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

Abstract

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.

Immunobiology of HLA Class-Ib Molecules in Transplantation

“…The same was true for peptide-induced HLA-E surface expression. In addition, the level of cell-surface expression was dependent on the sequence of the peptide presented by HLA-E [19]. Indeed, the CMV, HLA-B7 and HLA-B15 peptides induced higher expression of cell-surface HLA-E than those derived from Hsp60, and the induction of surface HLA-E is always significantly higher in patients with homozygous HLA-E*01:03 regardless of the peptide presented.…”

“…Indeed, the coding and non-coding allelic regions-respectively, exons and introns (plus 5' and 3' untranslated regions)-and the nature of the peptide presented affect the stability of the messenger (m) RNA and the mature protein. This leads to variable levels of protein expression [19,43,45,[62][63][64]. On the other hand, HLA-F is expressed on the cell surface of lymphocytes only upon activation [52] and can be released into the circulation in a soluble form [32]; however, there is not yet any evidence of the effect of specific genotypes of HLA-F or the level of cell-surface expression.…”

“…As part of adaptive immunity [18], HLA-E is capable of binding to peptides for antigen presentation in order to monitor cellular stress, integrity and function [19]. HLA-G is capable of modulating the adaptive immunity [20], as well as exerting antigen-presenting functions, although the latter is not its primary function [21,22].…”

“…Proinflammatory cytokines-e.g., interferon (IFN)-γ [28,39]-and the availability of several peptides [19] upregulate HLA-E expression. HLA-E acts as both an inhibitory and activating ligand for CD94/ NKG2 receptors on NK cells and T lymphocytes [31].…”

“…In addition, HLA-E can be released into the microenvironment of the allograft and in circulation as soluble HLA (sHLA)-E [32,33]. In contrast to HLA-Ia, HLA-E binds to a restricted set of peptides, commonly derived from the leader sequence of HLA-Ia and HLA-G molecules [34][35][36]; it can also bind to viral, bacterial and stress protein-derived peptides [15,19,37]. Two variants of HLA-E are well studied: the difference between them lies in one amino acid at position 107, located on a loop between β-strands in the α2 domain of the heavy chain, one with arginine at position 107-HLA-E*01:01 (HLA-E R107 )-and the other with glycine at the same position, HLA-E*01:03 (HLA-E G107 ) [38].…”