testing and appropriate clinical management.