Human leukocyte antigen (HLA)-E belongs, with HLA-G and HLA-F, to the nonclassic major histocompatibility complex (MHC) class I (Ib) molecules, broadly defined by a limited polymorphism and a restricted pattern of cellular expression. In contrast to HLA-G, the expression and function of HLA-E and HLA-F in physiologic and pathologic processes remain poorly established. In the present study, we show that HLA-E protein expression in normal human nonlymphoid organs is mainly restricted to endothelial cells (ECs). HLA-E is also basally expressed by B and T lymphocytes, natural killer (NK) cells and by macrophages. We demonstrate that tumor necrosis factor ␣ (TNF␣), interleukin-1␤ (IL-1␤), and interferon ␥ (IFN␥) up-regulate the cell-surface expression of HLA-E on ECs in vitro and induce the release of soluble HLA-E (sHLA-E). HLA-E up-regulation protects IFN␥-activated ECs from NK-mediated cell lysis, while sHLA-E protects bystander cells. Finally, sHLA-E is not detected in normal sera, and increased serum levels correlate with disease activity in patients with antineutrophil cytoplasmic antibody- IntroductionHuman leukocyte antigen (HLA)-E belongs, with HLA-G and HLA-F, to the nonclassical major histocompatibility complex (MHC) class I (Ib) molecules, broadly defined by a limited polymorphism and a restricted pattern of cellular expression. 1 HLA-G plays an important role in immune tolerance during pregnancy, and in the escape of tumors from immune control. By contrast, the expression and function of HLA-E and HLA-F in physiologic and pathologic processes remain poorly established. Among Ib molecules, HLA-E was initially characterized by a broad pattern of mRNA expression in different cell types. 2 Nevertheless, the surface expression of HLA-E requires the availability of a set of highly conserved nonameric peptides derived from the leader sequence of various HLA class I molecules, including HLA-A, HLA-B, HLA-C, and HLA-G, 3,4 suggesting that HLA-E cellsurface expression may be limited to some cell type and/or cell-activation processes.HLA-E has been identified as a ligand of CD94/NKG2A and CD94/NKG2C receptors expressed on natural killer (NK) cells and a subset of T cells. 5,6 The interaction of HLA-E with the inhibitory CD94/NKG2A receptor results in inhibition of NK cell-and cytotoxic T lymphocyte (CTL)-dependent lysis. 5,7 HLA-E molecules not only provide a protective pathway but also play a role in the regulation of T-cell function. HLA-E complexed with peptides can interact with ␣␤ and ␥␦ T-cell receptors (TCRs) expressed on CD8 ϩ T cells to trigger conventional CTL function. [8][9][10] HLA-E forms a heterodimer with ␤2-microglobulin, which binds and presents peptides derived from self-or foreign proteins 10 after infection, 11 immunization, or transplantation. 12 Finally, in vitro studies using human cells 10 and the demonstration that Qa-1 (homologous to HLA-E in mice)-deficient mice 13 have defects in immunoregulation mediated by CD8 ϩ T cells also provide evidence of the involvement of HLA-E-restricted CD8...

HLA-E are nonclassical MHC molecules with poorly characterized tissue distribution and functions. Because of their capacity to bind the inhibitory receptor, CD94/NKG2A, expressed by NK cells and CTL, HLA-E molecules might play an important role in immunomodulation. In particular, expression of HLA-E might favor tumor cell escape from CTL and NK immunosurveillance. To address the potential role of HLA-E in melanoma immunobiology, we assessed the expression of these molecules ex vivo in human melanoma biopsies and in melanoma and melanocyte cell lines. Melanoma cell lines expressed no or low surface, but significant intracellular levels of HLA-E. We also report for the first time that some of them produced a soluble form of this molecule. IFN-γ significantly increased the surface expression of HLA-E and the shedding of soluble HLA-E by these cells, in a metalloproteinase-dependent fashion. In contrast, melanocyte cell lines constitutively expressed HLA-E molecules that were detectable both at the cell surface and in the soluble form, at levels that were poorly affected by IFN-γ treatment. On tumor sections, a majority of tumor cells of primary, but a low proportion of metastatic melanomas (30–70 and 10–20%, respectively), expressed HLA-E. Finally, HLA-E expression at the cell surface of melanoma cells decreased their susceptibility to CTL lysis. These data demonstrate that HLA-E expression and shedding are normal features of melanocytes, which are conserved in melanoma cells of primary tumors, but become dependent on IFN-γ induction after metastasis. The biological significance of these findings warrants further investigation.

Acellular materials of xenogenic origin are used worldwide as xenografts and Phase I trials of viable pig pancreatic islets are currently being performed. However, limited information is available on transmission of porcine endogenous retrovirus (PERV) after xenotransplantation and on the long-term immune response of recipients to xenoantigens. We analyzed the blood of burn patients who had received living pig skin dressings for up to 8 weeks for the presence of PERV as well as for the level and nature of their long term (maximum 34 years) immune response against pig antigens. Whilst no evidence of PERV genomic material or anti PERV antibody response was found, we observed a moderate increase in anti αGal antibodies and a high and sustained anti non-αGal IgG response in those patients. Antibodies against the non-human sialic acid Neu5Gc constituted the anti non-αGal response with the recognition pattern on a sialogly can array differing from that of burn patients treated without pig skin. These data suggest that anti-Neu5Gc antibodies may represent a barrier for long-term acceptance of porcine xenografts. As anti-Neu5Gc antibodies can promote chronic inflammation, the long-term safety of living and acellular pig tissue implants in recipients warrants further evaluation.