Peptide-induced HLA-E expression in human PBMCs is dependent on peptide sequence and the HLA-E genotype.

Human Leukocyte Antigen (HLA)-E is a low-polymorphic non-classical HLA class I molecule which plays a crucial role in immune surveillance by presentation of peptides to T and natural killer (NK) cells. HLA-E polymorphism is related to HLA-E surface expression and is associated with patient outcome after stem cell transplantation. We aim to investigate the regulation of HLA-E expression level in peripheral blood mononuclear cells (PBMCs) of healthy individuals homozygous for HLA-E*01:01 or HLA-E*01:03, by using a panel of HLA-E binding peptides derived from CMV, Hsp60 and HLA class I. Basal and peptide-induced HLA-E surface expression levels were higher in PBMC from HLA-E*01:03 homozygous subjects as compared to PBMC from HLA-E*01:01 homozygous subjects. HLA-E mRNA levels were comparable between the two genotypes and remained constant after peptide stimulation. HLA-E surface expression seemed to be not only dependent on the HLA-E genotype, but also on the sequence of the peptide as evidenced by the profound difference in HLA-E upregulation with the Hsp60 and the B7 peptide. Our results showed that peptide-induced HLA-E expression is regulated at the posttranscriptional level as extracellular peptide stimulation did not influence RNA expression. This study provides new insights in the mechanism by which HLA-E expression is regulated and underlines a new role for extracellular peptides in inducing HLA-E translation, which may represent a defense mechanism against lytic viral infections and necrosis.

Full-length HLA-DPB1 diversity in multiple alleles of individuals from Caucasian, Black, or Oriental origin.

Despite DP antigens have been shown to be stimulators of the mixed lymphocyte reaction, human leukocyte antigen-DPB1 is not considered in the matching criteria for hematopoietic stem cell transplantation (HSCT). The role of DPB1 matching in HSCT remains inconclusive because of contradictory findings in different studies. The concept of permissible and non-permissible mismatches might clarify these contradictory results. Although several groups have attempted to identify immunogenic epitopes in exon 2 to establish permissive and non-permissive allele groups, the direct correlation between individual exon 2 amino acids and epitopes with DPB1 immunogenicity is still not evident. We hypothesize that polymorphism within the entire molecule, including polymorphic variability in different ethnic groups, is crucial to unravel the function of DPB1 polymorphism. Using an RNA-based approach, we sequenced all frequent and available non-frequent DPB1 alleles full length from 148 samples representing 28 different DPB1 alleles from either Black, Caucasian, or Oriental origin. We identified various DPB1 alleles with, in addition to the exon 2 polymorphism, polymorphisms in exons 1, 3, 4, and 5. Based on this polymorphism outside exon 2, we defined one new allele. Two alleles with identical exon 2 polymorphism but differing outside exon 2 were identified in individuals of different ethnic groups. As T cell binding is not restricted to the polymorphic groove and polymorphism in the ß2 domain of the DP molecule affects CD4 interaction, full-length polymorphism should be considered to determine immunogenicity. Eventually, this knowledge will provide new insights in the classification of DPB1 polymorphism and more importantly will add new perspectives to the concept of permissiveness in transplantation.