A Study of Soluble HLA-G1 Protecting Porcine Endothelial Cells Against Human Natural Killer Cell-Mediated Cytotoxicity

Genetically engineered pigs for xenotransplantation: Hopes and challenges

The shortage of donor resources has greatly limited the application of clinical xenotransplantation. As such, genetically engineered pigs are expected to be an ideal organ source for xenotransplantation. Most current studies mainly focus on genetically modifying organs or tissues from donor pigs to reduce or prevent attack by the human immune system. Another potential organ source is interspecies chimeras. In this paper, we reviewed the progress of the genetically engineered pigs from the view of immunologic barriers and strategies, and discussed the possibility and challenges of the interspecies chimeras.

Strategies to induce natural killer cell tolerance in xenotransplantation

Eliminating major xenoantigens in pig cells has drastically reduced human antibody-mediated hyperacute xenograft rejection (HXR). Despite these advancements, acute xenograft rejection (AXR) remains one of the major obstacles to clinical xenotransplantation, mediated by innate immune cells, including macrophages, neutrophils, and natural killer (NK) cells. NK cells play an ‘effector’ role by releasing cytotoxicity granules against xenogeneic cells and an ‘affecter’ role on other immune cells through cytokine secretion. We highlight the key receptor-ligand interactions that determine the NK cell response to target cells, focusing on the regulation of NK cell activating receptor (NKG2D, DNAM1) and inhibitory receptor (KIR2DL1-4, NKG2A, and LIR-1) signaling pathways. Inhibition of NK cell activity may protect xenografts from cytotoxicity. Recent successful approaches to reducing NK cell-mediated HXR and AXR are reviewed, including genetic modifications of porcine xenografts aimed at improving pig-to-human compatibility. Future directions to promote xenograft acceptance are discussed, including NK cell tolerance in pregnancy and NK cell evasion in viral infection.

Late-onset intrauterine growth restriction and HHV-6 infection: A pilot study

Late-onset Intrauterine growth restriction (IUGR) refers to impaired growth and development of the fetus, characterized by placental morphological abnormalities that affect the fetus's supply of nutrients. Human leukocyte antigen-G (HLA-G) is physiologically expressed during pregnancy, but decreased in normal placenta during the last weeks of gestation possibly inducing childbirth. Several viruses involved in congenital infection, such as herpesviruses, exploit HLA-G expression as an immune-escape mechanism. To date, despite different congenital herpetic infections having been associated with late IUGR, no direct implication of Human herpesvirus 6 (HHV-6) infection has been reported. We evaluated HLA-G expression and HHV-6 infection in 11 placentas from late-onset IUGR newborns and 11 placentas from uncomplicated pregnancies by histopathological and immunohistochemistry analysis. We found higher levels of HLA-G expression and HHV-6 presence in IUGR placenta samples compared with control placenta samples. We report HHV-6 staining in IUGR placenta samples, characterized by high HLA-G expression. These preliminary data suggest a possible involvement of HHV-6 infection in HLA-G deregulation that might affect vessel remodeling and prevent the correct pregnancy outcome in the IUGR condition.

Heterogeneity of HLA-G Expression in Cancers: Facing the Challenges

Phenotypic heterogeneity has been observed in most malignancies, which represents a considerable challenge for tumor therapy. In recent decades, the biological function and clinical significance of the human leukocyte antigen (HLA)-G have been intensively explored. It is now widely accepted that HLA-G is a critical marker of immunotolerance in cancer cell immune evasion and is strongly associated with disease progress and prognosis for cancer patients. Moreover, it has recently been emphasized that the signaling pathway linking HLA-G and immunoglobulin-like transcripts (ILTs) is considered an immune checkpoint. In addition, HLA-G itself can generate at least seven distinct isoforms, and intertumor and intratumor heterogeneity of HLA-G expression is common across different tumor types. Furthermore, HLA-G heterogeneity in cancers has been related to disease stage and outcomes, metastatic status and response to different therapies. This review focuses on the heterogeneity of HLA-G expression in malignant lesions, and clinical implications of this heterogeneity that might be relevant to personalized treatments are also discussed.

The Role of NK Cells in Pig-to-Human Xenotransplantation

Recruitment of human NK cells to porcine tissues has been demonstrated in pig organs perfused ex vivo with human blood in the early 1990s. Subsequently, the molecular mechanisms leading to adhesion and cytotoxicity in human NK cell-porcine endothelial cell (pEC) interactions have been elucidated in vitro to identify targets for therapeutic interventions. Specific molecular strategies to overcome human anti-pig NK cell responses include (1) blocking of the molecular events leading to recruitment (chemotaxis, adhesion, and transmigration), (2) expression of human MHC class I molecules on pECs that inhibit NK cells, and (3) elimination or blocking of pig ligands for activating human NK receptors. The potential of cell-based strategies including tolerogenic dendritic cells (DC) and regulatory T cells (Treg) and the latest progress using transgenic pigs genetically modified to reduce xenogeneic NK cell responses are discussed. Finally, we present the status of phenotypic and functional characterization of nonhuman primate (NHP) NK cells, essential for studying their role in xenograft rejection using preclinical pig-to-NHP models, and summarize key advances and important perspectives for future research.

Expression of the genes encoding human leucocyte antigens-A, -B, -DP, -DQ and -G in gastric cancer patients

This study compared the expression of the genes encoding human leucocyte antigens (HLA)-A, -B, -DP, -DR and -G in peripheral blood mononuclear cells (PBMCs) in gastric cancer patients and healthy controls. Using reverse transcription-polymerase chain reaction, levels of classical HLA-A, -B, -DP and -DR and non-classical HLA-G mRNA were studied in 43 gastric cancer patients and 22 controls. In addition, the levels of HLA-A,B,C and -G antigens on the surface of PBMCs were measured in 30 gastric cancer patients and 15 controls using flow cytometry. The mean fluorescence intensity of HLA-A,B,C antigen in the gastric cancer group was significantly lower than in controls. The HLA-G antigen was mainly present on CD4(+)CD8(-) T-lymphocytes. The percentage of CD4(+)CD8(-) T-lymphocytes positive for HLA-G antigen was significantly lower in the gastric cancer group compared with the healthy controls. Levels of HLA-A, -B and -G mRNA in the gastric cancer group were significantly lower than in controls. The HLA-G mRNA levels were significantly lower in gastric cancer of histological grades III and IV than in grades I and II. These data may provide a novel diagnostic and research tool for gastric cancer.

Suppression of human anti-porcine natural killer cell xenogeneic responses by combinations of monoclonal antibodies specific to CD2 and NKG2D and extracellular signal-regulated kinase kinase inhibitor

Natural killer (NK) cells can destroy xenogeneic tissues by antibody-dependent cell cytotoxicity (ADCC) and direct lysis. Unlike ADCC, activating interactions between human NK receptors and their cognate ligands in pigs are not fully elucidated. We set up this study to identify human NK activating receptors recognizing porcine cells isolated from distinct organs, e.g., aorta, cornea and liver, and to provide a molecular basis for effective immunosuppressive regimens. Among the array of NK receptors tested, NKp46, 2B4, CD49d, CD48, CD2 and NKG2D, only CD2 and NKG2D were shown to be involved in both cytotoxicity and cytokine (interferon-gamma and tumour necrosis factor-alpha) production against porcine targets. Simultaneous blocking of CD2 and NKG2D by combining its monoclonal antibodies further suppressed xenogeneic NK responses. Moreover, addition of a suboptimal dose of PD98059, an extracellular signal-regulated kinase (ERK) kinase inhibitor, to those cells maximally reduced NK cytotoxicity, suggesting that ERK plays an important role in NK-mediated xenoreactivity. These impairments in NK cells were tightly associated with defective intracellular calcium mobilization and the subsequent degranulation process. Therefore, our data demonstrate a distinct role of CD2 and NKG2D on human NK cells in recognizing porcine grafts and further provide a potentially efficacious combinational regimen using anti-CD2 and anti-NKG2D monoclonal antibodies with PD98059 in a pig-to-human transplantation model.

Down-regulation of HLA-G boosted natural killer cell-mediated cytolysis in JEG-3 cells cultured in vitro

OBJECTIVE

To determine how decidual natural killer (NK) cells interact with fetal trophoblasts in vitro.

DESIGN

Prospective study.

SETTING

University hospitals and IVF units.

PATIENT(S)

Not applicable.

INTERVENTION(S)

Not applicable.

MAIN OUTCOME MEASURE(S)

An adenovirus vector containing small interfering RNA (siRNA) specifically targeting the human lymphocyte antigen-G (HLA-G) gene was constructed and applied to diminish HLA-G mRNA expression. The steady-state levels of HLA-G messenger RNA (mRNA) were then checked by reverse transcriptase-polymerase chain reaction (RT-PCR) and protein levels by Western blot analysis. The NK-mediated cell cytotoxicity in the siRNA treated cells was studied by application of a nonradioactive cytotoxicity assay.

RESULT(S)

Steady-state levels of HLA-G mRNA and protein were significantly diminished by the targeting siRNA. In cells where HLA-G expression was thus reduced, a significant increase in NK cell-mediated lysis occurred.

CONCLUSION(S)

These results indicate that the recombinant adenoviral vectors used were efficient tools for studying HLA-G function. More important, this study reveals an important immunoprotective function for HLA-G in controlling NK cell-mediated lysis of trophoblasts, cells whose role in mediating normal pregnancy is important.

Soluble HLA-G and control of angiogenesis

In this review, we summarize the results of a number of our recent in vitro and in vivo experiments demonstrating that, in addition to the immunoregulatory functions, soluble HLA-G molecules also affect endothelial cell activity. We have found that soluble HLA-G1 (also designated HLA-G5) inhibits endothelial cell proliferation, migration and tubule formation, and this occurred through binding to the CD160 receptor and via an apoptotic pathway. Moreover, we have demonstrated that soluble HLA-G1 blocks in vivo rabbit corneal neoangiogenesis. Although it cannot be excluded that other soluble HLA class I molecules may have similar effects, as soluble forms of HLA-G are being produced by trophoblast cells at the maternal-fetal interface during early gestation, we discuss how such anti-angiogenic properties of soluble HLA-G1 may locally influence uterine vascular remodeling.