Definition of a novel complementation group in MHC class II deficiency.

In this study we analyzed fibroblasts derived from an MHC class II deficiency patient (type III bare lymphocyte syndrome). Northern blot analysis showed that upon induction with IFN-gamma these fibroblasts did not express HLA class II genes and displayed a strongly reduced level of HLA class I gene expression when compared with fibroblasts of a healthy individual. However, when analyzed by RT-polymerase chain reaction (PCR), residual expression could be detected for HLA-DRA, DPB, and DQA, but not for HLA-DRB, DPA, and DQB. The lack of HLA-DRB transcripts in the patient fibroblasts and the high degree of sequence polymorphism of HLA-DRB were exploited in the further analysis of these fibroblasts. Thus far, at least three, and probably four, complementation groups have been defined among patient-derived and experimentally-derived MHC class II-negative cell lines. Transient heterokaryons between the patient fibroblasts and representative B-lymphoblastoid cell lines from each of the complementation groups were analyzed by RT-PCR and Southern blotting, using HLA-DRB-specific primers and biotin-labeled sequence specific oligonucleotides, respectively. These analyses showed that the fibroblasts of this particular patient belonged to a novel complementation group in MHC class II deficiency.