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Although the expression of the non-classical HLA class I molecule HLA-G was first reported to be restricted to the fetal-maternal interface on the extravillous cytotrophoblasts, the distribution of HLA-G in normal tissues appears broader than originally described. HLA-G expression was found in embryonic tissues, in adult immune privileged organs, and in cells of the hematopoietic lineage. More interestingly, under pathophysiological conditions HLA-G antigens may be expressed on various types of malignant cells suggesting that HLA-G antigen expression is one strategy used by tumor cells to escape immune surveillance. In this article, we will focus on HLA-G expression in cancers of distinct histology and its association with the clinical course of diseases, on the underlying molecular mechanisms of impaired HLA-G expression, on the immune tolerant function of HLA-G in tumors, and on the use of membrane-bound and soluble HLA-G as a diagnostic or prognostic biomarker to identify tumors and to monitor disease stage, as well as on the use of HLA-G as a novel therapeutic target in cancer.

The expression of human leukocyte antigen (HLA)-G, a nonclassical HLA class I molecule, and its soluble forms (sHLA-G) are found to improve graft acceptance. In this study we investigated whether sHLA-G is the most biologically relevant molecule among all types of soluble HLA class I molecules for graft acceptance. We addressed this question in kidney-transplanted (n = 32) and kidney/pancreas-transplanted patients (n = 29). To this end we analyzed the levels of total soluble HLA class I (sHLA-I) in comparison to sHLA-G in 488 plasma samples procured before and serial after transplantation by specific enzyme-linked immunoabsorbent assay. Samples from 126 healthy individuals served as controls. Pretransplantation sHLA-I levels were significantly increased in patients (p < 0.001), whereas sHLA-G levels were in the range of those of healthy controls. Importantly, pretransplantation sHLA-I and sHLA-G levels did not differ between the two groups. Patients with biopsy-proven rejection (n = 15) revealed significantly lower sHLA-G levels before transplantation (mean +/- standard error of the mean, 12.9 +/- 1.8 vs. 20.1 +/- 1.9, p = 0.013) and after transplantation (p = 0.006, two-way analysis of variance) than patients without rejection (n = 46). In contrast, sHLA-I was slightly increased after but not before transplantation in patients with rejection (p < 0.05, two-way analysis of variance). Nonparametric determination analysis showed that pretransplantation levels of sHLA-G < 11.5 ng/ml (sensitivity, 60%; specificity, 80.4%) were related to rejection. Regarding antibody status, retransplantation, number of HLA mismatches, recipient age, and recipient body mass index, multivariate analysis showed that sHLA-G but not sHLA-I is an independent risk factor for graft rejection. Thus high levels of sHLA-G but not of sHLA-I seem to contribute to better graft acceptance after kidney or kidney/pancreas transplantation.

HLA-G appears to be involved in regulatory functions counteracting the cellular immune response of T and NK cells by several pathways. We here summarize the HLA-G expression patterns in leukaemia with emphasis on the clinical relevance of this expression for disease progression. Especially in patients with B-chronic lymphocytic leukaemia (B-CLL) the HLA-G expression on B-CLL cells was strongly associated with a reduced treatment-free survival. The corresponding immunological parameters point to a broad immunosuppression in these patients. Thus, HLA-G seems to contribute to the impaired immune response in B-CLL supporting disease progression.

Both, the expression of HLA-G (a non-classical HLA class I molecule) and the loss of classical HLA class I molecules enable tumor cells to evade from immunosurveillance of the host. Whereas HLA-G down-modulates the immune functions of all cells participating in the immune defence mechanisms, defects on HLA class I expression result in the resistance of tumor cells to cytotoxic T lymphocytes attacks. This contribution reviews the HLA-G expression pattern in malignant melanoma lesions, its correlation to the loss of classical HLA class I antigens, and new aspects of HLA-G regulation.

Major histocompatibility complex class I-related chain A (MICA) molecules are frequently expressed in lymphoproliferative malignancies including multiple myeloma (MM). MICA activates NK cells and co-stimulates T cells by interaction with its immunoreceptor NKG2D. In contrast, soluble MICA (sMICA) molecules impair the functions of NKG2D(+) T and NK cells, which may facilitate tumor cell escape from immunosurveillance. Here, we analyzed the clinical relevance of sMICA in 97 MM patients. sMICA (mean+/-SEM pg/ml) was significantly increased (p<0.0001) in patients (429+/-20) compared to controls (230+/-20; N=43). Serial determination showed a strong correlation between increments of sMICA and paraprotein levels (r=0.543, p<0.0001, N=49). sMICA levels >305 pg/ml are associated with a poor overall (p=0.004) and progression-free survival (p=0.002). Multivariate analysis revealed sMICA as an independent predictive factor for overall (p=0.007) and progression-free survival (p=0.002). Thus, our results suggest sMICA as a potent prognostic marker in MM, which may be useful to identify risk patients.

Human leukocyte antigen G (HLA-G) molecules are crucial for the maternal tolerance against the fetus during pregnancy. Thus, the presence of soluble HLA-G (sHLA-G) in embryo cultures is thought to be correlated to a successful pregnancy after assisted reproductive techniques (ART). Here, we established a rapid detection assay based on Luminex technology, which can be integrated into ART proceedings, allowing sHLA-G quantification in sample volumes of only 10 microl within 1.5 hours. Using this method, sHLA-G levels of 526 single-embryo cultures, 47 two-embryo cultures, and 15 three-embryo cultures were analyzed corresponding to 313 ART cycles. In 117 embryo cultures, sHLA-G was detectable. In single-embryo cultures, the sHLA-G levels were positively correlated to embryo quality (p = 0.048, r = 0.20, n = 100). The presence of sHLA-G in embryo cultures was significantly (p < 0.0001) associated with clinical pregnancy after intracytoplasmatic sperm injections (ICSI), especially in couples with male factor infertility, but not after in vitro fertilization (IVF) or in couples with female infertility. Importantly, in sHLA-G negative embryos, the abortion rate was increased threefold (p = 0.04). In conclusion, the results obtained by our novel method support strongly the diagnostic relevance of sHLA-G for predicting pregnancy outcome after ART. The ultimate conditions for this prediction have to be further investigated in a multicenter study.

We investigated potential correlations between soluble HLA-G (sHLA-G) and soluble HLA class I (sHLA-I) levels, respectively, and parameters of disease activity or genetic factors determined by HLA-DRB1 and HLA-DQB1 in patients with rheumatoid arthritis (RA). SHLA-G plasma concentrations from 106 RA patients (mean age 59.8 years, 80 women) were assessed by a sensitive enzyme-linked immunosorbent assay format. The mean sHLA-G levels were lower and sHLA-I levels higher in the RA patients than in healthy controls. Correlation coefficients of 0.248 to 0.344 (p < 0.01) between sHLA-G and rheumatoid factor, CRP, and EULAR joint swelling score were found. Patients with disease-associated HLA epitopes had higher sHLA-G levels than those without. Significantly lower sHLA-G was observed in groups of patients having HLA-DRB1*03 or HLA-DQB1*02 compared to groups without these genotypes. In contrast, HLA-DQB1*03 or disease-associated epitopes combined with HLA-DQB1*03 were associated with higher sHLA-G levels, whereas the inverse was observed in the combined presence of HLA-DRB1*03 and HLA-DQB1*02. SHLA-G as a percentage of sHLA-I was lower in patients positive for HLA-DQB1*02 and higher in patients positive for HLA-DQB1*03 and in its combined presence with disease-associated epitopes or with HLA-DRB1*07. As especially sHLA-G strongly inhibits T and natural killer (NK) cell functions, low sHLA-G suggests that T and NK cell activities are not efficiently restricted by sHLA-G molecules in rheumatoid arthritis. The sHLA-G levels, however, increase in correlation with parameters of disease activity and appear to be affected by the presence of disease-predisposing epitopes and other HLA-DRB1, DQB1 genotypes.

Previously we have reported on the development of antibodies against MICA alleles in kidney transplant recipients. These alloantibodies have now been determined using a new assay using Luminex beads bound to soluble recombinant MICA antigens produced in insect cells. In the present study we have analyzed sera from 85 kidney transplant recipients on the waiting list and 66 patients transplanted within the last 4 years and 59 acid eluates obtained from allograft nephrectomy specimens. Many of the patients in those groups were sensitized and some had previous transplants (waiting list: 15%; post-tx: 7.6%; eluates 22%) and their sera were found to contain anti-human leukocyte antigen (HLA) and anti-MICA antibodies. Anti-MICA antibodies were detected in 21/85 (24.7%) of the waiting list patients and in 15/66 (22.7%) of the transplanted recipients; 11 of the eluates (18.6%) were found to have MICA-specific antibodies (6 of them also had anti-HLA antibodies and 5 did not). These data suggested that immunization against mismatched MICA alleles induces development of anti-MICA antibodies. The finding of MICA allele-specific antibodies in eluates of kidney transplants suggests that anti-MICA antibodies can be involved in the pathogenesis of kidney allograft rejection. Further studies will be required to determine whether patients who produce alloantibodies against MICA alleles are at risk for transplant rejection even when no HLA antibodies are detected.

The human leukocyte antigen G (HLA-G) molecule exhibits limited tissue distribution and exerts multiple immunoregulatory functions. Recent studies indicate an ectopic up-regulation in tumor cells that may favor their escape from antitumor immune responses. The role of HLA-G in B-cell chronic lymphocytic leukemia (B-CLL) has not been defined. HLA-G expression was studied retrospectively in circulating B-CLL cells from 47 patients by flow cytometry using the anti-HLA-E specific monoclonal antibody MEM/G9. The proportion of leukemic cells expressing HLA-G varied from 1% to 54%. Patients with 23% or fewer HLA-G-positive cells (according to receiver operating characteristics [ROC] analysis; designated as HLA-G-negative group) had a significantly longer progression-free survival (PFS) time than patients with more than 23% positive cells (median PFS: 120 versus 23 months; P = .0001). In multivariate analysis, HLA-G expression (hazard ratio: 4.8; P = .002) was an even better independent prognostic factor than the zeta-associated protein 70 (ZAP-70) or CD38 status. Humoral and cellular immunosuppression were significantly more prominent in the HLA-G-positive compared with the HLA-G-negative patient group. In B-CLL, the level of HLA-G expression is correlated with the degree of immunosuppression and prognosis. HLA-G may contribute to the impairment of immune responses against tumor cells and infections. Thus, these findings need to be confirmed in a prospective study.

Soluble human leukocyte antigen-G molecules (HLA-G5) can be found in the peripheral blood of healthy females and males, and in other body fluids. To identify cells secreting HLA-G5 we generated a rabbit antiserum against a peptide motif encoded by intron-4 of HLA-G (RaHLA-G/I-4). Utilizing this antiserum as capture and antihuman beta 2-microglobulin as detection reagent, an enzyme-linked immunospot assay specific for HLA-G5 was developed. The results of this enzyme-linked immunospot assay format were proven by the HLA-G1 and soluble HLA-G5 (sHLA-G5) specific mAb MEM/G9 used in parallel as capture reagent. For the choriocarcinoma cell line JEG3 the number of HLA-G5 specific spots was found to be increased after stimulation with IFN-alpha, IFN-beta, and IFN-gamma at various concentrations. In contrast to this, no substantial variation of HLA-G5 specific spots was observed after incubation with lymphokines such as leukemia inhibitory factor, interleukin-10 (IL-10), IL-2, IL-4, and granulocyte-colony-stimulating factor. To clarify the cellular source of secreted HLA-G5 molecules, peripheral blood monocytes, CD4 and CD8 positive T and B cells from healthy donors (n = 14) were tested at a fixed cell number (5000/well) in the absence and presence of IFN-gamma (500 U/ml, 24 hours). In all experiments the number of HLA-G5 specific spots was significantly (p < 0.001) increased primarily in monocytes compared with T and B cells, which suggests that peripheral blood monocytes are the predominant cells secreting HLA-G5.

Our clinical studies revealed significantly increased soluble HLA-G (sHLA-G) plasma levels in patients suffering from malignant melanoma, glioma, breast and ovarian cancer. Specific ELISpot assays demonstrate that sHLA-G molecules expressing intron-4 sequences are preferentially secreted by peripheral blood monocytes. In vitro, the sHLA-G secretion of monocytes and tumor cells was strongly enhanced by TH1 cytokines like IFN-alpha, -beta, -gamma whereas TH2 cytokines (e.g. IL-4, -10) had minor effects. As sHLA-G can inhibit the functions of T and NK cells high concentration of these molecules should systemically or at the tumor side reduce the immune surveillance and thus favour the progression of cancer.

Purpose: Molecular approaches as supplements to cytological examination of malignant ascites may play an important role in the clinical management of cancer patients. HLA-G is a potential tumor-associated marker and that one of its isoforms, HLA-G5, produces a secretory protein. This study is to assess the clinical utility of secreted HLA-G levels in differential diagnosis of malignant ascites., Experimental Design: We used ELISA to assess whether secretory HLA-G (sHLA-G) could serve as a marker of malignant ascites in ovarian and breast carcinomas, which represent the most common malignant tumors causing ascites in women., Results: On the basis of immunohistochemistry, 45 (61%) of 74 ovarian serous carcinomas and 22 (25%) invasive ductal carcinomas of the breast demonstrated HLA-G immunoreactivity ranging from 2 to 100% of the tumor cells. HLA-G staining was not detected in a wide variety of normal tissues, including ovarian surface epithelium and normal breast tissue. Revese transcription-PCR demonstrated the presence of HLA-G5 isoform in all of the tumor samples expressing HLA-G. ELISA was performed to measure the sHLA-G in 42 malignant and 18 benign ascites supernatants. sHLA-G levels were significantly higher in malignant ascites than in benign controls (P < 0.001). We found that the area under the receiver-operating characteristic curve for sHLA-G was 0.95 for malignant versus benign ascites specimens. At 100% specificity, the highest sensitivity to detect malignant ascites was 78% (95% confidence interval, 68-88%) at a cutoff of 13 ng/ml., Conclusions: Our findings suggest that measurement of sHLA-G is a useful molecular adjunct to cytology in the differential diagnosis of malignant versus benign ascites.

The nonclassical HLA-G molecule exhibits a limited tissue distribution and exerts multiple immune regulatory functions. Recent studies indicate that HLA-G expression plays a key role in the induction of immune tolerance and may represent a novel immune escape mechanism of tumor cells. Despite a high frequency of tumor-infiltrating T lymphocytes in renal cell carcinoma (RCC) lesions, outgrowth of tumor cells occurs that might be attributable to abrogation-efficient antitumor responses. To delineate the potential role of HLA-G in RCC immunology, the HLA-G expression pattern and its functional consequences on immune responses were analyzed in cell lines and lesions derived from primary RCC lesions. A heterogeneous constitutive and IFN-gamma-inducible HLA-G mRNA and protein expression was found in 12.5% of RCC cell lines but not in autologous normal kidney cells. Western blot analysis of 37 primary RCC lesions revealed HLA-G protein expression in 27% of RCC lesions. Functional studies performed with alloreactive natural and lymphokine-activated killer cells as well as antigen-specific CD8(+) T-cell populations demonstrated that HLA-G expression inhibits lysis of RCC cells by these different immune effector cells, whereas HLA-G(-) normal kidney cells were recognized. Furthermore, the HLA-G-mediated counteraction of immune response could be restored by antibody blocking experiments. Thus, aberrant HLA-G expression is found at a relatively high frequency in RCC and might participate in evasion of these tumor cells from immunosurveillance.

Purpose: Because the expression of classic and nonclassic HLA antigens is crucial for the recognition and elimination of tumor cells by cytotoxic T and/or NK cells, we analyzed the HLA-A, -B, -C, and -G expression in uveal melanoma specimens from 18 patients., Methods: Tumor specimens and EDTA plasma samples from 18 patients treated by primary enucleation or tumor resection for primary uveal melanoma in the University Eye Clinic, Essen, Germany, were collected immediate after surgery. After solubilization of tumor tissue and specific immunoprecipitation of classic HLA-A, -B, and -C and nonclassic HLA-G antigens the tumor samples were analyzed by one-dimensional isoelectric focusing (1D-IEF) and Western blot analysis. In parallel, patients were typed for HLA-A, -B, and -C class I antigens by PCR with sequence-specific primers (PCR-SSP). In addition, HLA-A2 and -G expression was investigated by immunohistochemistry in paraffin-embedded tumor sections from these patients., Results: In 9 (50%) of 18 specimens, a full HLA-A and -B antigen expression pattern was detected by 1D-IEF. In six (33.3%) tumor specimens, an HLA class I allotype was missing (HLA-A2, -A28, -A29, -B18, -B35, and -B55), in two cases a haplotypic loss (HLA-A2, -B44 and HLA-A2, -B13) and in another case an allotype-specific loss combined with a haplotypic loss (HLA-A26, -A32, -B41) were observed. HLA-C and -G antigens were not detectable in any of the tumor samples by biochemical methods used., Conclusions: A considerable portion of the uveal melanomas tested showed a loss of classic HLA class I antigens, which may enable them to escape from the immunosurveillance of cytotoxic T cells. HLA-C and -G antigens were not found in uveal melanoma tissue implying a susceptibility for NK lysis.

One major materno-fetal interface in the human placenta is constituted by the syncytiotrophoblast, in contact with maternal blood of the intervillous space, which derives from differentiation and fusion of the villous cytotrophoblast (vct). In the present work, we purified vct from term placenta by depleting HLA class I- and class II-positive cells. We found by RT-PCR that both soluble intron 4-retaining HLA-G1 (sHLA-G1) and HLA-G2 isoforms were transcribed in purified vct. Using different HLA-G-specific mAb, we demonstrated by intracellular flow cytometry, Western blotting and ELISA, that sHLA-G1 but no soluble HLA class Ia molecule was secreted by vct. We then purified sHLA-G1 from vct culture supernatant and found that it exhibited an unusual glycosylation pattern. Finally, we showed that such trophoblast-derived sHLA-G1 triggered specific apoptosis of activated CD8+ T cells. Taken together, these results demonstrated that vct did secrete functional sHLA-G1 in primary culture and suggested that, in vivo, sHLA-G1 might be an important immunomodulatory molecule controlling the activity of maternal immune effector CD8+ cells circulating in the blood that immerses chorionic villi.