Biochemical complexity of serum HLA class I molecules

Allele-specific quantification of human leukocyte antigen transcript isoforms by nanopore sequencing

Introduction

While tens of thousands of HLA alleles have been identified by DNA sequencing, the contribution of alternative splicing to HLA diversity is not well characterized. In this study, we sought to determine if long-read sequencing could be used to accurately quantify allele-specific HLA transcripts in primary human lymphocytes.

Methods

cDNA libraries were prepared from peripheral blood lymphocytes from 12 donors and sequenced by nanopore long-read sequencing. HLA reads were aligned to donor-specific reference sequences based on the known type of each donor. Allele-specific exon utilization was calculated as the proportion of reads aligning to each allele containing known exons, and transcript isoforms were quantified based on patterns of exon utilization within individual reads.

Results

Splice variants were rare among class I HLA genes (median exon retention rate 99%-100%), except for several HLA-C alleles with exon 5 spliced out of up to 15% of reads. Splice variants were also rare among class II HLA genes (median exon retention rate 98%-100%), except for HLA-DQB1. Consistent with previous work, exon 5 of HLA-DQB1 was spliced out in alleles with a mutated splice acceptor site at rs28688207. Surprisingly, a 28% loss of exon 5 was also observed in HLA-DQB1 alleles with an intact splice acceptor site at rs28688207.

Discussion

We describe a simple bioinformatic workflow to quantify allele-specific expression of HLA transcript isoforms. Further studies are warranted to characterize the repertoire of HLA transcripts expressed in different cell types and tissues across diverse populations.

Open MHC Class I Conformers: A Look through the Looking Glass

Studies carried out during the last few decades have consistently shown that cell surface MHC class I (MHC-I) molecules are endowed with functions unrelated with antigen presentation. These include cis–trans-interactions with inhibitory and activating KIR and LILR, and cis-interactions with receptors for hormones, growth factors, cytokines, and neurotransmitters. The mounting body of evidence indicates that these non-immunological MHC-I functions impact clinical and biomedical settings, including autoimmune responses, tumor escape, transplantation, and neuronal development. Notably, most of these functions appear to rely on the presence in hematopoietic and non-hematopoietic cells of heavy chains not associated with β2m and the peptide at the plasma membrane; these are known as open MHC-I conformers. Nowadays, open conformers are viewed as functional cis-trans structures capable of establishing physical associations with themselves, with other surface receptors, and being shed into the extracellular milieu. We review past and recent developments, strengthening the view that open conformers are multifunctional structures capable of fine-tuning cell signaling, growth, differentiation, and cell communication.

Soluble HLA in the Aqueous Humour of Uveal Melanoma Is Associated with Unfavourable Tumour Characteristics

A high HLA expression in uveal melanoma (UM) is part of the prognostically unfavorable inflammatory phenotype. We wondered whether the presence of soluble HLA (sHLA) in the aqueous humour is associated with clinical, histopathological or genetic tumour characteristics, and represents tumour HLA expression and intratumoural inflammation. Aqueous humour from 108 UM patients was analysed for the presence of sHLA, using a Luminex assay specific for HLA Class I. Clinical and genetic parameters were compared between sHLA-positive and negative eyes. A qPCR analysis was performed on tumour tissue using a Fluidigm assay. In 19/108 UM-containing eyes, the sHLA level in the aqueous was above the detection limit. Tumours in sHLA-positive eyes were significantly larger, more frequently involved the ciliary body, and more often showed monosomy 3, gain of chromosome 8q and loss of BAP1 staining. Melanoma-related survival was worse in patients with sHLA-positive aqueous humour. sHLA in the aqueous did not represent the tumour's HLA expression and did not relate to immune cell infiltration in the tumour. We conclude that UM-containing eyes may contain sHLA in the aqueous humour, where it is a prognostically-unfavourable sign and may influence local immune responses.

Soluble major histocompatibility complex molecules in immune regulation: highlighting class II antigens

The involvement of major histocompatibility complex (MHC) antigens in the development and regulation of immune response has been well defined over the years, starting from maturation, antigenic peptide loading, migration to the cell membrane for recognition by the T-cell receptor and recycling for immune response cessation. During this intracellular trafficking, MHC antigens find a way to be excreted by the cells, because they can be found as soluble MHC class I (sMHC-I) and class II (sMHC-II) molecules in all body fluids. Although secretion mechanisms have not been sufficiently studied, sMHC molecules have been shown to display important immunoregulatory properties. Their levels in the serum have been shown to be altered in a variety of diseases, including viral infections, inflammation, autoimmunities and cancer, etc. while they seem to be involved in a number of physiological reactions, including maintenance of tolerance, reproduction, as well as mate choice vis-à-vis species evolution. The present review aims to present the thus far existing literature on sMHC molecules and point out the importance of these molecules in the maintenance of immune homeostasis.

Astroglial major histocompatibility complex class I following immune activation leads to behavioral and neuropathological changes

In the central nervous system, major histocompatibility complex class I (MHCI) molecules are mainly expressed in neurons, and neuronal MHCI have roles in synapse elimination and plasticity. However, the pathophysiological significance of astroglial MHCI remains unclear. We herein demonstrate that MHCI expression is up-regulated in astrocytes in the medial prefrontal cortex (mPFC) following systemic immune activation by an intraperitoneal injection of polyinosinic-polycytidylic acid (polyI:C) or hydrodynamic interferon (IFN)-γ gene delivery in male C57/BL6J mice. In cultured astrocytes, MHCI/H-2D largely co-localized with exosomes. To investigate the role of astroglial MHCI, H-2D, or sH-2D was expressed in the mPFC of male C57/BL6J mice using an adeno-associated virus vector under the control of a glial fibrillary acidic protein promoter. The expression of astroglial MHCI in the mPFC impaired sociability and recognition memory in mice. Regarding neuropathological changes, MHCI expression in astrocytes significantly activated microglial cells, decreased parvalbumin-positive cell numbers, and reduced dendritic spine density in the mPFC. A treatment with GW4869 that impairs exosome synthesis ameliorated these behavioral and neuropathological changes. These results suggest that the overexpression of MHCI in astrocytes affects microglial proliferation as well as neuronal numbers and spine densities, thereby leading to social and cognitive deficits in mice, possibly via exosomes created by astrocytes.

Relationship of the Content of Systemic and Endobronchial Soluble Molecules of CD25, CD38, CD8, and HLA-I-CD8 and Lung Function Parameters in COPD Patients

The definition of new markers of local and systemic inflammation of chronic obstructive pulmonary disease (COPD) is one of the priority directions in the study of pathogenesis and diagnostic methods improvement for this disease. We investigated 91 patients with COPD and 21 healthy nonsmokers. The levels of soluble CD25, CD38, CD8, and HLA-I-CD8 molecules in the blood serum and exhaled breath condensate (EBC) in moderate-to-severe COPD patients during exacerbation and stable phase were studied. An unidirectional change in the content of sCD25, sCD38, and sCD8 molecules with increasing severity of COPD was detected. The correlations between the parameters of lung function and sCD8, sCD25, and sHLA-I-CD8 levels in the blood serum and EBC were discovered in patients with severe COPD. The findings suggest a pathogenetic role of the investigated soluble molecules of the COPD development and allow considering the content of sCD8, sCD25, and sHLA-I-CD8 molecules as additional novel systemic and endobronchial markers of the progression of chronic inflammation of this disease.

Allele-Independent Turnover of Human Leukocyte Antigen (HLA) Class Ia Molecules

Major histocompatibility complex class I (MHCI) glycoproteins present cytosolic peptides to CD8+ T cells and regulate NK cell activity. Their heavy chains (HC) are expressed from up to three MHC gene loci (human leukocyte antigen [HLA]-A, -B, and -C in humans), whose extensive polymorphism maps predominantly to the antigen-binding groove, diversifying the bound peptide repertoire. Codominant expression of MHCI alleles is thus functionally critical, but how it is regulated is not fully understood. Here, we have examined the effect of polymorphism on the turnover rates of MHCI molecules in cell lines with functional MHCI peptide loading pathways and in monocyte-derived dendritic cells (MoDCs). Proteins were labeled biosynthetically with heavy water (2H2O), folded MHCI molecules immunoprecipitated, and tryptic digests analysed by mass spectrometry. MHCI-derived peptides were assigned to specific alleles and isotypes, and turnover rates quantified by 2H incorporation, after correcting for cell growth. MHCI turnover half-lives ranged from undetectable to a few hours, depending on cell type, activation state, donor, and MHCI isotype. However, in all settings, the turnover half-lives of alleles of the same isotype were similar. Thus, MHCI protein turnover rates appear to be allele-independent in normal human cells. We propose that this is an important feature enabling the normal function and codominant expression of MHCI alleles.

Behavior of soluble HLA-A, -B, -C and HLA-G molecules in patients with chronic hepatitis C virus infection undergoing pegylated interferon-α and ribavirin treatment: potential role as markers of response to antiviral therapy

The serum levels of soluble HLA class I antigens (sHLA-A, -B, -C and sHLA-G) were determined in 40 HCV genotype 1-infected patients before (T ₀), after 3, 6, and 12 months (T ₃, T ₆, and T ₁₂) of pegylated-IFN-α plus ribavirin therapy and 6 months (T ₁₈) after the end of treatment. Twenty patients were sustained virological responders (SVR), and 20 were non-responders (NR). sHLA-A, -B, -C levels at T ₀ were significantly higher in both SVR (mean 10.48 μg/ml) and NR (mean 11.87 μg/ml) patients as compared to healthy controls (mean 0.34 μg/ml, p < 0.0001) and HIV-infected subjects (mean 1.22 μg/ml, p < 0.0001). sHLA-G levels at T ₀ were significantly higher in SVR (mean 24.78 ng/ml) and NR (mean 24.93 ng/ml) patients as compared to healthy controls (mean 10.34 ng/ml, p = 0.015 and p = 0.014, respectively) but were lower as compared to HIV-infected subjects (mean 48.00 ng/ml, p < 0.0001). The levels of sHLA-A, -B, -C and sHLA-G significantly decreased in SVR from T ₀ to T ₁₈ (mean 1.64 and 1.43 ng/ml, respectively, p < 0.0001) and correlated with HCV-RNA, AST, ALT, γGT, and ALP levels. The determination of soluble HLA class I levels could be proposed as a surrogate marker to discriminate SVR and NR HCV-infected patients during PEG-IFN-α plus ribavirin therapy.

Soluble MHC-II proteins promote suppressive activity in CD4+ T cells

Soluble MHCII (sMHCII) molecules are present in body fluids of healthy individuals and are considered to be involved in the maintenance of self tolerance, and are also related to various diseases. Their concentration increases during in vivo antigen-specific tolerogenic stimulation and it was recently shown that exosome-mediated tolerance is MHCII dependent. At the cellular level, sMHCII proteins compete with membrane MHCII for T-cell receptor binding on CD4(+) T cells. Immunoaffinity purification techniques isolated sMHCII antigens from the serum of human serum albumin (HSA) -tolerant mice as a single highly glycosylated protein of ~ 60,000 molecular weight, specifically interacting with anti-class II antibodies in Western blotting and ELISA. Mass spectroscopy showed that these sMHCII proteins were loaded with the tolerogenic peptide as well as multiple self peptides. At the cellular level, sMHCII suppressed antigen-specific, and to a lesser degree antigen-non-specific, spleen cell proliferation and induced CD25 in naive T cells. In T cells activated by antigen-seeded macrophages, sMHCII decreased CD28 and increased CTLA-4 protein expression, while decreasing interleukin-2 and increasing interleukin-10 production. In this case, sMHCII proteins were shown to decrease ZAP-70 and LAT phosphorylation. The results presented here for the first time provide evidence for the role of sMHCII proteins in immune response suppression and maintenance of tolerance, revealing novel regulatory mechanisms for immune system manipulation.

HLA-G UTR haplotype conservation in the Malian population: association with soluble HLA-G

The HLA-G molecule plays an important role in immunomodulation. In a previous study carried out on a southern French population our team showed that HLA-G haplotypes, defined by SNPs in the coding region and specific SNPs located in 5'URR and 3'UTR regulatory regions, are associated with differential soluble HLA-G expression (sHLA-G). Furthermore, the structure of these HLA-G haplotypes appears to be conserved in geographically distant populations. The aim of our study is to confirm these expectations in a sub-Saharan African population and to explore additional factors, such as HLA-A alleles, that might influence sHLA-G expression. DNA and plasma samples were collected from 229 Malians; HLA-G and HLA-A genotyping were respectively performed by the Snap Shot® method and by Luminex™ technology. sHLA-G dosage was performed using an ELISA kit. HLA-G and HLA-A allelic and haplotypic frequencies were estimated using an EM algorithm from the Gene[Rate] program. Associations between genetic and non genetic parameters with sHLA-G were performed using a non-parametric test with GRAPH PAD Prism 5. Our results reveal a good conservation of the HLA-G UTR haplotype structure in populations with different origins and demographic histories. These UTR haplotypes appear to be involved in different sHLA-G expression patterns. Specifically, the UTR-2 haplotype was associated with low sHLA-G levels, displaying a dominant negative effect. Furthermore, an allelic effect of both HLA-G and HLA-A, as well as non genetic parameters, such as age and gender possibly linked to osteogenesis and sexual hormones, also seem to be involved in the modulation of sHLA-G. These data suggest that further investigation in larger cohorts and in populations from various ethnical backgrounds is necessary not only to detect new functional polymorphism in HLA-G regulatory regions, but also to reveal the extent of biological phenomena that influence sHLA-G secretion and this might therefore have an impact on transplantation practice.

MHC class I antigen processing and presenting machinery: organization, function, and defects in tumor cells

The surface presentation of peptides by major histocompatibility complex (MHC) class I molecules is critical to all CD8(+) T-cell adaptive immune responses, including those against tumors. The generation of peptides and their loading on MHC class I molecules is a multistep process involving multiple molecular species that constitute the so-called antigen processing and presenting machinery (APM). The majority of class I peptides begin as proteasome degradation products of cytosolic proteins. Once transported into the endoplasmic reticulum by TAP (transporter associated with antigen processing), peptides are not bound randomly by class I molecules but are chosen by length and sequence, with peptidases editing the raw peptide pool. Aberrations in APM genes and proteins have frequently been observed in human tumors and found to correlate with relevant clinical variables, including tumor grade, tumor stage, disease recurrence, and survival. These findings support the idea that APM defects are immune escape mechanisms that disrupt the tumor cells' ability to be recognized and killed by tumor antigen-specific cytotoxic CD8(+) T cells. Detailed knowledge of APM is crucial for the optimization of T cell-based immunotherapy protocols.

Systemic inhibition of myeloid dendritic cells by circulating HLA class I molecules in HIV-1 infection

BACKGROUND

HIV-1 infection is associated with profound dysfunction of myeloid dendritic cells, for reasons that remain ill-defined. Soluble HLA class I molecules can have important inhibitory effects on T cells and NK cells, but may also contribute to reduced functional properties of professional antigen-presenting cells. Here, we investigated the expression of soluble HLA class I isoforms during HIV-1 infection and assessed their functional impact on antigen-presenting characteristics of dendritic cells.

RESULTS

Soluble HLA class I molecules were highly upregulated in progressive HIV-1 infection as determined by quantitative Western blots. This was associated with strong increases of intracellular expression of HLA class I isoforms in dendritic cells and monocytes. Using mixed lymphocyte reactions, we found that soluble HLA class I molecules effectively inhibited the antigen-presenting properties of dendritic cells, however, there was no significant influence of HLA class I molecules on the cytokine-secretion properties of these cells. The immunomodulatory effects of soluble HLA class I molecules were mediated by interactions with inhibitory myelomonocytic MHC class I receptors from the Leukocyte Immunoglobulin Like Receptor (LILR) family.

CONCLUSIONS

During progressive HIV-1 infection, soluble HLA class I molecules can contribute to systemic immune dysfunction by inhibiting the antigen-presenting properties of myeloid dendritic cells through interactions with inhibitory myelomonocytic HLA class I receptors.

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.

Soluble human leukocyte antigen-G serum levels in patients with acquired immune deficiency syndrome affected by different disease-defining conditions before and after antiretroviral treatment

We have previously reported that the serum levels of soluble human leukocyte antigen (HLA)-A, -B, -C, and -G antigens are elevated in human immunodeficiency virus (HIV)-infected subjects and decrease after antiretroviral therapy. In this study, we measured soluble HLA-G serum levels in patients with acquired immune deficiency syndrome (AIDS) affected by different AIDS-defining conditions before and during antiretroviral therapy and correlated them with virologic and immunologic parameters of response to treatment. Soluble HLA-G levels were significantly higher in AIDS patients before treatment as compared with healthy controls and significantly decreased after 36 months of therapy. The decrease of soluble HLA-G correlated with the decrease of plasma HIV-RNA level and CD8(+) T-lymphocytes number and with the increase of CD4(+) T-lymphocytes number. Soluble HLA-G levels were significantly higher in patients with opportunistic infections and Kaposi's sarcoma compared with patients with the wasting syndrome. These data suggest that infections and neoplasms may trigger the shedding of soluble HLA-G molecules, and confirm that the level of soluble HLA-G in serum might represent a surrogate marker to monitor virologic response and immune reconstitution in HIV-positive individuals.

A potential role for shed soluble major histocompatibility class I molecules as modulators of neurite outgrowth

The neurobiological activities of classical major histocompatibility class I (MHCI) molecules are just beginning to be explored. To further examine MHCI's actions during the formation of neuronal connections, we cultured embryonic mouse retina explants a short distance from wildtype thalamic explants, or thalami from transgenic mice (termed “NSE-D^b”) whose neurons express higher levels of MHCI. While retina neurites extended to form connections with wildtype thalami, we were surprised to find that retina neurite outgrowth was very stunted in regions proximal to NSE-D^b thalamic explants, suggesting that a diffusible factor from these thalami inhibited retina neurite outgrowth. It has been long known that MHCI-expressing cells release soluble forms of MHCI (sMHCI) due to the shedding of intact MHCI molecules, as well as the alternative exon splicing of its heavy chain or the action proteases which cleave off it's transmembrane anchor. We show that the diffusible inhibitory factor from the NSE-D^b thalami is sMHCI. We also show that COS cells programmed to express murine MHCI release sMHCI that inhibits neurite outgrowth from nearby neurons in vitro. The neuroinhibitory effect of sMHCI could be blocked by lowering cAMP levels, suggesting that the neuronal MHCI receptor's signaling mechanism involves a cyclic nucleotide-dependent pathway. Our results suggest that MHCI may not only have neurobiological activity in its membrane-bound form, it may also influence local neurons as a soluble molecule. We discuss the involvement of complement proteins in generating sMHCI and new theoretical models of MHCI's biological activities in the nervous system.

Behavior of non-classical soluble HLA class G antigens in human immunodeficiency virus 1-infected patients before and after HAART: comparison with classical soluble HLA-A, -B, -C antigens and potential role in immune-reconstitution

We have reported that serum level of soluble HLA-A, -B, -C (sHLA-A,-B,-C) antigens is elevated in HIV-infected subjects and decreases after antiretroviral therapy (HAART). In this study, we measured the levels of soluble HLA-G (sHLA-G) antigens in a cohort of HIV-infected patients before and during HAART. sHLA-G and sHLA-A, -B, -C levels were significantly elevated in HIV-infected subjects as compared with controls before antiretroviral treatment and significantly decreased after 36 months of HAART. sHLA-G levels were correlated with sHLA-A, -B, -C levels, the decrease of plasma HIV-RNA level, the increase of CD4+ T-lymphocyte number and the decrease of CD8+ T-lymphocyte number. These results suggest that the measurement of sHLA-G and sHLA-A, -B, -C antigen serum levels might represent a useful surrogate marker to monitor virological response and immune reconstitution in HIV-positive subjects undergoing HAART treatment.

Tumor escape mechanisms: potential role of soluble HLA antigens and NK cells activating ligands

The crucial role played by human leukocyte antigen (HLA) antigens and natural killer (NK)-cell-activating ligands in the interactions of malignant cells with components of the host's immune system has stimulated interest in the characterization of their expression by malignant cells. Convincing evidence generated by the immunohistochemical staining of surgically removed malignant lesions with monoclonal antibodies recognizing HLA antigens and NK-cell-activating ligands indicates that the surface expression of these molecules is frequently altered on malignant cells. These changes appear to have clinical significance because in some types of malignant disease they are associated with the histopathological characteristics of the lesions as well as with disease-free interval and survival. These associations have been suggested to reflect the effect of HLA antigen and NK-cell-activating ligand abnormalities on the interactions of tumor cells with antigen-specific cytotoxic T lymphocytes (CTL) and with NK cells. Nevertheless, there are examples in which disease progresses in the face of appropriate HLA antigen and/or NK-cell-activating ligand as well as tumor antigen expression by malignant cells and of functional antigen-specific CTL in the investigated patient. In such scenarios, it is likely that the tumor microenvironment is unfavorable for CTL and NK cell activity and contributes to tumor immune escape. Many distinct escape mechanisms have been shown to protect malignant cells from immune recognition and destruction in the tumor microenvironment. In this article, following the description of the structural and functional characteristics of soluble HLA antigens and NK-cell-activating ligands, we will review changes in their serum level in malignant disease and discuss their potential role in the escape mechanisms used by tumor cells to avoid recognition and destruction.

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.

HLA-G and its role in implantation (review)

BACKGROUND

Human leukocyte antigen G (HLA-G) is thought to play a key role in implantation by modulating cytokine secretion to control trophopblastic cell invasion and to maintain a local immunotolerance.

METHOD OF STUDY

The literature is reviewed to provide a description of the genetic background, properties of the protein, and the function of HLA-G. Data are presented on potential clinical applications of HLA-G including the use of evaluation of HLA-G gene polymorphisms in the diagnosis of patients experiencing recurrent pregnancy loss and evaluation and testing of soluble HLA-G (sHLA-G) in embryo culture media for the selection of embryos for transfer after in vitro fertilization (IVF).

RESULTS

The literature supports a central role of HLA-G for successful implantation. Of couples experiencing recurrent pregnancy loss, 32% demonstrated the -1725G HLA-G polymorphism. Our data showed that when embryos were selected for transfer after IVF based on culture media concentrations of sHLA-G > or = 2 U/ml and good morphologic grade, a 65% pregnancy rate compared with a 0% pregnancy rate in those with <2 U/ml sHLA-G.

CONCLUSIONS

HLA-G is important for successful implantation in human beings. The HLA-G -725 promoter polymorphism is a risk factor for recurrent miscarriage. Measurement of sHLA-G in embryo culture media can help select embryos for transfer after IVF allowing fewer embryos to be transferred in an attempt to lower multiple gestation rates.

Quantification and identification of soluble HLA-G isoforms

In order to clarify the diagnostic relevance of soluble human leukocyte antigen-G (sHLA-G) molecules, reliable methods for the measurement of sHLA-G in various body fluids are of interest. Therefore, the aims of the 'Wet-Workshop for Quantification of Soluble HLA-G' held in Essen, Germany (at the Institute of Immunology, 18-20 October 2004) were to select and to validate HLA-G-specific enzyme-linked immunosorbent assay (ELISA) formats and purified standard HLA-G proteins, which can be easily generated and used as consensual references. We chose two ELISA formats, one for the simultaneous determination of shed HLA-G1 + sHLA-G5 (sHLA-G1 + G5) and one for the exclusive detection of HLA-G5 molecules. The first ELISA uses the antibody pair monoclonal antibody (mAb) MEM-G/9 + anti-beta2-microglobulin (beta2m), whereas the latter uses mAbs 5A6G7 + W6/32. Purified and well-defined HLA-G5 protein derived from insect SF9 cells transfected with HLA-G5 + human beta2m served as standard reagent. Twenty-five members of 13 international laboratories participated in the 3-day Wet-Workshop. The workshop demonstrated that the HLA-G5 protein was equally detected by both ELISA formats allowing direct comparison of quantitative results obtained by these two ELISA formats, and that sHLA-G1 + G5 and HLA-G5 molecules, respectively, were specifically and reproducibly quantified by the two ELISA formats. The comparison of the two ELISA results obtained allows the conclusion that sHLA-G1 and HLA-G5 molecules can exist in the blood of healthy donors. Moreover, there was evidence for a novel soluble HLA-G structure recognized by the mAbs 5A6G7 + W6/32 antibody combination but not by the one of mAb MEM-G/9 + anti-beta2m.

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.

Report of the Wet Workshop for Quantification of Soluble HLA-G in Essen, 2004

Membrane-anchored and soluble human leukocyte antigen HLA-G (sHLA-G) molecules exert strong inhibiting signals after interaction with their cognate receptors ILT2 (CD85j), ILT4 (CD85d), and KIR2DL4 (CD158d) that are differentially expressed by natural killer cells, T cells, and antigen-presenting cells. These inhibitory functions can become operative in conditions in which such immune cells try to attack viral infected or tumor cells. Recently, clinical studies showed that sHLA-G molecules are also relevant in the prediction of allograft acceptance after heart transplantation, liver-kidney cotransplantation, and the successful implantation and development of embryos after in vitro fertilization. In view of this diagnostic potential, reliable methods for the measurement of sHLA-G molecules in various body fluids are of interest. Thus, the aims of the Wet Workshop for measurement of sHLA-G held in Essen, Germany (at the Institute of Immunology October 18-20, 2004) were to select and validate HLA-G-specific enzyme-linked immunosorbent assay (ELISA) formats and purified standard HLA-G proteins, which can be easily generated and used as consensual references. To this end, the antibody combinations monoclonal antibody (mAb) MEM-G/9 (capture) + anti-beta2m (detection) and the mAb 5A6G7 (capture) + mAb W6/32 (detection) were chosen in an ELISA format for the simultaneous determination of shed HLA-G1 + soluble HLA-G5 (sHLA-G1 + HLA-G5) and for the exclusive detection of HLA-G5 molecules, respectively. As standard, protein HLA-G5 molecules were purified from insect SF9 cells coinfected by HLA-G5 + human beta2m and characterized for their antigenic determinants. A total of 24 members in 13 teams participated in the 3-day sHLA-G Wet Workshop. All workshop materials, protocols, standard reagents, and samples were provided to each team by the organizers. The Wet-Workshop results clearly demonstrated that (1) the HLA-G5 standard reagent was equally detected by both ELISA formats; (2) sHLA-G1 + G5 and HLA-G5 molecules, respectively, were specifically detected by the two ELISA formats; and (3) both ELISA formats measure reproducibly the amounts of sHLA-G. The comparison of the two ELISA results obtained evidenced that in healthy donors sHLA-G1 molecules can exist in body fluids besides HLA-G5. Moreover, a novel soluble HLA-G structure can be predicted that is recognized by the mAb 5A6G7 + mAb W6/32 antibody combination, but not by the one of mAb MEM-G/9 + anti-beta2m.

Soluble HLA measurement in saliva and cerebrospinal fluid in Caucasian patients with multiple sclerosis: a preliminary study

Background

Measurement of soluble HLA in body fluids has a potential role in assessing disease activity in autoimmune disorders.

Methods

We applied a solid phase, enzyme-linked immunoassay to measure soluble HLA class I (sHLA-I) and class II (sHLA-II) molecules in the saliva and cerebrospinal fluid (CSF) in 13 untreated patients with relapsing-remitting form of multiple sclerosis (MS). For comparison purposes, we also studied saliva from 53 healthy subjects.

Results

Saliva from normal controls had detectable sHLA-I levels in 41 of 53 individuals studied, with values ranging from 9–100 ng/ml (mean = 41 ± 2.8 ng/ml). sHLA-I was undetectable in the saliva in 11 of 13 MS patients, and in none of the CSF specimens. In contrast, mean sHLA-II concentration in the saliva of MS patients was significantly increased compared to controls (386 ± 52 unit/ml vs. 222 ± 18.4 unit/ml, t = 8.68, P < 0.005). The mean CSF sHLA-II level (369 ± 16 unit/ml) was equivalent to the mean sHLA-II concentration measured in saliva (mean = 386 ± 52 unit/ml) (P = 0.7). In patients with brain magnetic resonance imaging (MRI) enhancing lesions (n = 5), reflective of more active disease, CSF sHLA-II averaged 356 ± 26 unit/ml compared to 380 ± 51 in saliva. Similarly, in patients with non-enhancing lesions (n = 8), CSF sHLA-II averaged 377 ± 18 unit/ml compared to 390 ± 77 unit/ml in saliva. Thus, the mean sHLA-II concentration in saliva and CSF was essentially equivalent for MS patients with or without enhancing plaques.

Conclusion

Our data suggest that the measurement of soluble HLA in saliva, specifically sHLA-II, correlates with the level found in the CSF. Therefore, if sHLA correlates with disease activity in MS, as has been proposed, saliva measurements provide a noninvasive correlate of CSF measurement.

Detection in chick embryo of fetoproteins not recognized by the dam's immune system and of soluble alloantigens. Presumptive teratogenic and abortogenic capacity of their specific IgY

BACKGROUND

The aim of this work was to detect antigens, non-self to the dam, potentially present in chick embryo prior to organogenesis with a view to establishing the consequences of their neutralization on chick development. To this end, hens were immunized with the extract from embryos incubated for 53 h. Their eggs were either used to isolate immunoglobulins for dot and blot tests or incubated for variable lengths of time.

RESULTS

Immunoblot tests, using adsorbed primary and secondary antibodies against paternal serum, revealed the presence of at least four antigens of 32, 34, 70 and 200 kDa that can be classified as soluble alloantigens. The same antibodies against chick embryo extracts (between 53 h and 9) showed at least five aged antigens of 34, 52, 90, 200 and 250 kDa, not detected in cock serum, that can thus be considered as soluble, foreign to the immunized hens and transitory antigens. The abnormalities observed included arrested development and fetal death, as well as minor functional damage in the few chicks that were born alive. The ratio of abnormal to normal embryos was 2.85 in the experimental group and 0.43 in the control group. With regard to congenital anomalies it must be said that of the 81 eggs incubated only four chicks were born alive, and of these, only one had a healthy birth and subsequent growth. The other three showed a transitory ataxia and one of them presented adult lumbar scoliosis and asymmetric pelvis.

CONCLUSIONS

The problem of recurrent spontaneous abortions is revisited in the light of these results. Some recent data suggest that soluble alloantigens may be candidates for a new etiological entity in recurrent spontaneous abortions. They can also be the cause of some congenital anomalies. The soluble, foreign, transitory antigens may have a similar effect although there is no supportive data in the literature.

HLA and in vitro susceptibility to HIV infection

Since the beginning of the epidemic of acquired immunodeficiency syndrome (AIDS), evidence has accumulated that genetic factors are involved in the pathogenesis of the disease. Among the candidate molecules suspected to control susceptibility to human immunodeficiency virus (HIV) infection and fast or slow progression of AIDS are the human leukocyte antigens (HLA). Investigating HLA-HIV correlation was mainly performed by investigating cohorts of patients with HIV/AIDS for a period of time and then statistically correlating certain features of AIDS with particular HLA alleles. This type of in vivo investigations resulted in the generation of an immense literature on the subject with so many HLA alleles being found to correlate with certain features of AIDS. Because of the complexity of AIDS pathogenesis and the involvement of many factors in the disease process, the different investigators failed to agree on the involvement of certain HLA molecules in AIDS pathogenesis. Some reports deny, or fail to confirm the existence of association between HLA and HIV progression to AIDS. This is probably due to the different ethnic backgrounds of the populations studied, the complexity of the HLA system, and to the ways these studies were conducted. This article briefly addresses our current knowledge on the HLA-HIV correlation in vivo and discusses alternative in vitro studies.

Intercellular transfer of antigen-presenting cell determinants onto T cells: molecular mechanisms and biological significance

Upon physiological stimulation, receptors with tyrosine kinase activity (RTK) are rapidly internalized together with their soluble ligands. T cell activation is the consequence of recognition by the T cell receptor (TCR) of specific peptide-major histocompatibility protein complexes (peptide-MHC) present at the membrane of antigen-presenting cells (APC). The TCR belongs to the RTK family and is known to be endocytosed upon ligand recognition. It differs from most other RTK in that its ligand, the peptide-MHC complex, is membrane bound and the TCR-ligand interaction is quite weak. Recent experiments have shown that the TCR ligand becomes internalized by T cells upon stimulation. Here we review current knowledge on the molecular mechanisms by which the membrane-bound MHC molecules can be transferred onto T cells, and propose hypotheses on the role this phenomenon could play in physio-pathological situations involving T cells.

In vitro immunosuppressive activity of soluble HLA class I and Fas ligand molecules: do they play a role in autologous blood transfusion?

BACKGROUND

The immunomodulatory effects of allogeneic blood transfusion may contribute to a poor prognosis in patients with cancer who are undergoing surgery, and clinical trials have been carried out to investigate whether these patients would benefit from autologous blood donation. As the immunomodulatory effects of allogeneic blood transfusion have been related to soluble molecules released from residual WBCs during storage, the in vitro immunomodulatory activity of soluble molecules detected in supernatants from stored autologous blood was evaluated.

STUDY DESIGN AND METHODS

Blood was donated by four healthy volunteers. Packed WBC-reduced RBCs were obtained and stored for 30 days, and supernatants were collected. FFP and serum were also obtained. The concentration of soluble molecules was determined by immunoenzymatic assays. The in vitro immunomodulatory activity of undiluted blood component supernatant was assessed by antigen-specific cytotoxic T-cell activity and mixed lymphocyte reactions in autologous combinations and by apoptosis induction in Fas+ cells.

RESULTS

The concentrations of soluble Fas-ligand and HLA class I molecules were higher in packed RBCs than in WBC-reduced RBCs, FFP, and serum. Undiluted supernatants of packed RBCs strongly inhibited functional assays and induced apoptosis in Fas+ cells. The immunomodulatory effects were correlated with the amount of soluble Fas ligand and HLA class I molecules.

CONCLUSION

The results of the present study are comparable with those already reported in allogeneic blood components, and they indicate that undiluted supernatants of autologous blood components may exert immunosuppressive effects in vitro.

Association of soluble HLA-G plasma levels with HLA-G alleles

Soluble HLA-G (sHLA-G) molecules are found in the peripheral blood of healthy females and males, in cord blood and in amniotic fluids and discussed to be a mediator in maternal-fetal tolerance. In this study we investigated whether there are allele-specific differences in expression of sHLA-G molecules. For this, the sHLA-G plasma concentrations of 94 healthy unrelated individuals were measured by ELISA and correlated to their HLA-G genotypes, as determined by sequence analysis of exon 2 and 3 of the HLA-G gene. Mean sHLA-G levels in individuals with the most common HLA-G alleles G*01011 (27.0+/-2.1 SEM ng/ml, n=66), G*01012 (28.4+/-3.2 SEM ng/ml, n=34) were very similar. In contrast, individuals carrying the HLA-G*01013 (8.1+/-1.7 SEM ng/ml, n=17) or the "null" allele HLA-G*0105N (8.2+/-3.2 SEM ng/ml, n=7) presented significantly (P(c)=0.001 and P(c)<0.01, resp.) reduced sHLA-G levels. Furthermore, individuals with the HLA-G*01041 allele had significantly (P(c)=0.004) increased sHLA-G levels (42.5+/-4.6 SEM ng/ml, n=14). These results demonstrate that the generation of sHLA-G molecules is associated to certain HLA-G alleles and imply that sHLA-G levels are under genetic control. As low and high sHLA-G plasma levels did not segregate with HLA haplotypes including the HLA-G*01013 or *01041 allele, additional mechanisms may be involved in the regulation of the individual sHLA-G levels. Nevertheless, the existence of "low" and "high secretor" HLA-G alleles further suggests different levels of functionality in immune regulation.

Soluble beta2-mu-associated and beta2-mu-free HLA class I heavy chain serum levels in interferon-alpha nonresponder chronic hepatitis C patients. Markers of immune activation, and response to antiviral retreatment

The serum levels of soluble beta2-mu-associated and beta2-mu-free HLA class I heavy chains were determined in 28 interferon-alpha nonresponder chronic hepatitis C patients retreated with interferon-alpha plus ribavirin and in 70 healthy subjects. The baseline levels of beta2-mu-associated and beta2-mu-free HLA class I heavy chains were significantly higher in patients than in healthy controls (P = 0.001). The levels of beta2-mu-associated HLA class I heavy chains significantly increased in responder patients with respect to nonresponders at the third month of treatment (P = 0.03). At the sixth month of treatment and after 6 months of follow up the levels of beta2-mu-associated HLA class I heavy chains decreased in responder patients and increased in nonresponders. The levels of beta2-mu-free HLA class I heavy chains showed only minor changes during and after treatment. We suggest that the determination of hepatitis C virus RNA levels combined with soluble beta2-mu-associated HLA class I heavy chains, as a marker of immune activation, could identify interferon-alpha non responder chronic hepatitis C patients most likely to respond to a retreatment with interferon-alpha plus ribavirin.

Soluble HLA class I and Fas ligand molecules in blood components and their role in the immunomodulatory effects of blood transfusions

It has been known for many years that blood transfusions may have immunomodulatory effects, however an ultimate explanation of this phenomenon is lacking. In the present paper we report that the concentrations of soluble HLA class I (sHLA-I) and soluble Fas ligand (sFasL) molecules in supernatants of blood components which contain elevated numbers of residual donor leukocytes, like red blood cells and random-donor platelets, are significantly higher than in other blood components. Elevated amounts of sFasL molecules are also found in some commercial immunoglobulin preparations. sHLA-I and sFasL molecules in blood components and in immunoglobulin preparations are biologically active in vitro as they inhibit mixed lymphocyte responses and cytotoxic T cell activity in allogeneic and autologous combinations and induce apoptosis in Fas-positive cells. If these results are paralleled in vivo the amount of sHLA-I and sFasL molecules should be taken into account in clinical practice in order to select the blood component and the immunoglobulin preparation which could induce the desired immunomodulatory effect in the recipient.

Increased beta2-microglobulin-free HLA class I heavy chain serum levels in the course of immune responses to viral antigens and to mismatched HLA antigens

Besides being present in serum in association with beta2-mu, HLA class I heavy chains are also present in serum as beta2-micro-free moieties. The increase in serum levels of beta2-micro-associated HLA class I heavy chains in conditions associated with an activation of the immune system have prompted us to measure the serum levels of beta2-mu-free HLA class I heavy chains in the course of immune responses to viral antigens and to mismatched histocompatibility antigens. The serum level of beta2-mu-free HLA class I heavy chains, like that of beta2-mu-associated HLA class I heavy chains was significantly increased in patients affected by advanced HIV-1 infection or by chronic hepatitis C (CHC). In the latter group of patients an association was found between a reduction in the beta2-mu-free HLA class I heavy chain serum level and response to therapy with interferon alpha and ribavirin. Moreover, the beta2-mu-free HLA class I heavy chain serum level was increased more than that of beta2-mu-associated HLA class I heavy chains during episodes of liver ischemia following liver transplantation and in the course of acute graft rejection and of acute graft-versus-host-disease (GVHD) after allogeneic bone marrow transplantation (BMT). These results suggest that the serum levels of beta2-mu-free and beta2-mu-associated HLA class I heavy chains are independently regulated. Furthermore, beta2-mu-free HLA class I heavy chain serum level may be a useful marker to monitor response to therapy in CHC patients and the clinical course of liver and bone marrow grafts.

Liver transplant recipient sera derived soluble HLA mediates allele specific CTL apoptosis

BACKGROUND

Significant levels of donor soluble human leukocyte antigen (HLA) class I (sHLA) are present in patients after transplants. We investigated the possibility that sHLA may inhibit cytolytic T lymphocyte (CTL) activity by inducing apoptosis of the CTL, thereby serving as a mechanism for specific tolerance.

METHODS

sHLA-A2 and A3 were isolated from the sera of liver transplant recipients by affinity chromatography. T cell bulk lines directed against HLA-A2 and HLA-A3 were generated by stimulation with HLA-A2, A3+ peripheral blood leukocytes and B-lymphoblastoid cells. Induction of T cell apoptosis by sHLA was analyzed by adding sHLA to allospecific CTL 4 or for 24 hr before flow cytometric analysis of propidium iodide and fluorescein isothiocyanate-conjugated annexin V stained cells. T cell receptor (TCR) engagement by sHLA was demonstrated using a monoclonal antibody specific for the TCR.

RESULTS

sHLA-A3 inhibited CTL activity of a HLA-A3 T cell line by 53%, whereas sHLA-A2 had no effect. sHLA-A3 also increased T cell death by 77% over the control, whereas sHLA-A2 had no significant effect. However, sHLA-A2 induced 21% apoptosis of an anti-HLA-A2 T cell line, whereas sHLA-A3 caused only 3% apoptosis. The antibody complexed form of sHLA was ineffective in the induction of apoptosis. Preincubation of the T cells with anti-T cell receptor monoclonal antibody protected the T cells from sHLA-induced apoptosis, indicating that sHLA-TCR engagement is necessary for this process to occur.

CONCLUSION

TCR-mediated apoptosis of alloreactive CTL may serve as a mechanism by which sHLA can modulate the immune response.

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.

Monitoring of soluble HLA class I size variants after liver transplantation

To monitor soluble HLA class I (sHLA-I) and their size variants after liver transplantation (LTX) plasma samples from 22 LTX patients were studied by sHLA-I ELISA, SDS-PAGE, and densitometry. Samples collected were classified into three groups: Group 1 comprised samples taken during episodes without complications, group 2 during episodes of cholangitis/cholestasis (CC), and group 3 during episodes of acute rejection (AR). Compared to group 1 (0.27 +/- 0.03 SEM microg/ml) mean sHLA-I increments in groups 2 and 3 were with 0.53 +/- 0.05 SEM microg/ml and 0.47 +/- 0.04 SEM microg/ml increased (p < 0.001). The same samples were studied by SDS-PAGE and the 43, 39, and 35 kD sHLA-I variants were quantified densitometrically. In samples of group 1 ratios of 43 vs. 39 kD bands revealed a mean of 2.1 +/- 0.3, whereas in group 2 and 3 these were only 0.8 +/- 0.1 SEM and 0.9 +/- 0.1 SEM, respectively, (p < 0.001). For the relation between 43 and 35 kD variants a reduced ratio of 1.1 +/- 0.2 SEM was confined to group 3 samples (p < 0.001), as groups 1 and 2 had ratios of 13.4 +/- 2.3 SEM and 8.4 +/- 2.9 SEM, respectively. This indicates that elevated sHLA-I levels during CC or AR are mainly caused by increases of 39 and/or 35 kD sized molecules. Therefore, our study demonstrates, that after LTX the contribution of sHLA-I size variants to total sHLA-I amounts changes drastically during immune activation pointing to different mechanisms of sHLA-I release.

Detection and characterization of HLA class I molecules in the supernatant of an hepatocarcinoma cell line and of EBV-transformed B cell lines

Human leukocyte antigens (HLA) class I molecules can be detected in "soluble" form in the supernatant of cultured cell lines and in serum and plasma of humans. These "soluble" HLA class I molecules are assumed to play a role in liver transplantation. In order to define the nature and composition of HLA class I molecules found in solution, we studied the HLA class I production of an hepatoma carcinoma cell line (HepG2) and of EBV-transformed B-cell lines. Based on molecular weight (MW) analysis, it was demonstrated that different forms of HLA class I molecules were produced by HepG2 cells and EBV B-cells. Monoclonal antibodies (mAbs) specific for HLA class I alleles were able to recognize the mature 45 kDa form, but failed to interact with the 42 kDa and 39 kDa MW forms of HLA class I. Of these different MW forms of HLA class I molecules the mature 45 kDa product was found predominantly to be associated with subcellular vesicles whereas the alternative MW forms of 42 kDa and 39 kDa exist as truly free entities in supernatants.

Soluble HLA Class I, HLA Class II, and Fas Ligand in Blood Components: A Possible Key to Explain the Immunomodulatory Effects of Allogeneic Blood Transfusions

The immunomodulatory effect of allogeneic blood transfusions (ABT) has been known for many years. However, a complete understanding of the effects of ABT on the recipient’s immune system has remained elusive. Soluble HLA class I (sHLA-I), HLA class II (sHLA-II), and Fas ligand (sFasL) molecules may play immunoregulatory roles. We determined by double-determinant immunoenzymatic assay (DDIA) sHLA-I, sHLA-II, and sFasL concentrations in different blood components. sHLA-I and sFasL levels in red blood cells (RBCs) stored for up to 30 days and in random-donor platelets are significantly (P < .001) higher than in other blood components and their amount is proportionate to the number of residual donor leukocytes and to the length of storage. Blood components with high sHLA-I and sFasL levels play immunoregulatory roles in vitro as in allogeneic mixed lymphocyte responses (MLR) and antigen-specific cytotoxic T-cell (CTL) activity, and induce apoptosis in Fas-positive cells. These data suggest that soluble molecules in blood components are functional. If these results are paralleled in vivo, they should be taken into account in transfusion practice. Blood components that can cause immunosuppression should be chosen to induce transplantation tolerance, whereas blood components that lack immunosuppressive effects should be preferred to reduce the risk of postoperative complications and cancer recurrence.

Beta2-micro-free HLA class I heavy chain levels in sera of healthy individuals. Lack of association with beta2-micro-associated HLA class I heavy chain levels and HLA phenotype

We have applied a double-determinant immune assay (DDIA) to measure soluble beta2-microglobulin (beta2-micro)-free HLA class I heavy chains in serum. The mean concentration of beta2-micro-free HLA class I heavy chains in serum from 120 healthy subjects was 0.21+/-0.24 microg/ml. The individual serum levels of beta2-micro-free HLA class I heavy chains had a wide distribution, did not seem to be related with HLA phenotype, were stable over time and did not change with age. The serum levels of soluble beta2-micro-free HLA class I heavy chains did not correlate with those of soluble beta2-micro-associated HLA class I heavy chains, suggesting that their release is independently regulated. Three forms of soluble beta2-micro-free HLA class I heavy chains, with apparent molecular masses of 44, 39 and 37-35 kD, respectively, circulate in human serum. These results provide a useful background to assess the serum level of soluble beta2-micro-free HLA class I heavy chains in pathological conditions and to evaluate their putative immunoregulatory function.

Soluble HLA Class I, HLA Class II, and Fas Ligand in Blood Components: A Possible Key to Explain the Immunomodulatory Effects of Allogeneic Blood Transfusions

The immunomodulatory effect of allogeneic blood transfusions (ABT) has been known for many years. However, a complete understanding of the effects of ABT on the recipient’s immune system has remained elusive. Soluble HLA class I (sHLA-I), HLA class II (sHLA-II), and Fas ligand (sFasL) molecules may play immunoregulatory roles. We determined by double-determinant immunoenzymatic assay (DDIA) sHLA-I, sHLA-II, and sFasL concentrations in different blood components. sHLA-I and sFasL levels in red blood cells (RBCs) stored for up to 30 days and in random-donor platelets are significantly (P &lt; .001) higher than in other blood components and their amount is proportionate to the number of residual donor leukocytes and to the length of storage. Blood components with high sHLA-I and sFasL levels play immunoregulatory roles in vitro as in allogeneic mixed lymphocyte responses (MLR) and antigen-specific cytotoxic T-cell (CTL) activity, and induce apoptosis in Fas-positive cells. These data suggest that soluble molecules in blood components are functional. If these results are paralleled in vivo, they should be taken into account in transfusion practice. Blood components that can cause immunosuppression should be chosen to induce transplantation tolerance, whereas blood components that lack immunosuppressive effects should be preferred to reduce the risk of postoperative complications and cancer recurrence.

Detection of soluble HLA-G molecules in plasma and amniotic fluid

Although the cDNA sequence of HLA-G antigens is compatible with their expression as soluble molecules (sHLA-G), the determination of native sHLA-G levels in body fluids has not yet been described. The lack of this information is likely to reflect the difficulties in developing an assay suitable to measure sHLA-G antigens in the presence of soluble HLA-A, -B and -C (sHLA-I) antigens, since most of the available anti-HLA-G mAb do not detect soluble beta2-m associated HLA-G antigens or crossreact with sHLA-I antigens. Therefore, we have developed a two-step assay which eliminates the interference of classical HLA class I antigens. In the first step, the sample is depleted of sHLA-I antigens and of HLA-E antigens with mAb TP25.99. Then, HLA-G antigens are captured with mAb W6/32 and detected with anti-beta2-m mAb in ELISA. Utilizing this assay, sHLA-G antigen levels were measured in EDTA plasma from 92 controls with known HLA types, 28 women at delivery and the corresponding cord bloods and in 50 amniotic fluids. Mean sHLA-G plasma levels did not differ between males (24.9+/-3.0 SEM ng/ml; n=42) and females (20.1+/-2.1 SEM ng/ml; n = 50). However, sHLA-G levels in HLA-A11 positive probands (mean: 13.0+/-4.4 SEM ng/ml; n=12) were significantly (P<0.05) lower than in HLA-A11 negative ones (mean: 24.5+/-2.0 SEM ng/ml; n=80). sHLA-G levels in women at delivery (mean: 22.9+/-2.2 SEM ng/ml; n=28) were in the range of controls but were significantly (P<0.001) reduced in the corresponding cord bloods (mean: 13.8+/-1.5 SEM ng/ml; n=28). sHLA-G levels in amniotic fluids (mean: 15.5 + 1.0 SEM ng/ml; n=50) were significantly (P<0.001) lower than in plasma. sHLA-G levels were 5 and 11% of those of sHLA-I antigens in plasmas and amniotic fluids, respectively. Individual sHLA-G levels were not correlated with sHLA-I levels. SDS-PAGE analysis of plasma sHLA-G antigens revealed two molecular variants with a 35 kD and a 27 kD MW corresponding to the sizes of sHLA-G1 and -G2 isoforms. In conclusion, our study has shown that the two-step assay we have developed is reliable in measuring sHLA-G antigen levels. This assay will facilitate the analysis of the biological and clinical significance of sHLA-G antigens in plasma.

Soluble HLA-I in rheumatic diseases

OBJECTIVE

To study serum levels of Class I soluble HLA (sHLA-I) in patients with systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), polymyositis or dermatomyositis (PM/DM) or scleroderma and to assess the possible influence of ethnic factors on concentration in each disease group.

METHODS

Solid-phase enzyme linked immunoassay was used to measure sHLA-I in the serum of 385 patients with varied ethnic backgrounds (American-Caucasians, African-Americans, Georgian-Caucasians) with rheumatic diseases. Studies on patients were compared to similar measurements of 189 healthy individuals.

RESULTS

Mean sHLA-I levels were significantly higher in patients with SLE than those observed in healthy individuals or other rheumatic diseases. Highest concentrations were present in Georgian-Caucasian patients with SLE. American-Caucasian patients with RA or scleroderma had higher sHLA-I levels than normal Caucasian individuals. The majority of patients with PM/DM in all ethnic subgroups were low secretors of sHLA-I.

CONCLUSION

Mechanisms underlying the secretion of sHLA-I appear to differ among the rheumatic diseases studied and various ethnic groups. These genetic differences in sHLA-I secretion could be associated with ethnic and pathophysiologic differences among these rheumatic diseases.

Two distinct forms of soluble MHC class I molecules synthesized by different mechanisms in normal rat cells in vitro

Rat soluble MHC class I synthesis was studied at both RNA and protein levels to determine whether multiple forms of soluble MHC class I molecules are produced by different mechanisms. RT-PCR and sequencing of MHC class I transcripts identified an alternatively spliced nonclassical MHC class I gene product, lacking both exon 5 and 6, in both spleen and liver. Immunoprecipitation and SDS-PAGE identified two distinct soluble MHC class I proteins in both splenocyte- and hepatocyte-culture supernatants. The 36Kd classical soluble MHC class I protein (RT1.Aa) was precipitated by both allele-specific (MN4.91.6, R3/13, R2/15S) and pan-reactive (OX18) mAbs. The 39Kd non-RT1.A soluble MHC class I protein was precipitated only by OX18. The production of soluble RT1.Aa was inhibited by a metalloproteinase inhibitor, but not by serine/thiol protease inhibitors. None of these protease inhibitors interfered with the soluble non-RT1.A production, suggesting that it might be derived from an alternatively spliced MHC class I transcript. The soluble non-RT1.A was always associated with beta2m. However, soluble RT1.Aa molecule was cleaved in beta2m-free form and was reassociated with beta2m in culture supernatants. Thus two soluble MHC class I molecules, classical (36Kd RT1.Aa) and nonclassical (the alternatively spliced transcript), were produced from rat cells. Alternative splicing led to the nonclassical soluble MHC class I synthesis. Proteolytic cleavage by metalloproteinase led to the classical soluble MHC class I synthesis.

Increased level of serum HLA class I antigens in patients with systemic lupus in patients with systemic lupus erythematosus. Correlation with disease activity

The level of soluble beta2-mu-associated HLA Class I heavy chains (sHLA-I) and of soluble beta2-mu-free HLA Class I heavy chains (sHLA-FHC) was found to be significantly higher in sera from 58 patients with systemic lupus erythematosus (SLE) than in those from 82 age and sex-matched controls. The level of serum sHLA-I in patients with SLE was significantly correlated to disease activity. Western blotting analysis showed that the 44-kDa isoform represents the major component in the antigens immunoprecipitated by anti-beta2-mu mAb NAMB-1 and by anti-beta2-mu-free HLA Class I heavy chain mAb HC-10 from sera of patients with SLE. These results suggest that the increased serum levels of sHLA-I and of sHLA-FHC in patients with SLE reflect their increased shedding from cell membrane. In view of the ability of sHLA-I and of sHLA-FHC to induce apoptosis of activated T cells, it is suggested that their increased serum levels in patients with SLE is triggered by dysregulation of the immune system leading to T-cell activation. The increased serum levels of sHLA-I and of sHLA-FHC may be used by the immune system to control the pool of activated T cells by inducing apoptosis. If this possibility is proven to be correct, modulation of the serum level of sHLA-I and of sHLA-FHC may be utilized to develop strategies to treat SLE.

The functional transformation of cytotoxic lymphocytes into T-suppressors under the influence of two mediators

A set of CTL-lines in the system differing in the whole H-2 complex, as well as in distinct subregions of H-2, was established. These lines were routinely tested in Cr51-release assay to insure that they retained lytic activity and antigen-specificity. The result of the interaction of CTL with H-2Kb-molecule and alpha1-thymosine was transition of CTL into a reversible state of anergy. A high concentration of alpha1-thymosine is necessary for this transition. Anergic CTL caused specific suppressor activity that is functional if transformation of CTL into T-suppressors took place.

Soluble HLA class I antigens in patients with type I diabetes and their family members

Our objective was to study a possible contribution of MHC genes to S-HLA-I secretion in patients with Type I diabetes. Quantitatively, we used a highly sensitive enzyme-linked immunoassay to measure S-HLA-I in the serum of a total of 39 patients with Type I diabetes, as well as 36 kinships of 12 diabetic patients and 82 normal individuals with known HLA-phenotypes. S-HLA-I levels were abnormally elevated in patients or their non-diabetic relatives compared to normal controls (p < 0.0009). No complete HLA-haplotype had been identified to be correlated with high or low S-HLA-I secretion. Only the HLA-A23 or A24 (splits of HLA-A9) positive individuals sera were found to contain high S-HLA-I concentrations in all populations studied. The difference in S-HLA-I levels of HLA-A24 patients (n = 4) or their HLA-A24 positive non-diabetic relatives (n = 10) to the group of HLA-A24 normal controls (n = 15) was statistically highly significant (p < 0.0005 and p < 0.0009, respectively). The results suggests that HLA-A24 may confer additional independent risk for the disease expression in male children but not in female siblings. Nevertheless, the data implies that the patients or their non-diabetic relatives carrying the HLA-A24 have increased risk of developing ICA associated with high S-HLA-I levels compared to HLA-A24 negative probands or their kinships with low levels of S-HLA-I. This effect occurred irrespective to other diabetes related HLA-DR alleles. In summary, the results show a pronounced genetic heterogeneity of Type I diabetes with MHC control of the expression of S-HLA-I and possible involvement of hormonal factors that might potentiate a specific synthesis of S-HLA-I. The findings have implications for identifying individuals with a possible risk for developing the disease.