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HLA class I typing performed in parallel by molecular biology and serology has revealed cases where an HLA class I allele was identified but the corresponding antigen on the cell surface was not detected. In the present report, we describe three members of a family in whom an HLA-A24 allele identified at the molecular level was typed as A "blank" by lymphocytotoxicity. This serologically blank antigen was nevertheless faintly detectable by isoelectric focusing (IEF) and FACS analyses. Sequencing of the HLA-A*24 allele from the promoter region to the eighth exonic region revealed a point mutation in the acceptor site of the second intron as compared to the normal HLA-A*24 allele. This mutation could lead to incorrect processing of mRNA through a cryptic acceptor site located at the beginning of the third exon and hence to alternative splicing with a frame shift introducing an early stop codon into the fourth exon.

HLA class I typing performed in parallel by molecular biology and serology has revealed cases where an HLA class I allele was identified whereas the corresponding antigen was not detected on the cell surface. In the present report, we describe four members of a family in whom an HLA-A1 allele identified at the molecular level was typed as A "blank" by lymphocytotoxicity. This serologically blank antigen was undetectable by isoelectric focusing (IEF). Sequencing of the HLA-A*01 allele from the promoter region to the eighth exonic region revealed insertion of a "C" nucleotide at the beginning of the fourth exon as compared to the common HLA-A*0101 allele. This mutation causes a frame shift, giving rise to an early stop codon in the fourth exon.

A second HLA-A*68 null allele, HLA-A*6818 N, was identified in our laboratory after discrepant results were obtained between class I serological and molecular typing in a male patient suffering from narcolepsy. HLA-A*6818 N displays a sequence identical to that of the HLA-A*6802 allele, except in exon 2 where 20 nucleotides inserted at codon position 48 are a repeat of the 20 preceding nucleotides. This duplication creates a shift of the reading frame, which leads to a premature non-sense codon at position 59 of the null allele.

HLA-DP expression has been widely investigated on T lymphocytes activated under different conditions. In the present study, a double staining procedure was used in flow cytometric experiments to define DP expression on normal peripheral blood T lymphocytes. In about two-thirds of the case analyzed, DP was expressed on a higher percentage of normal peripheral T lymphocytes than DR was. This was particularly true for 1 of the 16 cases investigated in which the percentage of T lymphocytes expressing DP was 46%) and in which DP expression was mainly the prerogative of CD8+ and CD56+ lymphocytes.