A new HLA-B44 allele (B*44020102S) with a splicing mutation leading to a complete deletion of exon 5.

Using a combination of serology and polymerase chain reaction with sequence-specific primer (PCR-SSP), we have identified in a volunteer bone marrow donor a new HLA class I antigen within the B44 serotype. This human leukocyte antigen (HLA)-B44 variant was typed as 'blank' by microlymphocytotoxicity, whereas the B44020101 allele was identified by PCR-SSP. A family study confirmed the Mendelian segregation of this blank antigen identified on one of the maternal haplotype transmitted to her child. The DNA sequence of B44new, now referred to as B44020102S, performed from the promoter region to the 3' untranslated region revealed a single nucleotide difference (A/G) compared to B44020101 at the end of intron 4 in the acceptor-splicing site. This mutation leads to an incorrect splicing characterized by the deletion of exon 5 that encodes the transmembrane domain of the HLA antigen. Indeed, full-length complementary DNA sequencing revealed a complete absence of exon 5. Fluorescence-activated cell sorter analysis confirmed the absence of expression of HLA-B44 on the cell surface in the donor, compared to the HLA-B44 positive control. The isoelectric focusing analysis failed to reveal the presence of an HLA-B44 antigen in the donor, showing that no normal HLA-B44020101 allele was synthesized. The new B440201010102S allele is a soluble form of B44 without any detectable cell-surface expression. It can thus be considered as a soluble antigen, a form apparently inactive and unfit for antigen presentation.