Cell surface expression of HLA-E: interaction with human beta2-microglobulin and allelic differences

HLA-E*01:01 + HLA-E*01:01 genotype confers less susceptibility to COVID-19, while HLA-E*01:03 + HLA-E*01:03 genotype is associated with more severe disease

BACKGROUND

HLA-E interaction with inhibitory receptor, NKG2A attenuates NK-mediated cytotoxicity. NKG2A overexpression by SARS-CoV-2 exhausts NK cells function, whereas virus-induced down-regulation of MHC-Ia reduces its derived-leader sequence peptide levels required for proper binding of HLA-E to NKG2A. This leads HLA-E to become more complex with viral antigens and delivers them to CD8⁺ T cells, which facilitates cytolysis of infected cells. Now, the fact that alleles of HLA-E have different levels of expression and affinity for MHC Ia-derived peptide raises the question of whether HLA-E polymorphisms affect susceptibility to COVID-19 or its severity.

METHODS

104 COVID-19 convalescent plasma donors with/without history of hospitalization and 18 blood donors with asymptomatic COVID-19, all were positive for anti-SARS-CoV-2 IgG antibody as well as a group of healthy control including 68 blood donors with negative antibody were subjected to HLA-E genotyping. As a privilege, individuals hadn't been vaccinated against COVID-19 and therefore naturally exposed to the SARS-CoV-2.

RESULTS

The absence of HLA-E*01:03 allele significantly decreases the odds of susceptibility to SARS-CoV-2 infection [p = 0.044; OR (95 %CI) = 0.530 (0.286 - 0.983)], suggesting that HLA-E*01:01 + HLA-E*01:01 genotype favors more protection against SARS-CoV-2 infection. HLA-E*01:03 + HLA-E*01:03 genotype was also significantly associated with more severe COVID-19 [p = 0.020; 2.606 (1.163 - 5.844) CONCLUSION: Here, our observation about lower susceptibility of HLA-E*01:01 + HLA-E*01:01 genotype to COVID-19 could be clinical evidence in support of some previous studies suggesting that the lower affinity of HLA-E*01:01 to peptides derived from the leader sequence of MHC class Ia may instead shift its binding to virus-derived peptides, which then facilitates target recognition by restricted conventional CD8⁺ T cells and leads to efficient cytolysis. On the other hand, according to other studies, less reactivity of HLA-E*01:01 with NKG2A abrogates NK cells or T cells inhibition, which may also lead to a greater cytotoxicity against SARS-CoV-2 infected cells compared to HLA-E*01:03. Taken together given HLA-E polymorphisms, the data presented here may be useful in identifying more vulnerable individuals to COVID-19 for better care and management. Especially since along with other risk factors in patients, having HLA-E*01:03 + HLA-E*01:03 genotype may also be associated with the possibility of severe cases of the disease.

86 novel HLA-E alleles discovered through full-gene sequencing of 6227 hematopoietic cell transplant patients and unrelated donors

Until recently the number of alleles of the nonclassical HLA class I gene HLA-E documented in the IPD-IMGT/HLA Database was small and as a result, the gene was often not considered to be notably polymorphic. Here, we describe our work in identifying and submitting 86 novel HLA-E alleles after full-gene single-molecule real-time (SMRT) DNA sequencing of 6227 DNA samples. These samples were comprised of 2468 patients undergoing hematopoietic cell transplantation and 3759 unrelated potential donors. A total of 111 unique HLA-E alleles were detected in this cohort. The majority of novel alleles (79.1%) contained polymorphisms in intronic regions, highlighting the significant undiscovered variation present in the noncoding regions of the HLA-E gene.

Genetic Variants of the NKG2C/HLA-E Receptor-Ligand Axis Are Determinants of Progression-Free Survival and Therapy Outcome in Aggressive B-Cell Lymphoma

Simple Summary

NKG2C and its ligand HLA-E represent key molecules for NK cell-mediated immune responsiveness. However, the impact of genetic variants in NKG2C and HLA-E on clinical outcomes of aggressive B-cell non-Hodgkin lymphoma patients (B-NHL) has not been clarified. In this study, we analyzed the distribution of NKG2C deletion status and HLA-E variants in 441 patients and 192 healthy individuals. Homozygous deletion of NKG2C (NKG2C^−/−) was more often found in high-risk patients compared to patients with a lower risk and consequently was associated with reduced 2-year progression-free survival. The HLA-E*01:01 allele frequency was increased in B-NHL patients and was strongly related with complete remission. Our results show that absence of NKG2C and HLA-E allelic variations is predictive for B-NHL outcome; while carriers of HLA-E*01:01 are characterized by high, complete remission rates, NKG2C^−/− was rare, but associated with poorer outcome. Prospective validation of our results identifies patients that may benefit from risk-adapted therapy.

Abstract

Aggressive B-cell lymphomas account for the majority of non-Hodgkin lymphomas (B-NHL). NK cells govern the responses to anti-CD20 monoclonal antibodies and have emerged as attractive targets for immunotherapy in subtypes of B-NHL. NKG2C and its cognate ligand HLA-E represent key molecules for fine-tuning of NK cell-mediated immune responses. Here, we investigated the impact of genetic variants of NKG2C and HLA-E on clinical outcomes of 441 B-NHL patients. Homozygous deletion of NKG2C (NKG2C^−/−) was three-fold increased in patients compared to 192 healthy controls. Among studied patients, NKG2C^−/− was more abundant in International Prognostic Index (IPI) high-risk patients compared to patients with a lower IPI (p = 0.013). Strikingly, NKG2C^−/− was associated with a significantly reduced 2-year PFS (progression-free survival) (p = 0.0062) and represented an independent risk factor for 2-year PFS in multivariate analysis (p = 0.005). For HLA-E, the cognate ligand of NKG2C, the HLA-E*01:01 allele frequency was increased in B-NHL patients compared to controls (p = 0.033) and was associated with complete remission in univariate (p = 0.034) and multivariate (p = 0.018) analysis. Our data suggest that NKG2C and HLA-E genotyping is a promising tool for both defining risk groups of aggressive B-NHL and predicting response to immune therapeutic approaches.

HLA-G Polymorphisms Associated with HIV Infection and Preeclampsia in South Africans of African Ancestry

OBJECTIVES

HLA-G, part of the major histocompatibility complex (MHC), is associated with the risk of developing preeclampsia (PE). In this study, we determined the contribution of specific HLA-G polymorphisms on the risk of developing preeclampsia in HIV-infected and uninfected South Africans of African ancestry.

METHODS

One hundred and ninety-three women of African ancestry were enrolled (74 HIV-uninfected normotensive, 60 HIV-infected normotensive, 34 HIV-uninfected, and 25 HIV-infected preeclamptics). Sanger sequencing of the untranslated region was performed to genotype six SNPs, i.e., 14 bp Ins/Del of rs66554220, rs1710, rs1063320, rs1610696, rs9380142, and rs1707).

RESULTS

For rs66554220, we have the following results: (a) based on pregnancy type-the Ins/Ins and Del/Ins genotype frequency was higher in preeclampsia (PE) compared to normotensive pregnancies (Ins/Ins vs. Del/Ins, P = 0.02∗: OR (95%CI) = 13.44 (0.7222-249.9); Del/Del vs. Del/Ins, P = 0.03∗: OR (95%CI) = 2.95 (1.10-7.920)); (b) based on HIV status-the Ins/Ins showed both genotypic and allelic association with HIV infection. HIV-infected PE has higher Ins/Ins genotypic and allelic frequencies compared to HIV-uninfected PE (Ins/Ins vs. Del/Ins, P = 0.005∗∗: OR (95%CI) = 21.32 (1.71-4.17); Ins, P = 0.005∗∗; OR (95%IC) = 21.32 (1.71-4.17)). For rs1707, we have the following results: (a) based on pregnancy type-there were CT genotypic frequencies in PE, more especially LOPE compared to normotensive pregnancies (TT vs. CT, P = 0.0092∗∗: OR (95%CI) = 5.(1.39 - 25.64)), and no allelic association was noted; (b) based on HIV status-CT was higher in HIV-infected LOPE compared to uninfected LOPE (TT vs. TC, P = 0.0006∗∗∗: OR (95%CI) = 40.00 (2.89 - 555.1)). For rs1710 and rs1063320, no significant differences in the genotype and allele frequencies were noted based on pregnancy type and HIV status. For rs9380142, we have the following results: (a) based on pregnancy type-no significant differences were noted between normotensive compared to PE pregnancies; (b) based on HIV status-AA genotypes occurred more in the HIV-infected PE group (AA vs. GG, P = 0.02∗: OR (95%CI) = 13.97 (0.73 - 269.4)), while A allelic frequency occurred more in HIV-infected PE, especially LOPE compared to uninfected groups (A vs. G, P = 0.0003∗∗∗: OR (95%CI) = 10.72 (2.380 - 48.32); P = 0.02∗: OR (95%CI) = 9.00 (1.07 - 75.74)). For rs1610696, we have the following results: (a) based on pregnancy type-genotypic and allelic frequencies of CC were higher in PE compared to normotensive pregnancies (CC vs. GG, P = 0.0003∗∗∗: OR (95%CI) = 31.87 (1.861 - 545.9); C, P = 0.0001∗∗∗: OR (95%IC) = 21.91 (2.84 - 169.0)); (b) based on HIV status-GG frequencies were higher in the HIV-infected PE more especially LOPE groups (GG vs. GC, P = 0.02∗: OR (95%CI) = 16.87 (0.81 - 352.1); GG vs. CC, P = 0.0001∗∗∗: OR (95%CI) = 159.5 (13.10 - 1942)).

CONCLUSION

Selected HLA-G 14 bp polymorphisms (Ins/Ins) and genotypic and allelic differences in rs9380142, rs1610696, and rs1707 are associated with the pathogenesis of preeclampsia in HIV-infected South African women of African ancestry. More genetic studies evaluating the association between preeclampsia and HIV infection are needed to improve diagnosis and antenatal care.

The unconventional role of HLA-E: The road less traveled

Histocompatibility Leukocyte Antigens, or HLAs, are one of the most polymorphic molecules in humans. This high degree of polymorphism endows HLA molecules with the ability to present a vast array of peptides, an essential trait for responding to ever-evolving pathogens. Unlike classical HLA molecules (HLA-Ia), some non-classical HLA-Ib molecules, including HLA-E, are almost monomorphic. Several studies show HLA-E can present self-peptides originating from the leader sequence of other HLA molecules, which signals to our immune system that the cell is healthy. Therefore, it was traditionally thought that the chief role of HLA-E in the body was in immune surveillance. However, there is emerging evidence that HLA-E is also able to present pathogen-derived peptides to the adaptive immune system, namely T cells, in a manner that is similar to classical HLA-Ia molecules. Here we describe the early findings of this less conventional role of HLA-E in the adaptive immune system and its importance for immunity.

Subordinate Effect of -21M HLA-B Dimorphism on NK Cell Repertoire Diversity and Function in HIV-1 Infected Individuals of African Origin

Natural Killer (NK) cells play an important role in antiviral defense and their potent effector function identifies them as key candidates for immunotherapeutic interventions in chronic viral infections. Their remarkable functional agility is achieved by virtue of a wide array of germline-encoded inhibitory and activating receptors ensuring a self-tolerant and tunable repertoire. NK cell diversity is generated by a combination of factors including genetic determinants and infections/environmental factors, which together shape the NK cell pool and functional potential. Recently a genetic polymorphism at position -21 of HLA-B, which influences the supply of HLA-E binding peptides and availability of HLA-E for recognition by the inhibitory NK cell receptor NKG2A, was shown to have a marked influence on NK cell functionality in healthy human cytomegalovirus (HCMV) seronegative Caucasian individuals. In this study, -21 methionine (M)-expressing alleles supplying HLA-E binding peptides were largely poor ligands for inhibitory killer immunoglobulin-like receptors (KIRs), and a bias to NKG2A-mediated education of functionally-potent NK cells was observed. Here, we investigated the effect of this polymorphism on the phenotype and functional capacity of peripheral blood NK cells in a cohort of 36 African individuals with human immunodeficiency virus type 1 (HIV-1)/HCMV co-infection. A similarly profound influence of dimorphism at position -21 of HLA-B on NK cells was not evident in these subjects. They predominantly expressed African specific HLA-B and -C alleles that contribute a distinct supply of NKG2A and KIR ligands, and these genetic differences were compounded by the marked effect of HIV-1/HCMV co-infection on NK cell differentiation. Together, these factors resulted in a lack of correlation of the HLA-B -21 polymorphism with surface abundance of HLA-E and loss of the NK cell functional advantage in subjects with -21M HLA-B alleles. Instead, our data suggest that during HIV/HCMV co-infection exposure of NK cells to an environment that displays altered HLA-E ligands drives adaptive NKG2C+ NK cell expansions influencing effector responses. Increased efforts to understand how NK cells are functionally calibrated to self-HLA during chronic viral infections will pave the way to developing targeted therapeutic interventions to overcome the current barriers to enhancing immune-based antiviral control.

Donor HLA-E Status Associates with Disease-Free Survival and Transplant-Related Mortality after Non In Vivo T Cell-Depleted HSCT for Acute Leukemia

Previous studies have suggested that HLA-E may have a significant role in the outcome of matched unrelated hematopoietic stem cell transplantation (HSCT), especially for patients with acute leukemia. We used Center for International Blood and Marrow Transplant Research data and samples of 1840 adult patients with acute leukemia and their 10/10 HLA-matched unrelated donors to investigate the impact of HLA-E matching status as well as of donor/recipient (D/R) HLA-E genotype on post-HSCT outcome. Both patients and donors were HLA-E genotyped by next-generation sequencing. All patients received their first transplant in complete remission between 2000 and 2015. Median follow-up time was 90 months. Overall survival, disease-free survival (DFS), transplant-related mortality (TRM), and relapse incidence were primary endpoints with statistical significance set at .01. D/R HLA-E genotype analysis revealed a significant association of donor HLA-E*01:03/01:03 genotype with DFS (hazard ratio [HR] = 1.35, P = .0006) and TRM (HR = 1.41, P = .0058) in patients who received T cell replete (ie, without in vivo T cell depletion) transplants (n = 1297). As for D/R HLA-E matching, we did not identify any significant effect on any of the clinical outcome endpoints. In conclusion, this is the largest study to date reporting an improvement of DFS and TRM after matched unrelated HSCT by avoidance of HLA-E*01:03 homozygous donors in patients transplanted with T cell replete grafts for acute leukemia.

HLA-E Polymorphism Determines Susceptibility to BK Virus Nephropathy after Living-Donor Kidney Transplant

Human leukocyte antigen (HLA)-E is important for the regulation of anti-viral immunity. BK polyomavirus (BKPyV) reactivation after kidney transplant is a serious complication that can result in BKPyV-associated nephropathy (PyVAN) and subsequent allograft loss. To elucidate whether HLA-E polymorphisms influence BKPyV replication and nephropathy, we determined the HLA-E genotype of 278 living donor and recipient pairs. A total of 44 recipients suffered from BKPyV replication, and 11 of these developed PyVAN. Homozygosity of the recipients for the HLA-E*01:01 genotype was associated with the protection against PyVAN after transplant (p = 0.025, OR 0.09, CI [95%] 0.83-4.89). Considering the time course of the occurrence of nephropathy, recipients with PyVAN were more likely to carry the HLA-E*01:03 allelic variant than those without PyVAN (Kaplan-Meier analysis p = 0.03; OR = 4.25; CI (95%) 1.11-16.23). Our findings suggest that a predisposition based on a defined HLA-E genotype is associated with an increased susceptibility to develop PyVAN. Thus, assessing HLA-E polymorphisms may enable physicians to identify patients being at an increased risk of this viral complication.

HLA-E Peptide Repertoire and Dimorphism-Centerpieces in the Adaptive NK Cell Puzzle?

Adaptive Natural Killer (NK) cells, a heterogenous subpopulation of human NK cells with a unique phenotypic and functional signature, became arguably one of the central areas of interest in the field. While their existence seems closely associated with prior exposure to human cytomegalovirus (HCMV), many questions regarding their origin and regulation remain unanswered. However, a common denominator for the majority of adaptive NK cells is the expression of the activating heterodimeric receptor CD94/NKG2C that binds to HLA-E, a non-classical HLA molecule, that displays a comparably restricted expression pattern, very limited polymorphism and presents a distinct set of peptides. Recent studies suggest that-in analogy to T cell responses-peptides presented on HLA-E could play an unexpectedly decisive role for the biology of adaptive NK cells. Here, we discuss how this perspective on the CD94/NKG2C-HLA-E axis aligns with the existing literature and speculate about possible translational implication.

The Role of MHC-E in T Cell Immunity Is Conserved among Humans, Rhesus Macaques, and Cynomolgus Macaques

MHC-E is a highly conserved nonclassical MHC class Ib molecule that predominantly binds and presents MHC class Ia leader sequence-derived peptides for NK cell regulation. However, MHC-E also binds pathogen-derived peptide Ags for presentation to CD8+ T cells. Given this role in adaptive immunity and its highly monomorphic nature in the human population, HLA-E is an attractive target for novel vaccine and immunotherapeutic modalities. Development of HLA-E-targeted therapies will require a physiologically relevant animal model that recapitulates HLA-E-restricted T cell biology. In this study, we investigated MHC-E immunobiology in two common nonhuman primate species, Indian-origin rhesus macaques (RM) and Mauritian-origin cynomolgus macaques (MCM). Compared to humans and MCM, RM expressed a greater number of MHC-E alleles at both the population and individual level. Despite this difference, human, RM, and MCM MHC-E molecules were expressed at similar levels across immune cell subsets, equivalently upregulated by viral pathogens, and bound and presented identical peptides to CD8+ T cells. Indeed, SIV-specific, Mamu-E-restricted CD8+ T cells from RM recognized antigenic peptides presented by all MHC-E molecules tested, including cross-species recognition of human and MCM SIV-infected CD4+ T cells. Thus, MHC-E is functionally conserved among humans, RM, and MCM, and both RM and MCM represent physiologically relevant animal models of HLA-E-restricted T cell immunobiology.

Human leukocyte antigen-E mismatch is associated with better hematopoietic stem cell transplantation outcome in acute leukemia patients

The immunomodulatory role of human leukocyte antigen (HLA)-E in hematopoietic stem cell transplantation (HSCT) has not been extensively investigated. To this end, we genotyped 509 10/10 HLA unrelated transplant pairs for HLA-E, in order to study the effect of HLA-E as a natural killer (NK)-alloreactivity mediator on HSCT outcome in an acute leukemia (AL) setting. Overall survival (OS), disease free survival (DFS), relapse incidence (RI) and non-relapse mortality (NRM) were set as endpoints. Analysis of our data revealed a significant correlation between HLA-E mismatch and improved HSCT outcome, as shown by both univariate (53% vs 38%, P=0.002, 5-year OS) and multivariate (hazard ratio (HR)=0.63, confidence interval (CI) 95%=0.48-0.83, P=0.001) analyses. Further subgroup analysis demonstrated that the positive effect of HLA-E mismatch was significant and pronounced in advanced disease patients (n=120) (5-year OS: 50% vs 18%, P=0.005; HR=0.40, CI 95%=0.22-0.72, P=0.002; results from univariate and multivariate analyses, respectively). The study herein is the first to report an association between HLA-E incompatibility and improved post-transplant prognosis in AL patients who have undergone matched unrelated HSCT. Combined NK and T cell HLA-E-mediated mechanisms may account for the better outcomes observed. Notwithstanding the necessity for in vitro and confirmational studies, our findings highlight the clinical relevance of HLA-E matching and strongly support prospective HLA-E screening upon donor selection for matched AL unrelated HSCTs.

Association of high HLA-E expression during acute cellular rejection and numbers of HLA class I leader peptide mismatches with reduced renal allograft survival

Non-classical Human Leukocyte Antigen (HLA)-E preferentially presents leader peptides derived from classical HLA-class I molecules. HLA-E can trigger opposed immune responses by interacting with inhibitory NKG2A or by activating NKG2C receptors on NK and T-cells. We studied the impact of HLA-E on renal allograft survival during acute cellular rejection. HLA-E expression was up-regulated in acute cellular rejection (ACR) biopsies (n=12) compared to biopsies from 13 renal allografts with no rejection-signs. HLA-E up-regulation was correlated with numbers of HLA-class I leader peptide mismatches (p=0.04). CD8+ and CD56+ infiltrating cells correlated with HLA-E expression (p<0.0001 and p=0.0009, respectively). Activating NKG2C receptor dominated on effector cells in biopsies and peripheral blood during ACR potentially allowing HLA-E-mediated immune activation. Moreover, HLA-E expression correlated with deterioration in renal allograft function (p<0.008) and reduced allograft survival (p=0.002). Our findings provide evidence that during renal allograft rejection HLA-E along with high numbers of mismatched HLA-class I leader peptides might represent additional targets for immune-activating responses.

HLA-E polymorphism and soluble HLA-E plasma levels in chronic hepatitis B patients

Chronic hepatitis B virus (HBV) infection occurs in association to a deregulation of immune system. Human leukocyte antigen E (HLA-E) is an immune-tolerant nonclassical HLA class I molecule that could be involved in HBV progression. To measure soluble (s) HLA-E in patients with chronic HBV hepatitis (CHB). We tested the potential association of HLA-E*01:01/01:03 A > G gene polymorphism to CHB. Our cohort consisted of 93 Tunisian CHB patients (stratified in CHB with high HBV DNA levels and CHB with low HBV DNA levels) and 245 healthy donors. Plasma sHLA-E was determined using enzyme-linked immunosorbent assay (ELISA). Genotyping was performed using polymerase chain reaction sequence-specific primer. No association between HLA-E*01:01/01:03 A > G polymorphism and HBV DNA levels in CHB patients was found. G/G genotype is less frequent in CHB patients without significance. sHLA-E is significantly enhanced in CHB patients compared with healthy controls (P = 0.0017). Stratification according to HBV DNA levels showed that CHB patients with low HBV DNA levels have higher sHLA-E levels compared with CHB patients with high HBV DNA levels. CHB patients with G/G genotype have enhanced sHLA-E levels compared with other genotypes (P = 0.037). This significant difference is maintained only for CHB women concerning G/G genotypes (P = 0.042). Finally, we reported enhanced sHLA-E in CHB patients with advanced stages of fibrosis (P = 0.032). We demonstrate, for the first time, the association of sHLA-E to CHB. Owing to the positive correlation of HLA-E*01:01/01:03 A > G polymorphism and the association of sHLA-E to advanced fibrosis stages, HLA-E could be a powerful predictor for CHB progression. Further investigations will be required to substantiate HLA-E role as a putative clinical biomarker of CHB.

The positive prognostic effect of stromal CD8+ tumor-infiltrating T cells is restrained by the expression of HLA-E in non-small cell lung carcinoma

INTRODUCTION

Tumor-infiltrating CD8+ T cells are associated with improved clinical outcomes in non-small cell lung cancer (NSCLC). Here we studied their prognostic effect in the context of the expression of HLA molecules that are key in tumor recognition (HLA-A, B and C) or suppression of immunity (HLA-E) as this is still unknown.

METHODS

Tumor tissue of 197 patients with resected pulmonary adenocarcinoma was analyzed for the presence of CD8+ T cells and the expression of β2-microglobulin, HLA-A, HLA-B/C and HLA-E. The relation of these parameters with overall survival (OS) was assessed.

RESULTS

Loss and low expression of HLA-A or HLA-B/C was found in 44% and 75% of cases respectively. A high CD8+ tumor infiltration was strongly associated with clinical benefit only when the tumors retained good expression of HLA-A and HLA-B/C (p=0.004). In addition, more than 70% of the tumors were found to display a high expression of HLA-E. The expression of HLA-E by tumor cells was an independent negative prognostic factor for OS (p=0.031). Importantly, a dense stromal CD8+ T cell infiltration was strongly associated with improved OS only in HLA-E negative tumors (p=0.005) and its prognostic effect was completely abolished when tumors highly expressed HLA-E (p=0.989).

CONCLUSIONS

CD8+ T cell infiltration strongly contributes to a better prognosis in NSCLC when the tumor cells retain the expression of classical HLA class I and do not express HLA-E. Therefore, analysis of HLA-A, -B/C and HLA-E expression should be included as biomarkers to predict the response to immunotherapy.

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.

Do human leukocyte antigen E polymorphisms influence graft-versus-leukemia after allogeneic hematopoietic stem cell transplantation?

Hematopoietic-stem-cell transplantation (HSCT) is complicated by histocompatibility-dependent immune responses such as graft-versus-host disease, relapse, and graft rejection. The severity of these common adverse effects is directly related to the degree of human leukocyte antigen (HLA) incompatibility. In addition to the key role of classic HLA matching in influencing HSCT outcome, several lines of evidence suggest an important role for nonclassic major histocompatibility complex class I molecule, HLA-E. The interaction of HLA-E with NKG2A, its main receptor on natural killer cells, modulates cell-mediated cytotoxicity and cytokine production, an important role in innate immune responses. In addition, the HLA-E molecule can present peptides to different subtypes of T cells that may either support graft-versus-leukemia effects or be involved in bridging innate and acquired immunity. To date, the role of HLA-E and its polymorphisms in HSCT outcomes such as graft-versus-host disease, transplant-related mortality, and improved survival has been published by a number of groups. In addition, these data suggest an association between HLA-E polymorphisms and relapse. Whether the engagement of the HLA-E molecule in the modulation of donor T cells is involved in the graft-versus-leukemia effect, or whether a different mechanism of HLA-E dependent reduction of relapse is involved, requires further investigation.

HLA Class Ib Molecules and Immune Cells in Pregnancy and Preeclampsia

Despite decades of research, the highly prevalent pregnancy complication preeclampsia, “the disease of theories,” has remained an enigma. Indeed, the etiology of preeclampsia is largely unknown. A compiling amount of studies indicates that the pathological basis involves a complex array of genetic predisposition and immunological maladaptation, and that a contribution from the mother, the father, and the fetus is likely to be important. The Human Leukocyte Antigen (HLA)-G is an increasing focus of research in relation to preeclampsia. The HLA-G molecule is primarily expressed by the extravillous trophoblast cells lining the placenta together with the two other HLA class Ib molecules, HLA-E and HLA-F. Soluble isoforms of HLA-G have been detected in the early endometrium, the matured cumulus–oocyte complex, maternal blood of pregnant women, in umbilical cord blood, and lately, in seminal plasma. HLA-G is believed to be involved in modulating immune responses in the context of vascular remodeling during pregnancy as well as in dampening potential harmful immune attacks raised against the semi-allogeneic fetus. In addition, HLA-G genetic variants are associated with both membrane-bound and soluble forms of HLA-G, and, in some studies, with preeclampsia. In this review, a genetic contribution from the mother, the father, and the fetus, together with the presence and function of various immune cells of relevance in pregnancy are reviewed in relation to HLA-G and preeclampsia.

Mammalian non-classical major histocompatibility complex I and its receptors: Important contexts of gene, evolution, and immunity

The evolutionary conserved, less-polymorphic, nonclassical major histocompatibility complex (MHC) class I molecules: Qa-1 and its human homologue human leukocyte antigen-E (HLA-E) along with HLA-F, G and H cross-talk with the T-cell receptors and also interact with natural killer T-cells and other lymphocytes. Moreover, these nonclassical MHC molecules are known to interact with CD94/NKG2 heterodimeric receptors to induce immune responses and immune regulations. This dual role of Qa-1/HLA-E in terms of innate and adaptive immunity makes them more interesting. This review highlights the new updates of the mammalian nonclassical MHC-I molecules in terms of their gene organization, evolutionary perspective and their role in immunity.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

Worldwide HLA-E nucleotide and haplotype variability reveals a conserved gene for coding and 3' untranslated regions

The human leukocyte antigen-E (HLA-E) locus is a human major histocompatibility complex (MHC) gene associated with immune-modulation and suppression of the immune response by the interaction with specific natural killer (NK) and T cell receptors (TCRs). It is considered one of the most conserved genes of the human MHC; however, this low nucleotide variability seems to be a consequence of the scarce number of studies focusing on this subject. In this manuscript we assessed the nucleotide variability at the HLA-E coding and 3' untranslated regions (3'UTRs) in Brazil and in the populations from the 1000Genomes Consortium. Twenty-eight variable sites arranged into 33 haplotypes were detected and most of these haplotypes (98.2%) are encoding one of the two HLA-E molecules found worldwide, E*01:01 and E*01:03. Moreover, three worldwide spread haplotypes, associated with the coding alleles E*01:01:01, E*01:03:01 and E*01:03:02, account for 85% of all HLA-E haplotypes, suggesting that they arose early before human speciation. In addition, the low nucleotide diversity found for the HLA-E coding and 3'UTR in worldwide populations suggests that the HLA-E gene is in fact a conserved gene, which might be a consequence of its key role in the modulation of the immune system.

Dimorphic HLA-B signal peptides differentially influence HLA-E- and natural killer cell-mediated cytolysis of HIV-1-infected target cells

As a mechanism of self-protection, signal peptides cleaved from human leukocyte antigen (HLA) class I products bind to HLA-E before the complex interacts with the natural killer (NK) cell receptor CD94/NKG2A to inhibit NK-mediated cell lysis. Two types of the signal peptides differ in their position 2 (P2) anchor residue, with P2-methionine (P2-M) having higher HLA-E binding affinity than P2-threonine (P2-T). All HLA-A and HLA-C molecules carry P2-M, whereas HLA-B products have either P2-M or P2-T. Epidemiological evidence suggests that P2-M is unfavourable in the context of HIV-1 infection, being associated with accelerated acquisition of HIV-1 infection in two African cohorts. To begin elucidating the functional mechanism, we studied NK-mediated killing of CD4(+) T cells and monocyte-derived macrophages infected with two laboratory-adapted HIV-1 strains and two transmitted/founder (T/F) viruses. In the presence of target cells derived from individuals with the three HLA-B P2 genotypes (M/M, M/T and T/T), NK-mediated cytolysis was elevated consistently for P2-T in a dose-dependent manner for all cell and virus combinations tested (P = 0·008-0·03). Treatment of target cells with an anti-HLA-E monoclonal antibody restored NK-mediated cytolysis of cells expressing P2-M. Observations on cell lysis were also substantiated by measurements of HIV-1 p24 antigen in the culture supernatants. Overall, our experiments indicate that the anti-HIV-1 function mediated by NK cells is compromised by P2-M, corroborating the association of HLA-B genotype encoding P2-M with accelerated HIV-1 acquisition.

Role of non-classical MHC class I molecules in cancer immunosuppression

Growing neoplasms employ various mechanisms to evade immunosurveillance. The expression of non-classical MHC class I molecules by both immune and malignant cells in the tumor microenvironment constitute of the strategies used by tumors to circumvent the cytotoxic activity of effector cells of the immune system. The overexpression of HLA-G, -E, and -F is a common finding across a variety of malignancies. However, while the presence of HLA-G and HLA-E has been recently correlated with poor clinical outcome, information on the clinicopathological significance of HLA-F is limited. In the present review, we summarize studies on non-classical MHC class I molecules with special emphasis on their role in the modulation of anticancer immune responses.

The major histocompatibility complex: a model for understanding graft-versus-host disease

Acute graft-versus-host disease (GVHD) afflicts as much as 80% of all patients who receive an unrelated donor hematopoietic cell transplant (HCT) for the treatment of blood disorders, even with optimal donor HLA matching and use of prophylactic immunosuppressive agents. Of patients who develop acute GVHD, many are at risk for chronic GVHD and bear the burden of considerable morbidity and lowered quality of life years after transplantation. The immunogenetic basis of GVHD has been the subject of intensive investigation, with the classic HLA genetic loci being the best-characterized determinants. Recent information on the major histocompatibility complex (MHC) region of chromosome 6 as an important source of untyped genetic variation has shed light on novel GVHD determinants. These data open new paradigms for understanding the genetic basis of GVHD.

HLA-E variants are associated with sustained virological response in HIV/hepatitis C virus-coinfected patients on hepatitis C virus therapy

OBJECTIVES

To analyze whether human leukocyte antigen (HLA)-E allelic variants are associated with and may predict response to peg-interferon (IFN) alpha and ribavirin treatment in HIV/hepatitis C virus (HCV)-coinfected patients.

DESIGN

Retrospective follow-up study.

METHODS

We studied 321 naive patients who started HCV treatment. HLA-E genotyping was performed by restriction fragment length polymorphism. A sustained virological response (SVR) was defined as undetectable plasma HCV-RNA up through 24 weeks after the end of HCV treatment.

RESULTS

The HLA-E*0101 allele increased the odds of achieving SVR for all patients [adjusted odds ratio (aOR) = 2.03 (95% confidence interval, 95% CI = 1.35-3.06); P = 0.001], for HCV genotype (GT) 1/4 patients (aOR = 1.62 (95% CI = 1.03-2.54), P = 0.035), and for GT2/3 patients [aOR = 9.87 (95% CI = 2.47-31.89), P = 0.001]. For decision tree analysis, the SVR rate increased from 0 to 82.6% and then to 92.5% in GT2/3 patients when the count of HLA-E*0101 alleles increased. In GT1/4 patients with rs8099917 TT genotype, the SVR rate increased from 33.3 to 54.8% and then to 61.8% when the count of HLA-E*0101 alleles increased. In GT1/4 patients with rs8099917 GT/GG genotype, the SVR rate increased from 15.4 to 22% and then to 44% when the count of HLA-E*0101 alleles increased. The overall percentage of patients correctly classified was 73.2% and the area under the receiver operating characteristic curve (AUROC) was 0.803 ± 0.024.

CONCLUSION

The HLA-E*0101 allele was associated with increased odds of HCV clearance and could help to predict SVR among HIV/HCV-coinfected patients on HCV therapy. This would be helpful to avoid treatment in those less likely to respond to pegylated-interferon-alpha and ribavirin treatment.

Effective RNAi-mediated β2-microglobulin loss of function by transgenesis in Xenopus laevis

To impair MHC class I (class I) function in vivo in the amphibian Xenopus, we developed an effective reverse genetic loss of function approach by combining I-SceI meganuclease-mediated transgenesis with RNAi technology. We generated transgenic outbred X. laevis and isogenetic laevis/gilli cloned lines with stably silenced expression of β2-microglobulin (b2m) critical for class I function. Transgenic F₁ frogs exhibited decreased surface class I expression on erythrocytes and lymphocytes, decreased frequency of peripheral CD8 T cells and impaired CD8 T cell-mediated skin allograft rejection. Additionally, b2m knockdown increased susceptibility to viral infection of F₀ transgenic larvae. This loss of function strategy offers new avenues for studying ontogeny of immunity and other developmental processes in Xenopus.

HLA-E contributes to an immune-inhibitory phenotype of glioblastoma stem-like cells

Cancer stem cells are an attractive target for immunotherapeutic approaches to glioblastoma. However, an immune inhibitory phenotype of cells currently classified as "glioma-initiating cells" (GIC) might counteract recognition by immune effector cells. Here, we investigate the contribution of the non-classical MHC molecule HLA-E to the immunosuppressive phenotype of GIC. HLA-E is expressed in GIC lines and its expression is reduced upon differentiation of GIC in serum-containing culture conditions. Constitutive HLA-E inhibits natural killer (NK) cell-mediated lysis of GIC since small-interfering RNA-mediated HLA-E gene silencing enhances the immunogenicity of GIC. Increased GIC lysis was observed both in the CD133+ and in the CD133- compartment. Furthermore, the use of interferon-γ as a possible agent to boost an immune response against glioblastoma cells might be limited by the concurrent upregulation of HLA-E.

Augmentation of anti-HLA-E antibodies with concomitant HLA-Ia reactivity in IFNγ-treated autologous melanoma cell vaccine recipients

HLA-E expressed on the surface of melanoma cells and shed into circulation are known to inhibit killing of tumor cells by binding to CD94/NKGA2 receptors on cytotoxic T- and NKT cells. Interferon (IFN)-γ is known to promote HLA-E over-expression on the cell surface and shedding. The shed HLA-E heavy chain may expose cryptic epitopes to elicit antibodies (Abs). The anti-HLA-E Abs may bind to shed HLA-E or to the tumor cell surface to block its interaction with CTL/NKT cells. This is the basis for a melanoma cell vaccine that will generate anti-HLA-E Abs. The objective of this study was to characterize the antibody response and characterize the cross-reactivity of the antibodies produced in melanoma patients immunized with autologous melanoma cells treated with IFNγ. Anti-HLA-E murine mAbs and serum anti-HLA-E Abs in healthy individuals were known to react with HLA-Ia alleles, which is attributed to the presence of peptide sequences shared between HLA-E and HLA-Ia. Therefore, pre- and post-immune (weeks 4 and 24) serum Abs reacting to both HLA-E and HLA-Ia alleles were measured by multiplex Luminex®-based immunoassay. To ascertain whether the reactivity of the serum Abs to HLA-Ia was due to anti-HLA-E Abs, the shared-peptides were used to inhibit anti-HLA-E and HLA-Ia reactivities. The level of anti-HLA-E IgG in sera has increased post-immunization from its pre-immune level. Concomitantly, the HLA-Ia reactivity of the sera was also augmented. The reactivity of both anti-HLA-E Abs and HLA-Ia were inhibited by the shared-peptides. The HLA-Ia reactivity of the anti-HLA-E Abs in patients' sera is similar to the HLA-Ia reactivity of the anti-HLA-E mAbs and anti-HLA-E Abs in normal sera. The results establish the immunogenicity of HLA-E and also ascertain that the HLA-Ia reactivity of the anti-HLA-E Abs is due to shared-peptide epitopes.

HLA-B signal peptide polymorphism influences the rate of HIV-1 acquisition but not viral load

Human leukocyte antigen alleles influence the immune response to HIV-1. Signal peptides cleaved from those alleles bind to HLA-E and mediate natural killer cell function. Signal peptides of HLA-A and HLA-C proteins carry methionine (Met) at anchor position 2 (P2); those of HLA-B carry Met or threonine (Thr). Different P2 residues alter HLA-E binding to its cognate receptors and may impact HIV-1 acquisition. Among Zambian couples (N = 566) serodiscordant for HIV-1, P2-Met accelerated acquisition in the HIV-1-negative partner (relative hazard [RH], 1.79). Among seroconverting Zambian (n = 240) and Rwandan (n = 64) partners, P2-Met also accelerated acquisition (RH, 1.47 and RH, 1.83 respectively). HLA-B alleles displaying the reportedly protective Bw4 epitope carry P2-Thr. Bw4/P2-Thr and Bw6/P2-Thr showed similar protective effects compared with Bw6/P2-Met. Neither motif was associated with viral load. The influence of HLA-B alleles on HIV/AIDS may derive from multiple motifs in and beyond the mature proteins.

HLA-E polymorphisms in hematopoietic stem cell transplantation

Human leukocyte antigen (HLA)-E is an inhibitory ligand of natural killer cells and γ/δ T-cells. Differential expression of HLA-E alleles on the cell surface has been reported to influence outcome of hematopoietic stem cell transplantation (HSCT). We performed HLA-E genotyping in 116 HSCT patients and their HLA-matched unrelated donors. The impact of HLA-E genotypes on patient's overall survival (OS), disease free survival (DFS), cumulative incidences for relapse, transplant-related mortality (TRM) and acute graft vs host disease (aGvHD) was assessed. Neither univariate nor multivariate analysis showed any influence of HLA-E polymorphisms on the investigated endpoints of HSCT in our cohort. We could not confirm any of the previous observations in our cohort and consider it unlikely that HLA-E polymorphisms affect outcome of HSCT.

Non-classical HLA-E gene variability in Brazilians: a nearly invariable locus surrounded by the most variable genes in the human genome

The non-classical human leukocyte antigen (HLA) class I genes present a very low rate of variation. So far, only 10 HLA-E alleles encoding three proteins have been described, but only two are frequently found in worldwide populations. Because of its historical background, Brazilians are very suitable for population genetic studies. Therefore, 104 bone marrow donors from Brazil were evaluated for HLA-E exons 1-4. Seven variation sites were found, including two known single nucleotide polymorphisms (SNPs) at positions +424 and +756 and five new SNPs at positions +170 (intron 1), +1294 (intron 3), +1625, +1645 and +1857 (exon 4). Haplotyping analysis did show eight haplotypes, three of them known as E*01:01:01, E*01:03:01 and E*01:03:02:01 and five HLA-E new alleles that carry the new variation sites. The HLA-E*01:01:01 allele was the predominant haplotype (62.50%), followed by E*01:03:02:01 (24.52%). Selective neutrality tests have disclosed an interesting pattern of selective pressures in which balancing selection is probably shaping allele frequency distributions at an SNP at exon 3 (codon 107), sequence diversity at exon 4 and the non-coding regions is facing significant purifying pressure. Even in an admixed population such as the Brazilian one, the HLA-E locus is very conserved, presenting few polymorphic SNPs in the coding region.

Antibodies to HLA-E may account for the non-donor-specific anti-HLA class-Ia antibodies in renal and liver transplant recipients

The non-donor-specific anti-HLA-Ia antibodies correlate significantly with lower graft survival in organ transplant patients. Based on our earlier findings that anti-HLA-E murine monoclonal antibodies (MEM-E/02 and 3D12) reacted with different HLA-Ia alleles and the peptides shared by HLA-E and HLA class, Ia alleles inhibited the HLA-Ia reactivity of the anti-HLA-E antibodies in normal non-alloimmunized males, the possibility of that anti-HLA-E IgG may account for the non-donor-specific anti-HLA-Ia antibodies in the allograft recipients was examined by multiplex-Luminex®-immunoassay. About 73% of renal and 53% of liver transplant patients' sera with high level of anti-HLA-E IgG showed reactivity to different non-donor HLA-Ia alleles. About 50% renal and 52% liver allograft recipients' sera with low level of anti-HLA-E IgG had no reactivity to any HLA-Ia alleles; however, the IgG isolated from the same sera with protein-G columns showed the presence of anti-HLA-E IgG with HLA-Ia reactivity. Furthermore, both recombinant HLA-E and the IgG-free serum containing soluble HLA-E (sHLA-E) inhibited HLA-Ia reactivity of anti-HLA-E murine monoclonal IgG significantly. The data suggest that the HLA-Ia reactivity of the anti-HLA-E antibody accounts for the non-donor-specific anti-HLA-Ia antibodies. It is proposed that the sHLA-E heavy chain, shed in circulation after organ transplantation, may expose cryptic epitopes of HLA-E to elicit anti-HLA-E IgG antibodies, which may cross react with HLA-Ia alleles due to the peptide sequences shared between them. This study provides a new explanation for the presence of non-donor-specific antibodies for non-existing HLA-Ia alleles, frequently observed and correlated with survival in organ transplant recipients.

Molecular typing of HLA-E

Human leukocyte antigen-E (HLA-E) is a non-classical HLA class I gene that shows a limited degree of polymorphism compared to the classical HLA genes. The HLA-E molecule can bind peptides derived from the leader sequence of various HLA class I alleles and some viral homologues, including CMV. The HLA-E peptide complex can act as a ligand for the CD94/NKG2 receptors expressed on the surface of natural killer cells and T cell subsets. Differences in expression levels between the different HLA-E alleles have been reported and a role for HLA-E polymorphism in stem cell transplantation has been postulated. This chapter focuses on routine technologies for HLA-E typing: the sequence-specific primer-PCR method that uses sequence-specific primers, the PCR sequence-specific oligonucleotides Luminex method, using sequence-specific probes attached to beads and the sequencing-based typing method, where sequencing of the alleles is performed.

Human leukocyte antigen E contributes to protect tumor cells from lysis by natural killer cells

The nonclassic class I human leukocyte antigen E (HLA-E) molecule engages the inhibitory NKG2A receptor on several cytotoxic effectors, including natural killer (NK) cells. Its tissue distribution was claimed to be wider in normal than in neoplastic tissues, and surface HLA-E was undetectable in most tumor cell lines. Herein, these issues were reinvestigated taking advantage of HLA-E-specific antibodies, immunohistochemistry, and biochemical methods detecting intracellular and surface HLA-E regardless of conformation. Contrary to published evidence, HLA-E was detected in a few normal epithelia and in a large fraction (approximately 1/3) of solid tumors, including those derived from HLA-E-negative/low-normal counterparts. Remarkably, HLA-E was detected in 30 of 30 tumor cell lines representative of major lymphoid and nonlymphoid lineages, and in 11 of 11, it was surface-expressed, although in a conformation poorly reactive with commonly used antibodies. Coexpression of HLA-E and HLA class I ligand donors was not required for surface expression but was associated with NKG2A-mediated protection from lysis by the cytotoxic cell line NKL and polyclonal NK cells from healthy donors, as demonstrated by antibody-mediated relief of protection in 10% to 20% of the tested target-effector combinations. NKG2A-mediated protection of additional targets became evident on NK effector blocking with antibodies to activating receptors (DNAM-1, natural cytotoxicity receptors, and NKG2D). Thus, initial evidence that the long-elusive HLA-E molecule is enhanced by malignant transformation and is functional in tumor cells is presented here, although its importance and precise functional role remain to be addressed in the context of a general understanding of the NK ligand-receptor network.

Association between HLA-E *0101 homozygosity and recurrent miscarriage in Egyptian women

The objective was to investigate the frequency of human leucocyte antigen (HLA)-E alleles in Egyptian women with and without recurrent miscarriage (RM) to evaluate their role on the maintenance of pregnancy. A case-control study was adopted. HLA-E gene polymorphism typing was carried out by restriction fragment length polymorphism for 108 women with RM and 120 fertile female controls. The frequency of HLA-E *0101 allele was higher in patients with RM and HLA-E*0103 allele was higher in fertile controls, and the difference was statistically significant (P=0.003, P(c)=0.006). HLA-E*0101/0101 genotype was the most frequent genotype in patients (45.4%), followed by HLA-E*0101/0103 (44.4%) and finally HLA-E*0103/0103 genotype (10.2%). The difference in the frequency of HLA-E*0101/0101 homozygous genotype in patients with RM compared with that in the fertile controls was statistically significant (OR=2.02, 95% CI=1.13-3.62, P=0.011, P(c)=0.033). We found an increased frequency of homozygosity for HLA-E*0101 in Egyptian women with RM. HLA-E*0101 homozygosity may thus be a risk factor for RM.

The emerging role of HLA-E-restricted CD8+ T lymphocytes in the adaptive immune response to pathogens and tumors

Human leukocyte antigen (HLA)-E is a nonclassical major histocompatibility complex (MHC) class I molecule of limited sequence variability that is expressed by most tissues albeit at low levels. HLA-E has been first described as the ligand of CD94/NKG2 receptors expressed mainly by natural killer (NK) cells, thus confining its role to the regulation of NK-cell function. However, recent evidences obtained by our and other groups indicate that HLA-E complexed with peptides can interact with alphabeta T-cell receptor (TCR) expressed on CD8(+) T cells. Although, HLA-E displays a selective preference for nonameric peptides, derived from the leader sequence of various HLA class I alleles, several reports indicate that it can present also "noncanonical" peptides derived from both stress-related and pathogen-associated proteins. Because HLA-E displays binding specificity for innate CD94/NKG2 receptors, as well as all the features of an antigen-presenting molecule, its role in both natural and acquired immune responses has recently been re-evaluated.

HLA-E/human beta2-microglobulin transgenic pigs: protection against xenogeneic human anti-pig natural killer cell cytotoxicity

BACKGROUND

Natural killer (NK) cells participate in pig-to-primate xenograft rejection both by antibody-dependent and -independent mechanisms. A majority of human NK cells express the inhibitory receptor CD94/NKG2A, which binds specifically to human leukocyte antigen (HLA)-E, a trimeric complex consisting of the HLA-E heavy chain, beta2-microglobulin (beta2m), and a peptide derived from the leader sequence of some major histocompatibility complex class I molecules.

METHODS

To use this mechanism for protection of pig tissues against human NK cell-mediated cytotoxicity, we generated transgenic pigs by pronuclear microinjection of genomic fragments of HLA-E with an HLA-B7 signal sequence and of human beta2-microglobulin (hubeta2m) into zygotes.

RESULTS

Three transgenic founder pigs were generated. Northern blot analysis of RNA from peripheral blood mononuclear cells revealed the presence of the expected transcript sizes for both transgenes in two of the three founders. The founder with the highest expression and his offspring were characterized in detail. Fluorescence-activated cell sorting (FACS) and Western blot analyses demonstrated consistent expression of HLA-E and hubeta2m in peripheral blood mononuclear cells. Immunohistochemistry revealed the presence of HLA-E and hubeta2m on endothelial cells of many organs, including heart and kidney. In vitro studies showed that lymphoblasts and endothelial cells derived from HLA-E/hubeta2m transgenic pigs are effectively protected against human NK cell-mediated cytotoxicity, depending on the level of CD94/NKG2A expression on the NK cells. Further, HLA-E/hubeta2m expression on porcine endothelial cells inhibited the secretion of interferon (IFN)-gamma by co-cultured human NK cells.

CONCLUSIONS

This novel approach against cell-mediated xenogeneic responses has important implications for the generation of multitransgenic pigs as organ donors for clinical xenotransplantation.

HLA-E: strong association with beta2-microglobulin and surface expression in the absence of HLA class I signal sequence-derived peptides

The nonclassical class I HLA-E molecule folds in the presence of peptide ligands donated by the signal sequences of permissive class I HLA alleles, with the aid of TAP and tapasin. To identify HLA-E-specific Abs, four monoclonals of the previously described MEM series were screened by isoelectric focusing (IEF) blot and immunoprecipitation/IEF on >30 single-allele class I transfectants and HLA-homozygous B lymphoid cells coexpressing HLA-E and HLA-A, -B, -C, -F, or -G. Despite their HLA-E-restricted reactivity patterns (MEM-E/02 in IEF blot; MEM-E/07 and MEM-E/08 in immunoprecipitation), all of the MEM Abs unexpectedly reacted with beta(2)-microglobulin (beta(2)m)-free and denatured (but not beta(2)m-associated and folded) HLA-E H chains. Remarkably, other HLA-E-restricted Abs were also reactive with free H chains. Immunodepletion, in vitro assembly, flow cytometry, and three distinct surface-labeling methods, including a modified (conformation-independent) biotin-labeling assay, revealed the coexistence of HLA-E conformers with unusual and drastically antithetic features. MEM-reactive conformers were thermally unstable and poorly surface expressed, as expected, whereas beta(2)m-associated conformers were either unstable and weakly reactive with the prototypic conformational Ab W6/32, or exceptionally stable and strongly reactive with Abs to beta(2)m even in cells lacking permissive alleles (721.221), TAP (T2), or tapasin (721.220). Noncanonical, immature (endoglycosidase H-sensitive) HLA-E glycoforms were surface expressed in these cells, whereas mature glycoforms were exclusively expressed (and at much lower levels) in cells carrying permissive alleles. Thus, HLA-E is a good, and not a poor, beta(2)m assembler, and TAP/tapasin-assisted ligand donation is only one, and possibly not even the major, pathway leading to its stabilization and surface expression.

Immune-refractory cancers and their little helpers--an extended role for immunetolerogenic MHC molecules HLA-G and HLA-E?

There is strong evidence to support a role for non-classical MHC class I (class Ib) molecules, most notably HLA-E and HLA-G in tumour immune escape. In this article, we summarize the current knowledge on their expression, regulation and functional relevance in various malignancies, particularly brain tumours. Special emphasis is devoted to the phenomenon that these tolerogenic molecules are expressed by non-transformed cells that are found in close neighborhood to tumour cells representing either parenchymal cells or immune cells attracted to the tumour microenvironment. Here they may act as "natural" or "inducible" suppressors of anti-tumoural immune responses. We thus speculate about the role of HLA-G expressing T cells, a novel population of natural regulatory cells that was identified recently. It is suggested that various cell types within a tumour cooperate in order to inhibit anti-tumour immunity-and that immunetolerogenic HLA-G may play a major role in this context.

HLA-E*0101 allele in homozygous state favors severe bacterial infections in sickle cell anemia

Severe bacterial infections are the major causes of morbidity and mortality in sickle cell anemia (SCA) but are poorly explained. The distribution of a bi-allelic polymorphism (Arg107Gly) of human leukocyte antigen-E (HLA-E) locus was investigated in 144 SCA patients, most of whom originated from from sub-Saharan Africa. Among them, 73 presented with at least one severe bacterial infection, whereas 71 did not. The HLA-E*0101/E*0101 genotype was more frequent among the group with infections than their counterparts (47% vs 21%; p corrected = 0.003). This genetic association is of relevance, given the emerging evidence for the involvement of HLA-E molecules in host response to pathogens.

Association of CD94/NKG2A, CD94/NKG2C, and its ligand HLA-E polymorphisms with Behcet's disease

Inhibitory CD94/NKG2A and activating CD94/NKG2C receptors are expressed on natural killer, CD4, and CD8 T cells and recognize human leukocyte antigen (HLA)-E, resulting in the modulation of cytotoxic activity and cytokine production. An imbalance in cytotoxic activity and cytokine production has been implicated in Behcet's disease (BD). The results of this study showed that the NKG2A c.-4258*C, c.338-90*G, and CD94 c.-134*T alleles (P= 0.015, OR = 0.8; P < 0.0001, OR = 0.5; and P= 0.034, OR = 0.8, respectively) were associated with decreased risk and that NKG2A c.284-67_-62del, c.1077*C, and the activating receptor, NKG2C c.305*T were not associated with 345 patients with BD. But a significant difference in NKG2C c.305*T was detected among BD patients with ocular lesions and arthritis (P < 0.0001, OR = 2.1 and P= 0.0001, OR = 1.8, respectively). We already showed in our previous research that HLA-E*0101 also appears to contribute to a reduction in risk through the inhibitory CD94/NKG2A-mediated immune response. This result led us to the analyses of the combined risk of the HLA-E and the NKG2A for BD. Individuals harboring HLA-E*0101, NKG2A c.-4258*C, and c.338-90*G evidenced a reduced risk of BD compared with healthy controls (21.1% vs 40.1%, P < 0.0001, OR = 0.4). By way of contrast, individuals without the HLA-E*0101, NKG2A c.-4258*C, and c.338-90*G alleles evidenced a twofold increased risk of BD (P= 0.014, OR = 2.0). Individuals without HLA-E*0101, NKG2A c.-4258*G/*G, and c.338-90*G evidenced a 4.8-fold increase in BD risk (P= 0.0002, OR = 4.8). Although the effects of these single nucleotide polymorphisms (SNPs) remain unclear, our results indicate that the SNPs of the inhibitory receptor CD94/NKG2A and its haplotypes, as well as its ligand HLA-E, are associated with BD immune systems.

HLA-E*0101 and HLA-G*010101 reduce the risk of Behcet's disease

The nonclassical human leukocyte antigen (HLA)-E and -G molecules have previously been shown to inhibit natural killer- and cytotoxic T-lymphocyte-mediated cell lysis and have also been shown to prevent the proliferation of CD4 T cells and secrete cytokines that appear to be important in the modulation of the Behcet's disease (BD) immune systems. Polymorphisms in the HLA-E and HLA-G genes have been associated with differential expression and function. Thus, we conducted an analysis of the HLA-E and HLA-G alleles using Amplification Refractory Mutation System-polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism techniques in a study comprising 312 patients with BD and 486 controls. The HLA-E*0101 and HLA-G*010101 alleles were associated with a reduced risk of BD (P = 0.0002, odds ratio (OR) = 0.7 and P = 0.002, OR = 0.7, respectively). By way of contrast, the variants HLA-E*010302, HLA-G*010102, G*0105N alleles and 3741_3754ins14bp were all associated with an increased risk of BD (P < 0.0001, OR = 1.6; P = 0.002, OR = 1.8; P = 0.024, OR = 2.0 and P = 0.003, OR = 1.4, respectively). Individuals carrying both the HLA-E*0101 and the HLA-G*010101 alleles evidenced significantly lower frequency in the patients than in the controls (35.6% vs 49.6%; P < 0.0001, OR = 0.6). These results indicate that variant HLA-E and HLA-G molecules appear to function independently and synergistically, increasing the risk of BD, and may result in an imbalance of lymphocytic functions, which may culminate in the development of BD.

Homozygous status for HLA-E*0103 confers protection from acute graft-versus-host disease and transplant-related mortality in HLA-matched sibling hematopoietic stem cell transplantation

BACKGROUND

The posttransplant period following hematopoietic stem cell transplantation (HSCT) is potentially high risk for developing survival-compromising complications, many of which are known to be under the control of immunogenetic factors. Given the dual role of human leukocyte antigen (HLA)-E molecules in innate and adaptive immune processes, we analyzed the impact of HLA-E polymorphism in genoidentical HSCT setting.

METHODS

We analyzed 187 HLA-genoidentical sibling pairs for HLA-E polymorphism. To explore its potential association with the incidence of acute and chronic graft versus host disease (aGVHD, cGVHD), severe infections, risk for transplant-related mortality (TRM), and overall survival, HLA-E locus was genotyped by a polymerase chain-reaction-sequence-specific primer (PCR-SSP) strategy.

RESULTS

Multivariate analysis, taking into account the patient-, donor- and transplant-related factors, showed that the incidence of aGVHD and TRM at day 180 were low when the genotype was HLA-E*0103/E*0103, either in the donor or in the recipient, the pairs being identical for HLA-E alleles (hazard ratio [HR]=0.71, P=0.009; and HR=0.42, P=0.04, respectively). We also found a trend towards association between E*0103 homozygosity and improved survival (P=0.05). There was no association between HLA-E polymorphism and incidence of severe infections.

CONCLUSIONS

These data suggest that the homozygous state for HLA-E*0103 allele behaves as a protective genetic factor against aGVHD and TRM and likely contributes to improved survival in HLA-genoidentical bone marrow transplantation.

Expression and release of HLA-E by melanoma cells and melanocytes: potential impact on the response of cytotoxic effector cells

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-gamma 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-gamma 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-gamma induction after metastasis. The biological significance of these findings warrants further investigation.

HLA-E expression on porcine cells: protection from human NK cytotoxicity depends on peptide loading

Human NK cells lyse porcine cells and may play an important role in the cell-mediated rejection of pig-to-human xenografts. Lysis is probably a consequence of the failure of human MHC-specific killer inhibitory receptors to recognize porcine MHC class I molecules. A majority of activated human NK cells express the HLA-E-specific inhibitory receptor CD94/NKG2A. The aim of this study was therefore to test the hypothesis that stable surface expression of HLA-E on porcine cells protects against xenogeneic NK-mediated cytotoxicity. Porcine lymphoblastoid (13 271) and endothelial (pEC) cell lines were transfected with constructs coding for HLA-E together with the leader sequence of HLA-B7 or -A2. HLA-E was correctly expressed on 13 271 cells while pEC required peptide-pulsing and/or IFN-gamma stimulation to express the HLA-E complex on the cell surface. HLA-E-expressing porcine cells were partially protected from lysis mediated by human polyclonal NK populations and completely protected from killing by NKG2Abright NK clones. In conclusion, the capability of different porcine cell types to express HLA-E on the cell surface can differ considerably depending decisively on the availability of peptides. These findings are important for the applicability of transgenic HLA-E expression as an approach to protect porcine tissues from human NK cytotoxicity.

Association of HLA-E polymorphism with severe bacterial infection and early transplant-related mortality in matched unrelated bone marrow transplantation

Despite prophylactic measures, susceptibility to severe infections in patients who had undergone bone marrow transplantation (BMT) is quite variable. To evaluate the potential role of human leukocyte antigen (HLA)-E polymorphism on the incidence of early infections, we analyzed 77 unrelated-donor (UD) BMT pairs identically matched for classical HLA class I and class II alleles. Multivariate analysis taking into account the patient-, donor- and transplant-related factors showed that bacterial infections and transplant-related mortality (TRM) at day 180 were high when the donor genotype was HLA-E*0101/E*0101 (hazard ratio [HR]=2.20; P=0.03 and HR=2.12, P=0.048, respectively), suggesting that homozygous state for HLA-E*0101 allele is a risk factor for early severe bacterial infections and TRM in UD-BMT.

HLA-E surface expression is independent of the availability of HLA class I signal sequence-derived peptides in human tumor cell lines

Human leukocyte antigen (HLA)-E is a nonclassic HLA class I molecule whose expression at the cell surface of tumor cells might allow them to escape T- and natural killer (NK)-cell immune surveillance. In this study, we analyzed HLA-E expression in a panel of human HLA-typed tumor cell lines of different histotypes by flow cytometry with anti-HLA-E monoclonal antibodies and by reverse transcriptase-polymerase chain reaction. Although specific HLA-E transcripts were detected in all cell lines, except in HELA, surface expression was detected at different intensities on seven (23%) of 30 cell lines with higher frequency and intensity among osteosarcoma cell lines. HLA-E-positive tumor cell lines mainly expressed the HLA-A*02 class I allele. Some tumor cell lines demonstrating HLA class I A* or Cw* alleles, which we expected to allow HLA-E surface expression on the basis of reported data on lymphoid cells, instead were HLA-E negative. All tumor cell lines were either tapasin and TAP-1 positive by flow cytometry, except two osteosarcoma cell lines, a finding that suggests an intact assembly machinery for peptide loading. We conclude that the concomitant presence of the appropriate HLA class I alleles with leader sequence-derived peptides and HLA-E heavy chain may not be sufficient to allow HLA-E surface expression in tumor cell lines as opposed to lymphoid cells.