MHC class II gene silencing in trophoblast cells is caused by inhibition of CIITA expression

Molecular evolution of elements controlling HLA-C expression: Adaptation to a role as a killer-cell immunoglobulin-like receptor ligand regulating natural killer cell function

The regulatory elements controlling the transcription of the HLA-A, HLA-B, and HLA-C genes have been extensively studied and compared. However, few studies have considered regulatory differences in the HLA genes from the perspective of their role as ligands for the killer-cell immunoglobulin-like receptor (KIR) family of HLA receptors expressed by natural killer (NK) cells. HLA-C is the most recently evolved gene, and there is considerable evidence pointing to its emergence as a specialized KIR ligand playing a major role in the missing-self recognition system of NK cells. Here I evaluate gene-specific differences in regulatory elements of the HLA genes, showing alterations that are consistent with the adaptation of HLA-C to a role in NK cell regulation.

Identification of trophoblast-specific elements in the HLA-C core promoter

There are several aspects of HLA-C gene expression that distinguish it from the HLA-A and HLA-B genes. First, HLA-C is expressed by extravillous trophoblasts, whereas HLA-A and HLA-B are not. Second, its cell-surface expression is much lower, which has been linked to changes in transcription and efficiency of peptide loading and export. Third, HLA-C possesses a NK cell-specific promoter and a complex alternative splicing system that regulates expression during NK cell development. In this study, we investigate the contribution of the HLA-C core promoter to trophoblast-specific expression. Analysis of transcription start sites showed the presence of a trophoblast-associated start site and additional upstream TATA and CCAAT-box elements in the HLA-C promoter, suggesting the presence of an overlapping trophoblast-specific promoter. A comparison of in vitro promoter activity showed that the HLA-C promoter was more active in trophoblast cell lines than either the HLA-A or HLA-B promoters. Enhanced trophoblast activity was mapped to the central enhanceosome region of the promoter, and mutational analysis identified changes in the RFX-binding region that generated a trophoblast-specific enhancer.

Genomewide association study of HLA alloimmunization in previously pregnant blood donors

BACKGROUND

Alloimmunization through blood transfusion, transplantation, or circulating fetal cells during pregnancy is a significant concern. Some exposed individuals make alloantibodies while others do not, implying variation in genetic risk factors.

STUDY DESIGN AND METHODS

We conducted a genomewide association study (GWAS) of 9,427,497 single-nucleotide polymorphisms (SNPs) to identify genetic variants for HLA alloimmunization in previously pregnant blood donors with (n = 752) and without (n = 753) HLA Class I or II alloantibodies.

RESULTS

A SNP in the neurexophilin 2 (NXPH2) gene surpassed genome-wide significance (p = 2.06 × 10^-8 ), with multiple adjacent markers p < 10^-6 , for women with anti-Class I alloantibodies only. Little is currently known about the function of NXPH2, although gene family members have been shown to impact immunity. SNPs in the E2F7 gene, a transcription factor related to cell cycle control and cellular proliferation, also approached genomewide significance (p = 2.5 × 10^-7 ).

CONCLUSION

Further work to extend the GWAS approach and to characterize variants in NXPH2 and E2F7 in the context of alloantibody formation is warranted.

Histone deacetylase inhibitors activate CIITA and MHC class II antigen expression in diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL), the most common form of non-Hodgkin's lymphoma (NHL) diagnosed in the USA, consists of at least two distinct subtypes: germinal centre B (GCB) and activated B-cell (ABC). Decreased MHC class II (MHCII) expression on the tumours in both DLBCL subtypes directly correlates with significant decreases in patient survival. One common mechanism accounting for MHCII down-regulation in DLBCL is reduced expression of the MHC class II transactivator (CIITA), the master regulator of MHCII transcription. Furthermore, reduced CIITA expression in ABC DLBCL correlates with the presence of the transcriptional repressor positive regulatory domain-I-binding factor-1 (PRDI-BF1). However, the mechanisms underlying down-regulation of CIITA in GCB DLBCL are currently unclear. In this study, we demonstrate that neither PRDI-BF1 nor CpG hypermethylation at the CIITA promoters are responsible for decreased CIITA in GCB DLBCL. In contrast, histone modifications associated with an open chromatin conformation and active transcription were significantly lower at the CIITA promoters in CIITA(-) GCB cells compared with CIITA(+) B cells, which suggests that epigenetic mechanisms contribute to repression of CIITA transcription. Treatment of CIITA(-) or CIITA(low) GCB cells with several different histone deacetylase inhibitors (HDACi) activated modest CIITA and MHCII expression. However, CIITA and MHCII levels were significantly higher in these cells after exposure to the HDAC-1-specific inhibitor MS-275. These results suggest that CIITA transcription is repressed in GCB DLBCL cells through epigenetic mechanisms involving HDACs, and that HDACi treatment can alleviate repression. These observations may have important implications for patient therapy.

Immunohistochemical analysis reveals an influx of regulatory T cells and focal trophoblastic STAT-1 phosphorylation in chronic villitis of unknown etiology

Maternal T cells and fetal macrophages constitute the primary infiltrate of chronic villitis of unknown etiology (CVUE), but the role of CD25(+)/FOXP3(+) regulatory T (Treg) cells in CVUE has not been examined. Moreover, little is known about the expression of immune markers, such as the major histocompatibility complex (MHC) class II antigen, human leukocyte antigen-DR (HLA-DR), in trophoblasts in this disease. We, therefore, examined CVUE placentas for the presence of Treg cells and aberrant activation of HLA-DR in trophoblasts. Sequential formalin-fixed, paraffin-embedded tissue sections from 8 CVUE placentas and 10 control placentas were stained by immunohistochemistry with antibodies for CD3, CD4, CD8, CD20, CD25, FOXP3, CD56, CD68, HLA-DR, STAT-1, and phosphorylated STAT-1 [P-(Y701)-STAT-1]. T cells and histiocytes were confirmed as the inflammatory infiltrate in CVUE. In areas of CVUE, histiocytes strongly expressed HLA-DR and nuclear P-(Y701)-STAT-1, and the relative numbers of CD25(+)/FOXP3(+) Treg cells were increased, compared with control placentas. In 5 of 8 CVUE cases, there was patchy nuclear expression of P-(Y701)-STAT-1 in syncytiotrophoblast most extensively involved by villitis, but no other marker examined was detected in the trophoblast cell layer. We confirmed the influx of T cells and histiocytes in CVUE. Our results are the 1st, to our knowledge, to identify increased numbers of Treg cells in CVUE vs noninflamed placentas. However, we were unable to verify HLA-DR expression in trophoblasts of placentas with CVUE, suggesting that this does not contribute to the influx of T cells. Our observation that P-(Y701)-STAT-1 expression in a syncytiotrophoblast is restricted to regions of inflammation suggests that the JAK-STAT-1 pathway is aberrantly activated in these cells.

Fetomaternal immunotolerance

Implantation of mammalian conceptus in uterine cavity is the result of evolutionary adaptation, through high level of physiological procedures to ensure its success. However the majority of pregnancy losses occur before or during implantation. It is expected that exploring and defining the molecular and physiological road map during the crucial time of implantation will enable us to decode and effectively treat fertility defects. Immunological, hormonal and molecular factors participate in the feto-maternal cross talk during implantation and designate the effectiveness of the process. The atypical expression of major histocompatibility complex and other protein-antigens, such as Fas/FasL and petformin in human trophoblast, the modified function of cellular constituents of the feto-maternal interface, as well as the specific role of some hormones and cytokines, represent substantive parameters of feto-maternal immunotolerance during implantation.

Human leucocyte antigen (HLA) expression of primary trophoblast cells and placental cell lines, determined using single antigen beads to characterize allotype specificities of anti-HLA antibodies

Human trophoblast cells express an unusual repertoire of human leucocyte antigen (HLA) molecules which has been difficult to define. Close homology between and extreme polymorphism at the classical HLA class-I (HLA-I) loci has made it difficult to generate locus-specific monoclonal antibodies (mAbs). The problem of defining an antibody's reactivity against the thousands of existing HLA-I allotypes has often made it impossible to determine the HLA bound by a mAb in biological samples from a normal outbred population. Here we have used commercially available beads coated with individual HLA-I to characterize experimentally the reactivity of nine mAb against 96 common HLA-I allotypes. In conjunction with donor HLA-I genotyping, we could then define the specific HLA molecules bound by these antibodies in normal individuals. We used this approach to analyse the HLA expression of primary trophoblast cells from normal pregnancies; the choriocarcinoma cells JEG-3 and JAR; and the placental cell lines HTR-8/SVneo, Swan-71 and TEV-1. We confirm that primary villous trophoblast cells are HLA null whereas extravillous trophoblast cells express HLA-C, HLA-G and HLA-E, but not HLA-A, HLA-B or HLA-DR molecules in normal pregnancy. Tumour-derived JEG-3 and JAR cells reflect extravillous and villous trophoblast HLA phenotypes, respectively, but the HLA repertoire of the in vitro derived placental cell lines is not representative of either in vivo trophoblast phenotype. This study raises questions regarding the validity of using the placental cell lines that are currently available as model systems for immunological interactions between fetal trophoblast and maternal leucocytes bearing receptors for HLA molecules.

Histone deacetylases inhibit IFN-gamma-inducible gene expression in mouse trophoblast cells

Trophoblast cells are the first cells to differentiate from the developing mammalian embryo, and they subsequently form the blastocyst-derived component of the placenta. IFN-gamma plays critical roles in activating innate and adaptive immunity, as well as apoptosis. In mice, IFN-gamma is produced in the pregnant uterus, and is essential for formation of the decidual layer of the placenta and remodeling of the uterine vasculature. Responses of mouse trophoblast cells to IFN-gamma appear to be selective, for IFN-gamma activates MHC class I expression and enhances phagocytosis, but fails to activate either MHC class II expression or apoptosis in these cells. To investigate the molecular basis for the selective IFN-gamma responsiveness of mouse trophoblast cells, IFN-gamma-inducible gene expression was examined in the trophoblast cell lines SM9 and M-11, trophoblast stem cells, and trophoblast stem cell-derived giant cells. IFN-gamma-inducible expression of multiple genes, including IFN regulatory factor-1 (IRF-1), was significantly reduced in trophoblast cells compared with fibroblast cells. Decreased IRF-1 mRNA expression in trophoblast cells was due to a reduced rate of IRF-1 transcription relative to fibroblast cells. However, no impairment of STAT-1 tyrosine phosphorylation or DNA-binding capacity was observed in IFN-gamma-treated mouse trophoblast cells. Importantly, histone deacetylase (HDAC) inhibitors significantly enhanced IFN-gamma-inducible gene expression in trophoblast cells, but not fibroblasts. Our collective studies demonstrate that IFN-gamma-inducible gene expression is repressed in mouse trophoblast cells by HDACs. We propose that HDAC-mediated inhibition of IFN-gamma-inducible gene expression in mouse trophoblast cells may contribute to successful pregnancy by preventing activation of IFN-gamma responses that might otherwise facilitate the destruction of the placenta.

Interferon gamma in successful pregnancies

Interferon gamma (IFNG) is a proinflammatory cytokine secreted in the uterus during early pregnancy. It is abundantly produced by uterine natural killer cells in maternal endometrium but also by trophoblasts in some species. In normal pregnancies of mice, IFNG plays critical roles that include initiation of endometrial vasculature remodeling, angiogenesis at implantation sites, and maintenance of the decidual (maternal) component of the placenta. In livestock and in humans, deviations in these processes are thought to contribute to serious gestational complications, such as fetal loss or preeclampsia. Interferon gamma has broader roles in activation of innate and adaptive immune responses to viruses and tumors, in part through upregulating transcription of genes involved in cell cycle regulation, apoptosis, and antigen processing/presentation. Despite this, rodent and human trophoblast cells show dampened responses to IFNG that reflect the resistance of these cells to IFNG-mediated activation of major histocompatibility complex (MHC) class II transplantation antigen expression. Lack of MHC class II antigens on trophoblasts is thought to facilitate survival of the semiallogeneic conceptus in the presence of maternal lymphocytes. This review describes the dynamic roles of IFNG in successful pregnancy and briefly summarizes data on IFNG in gestational pathologies.

Dampening of IFN-gamma-inducible gene expression in human choriocarcinoma cells is due to phosphatase-mediated inhibition of the JAK/STAT-1 pathway

Trophoblast cells (TBCs) form the blastocyst-derived component of the placenta and play essential roles in fetal maintenance. The proinflammatory cytokine IFN-gamma plays a central role in activating cellular immunity, controlling cell proliferation, and inducing apoptosis. IFN-gamma is secreted by uterine NK cells in the placenta during pregnancy and in mice is required for proper formation of the decidual layer and remodeling of the uterine vasculature. Despite the presence of IFN-gamma in the placenta, TBCs do not express either MHC class Ia or class II Ags, and are resistant to IFN-gamma-mediated apoptosis. In this study, we demonstrate that IFN-gamma-induced expression of multiple genes is significantly reduced in human trophoblast-derived choriocarcinoma cells relative to HeLa epithelial or fibroblast cells. These results prompted us to investigate the integrity of the JAK/STAT-1 pathway in these cells. Choriocarcinoma cells and HeLa cells express comparable levels of the IFN-gamma receptor. However, tyrosine phosphorylation of JAK-2 is compromised in IFN-gamma-treated choriocarcinoma cells. Moreover, phosphorylation of STAT-1 at tyrosine 701 is substantially reduced in both IFN-gamma-treated human choriocarcinoma and primary TBCs compared with HeLa cells or primary foreskin fibroblasts. A corresponding reduction of both IFN regulatory factor 1 mRNA and protein expression was observed in IFN-gamma-treated TBCs. Treatment of choriocarcinoma cells with the tyrosine phosphatase inhibitor pervanadate significantly enhanced IFN-gamma-inducible JAK and STAT-1 tyrosine phosphorylation and select IFN-gamma-inducible gene expression. We propose that phosphatase-mediated suppression of IFN-gamma signaling in TBCs contributes to fetal maintenance by inhibiting expression of genes that could be detrimental to successful pregnancy.

Inhibition of MHC class II gene expression in uveal melanoma cells is due to methylation of the CIITA gene or an upstream activator

Most cells with an intact interferon-gamma receptor and signaling pathway are able to express MHC class II molecules when treated with cytokines such as interferon-gamma and tumor necrosis factor-a. Interestingly, primary uveal melanocytes and most ocular melanoma cells are resistant to interferon-gamma mediated induction of class II MHC genes. This unusual phenotype is hypothesized to be germane to the immune-privileged status to the eye. Via a series of experiments, we have probed the molecular basis of this class II MHC resistant phenotype. We have analyzed the methylation status of the gene encoding the class II transactivator (CIITA), and asked whether treatment of class II MHC resistant ocular melanoma cells with the demethylating agent 5'-azacytidine can restore interferon-gamma inducibility of these class II MHC genes in these cells. The data obtained suggest that the specific blockade in cytokine-induced class II MHC gene expression is due to a suppression of the gene encoding the class II transactivator (CIITA). Treatment with 5' azacytidine restores the ability of these cells to express class II MHC genes upon interferon-gamma treatment. Whilst this is reminiscent of what occurs in another immune-privileged tissue--the placental trophoblast--we show here that silencing of the CIITA gene in uveal melanocytes either involves methylation of distinct nucleotides from those detected in trophoblasts, or involves an upstream activator of CIITA gene expression.

Histone acetylation regulates the cell type specific CIITA promoters, MHC class II expression and antigen presentation in tumor cells

The regulation of MHC class II expression by the class II transactivator (CIITA) is complex and differs in various cell types depending on the relative activity of three CIITA promoters. Here we show that, in plasma cell tumors, the deacetylase inhibitor trichostatin A (TSA) elicits PIII-CIITA but does not activate the IFN-gamma-inducible PIV-CIITA promoter. In trophoblast cells, all CIITA promoter types are constitutively silent and not induced by IFN-gamma or TSA treatment. TSA induction of PI-CIITA was restricted to macrophage and dendritic cell lines. In the Colon 26 tumor IFN-gamma induced endogenous PIV-CIITA but not PIII-CIITA while TSA activated class II in the apparent absence of CIITA. Reporter assays in Colon 26 showed that TSA induced PIII-CIITA but not PIV-CIITA. Transfection of a dominant negative CIITA plasmid in Colon 26 inhibited induction of class II by IFN-gamma but not TSA. Thus, the potential for both CIITA-dependent and -independent pathways of MHC induction exists within a single cell. Further evidence of CIITA-independent class II expression elicited by TSA was obtained using knockout mice with defects in CIITA, STAT-1alpha and IRF-1 expression. TSA treatment can also activate class II expression in mutant cell lines with deficiencies in signaling molecules, transcription factors and the BRG-1 cofactor that are required for IFN-gamma-induced CIITA expression. Importantly, after epigenetic activation by the deacetylase inhibitor, MHC class II is transported and displayed on the cell surface of a plasma cell tumor and it is converted to an efficient antigen presenting cell for protein and class II-peptide presentation.

Inhibition of alloresponse by a human trophoblast non-coding RNA suppressing class II transactivator promoter III and major histocompatibility class II expression in murine B-lymphocytes

BACKGROUND

Trophoblasts are a model of natural allograft tolerance. A unique characteristic is the complete lack of expression of all classic major histocompatibility (MHC) antigens. We cloned a human trophoblast non-coding RNA (TncRNA) that suppresses MHC class II expression through inhibition of the class II transactivator (CIITA) promoter. We assessed the functional affects of TncRNA on an alloresponse and dissected the functional domain on CIITA promoter III.

METHODS

Murine B-cell line A20 was transfected with TncRNA. Class II suppressed clones were selected and characterized by flow cytometry and Northern analysis. The clones were then subjected to lymphocyte proliferation assay to assess the stimulation of T-lymphocytes. CIITA promoter III-luciferase reporter plasmids were used with TncRNA plasmids in co-transfection assays; 5'-end deletion plasmids were used to dissect the promoter.

RESULTS

Significant suppression of I-Ad expression was seen. Northern blot scans demonstrated 84% to 93% suppression of class II transcripts. Lymphocyte proliferation assay demonstrated a 50% and 64% inhibition of lymphocyte stimulation in the 2 clones, compared to A20 wild type. Dissection of promoter III indicated that an area between bp -152 to -107 contains the functional site of TncRNA.

CONCLUSION

Human TncRNA is active across species lines and significantly inhibits allogenic response to B-cells. There is concurrent suppression of constitutive class II expression in TncRNA clones mediated through a defined region of CIITA promoter III.

Regulation of major histocompatibility complex class II gene expression in trophoblast cells

Trophoblast cells are unique because they are one of the few mammalian cell types that do not express major histocompatibility complex (MHC) class II antigens, either constitutively or after exposure to IFN-gamma. The absence of MHC class II antigen expression on trophoblast cells has been postulated to be one of the essential mechanisms by which the semi-allogeneic fetus evades immune rejection reactions by the maternal immune system. Consistent with this hypothesis, trophoblast cells from the placentas of women suffering from chronic inflammation of unknown etiology and spontaneous recurrent miscarriages have been reported to aberrantly express MHC class II antigens. The lack of MHC class II antigen expression on trophoblast cells is due to silencing of expression of the class II transactivator (CIITA), a transacting factor that is essential for constitutive and IFN-gamma-inducible MHC class II gene transcription. Transfection of trophoblast cells with CIITA expression vectors activates both MHC class II and class Ia antigen expression, which confers on trophoblast cells both the ability to activate helper T cells, and sensitivity to lysis by cytotoxic T lymphocytes. Collectively, these studies strongly suggest that stringent silencing of CIITA (and therefore MHC class II) gene expression in trophoblast cells is critical for the prevention of immune rejection responses against the fetus by the maternal immune system. The focus of this review is to summarize studies examining the novel mechanisms by which CIITA is silenced in trophoblast cells. The elucidation of the silencing of CIITA in trophoblast cells may shed light on how the semi-allogeneic fetus evades immune rejection by the maternal immune system during pregnancy.

DNA methylation and expression of major histocompatibility complex class I and class II transactivator genes in human developmental tumor cells and in T cell malignancies

Major histocompatibility complex (MHC) class I and class II molecules play essential roles in the immune response by virtue of their ability to present peptides to T lymphocytes. Given their central role in adaptive immunity, the genes encoding these peptide-presenting molecules are regulated in a tight fashion to meet with local requirements for an adequate immune response. In contrast to MHC class I gene products, which are expressed on almost all nucleated cells, constitutive expression of MHC class II molecules is found in specialized antigen presenting cells of the immune system only. Transcription of both MHC class I and class II genes can be induced by immune regulators and upon cell activation. Transcription of MHC class I genes is mediated by a set of conserved cis acting regulatory elements in their promoters. Of these regulatory elements, MHC class II promoters share the SXY-module. Essential for activation of MHC class II promoters is the class II transactivator (CIITA), which acts through protein/protein interactions with regulatory factors bound to the SXY module. In this review, we discuss the role of DNA methylation in relation to altered expression of MHC class I and CIITA genes as observed in malignancies and in development.

Novel mechanisms of class II major histocompatibility complex gene regulation

Class II MHC molecules present processed peptides from exogenous antigens to CD4+ helper T lymphocytes. In so doing, they are central to immunity, driving both the humoral and cell mediated arms of the immune response. Class II MHC molecules, and the genes encoding them, are expressed primarily in cells of the immune system (B cells, thymic epithelial cells, activated T cells and professional antigen presenting cells). The expression is also under developmental control. Research over the past 20 years have provided a clear understanding of the cis-elements and transcription factors that regulate the expression of Class II MHC genes. Perhaps the most critical advance has been the discovery of CIITA, a non- DNA binding activator of transcription that is a master control gene for class II gene expression. Current research is focused on understanding the situations where class II MHC gene expression occurs in a CIITA-independent pathway, and the molecular basis for this expression. Finally, significant emphasis is being placed on targeting class II MHC transcription factors to either inhibit or stimulate the immune response to transplanted tissue or in cell based vaccines. This communication outlines recent advances in this field and discusses likely areas for future research.

Class II transactivator (CIITA) promoter methylation does not correlate with silencing of CIITA transcription in trophoblasts

Trophoblast cells are unique because they do not express major histocompatibility complex (MHC) class II antigens, either constitutively or after exposure to interferon-gamma (IFN-gamma). The absence of MHC class II antigens on trophoblasts is thought to play a critical role in preventing rejection of the fetus by the maternal immune system. The inability of trophoblasts to express MHC class II genes is primarily due to lack of the class II transactivator (CIITA), a transacting factor that is required for constitutive and IFN-gamma-inducible MHC class II transcription. We, therefore, investigated the silencing of CIITA expression in trophoblasts. In transient transfection assays, transcription from the IFN-gamma-responsive CIITA type IV promoter was upregulated by IFN-gamma in trophoblasts, which suggests that CIITA is silenced by an epigenetic mechanism in these cells. Polymerase chain reaction analysis demonstrated that the CIITA type IV promoter is methylated in both the human choriocarcinoma cell lines JEG-3 and Jar and in 2fTGH fibrosarcoma cells, which are IFN-gamma inducible for CIITA. Conversely, methylation of the CIITA type IV promoter was not observed in human primary cytotrophoblasts isolated from term placentae or in mouse or rat trophoblast cell lines. Simultaneous treatment with IFN-gamma and the histone deacetylase inhibitor trichostatin A weakly activated CIITA transcription in mouse trophoblasts. Stable hybrids between human choriocarcinoma and fibrosarcoma cells and between mouse trophoblasts and fibroblasts expressed CIITA following treatment with IFN-gamma. These results suggest that silencing of CIITA transcription is recessive in trophoblasts and involves an epigenetic mechanism other than promoter methylation. The fact that CIITA is expressed in the stable hybrids implies that trophoblasts may be missing a factor that regulates chromatin structure at the CIITA promoter.

Human trophoblast noncoding RNA suppresses CIITA promoter III activity in murine B-lymphocytes

Trophoblasts lack expression of all classical MHC antigens. Determination of the mechanism involved could provide insight into selective gene suppression and allograft tolerance. Suppression of class II expression in trophoblasts is secondary to dominant negative trans-acting factors that suppress class II transactivator (CIITA) transcription. We recently described a trophoblast-derived noncoding RNA (TncRNA) that suppresses class II expression. Murine B-cells CH27 were stably transfected with TncRNA and analyzed for MHC class II and CIITA expression by FACS and Northern blots. Functional assessment of CIITA promoter III (pIII) was performed by transient transfection of promoter-reporter constructs. Methylation of pIII was assessed by Southern blots and FACS. TncRNA suppressed constitutive I-Ak and CIITA expression in murine B-cells CH27. The mechanism involves inhibition of CIITA pIII activity. The mechanism does not involve methylation of the promoter.

DNA alkylating agents alleviate silencing of class II transactivator gene expression in L1210 lymphoma cells

MHC class II (Ia) Ag expression is inversely correlated with tumorigenicity and directly correlated with immunogenicity in clones of the mouse L1210 lymphoma (1 ). Understanding the mechanisms by which class II Ag expression is regulated in L1210 lymphoma may facilitate the development of immunotherapeutic approaches for the treatment of some types of lymphoma and leukemia. This study demonstrates that the variation in MHC class II Ag expression among clones of L1210 lymphoma is due to differences in the expression of the class II transactivator (CIITA). Analysis of stable hybrids suggests that CIITA expression is repressed by a dominant mechanism in class II-negative L1210 clones. DNA-alkylating agents such as ethyl methanesulfonate and the chemotherapeutic drug melphalan activate CIITA and class II expression in class II negative L1210 cells, and this effect appears to be restricted to transformed cell lines derived from the early stages of B cell ontogeny. Transient transfection assays demonstrated that the CIITA type III promoter is active in class II(-) L1210 cells, despite the fact that the endogenous gene is not expressed, which suggests that these cells have all of the transacting factors necessary for CIITA transcription. An inverse correlation between methylation of the CIITA transcriptional regulatory region and CIITA expression was observed among L1210 clones. Furthermore, 5-azacytidine treatment activated CIITA expression in class II-negative L1210 cells. Collectively, our results suggest that 1) CIITA gene expression is repressed in class II(-) L1210 cells by methylation of the CIITA upstream regulatory region, and 2) treatment with DNA-alkylating agents overcomes methylation-based silencing of the CIITA gene in L1210 cells.

Kinetics of a gamma interferon response: expression and assembly of CIITA promoter IV and inhibition by methylation

Chromatin immunoprecipitation assays were employed to assess the kinetics of transcription factor assembly and histone modifications that occur during gamma interferon (IFN-gamma) induction of CIITA gene expression. CIITA is the master regulator of major histocompatibility complex class II transcription. Promoter IV (PIV), the major IFN-gamma responsive promoter for CIITA expression, requires both STAT1 and IFN regulatory factor 1 (IRF-1) for induction by IFN-gamma. STAT1 binding to PIV was detected first and was accompanied by a modest acetylation of histones H3 and H4 that were associated with the region. Despite these changes, which occurred within 30 min of IFN-gamma treatment, CIITA mRNA was not detected until IRF-1 protein was synthesized and bound to its site, a process that required >120 min. In contrast to these events, fetal trophoblast-like cell lines, which are refractory to CIITA induction by IFN-gamma, failed to assemble the above factors or modify their chromatin, suggesting that accessibility to the promoter is blocked. Bisulfite sequencing of PIV showed strong hypermethylation of PIV, providing a link between methylation, chromatin structure, and factor binding. Together, this analysis provides a kinetic view of the activation of the CIITA gene in response to IFN-gamma and shows that regulatory factor assembly, chromatin modification, and gene expression proceed in discrete steps.

Gamma interferon fails to induce expression of indoleamine 2,3-dioxygenase and does not control the growth of Chlamydophila abortus in BeWo trophoblast cells

The BeWo trophoblast cell line does not constitutively express the tryptophan degrading enzyme indolamine 2,3-dioxygenase (IDO), nor can IDO expression be induced by gamma interferon. This correlates with the inability of BeWo cells to control the growth of Chlamydophila abortus, in contrast to effects observed in HeLa cells treated with gamma interferon.

Brucella abortus siderophore 2,3-dihydroxybenzoic acid (DHBA) facilitates intracellular survival of the bacteria

Siderophores are low molecular weight molecules that allow bacteria to acquire iron from host cell proteins. 2,3-dihydroxybenzoic acid (DHBA) is the only known siderophore produced by the intracellular pathogen Brucella abortus. Here its role in virulence was assessed by evaluating the ability of a mutant with a disruption of the entC gene to survive and replicate in vitro in murine and bovine cells and in vivo in resistant and susceptible murine hosts. It was hypothesized that DHBA is vital for bacterial virulence by its ability to chelate intracellular iron thereby preventing generation of anti-bacterial hydroxyl radicals via the Haber-Weiss reaction, to scavenge reactive oxygen intermediates and for acquisition of iron needed for nutritional purposes. The data showed DHBA played a significant role for bacterial survival in host cells after infection including in murine macrophages cultured in the presence and absence of exogenous interferon-gamma (IFN-gamma) and in bovine trophoblasts supplemented with erythritol. In severely iron-depleted conditions, DHBA was also found to be essential for growth in murine macrophages. Despite these deficiencies, the absence of DHBA had no long-term significant effect on the number of CFU recovered in vivo from either the Brucella-resistant C57BL/6 mice or Brucella-susceptible IFN-gamma knock-out C57BL/6 mice.

Class II transactivator is required for maximal expression of HLA-DOB in B cells

HLA-DO, encoded by the HLA-DOA and HLA-DOB genes, has been shown to function as a modulator of Ag presentation. DNA microarray comparisons between B cells wild-type and mutant for the master regulator of MHC class II transcription, class II transactivator (CIITA), identified HLA-DOA and HLA-DOB as being up-regulated by CIITA. Although HLA-DOA had been shown previously to be regulated by CIITA, HLA-DOB expression was suggested to be independent of CIITA. A series of assays including quantitative RT-PCR, promoter-reporter assays, chromatin immunoprecipitations, and intracellular staining were performed to corroborate the DNA microarray analysis. The combined data demonstrate that HLA-DOB levels are increased by CIITA, and that this difference has an impact on the overall level of HLA-DO expression. Additionally, unlike the classical MHC class II genes, HLA-DOB expression was present in the absence of CIITA, indicating that additional factors mediate HLA-DOB expression in B cells.

Transcriptional control of MHC genes in fetal trophoblast cells

Tight control of MHC expression is essential for the outcome of a successful pregnancy. The lack of MHC class II and class I mediated antigen presentation by fetal trophoblast cells is an important mechanism to evade maternal immune recognition. Interestingly, the deficient expression of MHC class II molecules (HLA-DR, -DQ and -DP) and of the classical MHC class I molecules HLA-A and HLA-B is also noted after IFN-gamma treatment in trophoblast-derived cell lines. Our studies show that in trophoblast cell lines the IFN-gamma induced transactivation of HLA-A and HLA-B promoters is repressed. Furthermore, it was found that trophoblast cells lacked IFN-gamma mediated induction of the class II transactivator (CIITA). This lack of CIITA expression in trophoblast cells is due to CIITA promoter hypermethylation. In addition to lack of CIITA expression, trophoblast cells also displayed a repressed expression of RFX5. Together, these observations reveal a silencing of multiple activation pathways that are critical to the transcriptional control of MHC class II and class I antigen presentation functions by trophoblast cells.

A role for tryptophan in immune control of chlamydial abortion in sheep

Tryptophan (Trp) catabolism appears to be an important mechanism for regulation of inflammatory responses, resulting in T-cell tolerance and survival of semi-allogeneic concepti during pregnancy. Trp catabolism can be induced by IFN-gamma, and is therefore an important host defence mechanism against intracellular pathogens. Chlamydophila abortus is a bacterial pathogen that can cause persistent infection in non-pregnant sheep, but invades the placenta and causes abortion in late pregnancy. IFN-gamma was found to control the growth of Chlamydophila abortus in ovine cells in a highly dose-dependent manner. Addition of 200U/ml IFN-gamma eradicated all traces of infection from the cultures, whereas concentrations less than 50U/ml failed to control the growth of the organism, resulting in cell lysis. However, concentrations in the range of 50-100U/ml were found to restrict growth to an extent that a persistent infection was established, allowing survival of the organism in tissue culture for several months. Removal of IFN-gamma resulted in the re-appearance of infectious organisms. Addition of exogenous Trp to the cells treated with 50-100U/ml IFN-gamma prevented the establishment of persistence. These effects in tissue culture are analogous to the persistent infection observed in pregnant sheep prior to abortion. These data suggest that control of C. abortus growth in the periphery is linked to the balance of pro-inflammatory cytokine production and availability of Trp during pregnancy.

The bare lymphocyte syndrome and the regulation of MHC expression

The bare lymphocyte syndrome (BLS) is a hereditary immunodeficiency resulting from the absence of major histocompatibility complex class II (MHCII) expression. Considering the central role of MHCII molecules in the development and activation of CD4(+) T cells, it is not surprising that the immune system of the patients is severely impaired. BLS is the prototype of a "disease of gene regulation." The affected genes encode RFXANK, RFX5, RFXAP, and CIITA, four regulatory factors that are highly specific and essential for MHCII genes. The first three are subunits of RFX, a trimeric complex that binds to all MHCII promoters. CIITA is a non-DNA-binding coactivator that functions as the master control factor for MHCII expression. The study of RFX and CIITA has made major contributions to our comprehension of the molecular mechanisms controlling MHCII genes and has made this system into a textbook model for the regulation of gene expression.

[HLA molecules, immunity and gestation]

During pregnancy, the fetus develops particularly efficient molecular regulatory mechanisms to prevent possible maternal anti-paternal alloimmune response and avoid viral spreading from maternal tissue. Among the different mechanisms, there has been noted a selective expression of HLA molecules on trophoblast cells: the absence of HLA class II and of polymorphic HLA-A and HLA-B expression but presence of both non polymorphic HLA-G and HLA-E class Ib as well as of HLA-C class Ia. The functional consequences of such a particular pattern of HLA expression in gestation are examined here.

Activation of MHC class I, II, and CD40 gene expression by histone deacetylase inhibitors

Epigenetic mechanisms are involved in regulating chromatin structure and gene expression through repression. In this study, we show that histone deacetylase inhibitors (DAIs) that alter the acetylation of histones in chromatin enhance the expression of several genes on tumor cells including: MHC class I, II, and the costimulatory molecule CD40. Enhanced transcription results in a significant increase in protein expression on the tumor cell surface, and expression can be elicited on some tumors that are unresponsive to IFN-gamma. The magnitude of induction of these genes cannot be explained by the effect of DAIs on the cell cycle or enhanced apoptosis. Induction of class II genes by DAIs was accompanied by activation of a repressed class II transactivator gene in a plasma cell tumor but, in several other tumor cell lines, class II was induced in the apparent absence of class II transactivator transcripts. These findings also suggest that the abnormalities observed in some tumors in the expression of genes critical to tumor immunity may result from epigenetic alterations in chromatin and gene regulation in addition to well-established mutational mechanisms.

Transcriptional regulation of the MHC class Ib genes HLA-E, HLA-F, and HLA-G

The restricted tissue expression of the MHC class Ib molecules HLA-E, HLA-F, and HLA-G has suggested specialized functions and tight transcriptional control of their genes. Transactivation of classical MHC class I genes is mediated by two groups of juxtaposed cis-acting elements, which can be viewed as regulatory modules. The most upstream module consists of the enhancer A and ISRE, and mediates constitutive and cytokine induced expression, whereas the SXY module is important for the constitutive and CIITA-mediated transactivation of MHC class I genes. Nucleotide sequence divergence in these regulatory elements in the promoters of HLA-E, HLA-F, or HLA-G determines their differential responsiveness to NF-kappaB, IRF1, and CIITA-mediated induction. HLA-E is not inducible by NF-kappaB or IRF1, but is responsive to IFN-gamma through an upstream STAT1 binding site. Furthermore, HLA-E is inducible by CIITA through the SXY regulatory module. HLA-F is inducible by NF-kappaB through the kappaB1 site of enhancer A, is responsive to IFN-gamma through the ISRE, and is inducible by CIITA. Both regulatory modules are divergent in HLA-G rendering this gene unresponsive to NF-kappaB, IRF1, and CIITA-mediated induction. This implies a unique regulation of HLA-G transcription amongst the MHC class Ib genes.

Associations and interactions between bare lymphocyte syndrome factors

The bare lymphocyte syndrome, a severe combined immunodeficiency due to loss of major histocompatibility complex (MHC) class II gene expression, is caused by inherited mutations in the genes encoding the heterotrimeric transcription factor RFX (RFX-B, RFX5, and RFXAP) and the class II transactivator CIITA. Mutagenesis of the RFX genes was performed, and the properties of the proteins were analyzed with regard to transactivation, DNA binding, and protein-protein interactions. The results identified specific domains within each of the three RFX subunits that were necessary for RFX complex formation, including the ankyrin repeats of RFX-B. DNA binding was dependent on RFX complex formation, and transactivation was dependent on a region of RFX5. RFX5 was found to interact with CIITA, and this interaction was dependent on a proline-rich domain within RFX5. Thus, these studies have defined the protein domains required for the functional regulation of MHC class II genes.

Lack of CIITA expression is central to the absence of antigen presentation functions of trophoblast cells and is caused by methylation of the IFN-gamma inducible promoter (PIV) of CIITA

Lack of MHC-mediated antigen presenting functions of fetal trophoblast cells is an important mechanism to evade maternal immune recognition. In this study we demonstrated that the deficiency in MHC expression and antigen presentation in the trophoblast cell lines JEG-3 and JAR is caused by lack of class II transactivator (CIITA) expression due to hypermethylation of its interferon-gamma (IFN-gamma)-responsive promoter (PIV). Circumvention of this lack of CIITA expression by introduction of exogenous CIITA induced cell surface expression of HLA-DR, -DP, and -DQ, leading to an acquired capacity to present antigen to antigen-specific T cells. Transfection of CIITA in JEG-3 cells also upregulated functional HLA-B and HLA-C expression. Noteworthy, this lack of IFN-gamma-mediated induction of CIITA was also found to exist in normal trophoblast cells expanded from chorionic villus biopsies. Together, these observations demonstrate that lack of CIITA expression is central to the absence of antigen presentation functions of trophoblast cells.

Chromatin-immune connections: regulation of MHC and other genes

In cells, genes are contained within chromatin - a highly structured array of DNA wrapped around core histone proteins. Packaged genes are subject to a variety of regulatory modifications including, CpG methylation, histone acetylation and phosphorylation. These epigenetic mechanisms of gene regulation involve higher ordered protein complexes possessing enzymatic activities such as ATP hydrolysis and acetylation that are targeted to specific genes by transcription factors, coactivatorsand coreptessors. In this article, we endeavor to providean overview of current research on mechanisms of transcriptional regulation by chromatin remodeling of MHC and other genes that are of interest in reproductive immunology.

Immunology at the maternal-fetal interface: lessons for T cell tolerance and suppression

Mammalian reproduction poses an immunological paradox because fetal alloantigens encoded by genes inherited from the father should provoke responses by maternal T cells leading to fetal loss. Current understanding of T cell immunobiology and the critical role of inflammatory processes during pregnancy is reviewed and discussed. Lessons derived from studies on the regulation of T cell responsiveness during mammalian gestation are considered in the wider context of T cell tolerance toward some microbial infections and tumors, avoidance of autoimmunity, and tissue allograft rejection.

Methylation of class II trans-activator promoter IV: a novel mechanism of MHC class II gene control

Inhibition of class II trans-activator (CIITA) expression prevents embryonic trophoblast cells from up-regulating MHC class II genes in response to IFN-gamma. This is thought to be one mechanism of maternal tolerance to the fetal allograft. The CIITA gene is regulated by four distinct promoters; promoter III directs constitutive (B cell) expression, and promoter IV regulates IFN-gamma-inducible expression. Using in vivo genomic footprinting, promoter-reporter analysis, Southern blot analysis, and RT-PCR, we have examined the cause of CIITA silencing in a trophoblast-derived cell line. We report here that methylation of promoter IV DNA at CpG sites in Jar cells prevents promoter occupancy and IFN-gamma-inducible transcription. The inhibition of CpG methylation in Jar cells by treatment with 5-aza-2'-deoxycytidine restores IFN-gamma inducibility to CIITA. This is the first description of an epigenetic mechanism involved in regulation of CIITA and MHC class II gene expression.