Soluble form of an HLA-B7 class I antigen specifically suppresses humoral alloimmunization

Uncovering the Tumor Antigen Landscape: What to Know about the Discovery Process

According to the latest available data, cancer is the second leading cause of death, highlighting the need for novel cancer therapeutic approaches. In this context, immunotherapy is emerging as a reliable first-line treatment for many cancers, particularly metastatic melanoma. Indeed, cancer immunotherapy has attracted great interest following the recent clinical approval of antibodies targeting immune checkpoint molecules, such as PD-1, PD-L1, and CTLA-4, that release the brakes of the immune system, thus reviving a field otherwise poorly explored. Cancer immunotherapy mainly relies on the generation and stimulation of cytotoxic CD8 T lymphocytes (CTLs) within the tumor microenvironment (TME), priming T cells and establishing efficient and durable anti-tumor immunity. Therefore, there is a clear need to define and identify immunogenic T cell epitopes to use in therapeutic cancer vaccines. Naturally presented antigens in the human leucocyte antigen-1 (HLA-I) complex on the tumor surface are the main protagonists in evocating a specific anti-tumor CD8+ T cell response. However, the methodologies for their identification have been a major bottleneck for their reliable characterization. Consequently, the field of antigen discovery has yet to improve. The current review is intended to define what are today known as tumor antigens, with a main focus on CTL antigenic peptides. We also review the techniques developed and employed to date for antigen discovery, exploring both the direct elution of HLA-I peptides and the in silico prediction of epitopes. Finally, the last part of the review analyses the future challenges and direction of the antigen discovery field.

Human Leucocyte Antigen Sensitisation and Its Impact on Transfusion Practice

Human leucocyte antigen (HLA) sensitisation, including the formation of antibodies against HLA, can cause serious effects in patients receiving blood. Under certain circumstances, donor HLA antibodies in the blood product can trigger the patient's granulocytes to release mediators that cause transfusion-associated lung injury (TRALI), a serious complication of transfusion. The HLA systems of both donor and patient are involved in transfusion-associated graft-versus-host disease, which is a rare disease with a high mortality. Patient HLA antibodies can destroy incompatible platelets and may cause refractoriness to platelet transfusion. Identification of a patient's HLA antibody specificities is necessary for issuing compatible platelets to overcome refractoriness. Many techniques for the detection and identification of HLA antibodies have been developed, including complement-dependent cytotoxicity assay, bead-based assays, the platelet adhesion immunofluorescence test, and the monoclonal antibody-specific immobilisation of platelet antigens assay. Different strategies for the selection of HLA-compatible platelets are applied. These strategies depend on the breadth of antibody reactivity and range from avoiding single HLA antigens in the platelet concentrates issued to apheresis of platelets from HLA-identical donors. The mechanisms of HLA sensitisation and the efforts made to provide compatible blood products to sensitised patients are reviewed in this article from the perspective of clinical transfusion medicine.

Soluble MHC I and soluble MIC molecules: potential therapeutic targets for cancer

It has become clear that soluble MHC I (sMHC I) and soluble MIC (sMIC), which are highly elevated in sera of cancer patients, can be viewed to be tolerogenic, and that metalloproteinases are involved in their generation process. In this review, an overview is provided of the recent progress made in the sMHC I and sMIC fields, with emphasis on their structure, formation, and function, and the key-questions that still await answers are addressed. Understanding better their formation mechanism, it will become more feasible to modulate the immune responses in cancer patients by targeting molecules involved in their generation process.

Behavior of serum human major histocompatibility complex class I antigen levels in human immunodeficiency virus-infected patients during antiretroviral therapy: correlation with clinical outcome

Human major histocompatibility complex class I antigens (HLA-A, -B, and -C) are heterodimeric molecules composed of a alpha heavy chain noncovalently associated with an invariant protein known as beta(2)-microglobulin. Beside being expressed on the membrane of the large majority of nucleated cells, HLA class I antigens are evident in serum (sHLA-I). We have previously detected a significant increase in the serum level of beta(2)-microglobulin-associated HLA-I antigens in human immunodeficiency virus (HIV)-infected patients compared with HIV-negative controls. The introduction of highly active antiretroviral therapy (HAART) modified the clinical course of the disease and decreased the acquired immunodeficiency syndrome-related morbidity and mortality. Therefore, we measured the levels of sHLA-I antigens in 64 HIV-infected patients before and during HAART treatment and correlated them with the immunological and virological response to antiretroviral treatment. Serum sHLA-I antigen level was elevated in all HIV-infected patients before and significantly decreased after 36 months of HAART treatment, correlating with the decrease of plasma HIV-RNA level and with the increase of CD4+ T-lymphocyte number. These results suggest that the measurement of sHLA-I antigens serum level might represent a useful surrogate marker to monitor HIV-positive patients undergoing HAART treatment.

Hepatocyte expression of soluble donor MHC class I antigen via gene transfer inhibits multiple aspects of the antidonor immune response in fully sensitized rat transplant recipients

Sensitized organ transplant recipients face an increased risk of hyperacute rejection (HAR) due to donor major histocompatibility complex (MHC) class I antigen (Ag) preexposure. We recently reported a novel donor-specific strategy to address this problem, whereby soluble donor MHC class I Ag gene therapy prevented HAR of heart allografts in passively sensitized rats. Here, we tested this same approach in presensitized rat recipients with a fully preactivated humoral and cellular immune response. Our gene therapy method involved liposomal transfection of cultured recipient (Lewis-RT1.A(1)) hepatocytes with DNA encoding secretable donor MHC class I Ag, RT1.A(a). Control-transfected or RT1.A(a)-transfected hepatocytes were implanted intrasplenically into Lewis rats presensitized with three skin transplants. Subsequently, antidonor antibody, cytotoxic T lymphocyte (CTL), and helper T lymphocyte (HTL) assays were performed. Additionally, the effectiveness of our gene therapy on the prevention of ACI (RT1(a)) heart HAR was evaluated. Results indicated that soluble MHC not only decreased cytotoxic antibody levels, but also suppressed antidonor CTL and HTL responses; furthermore, HAR of heart allografts was prevented in all recipients. Therefore, soluble donor MHC class I gene therapy can inhibit multiple aspects of the primed antidonor immune response in actively presensitized rats. Development of this strategy in presensitized humans could improve organ transplant outcome.

Analysis of endogenous peptides bound by soluble MHC class I molecules: a novel approach for identifying tumor-specific antigens

The Human MHC Project aims at comprehensive cataloging of peptides presented within the context of different human leukocyte antigens (HLA) expressed by cells of various tissue origins, both in health and in disease. Of major interest are peptides presented on cancer cells, which include peptides derived from tumor antigens that are of interest for immunotherapy. Here, HLA-restricted tumor-specific antigens were identified by transfecting human breast, ovarian and prostate tumor cell lines with truncated genes of HLA-A2 and HLA-B7. Soluble HLA secreted by these cell lines were purified by affinity chromatography and analyzed by nano-capillary electrospray ionization-tandem mass spectrometry. Typically, a large peptide pool was recovered and sequenced including peptides derived from MAGE-B2 and mucin and other new tumor-derived antigens that may serve as potential candidates for immunotherapy.

Donor-derived soluble MHC antigens plus low-dose cyclosporine induce transplantation unresponsiveness independent of the thymus by down-regulating T cell-mediated alloresponses in a rat transplantation model

BACKGROUND

In vitro, soluble MHC (sMHC) antigens modulate and induce apoptosis in alloreactive and antigen-specific T cells, demonstrating their potency to regulate T cell-mediated immune responses. However, their efficacy to regulate immunological responses in vivo remains unclear. Here, we report that repetitive intraperitoneal injection of recombinant Lewis rat-derived MHC class I antigens in Dark Agouti (DA) rats modulates alloreactivity.

METHODS

RT1.A1 (Lewis derived) genes were cloned into mammalian expression vectors, and RT1.Aa (DA derived) genes were used to transfect a rat myeloma cell line. RT1.A1 molecules were injected intraperitoneally in DA recipients that subsequently underwent transplantation with Lewis-derived cardiac allografts.

RESULTS

Soluble class I antigens were secreted by the transfected cells and were shown to be heterodimeric, peptide-loaded, and conformationally folded. Injection of donor-derived soluble MHC significantly reduced the ability of recipient animals to mount a cytotoxic T-cell response to donor-derived tissue. More interestingly, this treatment significantly prolonged donor-graft survival and allowed 60% of treated animals to develop graft tolerance (>120 days), when donor sMHC were combined with a single subtherapeutic dosage of cyclosporine. Thymectomy of recipient animals before transplantation did not interfere with induction of peripheral tolerance.

CONCLUSIONS

Donor-derived sMHC are potential tolerogens for down-regulating the cytotoxic T-cell response of animals that undergo transplantation. Thus, these data provide for the first time a rationale for the application of directly injected sMHC in vivo to down-regulate immunological responses and aid the induction of graft tolerance.

Clinical significance of soluble form of HLA class I molecule in Japanese patients with pancreatic cancer

In recent studies a soluble form of human leukocyte antigen class I (sHLA-I) has been found in blood, urine, ascitic fluid, and various other tissues. Research has been focused on the role of sHLA-I in the induction of immunotolerance in organ transplantation. To examine the role of sHLA-I in the immune system of patients with malignancy, we examined serum sHLA-I levels in patients with pancreatic, biliary, hepatic malignancy, and other diseases. We examined sHLA-I levels in the sera of patients with pancreatic cancer (n = 19), benign biliary disease and chronic pancreatitis (n = 20), hepatocellular carcinoma (n = 51), gallbladder cancer (n = 6), cholangiocellular carcinoma (n = 6), and in normal controls (n = 22), using enzyme-linked immunosorbent assay (ELISA). In patients with pancreatic cancer we also analyzed the relationship between sHLA-I and CA19-9, and the specificity and sensitivity of sHLA-I. When patients with acute or chronic hepatitis were excluded from analysis, the mean sHLA-I level in patients with pancreatic cancer was significantly higher than that of normal controls (p < 0.01) and patients with benign disease (p < 0.01), hepatocellular carcinoma (p < 0.01), gallbladder cancer (p < 0.05), and cholangiocarcinoma (p < 0.05). We determined a serum sHLA-I cutoff level for normal controls of 2000 ng/ml; serum levels of sHLA-I were higher than the cutoff in ten patients with pancreatic cancer, and serum levels of CA19-9 were lower than 37 IU/l in 9 of 14 patients; sensitivity and specificity were 88.2% and 85.5%, respectively. Serum levels of sHLA-I in pancreatic cancer patients were higher than in the other diseases, although we found that pancreatic cancer cell lines did not produce the sHLA-I. The evaluation of serum sHLA-I levels could have clinical significance in pancreatic cancer.

Immunologic considerations for therapeutic strategies utilizing allogeneic hepatocytes: hepatocyte-expressed membrane-bound major histocompatibility complex class I antigen sensitizes while soluble antigen suppresses the immune response in rats

Understanding the immunologic effects of hepatocytes is critical because of the potential to use these cells for bioartificial livers, as a vehicle for gene transfer, and as a means to induce donor-specific immunosuppression in organ transplantation. However, this understanding is complicated by the fact that hepatocytes express membrane-bound and soluble forms of major histocompatibility complex (MHC) class I antigen, each with the potential to induce different immune responses. In the present study we first determined the immunologic effect of normal donor-derived hepatocytes in a rat heart transplant model. We then used ex vivo hepatocyte gene transfer to examine the immunologic effects of different forms of hepatocyte-expressed MHC class I antigen. Results showed that intrasplenic injection of purified, donor-strain-specific hepatocytes into recipients primes alloimmunity, as evidenced by acceleration of heart allograft rejection. Interestingly, injection of autologous hepatocytes transfected ex vivo with DNA encoding only membrane-bound donor MHC class I antigen (RT1.A(a)) also accelerated allograft rejection. However, hepatocytes transfected to express only secreted donor MHC antigen prolonged transplant survival. Limiting-dilution analysis of lymphocytes from animals treated with hepatocytes producing only secreted alloantigen showed an antigen-specific reduction in cytotoxic T lymphocyte (CTL) and helper T lymphocyte (HTL) precursors. Further analysis of CTL populations by flow cytometry revealed a relatively high percentage of nonviable cells, implying that soluble antigen promotes allospecific CTL death. In summary, this study suggests that hepatocyte-expressed MHC class I molecules have opposing immunologic effects, with the membrane-bound antigen inducing immunologic sensitization, and the soluble antigen promoting donor-specific immunosuppression.

A three-colour flow cytometry technique for measuring trophoblast intracellular antigens: the relative expression of TAP1 in human cytotrophoblast and decidual cells

Flow cytometry is conventionally used to measure cell-surface antigen expression. However, many antigens are found within the cytoplasm, and it is necessary to fix and permeabilize cells to enable antibodies to gain access to them. In this study we have established the conditions for studying intracellular antigens in human trophoblast cells by flow cytometry using an antibody to TAP1 (a key molecule in the process of Class I MHC assembly). We have previously shown by immunocytochemistry that TAP1 expression is apparently greater on Class 1 positive extravillous cytotrophoblast than on any other fetal or maternal tissue. However, as immunohistochemistry is not quantitative we have used three-colour flow cytometry to measure the expression of TAP1 in different trophoblast populations. Villous and extravillous cytotrophoblast were identified in first trimester and term placental and decidual digests on the basis of their expression of cytokeratin and Class I MHC antigens. The level of expression of TAP1 for each population was investigated using a commercial kit that determines the number of antibody-binding sites per cell. TAP expression was found to be three- to fivefold higher in extravillous cytotrophoblast, confirming our previous findings. The techniques developed here are directly applicable to the measurement of other intracellular molecules in trophoblast, in particular cytokines.

Cutting edge: soluble HLA-G1 triggers CD95/CD95 ligand-mediated apoptosis in activated CD8+ cells by interacting with CD8

The nonpolymorphic soluble HLA-G1 (sHLA-G1) isoform has been reported to be secreted by trophoblast cells at the materno-fetal interface, suggesting that it may act as immunomodulator during pregnancy. In this paper, we report that affinity-purified beta2-microglobulin-associated sHLA-G1 triggered apoptosis in activated, but not resting CD8+ peripheral blood cells. We demonstrate by Western blotting that sHLA-G1 enhanced CD95 ligand expression in activated CD8+ cells. Cytotoxicity was inhibited by preincubation of the cells with a CD95 antagonist mAb (ZB4) or a soluble recombinant CD95-Fc, indicating that apoptosis is mediated through the CD95/CD95 ligand pathway. Finally, we show that such sHLA-G1-induced apoptosis depends on the interaction with CD8 molecules, with cell death being blocked by various CD8 mAbs.

Comparative reactivity of different HLA-G monoclonal antibodies to soluble HLA-G molecules

Different HLA-G monoclonal antibodies (mAbs) were first evaluated for their capability to identify soluble HLA-G (sHLA-G) in ELISA. Three of them, namely 87G, BFL.1 and MEM-G/9, when used as coating mAbs together with W6/32 capture mAb, identified beta2-microglobulin (beta2m)-associated-sHLA-G but not soluble HLA-B7 (sHLA-B7) in cell culture supernatants from transfected cells. By comparison, the anti-HLA class I mAb 90 did recognize both sHLA-G and sHLA-B7. By using these HLA-G mAbs, sHLA-G was identified in amniotic fluids as well as in culture supernatants of first trimester and term placental explants but not in cord blood. Intron 4-retaining sHLA-G isoforms were identified in some amniotic fluids by the use of an intron 4-specific mAb (16G1). Reactivity of these different HLA-G mAbs was then compared to determine their respective binding sites on soluble and membrane-bound HLA-G. Using both ELISA and flow cytometry analysis, we showed that they did not compete with each other, which suggested that they did not recognize the same determinants. Finally, we report that two mAbs directed against the alpha1 domain of HLA class I heavy chain (mAb 90 and YTH 862) did compete with 87G, therefore demonstrating that this latter mAb recognized an epitope localized on this external domain of HLA-G.

Inhibition of Epstein-Barr virus (EBV)-specific CD8+ cytotoxic T lymphocyte (CTL) activity by soluble HLA class I in vitro

In the present study, the effects of soluble HLA (sHLA) class I molecules against EBV-specific CTL were examined. Two different sources of sHLA class I, either bioengineered spliced form of HLA-B7 (sB7) or natural production from EBV-transformed B cells (natural sHLA), were added during the induction of CTL or incubated with MHC-restricted CD8+ CTL, which were selected by immunobeads just before testing for their cytotoxic activity. Both sB7 and natural sHLA class I blocked the generation of CD8+ CTL and also inhibited the cytotoxic activity of established CTL in a dose-dependent manner. In both ways, natural sHLA class I was effective in 10-fold lower concentrations compared with sB7. The inhibitory effect did not require a sharing of the HLA allotypes between sHLA and the CTL. CTL, after being treated with sHLA, underwent apoptosis, which was considered here as the main mechanism.

Implication of soluble and membrane HLA class I and serum IL-10 in liver graft acceptance

Membrane HLA class-I expression (mHLA-I), soluble HLA class-I antigens (sHLA-I) and interleukin (IL)-10 are different factors implicated in the special acceptance of liver allograft. In this study, pre- and post-operative levels of mHLA-I in peripheral blood lymphocytes (PBL) and serum sHLA-I were analyzed in 86 liver transplants, immunosuppressed with Cyclosporine-A, methylprednisolone and azathioprine, and classified into acute-rejection (AR, n = 28) and non-acute-rejection (NAR, n = 58) groups. Serum IL-10 was studied in 47 recipients (AR-group, n = 16 and NAR-group, n = 31). Pre-transplant values of mHLA-I and sHLA-I showed a bimodal distribution (high/low) in NAR-recipients, but in AR-patients were mainly included in the low expression/secretion zone (mHLA-I, p < 0.02 and sHLA-I, p < 0.05). Consequently, average pre-transplant mHLA-I (868 +/- 109 versus 998 +/- 123, p < 0.05) and sHLA-I (1.3 +/- 0.4 versus 2.02 +/- 0.7 microg/ml, p < 0.01) was lower in the AR- than in the NAR-group. After transplant both parameters decreased in the NAR-group, but increased in AR-recipients previous to and on rejection diagnosis day. Additionally, serum IL-10 levels were significantly higher (p < 0.01) in the NAR than in the AR-group during the first 24 h post-transplant. In conclusion, low pre-transplant mHLA-I and sHLA-I levels pre-dispose liver recipients to acute rejection, whereas early post-transplant increases of serum IL-10 appear to be related to a good liver allograft acceptance.

HLA (A*0201) Mimicry by Anti-Idiotypic Monoclonal Antibodies

Soluble MHC Ags and anti-Id (anti-anti-MHC) Abs have both been shown to inhibit MHC alloantigen-specific B cell responses in vivo. We hypothesized that some anti-idiotypic Abs function as divalent molecular mimics of soluble HLA alloantigen. To test this idea, we studied two well-defined anti-idiotypic mAbs, T10-505 and T10-938, elicited in syngeneic BALB/c mice by immunization with CRll-351, an HLA-A2,24,28-specific mAb. Each anti-Id induced “Ab-3” Abs in rabbits that cross-reacted with HLA-A2 but not with HLA-B Ags. Furthermore, each anti-Id could bind to and block Ag recognition by Ha5C2.A2, a human homologue of mAb CRll-351. Both anti-Id mAb displayed weak reactivity with the human mAb SN66E3, which recognized an overlapping but distinct determinant of HLA-A2 Ags; neither reacted with human mAb MBW1, which recognized a nonoverlapping HLA-A2 determinant. Amino acid sequence comparison of mAb CRll-351 heavy and light chain variable region complementarity-determining regions (CDRs) with those of mAb Ha5C2.A2 and SN66E3 revealed short regions of homology with both human mAb; a large insert in the light chain CDR1 of mAb SN66E3 distinguished it from both CRll-351 and Ha5C2.A2. The amino acid sequences of mAb T10-505 and T10-938, which differed markedly from each other, revealed no homology to the α2 domain sequence of HLA-A*0201 that contains the CRll-351 mAb-defined epitope. We conclude that structurally different anti-Id Abs can mimic a polymorphic conformational epitope of an HLA Ag. In the case of T10-505 and T10-938 mimicry was not based on exact replication of the epitope by the hypervariable loops of the anti-Id mAb.

The metalloproteinase-mediated pathway is essential for generation of soluble HLA class I proteins by activated cells in vitro: proposed mechanism for soluble HLA release in transplant rejection

We and others have found donor-derived soluble beta2m-associated HLA class I proteins (sHLA/beta2m) in the serum of allograft recipients with acute and chronic rejection. Whether appearance of sHLA/beta2m and upregulated expression of donor cell-bound HLA/beta2m during allograft rejection are related events is unknown. Activation-induced upregulation of in vitro HLA/beta2m expression correlates with the surface expression of another form of HLA class I, namely beta2m-free HLA heavy chains (beta2m-free HC). We have shown that beta2m-free HC, but not beta2m-associated HC, are then cleaved by a specific membrane-bound metalloproteinase and released into supernatants as soluble 36 kDa proteins. We show now that activated peripheral blood lymphocytes produce predominantly the 36 kDa form of sHLA proteins which is present in supernatants as both beta2m-free HC and sHLA/beta2m. Importantly, the metalloprotease inhibitor BB-94 blocked not only the release of soluble beta2m-free HC, but also the appearance of sHLA/beta2m in cell supernatants. Low levels of 36 kDa beta2m-free HC were also present in human plasma of healthy donors. These data suggest an important role for the HLA class I-specific metalloproteinase in vivo in healthy individuals and during allograft rejection in the generation of soluble beta2m-free and beta2m-associated HLA proteins.

Immunologic suppression mediated by genetically modified hepatocytes expressing secreted allo-MHC class I molecules

Studies suggest that immunosuppression associated with liver transplantation may be related to the secretion of MHC class I antigen (Ag) by hepatocytes. To investigate this possibility, we developed a culture system whereby naive Lewis (RT1.A1) splenocytes were cocultured with autologous hepatocytes transfected with plasmids encoding either the membrane-bound or secreted allogeneic MHC class I Ag, RT1.Aa. Cytotoxic T lymphocyte (CTL) and helper T lymphocyte (HTL) limiting dilution assays were subsequently performed on preconditioned lymphocytes. Lymphocytes preconditioned with hepatocytes secreting RT1.Aa showed an alloantigen specific inhibition of CTL precursors (CTLp). In contrast, exposure of splenocytes to hepatocyte-expressed membrane-bound RT1.Aa resulted in Ag-specific CTLp priming. This CTLp priming effect by hepatocyte-expressed membrane-bound Ag could be effectively blocked when splenocytes were first preincubated with hepatocytes secreting RT1.Aa, before being exposed to hepatocytes expressing membrane-bound RT1.Aa. In contrast to CTLp, HTLp frequency, as determined by IL-2 production, was unaffected by either hepatocyte-expressed membrane-bound or secreted RT1.Aa. Further studies on splenocytes conditioned with hepatocytes expressing secreted allo-MHC Ag suggest the possibility of suppressor cell development. This was demonstrated by prolongation of ACI (RT1a) heart allograft survival in Lewis recipients following adoptive transfer of splenocytes that were preconditioned in vitro with hepatocytes secreting alloantigen.

Soluble HLA class I molecules: biological significance and clinical implications

Soluble class I human leukocyte antigens (sHLAs) have been detected in serum, sweat, lymphatic fluid, urine and cerebrospinal fluid. Their biological function has, however, remained a puzzle. The physiological concentration of sHLA varies more than tenfold depending on the phenotype of the individual, and is significantly upregulated in various diseases and during inflammation. This suggests that sHLAs might serve as a marker of pathological changes. Recent experiments have shown that, in vitro, sHLAs can modulate T-cell reactivity and induce cell-activated apoptosis, implicating sHLAs in the induction and maintenance of peripheral tolerance. Therefore, sHLAs have the therapeutic potential to induce tolerance to transplants.

Serum soluble human leucocyte antigen class I in paediatric liver transplantation with live, related donors

Serum soluble human leucocyte antigen (HLA) class-I is a useful marker for predicting immunological events in organ transplantation. In cadaver liver transplant cases it is especially the case that high amounts of soluble HLA-I are excreted from the grafts. In Japan, almost all liver transplants have been performed from living parent donors to their children. Therefore, it is interesting to know how soluble HLA-I changes in relation to clinical course. As part of this study we first examined serum concentrations of soluble HLA-I in 33 paediatric patients using enzyme-linked immunosorbent assay. Soluble HLA-I is composed of three different sized molecules (45, 39 and 34-36 kDa); then the change of distribution of these three molecules was demonstrated by Western blot analysis. When donor and recipient have different soluble HLA-I band patterns, the origin of the antigen can be assumed by this method. We found that in a comparison between pre- and post-transplants, the six out of eight (75%) patients that suffered episodes of acute rejection showed a significant elevation of soluble HLA-I, and all patients with infectious episodes had an elevated soluble HLA-I. Meanwhile, 10 out of 22 (45%) patients without any clinical complications still showed increased soluble HLA-I. The Western blot analysis showed that the soluble HLA-I molecules were considerably derived from the grafted liver, from one week to 24 months after grafting. In acute rejection, the band signals of donor origin were significantly increased. These signals were attenuated after immunosuppressive therapy. The grafted liver appears to contribute to the increase of soluble HLA-I following liver transplantation, and this increase is greater with the effects of the host immune system.

Soluble HLA class I in epithelial lining fluid of lung transplants: associations with graft outcome

We hypothesized that the small amounts of donor HLA-A and HLA-B proteins detected in the serum during organ allograft rejection are indicative of higher local releases within the graft itself. We determined the concentrations of total HLA class I (HLA-I) and, in selected cases, specific donor and host HLA-A and HLA-B proteins, in the epithelial lining fluid (ELF) sampled by bronchoalveolar lavage (BAL) of lung transplant recipients (n = 37) and of normal controls (n = 25). We found that 1) HLA-I proteins were enriched in the lung ELF relative to other proteins; 2) the concentration of HLA-I in the ELF of well-functioning transplants was similar to that in normal lungs; 3) HLA-I proteins and total proteins were elevated in the ELF of patients who developed chronic rejection or refractory acute rejection; 4) the concentration of HLA-I was correlated with the percentage of neutrophils but not with the percentage of lymphocytes in the ELF of transplanted lungs; and 5) only the percentage of lymphocytes was elevated in the ELF of transplant patients with active CMV infections. Total HLA-I from the ELF was found to contain a mixture of both donor- and recipient-type HLA-A and HLA-B proteins and the donor-type HLA-A2 was found to be highly enriched in the ELF relative to serum.

Distribution of HLA-G alternative mRNAs including soluble forms in normal lymphocytes and in lymphoid cell-derived leukemia

The non-classical HLA-G gene is the only class I antigen expressed in trophoblasts at the maternofetal interface. In placenta, the HLA-G gene produces several alternatively spliced isoforms encoding bound-membrane proteins (G1, G2, G3 and G4) lacking, respectively, exon 7; exons 7 and 3; exons 7, 3 and 4, and exons 7 and 4. In addition, two isoforms (G1s and G2s) containing an intron 4 sequence are able to encode soluble antigens. We have recently reported that the HLA-G gene is transcriptionally active in lymphocytes and is not transcribed in CD34+ cells, polynuclear cells or monocytes. To investigate the functional significance of the different isoforms in lymphocytes, we studied their distribution in normal T and B lymphocytes and in malignant lymphoid cells by using the RT-PCR technique followed by hybridization with exon-specific probes and sequencing assays. In transcriptionally active lymphocytes, the HLA-G primary transcript is the major form and is differentially spliced in B and T lymphocytes: (i) G1s is found in several samples of T and B cells whereas G2s is only transcribed in T lymphocytes, (ii) the G4 isoform is never detected in B lymphocytes. In addition, we have shown that HLA-G is inactive in some samples of lymphocytes. Our data suggest that HLA-G transcription is regulated at the initiation level and at the subsequent splicing. These two levels of regulation may be dysregulated in some cases of T-ALL and CLL. The potential functions of the HLA-G alternative forms in lymphocytes, such as peptide binding and modulation of the immune response, are discussed.

Soluble HLA-G molecule. An alternatively spliced HLA-G mRNA form candidate to encode it in peripheral blood mononuclear cells and human trophoblasts

The HLA-G nonclassic MHC class I gene expressed at the maternal-fetal interface may be involved in cell protection against NK cell lysis. HLA-G mRNA is observed in different adult or fetal human cells and exhibits four alternative forms: HLA-G1, HLA-G2, HLA-G3, and HLA-G4 lacking, respectively, exon 7; exons 7 and 3, exons 7, 3, and 4; exons 7 and 4. Because exon 5 encodes the transmembrane domain of the HLA-G antigen, none of these transcripts could give a soluble form as detected in supernatant of trophoblasts. In this report, we describe an additional alternatively spliced for of HLA-G transcript (HLA-G5) present in adult PBMCs and first-trimester trophoblasts that contains intron 4. Moreover, as with all other transcripts, HLA-G5, is devoid of exon 7. Its relative frequency is, respectively, approximately 1:8 and approximately 1:26 in adult PBMCs and first-trimester trophoblasts. The presence of intron 4 generates a stop codon that excludes transmembrane region (exon 5) of the HLA-G molecule and therefore might produce a soluble antigen. These results are discussed with regard to data on soluble forms of classic HLA antigens and the possible role of HLA-G.