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.

Type II bare lymphocyte syndrome (BLS II) group A is a rare primary severe immunodeficiency caused by defects in CIITA, one of genes encoding transcriptional regulatory factors for MHC II molecules. To report a Chinese boy with mutation of CIITA. By reviewing the clinical data of the child and performing a literature search of BLS II group A. The patient was presented with persistent pneumonia, chronic diarrhea, urinary tract infection, rash, failure to thrive and special facial characteristics. The patient carried novel mutations in CIITA (c.1243delC, p.R415fs*2 and c.3226C>T, p.R1076W) which were identified by next-generation sequencing and confirmed by Sanger sequencing. This study found novel mutations in the CIITA gene of BLS II, which complemented the mutation spectrum and contributed to the diagnosis, treatment, genetic counseling and prenatal diagnosis of BLS II.

Major histocompatibility complex class II (MHC II) is an essential immune regulatory molecule that plays an important role in antigen presentation and T-cell development. Abnormal MHC II expression can lead to immunodeficiency, clinically termed as type II bare lymphocyte syndrome (BLS), which usually results from mutations in the MHC II transactivator (CIITA) and other coactivators. Here, we present a new paradigm for MHC II deficiency in mice that involves a spontaneous point mutation on H2-Aa. A significantly reduced population of CD4+ T cells was observed in mice obtained from the long-term homozygous breeding of autophagy-related gene microtubule-associated protein 1 light chain 3 β (Map1lc3b, Lc3b) knockout mice; this phenotype was not attributed to the original knocked-out gene. MHC II expression was generally reduced, together with a marked deficiency of H2-Aa in the immune cells of these mice. Using cDNA and DNA sequencing, a spontaneous H2-Aa point mutation that led to false pre-mRNA splicing, deletion of eight bases in the mRNA, and protein frameshift was identified in these mice. These findings led to the discovery of a new type of spontaneous MHC II deficiency and provided a new paradigm to explain type II BLS in mice.

A patient presented severe combined immunodeficiency (SCID)-like symptoms. The presence of a substantial number of CD4+ T-cells in the peripheral blood was not explained by maternal engraftment. Genetic analysis revealed a novel RFXANK mutation, c.232C > T, resulting in a stop codon, with consequently defective transcription of MHC class II resulting in bare lymphocyte syndrome (BLS) type II. The initial unawareness of complete absence of MHC class II expression and normal T-cell receptor excision circles (TREC)-levels delayed the final diagnosis. After identification of the genetic defect the patient was scheduled for hematopoietic stem cell transplantation (HSCT). Here, we present and discuss the diagnostic and therapeutic approach of a novel case of BLS type II in relation to T-cell development.