HLA class I expression depends on the formation of a peptide-loading complex composed of class I heavy chain; ␤ 2 -microglobulin; the transporter associated with antigen processing (TAP); and tapasin, which links TAP to the heavy chain. Defects in TAP result in a class I deficiency called the type I bare lymphocyte syndrome (BLS). In the present study, we examined a subject with a novel type I BLS who does not exhibit apparent TAP abnormalities but who has a tapasin defect. The subject's TAPASIN gene has a 7.4-kilobase deletion between introns 3 and 7; an Alu repeat-mediated unequal homologous recombination may be the cause of the deletion. No tapasin polypeptide was detected in the subject's cells. IntroductionType I bare lymphocyte syndrome (BLS) is characterized by the lack of cell surface HLA class I. 1,2 Patients with this syndrome showed a reduced number of CD8 ϩ T cells and the lack of natural killer (NK) activities. 3,4 Type I BLS is caused by a deficiency in the transporter associated with antigen processing (TAP). 4-7 TAP transports peptides from the cytoplasm into the inner lumen of the endoplasmic reticulum (ER), and defects in TAP induce poor peptide loading on class I heavy chains (HCs), resulting in a reduction in the number of mature class I molecules on the cell surface. Peptide binding of HCs occurs on the ER by the formation of a complex that is composed of HCs, ␤ 2 -microglobulin, TAP, and tapasin. 8 Tapasin is a 48-kd transmembrane protein that has 2 immunoglobulinlike domains and the ER-retention motif. 9,10 The TAPASIN gene is located at the HLA region. 11 Tapasin binds with TAP, forms a bridge between TAP and HCs, enhances peptide loading, and retains immature HCs in the ER. 12,13 The importance of tapasin in the class I assembly has been indicated by studies using a human mutant cell line, 721.220, which has a truncated tapasin by aberrant splicing. 9,14 TAPASIN-gene-targeted mice had an impaired class I assembly, reduced level of cell surface class I expression, and impaired immune responses. 15,16 Here we analyzed a subject with a novel type I BLS who did not manifest the symptoms commonly observed in TAP-deficient subjects and found that the subject has tapasin deficiency. Study design AntibodiesAntitapasin (peptides at 313-330) and anti-TAP1 rabbit antibodies 5 were provided by K. Tanaka Cell preparationsPeripheral blood mononuclear cells (PBMCs) were isolated from a subject S.M. The B-cell line SM-LCL was established from the patient's PBMCs. The TAP1-deficient KMW-B2 cells have been described previously. 5 Polymerase chain reaction amplification and sequencingGenomic DNA and complementary DNA (cDNA) were prepared as described previously. 5 DNA sequencing was performed by means of ABI 310 (PE Applied Biosystems, Foster City, CA). Flow cytometric analysisCells were stained by fluorescein isothiocyanate (FITC)-conjugated antibodies and analyzed by means of EPICS Elite flow cytometer (Beckman Coulter, Fullerton, CA) Results and discussion S.M. was a 54-year-old woman suffering fr...