Differential selectivity of CIITA promoter activation by IFN-gamma and IRF-1 in astrocytes and macrophages: CIITA promoter activation is not affected by TNF-alpha

Epigenomic priming of immune genes implicates oligodendroglia in multiple sclerosis susceptibility

Multiple sclerosis (MS) is characterized by a targeted attack on oligodendroglia (OLG) and myelin by immune cells, which are thought to be the main drivers of MS susceptibility. We found that immune genes exhibit a primed chromatin state in single mouse and human OLG in a non-disease context, compatible with transitions to immune-competent states in MS. We identified BACH1 and STAT1 as transcription factors involved in immune gene regulation in oligodendrocyte precursor cells (OPCs). A subset of immune genes presents bivalency of H3K4me3/H3K27me3 in OPCs, with Polycomb inhibition leading to their increased activation upon interferon gamma (IFN-γ) treatment. Some MS susceptibility single-nucleotide polymorphisms (SNPs) overlap with these regulatory regions in mouse and human OLG. Treatment of mouse OPCs with IFN-γ leads to chromatin architecture remodeling at these loci and altered expression of interacting genes. Thus, the susceptibility for MS may involve OLG, which therefore constitutes novel targets for immunological-based therapies for MS.

Inhibition of the JAK/STAT Pathway Protects Against α-Synuclein-Induced Neuroinflammation and Dopaminergic Neurodegeneration

Parkinson's Disease (PD) is an age-related, chronic neurodegenerative disorder. At present, there are no disease-modifying therapies to prevent PD progression. Activated microglia and neuroinflammation are associated with the pathogenesis and progression of PD. Accumulation of α-synuclein (α-SYN) in the brain is a core feature of PD and leads to microglial activation, inflammatory cytokine/chemokine production, and ultimately to neurodegeneration. Given the importance of the JAK/STAT pathway in activating microglia and inducing cytokine/chemokine expression, we investigated the therapeutic potential of inhibiting the JAK/STAT pathway using the JAK1/2 inhibitor, AZD1480. In vitro, α-SYN exposure activated the JAK/STAT pathway in microglia and macrophages, and treatment with AZD1480 inhibited α-SYN-induced major histocompatibility complex Class II and inflammatory gene expression in microglia and macrophages by reducing STAT1 and STAT3 activation. For in vivo studies, we used a rat model of PD induced by viral overexpression of α-SYN. AZD1480 treatment inhibited α-SYN-induced neuroinflammation by suppressing microglial activation, macrophage and CD4(+) T-cell infiltration and production of proinflammatory cytokines/chemokines. Numerous genes involved in cell-cell signaling, nervous system development and function, inflammatory diseases/processes, and neurological diseases are enhanced in the substantia nigra of rats with α-SYN overexpression, and inhibited upon treatment with AZD1480. Importantly, inhibition of the JAK/STAT pathway prevented the degeneration of dopaminergic neurons in vivo These results indicate that inhibiting the JAK/STAT pathway can prevent neuroinflammation and neurodegeneration by suppressing activation of innate and adaptive immune responses to α-SYN. Furthermore, this suggests the feasibility of targeting the JAK/STAT pathway as a neuroprotective therapy for neurodegenerative diseases.

SIGNIFICANCE STATEMENT

α-SYN plays a central role in the pathophysiology of PD through initiation of neuroinflammatory responses. Using an α-SYN overexpression PD model, we demonstrate a beneficial therapeutic effect of AZD1480, a specific inhibitor of JAK1/2, in suppressing neuroinflammation and neurodegeneration. Our findings document that inhibition of the JAK/STAT pathway influences both innate and adaptive immune responses by suppressing α-SYN-induced microglia and macrophage activation and CD4(+) T-cell recruitment into the CNS, ultimately suppressing neurodegeneration. These findings are the first documentation that suppression of the JAK/STAT pathway disrupts the circuitry of neuroinflammation and neurodegeneration, thus attenuating PD pathogenesis. JAK inhibitors may be a viable therapeutic option for the treatment of PD patients.

Polycomb recruitment at the Class II transactivator gene

The Class II Transactivator (CIITA) is the master regulator of Major Histocompatibility Class II (MHC II) genes. Transcription of CIITA through the IFN-γ inducible CIITA promoter IV (CIITA pIV) during activation is characterized by a decrease in trimethylation of histone H3 lysine 27 (H3K27me3), catalyzed by the histone methyltransferase Enhancer of Zeste Homolog 2 (EZH2). While EZH2 is the known catalytic subunit of the Polycomb Repressive Complex 2 (PRC2) and is present at the inactive CIITA pIV, the mechanism of PRC2 recruitment to mammalian promoters remains unknown. Here we identify two DNA-binding proteins, which interact with and regulate PRC2 recruitment to CIITA pIV. We demonstrate Yin Yang 1 (YY1) and Jumonji domain containing protein 2 (JARID2) are binding partners along with EZH2 in mammalian cells. Upon IFN-γ stimulation, YY1 dissociates from CIITA pIV while JARID2 binding to CIITA pIV increases, suggesting novel roles for these proteins in regulating expression of CIITA pIV. Knockdown of YY1 and JARID2 yields decreased binding of EZH2 and H3K27me3 at CIITA pIV, suggesting important roles for YY1 and JARID2 at CIITA pIV. JARID2 knockdown also results in significantly elevated levels of CIITA mRNA upon IFN-γ stimulation. This study is the first to identify novel roles of YY1 and JARID2 in the epigenetic regulation of the CIITA pIV by recruitment of PRC2. Our observations indicate the importance of JARID2 in CIITA pIV silencing, and also provide a novel YY1-JARID2-PRC2 regulatory complex as a possible explanation of differential PRC2 recruitment at inducible versus permanently silenced genes.

Transcription factor IRF1 is responsible for IRF8-mediated IL-1β expression in reactive microglia

Interferon regulatory factor-8 (IRF8) plays a crucial role in the transformation of microglia to a reactive state by regulating the expression of various genes. In the present study, we show that IRF1 is required for IRF8-induced gene expression in microglia. Peripheral nerve injury induced IRF1 gene upregulation in the spinal microglia in an IRF8-dependent manner. IRF8 transduction in cultured microglia induced de novo gene expression of IRF1. Importantly, knockdown of the IRF1 gene in IRF8-transduced microglia prevented upregulation of interleukin-1β (IL-1β). Therefore, our findings suggest that expression of IL-1β is dependent on IRF1 in IRF8-expressing reactive microglia.

Self-recognition of the endothelium enables regulatory T-cell trafficking and defines the kinetics of immune regulation

Localization of CD4(+)CD25(+)Foxp3(+) regulatory T (Treg) cells to lymphoid and non-lymphoid tissue is instrumental for the effective control of immune responses. Compared with conventional T cells, Treg cells constitute a minute fraction of the T-cell repertoire. Despite this numeric disadvantage, Tregs efficiently migrate to sites of immune responses reaching an optimal number for the regulation of T effector (Teff) cells. The array and levels of adhesion and chemokine receptor expression by Tregs do not explain their powerful migratory capacity. Here we show that recognition of self-antigens expressed by endothelial cells in target tissue is instrumental for efficient Treg recruitment in vivo. This event relies upon IFN-γ-mediated induction of MHC-class-II molecule expression by the endothelium and requires optimal PI3K p110δ activation by the T-cell receptor. We also show that, once in the tissue, Tregs inhibit Teff recruitment, further enabling a Teff:Treg ratio optimal for regulation.

Immune players in the CNS: the astrocyte

In the finely balanced environment of the central nervous system astrocytes, the most numerous cell type, play a role in regulating almost every physiological system. First found to regulate extracellular ions and pH, they have since been shown to regulate neurotransmitter levels, cerebral blood flow and energy metabolism. There is also growing evidence for an essential role of astrocytes in central immunity, which is the topic of this review. In the healthy state, the central nervous system is potently anti-inflammatory but under threat astrocytes readily respond to pathogens and to both sterile and pathogen-induced cell damage. In response, astrocytes take on some of the roles of immune cells, releasing cyto- and chemokines to influence effector cells, modulating the blood-brain barrier and forming glial scars. To date, much of the data supporting a role for astrocytes in immunity have been obtained from in vitro systems; however data from experimental models and clinical samples support the suggestion that astrocytes perform similar roles in more complex environments. This review will discuss some aspects of the role of astrocytes in central nervous system immunity.

TNF-alpha induces macroautophagy and regulates MHC class II expression in human skeletal muscle cells

Macroautophagy, a homeostatic process that shuttles cytoplasmic constituents into endosomal and lysosomal compartments, has recently been shown to deliver antigens for presentation on major histocompatibility complex (MHC) class II molecules. Skeletal muscle fibers show a high level of constitutive macroautophagy and express MHC class II molecules upon immune activation. We found that tumor necrosis factor-α (TNF-α), a monokine overexpressed in inflammatory myopathies, led to a marked up-regulation of macroautophagy in skeletal myocytes. Furthermore, TNF-α augmented surface expression of MHC class II molecules in interferon-γ (IFN-γ)-treated myoblasts. The synergistic effect of TNF-α and IFN-γ on the induction of MHC class II surface expression was not reflected by higher intracellular human leukocyte antigen (HLA)-DR levels and was reversed by macroautophagy inhibition, suggesting that TNF-α facilitates antigen processing via macroautophagy for more efficient MHC class II loading. Muscle biopsies from patients with sporadic inclusion body myositis, a well defined myopathy with chronic inflammation, showed that over 20% of fibers that contained autophagosomes costained for MHC class II molecules and that more than 40% of double-positive muscle fibers had contact with CD4(+) and CD8(+) immune cells. These findings establish a mechanism through which TNF-α regulates both macroautophagy and MHC class II expression and suggest that macroautophagy-mediated antigen presentation contributes to the immunological environment of the inflamed human skeletal muscle.

CNS-specific expression of C3a and C5a exacerbate demyelination severity in the cuprizone model

Demyelination in the central nervous system (CNS) is known to involve several immune effector mechanisms, including complement proteins. Local production of complement by glial cells in the brain can be both harmful and protective. To investigate the roles of C3a and C5a in demyelination and remyelination pathology we utilized the cuprizone model. Transgenic mice expressing C3a or C5a under the control of the glial fibrillary acidic protein (GFAP) promoter had exacerbated demyelination and slightly delayed remyelination in the corpus callosum compared to WT mice. C3a and C5a transgenic mice had increased cellularity in the corpus callosum due to increase activation and/or migration of microglia. Oligodendrocytes migrated to the corpus callosum in higher numbers during early remyelination events in C3a and C5a transgenic mice, thus enabling these mice to remyelinate as effectively as WT mice by the end of the 10 week study. To determine the effects of C3a and/or C5a on individual glial subsets, we created murine recombinant C3a and C5a proteins. When microglia and mixed glial cultures were stimulated with C3a and/or C5a, we observed an increase in the production of proinflammatory cytokines and chemokines. In contrast, astrocytes had decreased cytokine and chemokine production in the presence of C3a and/or C5a. We also found that the MAPK pathway proteins JNK and ERK1/2 were activated in glia upon stimulation with C3a and C5a. Overall, our findings show that although C3a and C5a production in the brain play a negative role during demyelination, these proteins may aid in remyelination.

Common cellular and diverse genetic basis of thymoma-associated myasthenia gravis: role of MHC class II and AIRE genes and genetic polymorphisms

Generation of autoreactive CD4(+) effector T cells and defective production of regulatory CD4(+) T cells inside thymomas contribute to the development of myasthenia gravis (MG) in >90% of MG(+) thymomas. The molecular basis of these abnormalities is unknown. We report here that a) expression levels of class II major histocompatibility complex (MHCII) genes are variably decreased in thymomas, most prominently in histological WHO types A and AB; b) epithelial cells of type A and AB thymomas exhibit signal transducer and activator of transcription (STAT-1)-related defects of interferon-gamma (IFN-gamma) signaling and human leukocyte antigen (HLA)-DR expression in vitro; c) the promoter III (pIII)- and pIV-driven splice variants of the MHCII transactivator (CIITA) play a key role in MHCII gene expression in thymus and thymomas; and d) the pIV CIITA promoter is heavily methylated in thymomas. Recently, we also found that expression of the autoimmune regulator (AIRE) gene is absent from approximately 95% of thymomas. Among all theses abnormalities, only better preserved expression levels of MHCII (P < 0.001) in thymomas were significantly associated with the presence of MG. Taking the association of a gain-of-function polymorphism of the CTLA-4 and PTPN22 gene with MG in thymomas into account, we conclude that these acquired cellular abnormalities of the thymoma microenvironment in concert with inherited genetic high-risk polymorphisms of immunoregulatory genes have an impact on intratumorous thymopoiesis and appear to tip the balance toward central tolerance failure and development of MG. The findings imply that IFN-gamma and STAT-1 signaling play a role in MHCII expression in the human thymus and in the pathogenesis of paraneoplastic MG.

A new inducible adenoviral expression system that responds to inflammatory stimuli in vivo

BACKGROUND

Gene transfer using inducible promoters, which control expression of transgenic proteins in response to physiological conditions, may have significant advantages. In this study, we tried to achieve an inducible adenoviral expression system for physiologically responsive gene therapy of autoimmune or inflammatory diseases.

METHODS

A luciferase reporter vector with a hybrid promoter containing the human IL-1beta enhancer region (-3690 to - 2720) and the human CIITA promoter IV (-399 to + 2) was constructed. A replication-deficient adenovirus was engineered with luciferase controlled by the IL1beta/CIITApIV promoter (Ad-IL1beta/CIITApIV-Luc). The reporter vector or adenovirus was transfected to C57Bl/6 myeloid dendritic cells (DCs), RAW264.7, and Hep G2 to study the in vitro characteristics of this hybrid promoter. An inflammation model was prepared by injecting lipopolysaccharide (LPS) into Balb/c mice intraperitoneally (i.p.), and infected with Ad-IL1beta/CIITApIV-Luc or Ad-CMV-Luc to study the in vivo characteristics of the IL1beta/CIITApIV promoter.

RESULTS

The IL1beta/CIITApIV hybrid promoter has pronounced promoter activity, broad-range responsiveness to cytokines or LPS, and can be rechallenged after first induction. In the inflammation model, IL1beta/CIITApIV could drive hepatic luciferase expression increasedly rapidly after LPS challenge and in a LPS dose-dependent manner.

CONCLUSIONS

Using the IL1beta/CIITApIV hybrid promoter in gene transfer vectors may make it possible to produce transgenic proteins in vivo in direct relationship with the intensity and duration of an individual's status. By providing endogenously controlled production of transgenic proteins, this approach might limit the severity of autoimmune or inflammatory response without interfering with the beneficial components of host defense and immunity.

Sequential Modifications in Class II Transactivator Isoform 1 Induced by Lipopolysaccharide Stimulate Major Histocompatibility Complex Class II Transcription in Macrophages

By presenting antigenic peptides on major histocompatibility complex class (MHC) II determinants to CD4(+) T cells, macrophages help to direct the establishment of adaptive immunity. We found that in these cells, lipopolysaccharide stimulates the expression of MHC II genes via the activation of Erk1/2, which is mediated by Toll-like receptor 4. Erk1/2 then phosphorylates the serine at position 357, which is located in a degron of CIITA isoform 1 that leads to its monoubiquitylation. Thus modified, CIITA isoform 1 binds P-TEFb, which mediates the elongation of RNA polymerase II and co-transcriptional processing of nascent transcripts. This induction leads to the expression of MHC II genes. Subsequent polyubiquitylation results in the degradation of CIITA isoform 1. Thus, the signaling cascade from Toll-like receptor 4 to CIITA isoform 1 represents one connection between innate and adaptive immunity in macrophages.

Genetic analysis of intracranial tumors in a murine model of glioma demonstrate a shift in gene expression in response to host immunity

For the study of malignant glioma, we have previously characterized a highly tumorigenic murine astrocytoma, SMA-560, which arose spontaneously in an inbred, immunocompetent VM/Dk mouse. Using this cell line as a model of murine glioma, we performed DNA microarray analysis of autologous normal murine astroctyes (NMA) and SMA-560 tumor cells grown in monolayer culture or intracranially in syngeneic immunocompetent or immunocompromised hosts in order to determine whether tumors grown in vitro recreate the complex genetic regulation that occurs in vivo. Our findings support our hypothesis that glioma phenotype in vitro may be quite different in vivo and significantly altered by in situ growth factors and other invading cell populations.

Mycobacterium tuberculosis 19-kDa lipoprotein inhibits IFN-gamma-induced chromatin remodeling of MHC2TA by TLR2 and MAPK signaling

During infection of macrophages, prolonged signaling by Mycobacterium tuberculosis (Mtb) or its 19-kDa lipoprotein (LpqH; Rv3763) inhibits IFN-gamma-induced expression of several immune function genes, including class II transactivator (CIITA), which regulates class II MHC. Mtb does not inhibit early IFN-gamma signaling events, e.g., Stat1alpha activation. This study analyzed downstream mechanisms that regulate the transcription of MHC2TA, the gene encoding CIITA. Chromatin immunoprecipitation showed that IFN-gamma induced acetylation of histones H3 and H4 at the CIITA promoter IV (pIV). In contrast, IFN-gamma-dependent histone acetylation at CIITA pIV was inhibited by Mtb or 19-kDa lipoprotein. Mtb 19-kDa lipoprotein also inhibited IFN-gamma-dependent recruitment of Brahma-related gene 1, a chromatin remodeling protein, to CIITA pIV. Mtb 19-kDa lipoprotein did not inhibit histone acetylation in TLR2(-/-) macrophages. Furthermore, 19-kDa lipoprotein did not inhibit CIITA expression or IFN-gamma-dependent histone acetylation of CIITA pIV in macrophages treated with inhibitors of MAPKs p38 or ERK. Thus, CIITA expression was inhibited by TLR2-induced MAPK signaling that caused histone hypoacetylation at CIITA pIV and suppression of CIITA transcription. Chromatin remodeling at MHC2TA is a novel target of inhibition by Mtb. These mechanisms may diminish class II MHC expression by infected macrophages, contributing to immune evasion by Mtb.

Glial grafting for demyelinating disease

Remyelination of demyelinated central nervous system (CNS) axons is considered as a potential treatment for multiple sclerosis, and it has been achieved in experimental models of demyelination by transplantation of pro-myelinating cells. However, the experiments undertaken have not addressed the need for tissue-type matching in order to achieve graft-mediated remyelination since they were performed in conditions in which the chance for graft rejection was minimized. This article focuses on the factors determining survival of allogeneic oligodendrocyte lineage cells and their contribution to the remyelination of demyelinating CNS lesions. The immune status of the CNS as well as the suitability of different models of demyelination for graft rejection studies are discussed, and ways of enhancing allogeneic oligodendrocyte-mediated remyelination are presented. Finally, the effects of glial graft rejection on host remyelination are described, highlighting the potential benefits of the acute CNS inflammatory response for myelin repair.

Interferon-gamma induces major histocompatibility class II transactivator (CIITA), which mediates collagen repression and major histocompatibility class II activation by human aortic smooth muscle cells

Chronic inflammation in atherosclerosis is responsible for plaque instability through alterations in extracellular matrix. Previously, we demonstrated that major histocompatibility class II (MHC II) transactivator (CIITA) in a complex with regulatory factor for X box 5 (RFX5) is a crucial protein mediating interferon (IFN)-gamma-induced repression of collagen type I gene transcription in fibroblasts. This article demonstrates that, in smooth muscle cells (SMCs), IFN-gamma dramatically increases the expression of CIITA isoforms III and IV, with no increase in expression of CIITA isoform I. Expression of CIITA III and IV correlates with decreased collagen type I and increased MHC II gene expression. Exogenous expression of CIITA I, III, and IV, in transiently transfected SMCs, represses collagen type I promoters (COL1A1 and COL1A2) and activates MHC II promoter. Levels of CIITA and RFX5 increase in the nucleus of cells treated with IFN-gamma. Moreover, simvastatin lowers the IFN-gamma-induced expression of RFX5 and MHC II in addition to repressing collagen expression. However, simvastatin does not block the IFN-gamma-induced expression of CIITA III and IV, suggesting a CIITA-independent mechanism. This first demonstration that RFX5 and CIITA isoforms are expressed in SMCs after IFN-gamma stimulation suggest that CIITA could be a key factor in plaque stability in atherosclerosis.

Regulation of MHC class II gene expression by the class II transactivator

MHC class II molecules are pivotal for the adaptive immune system, because they guide the development and activation of CD4+ T helper cells. Fulfilling these functions requires that the genes encoding MHC class II molecules are transcribed according to a strict cell-type-specific and quantitatively modulated pattern. This complex gene-expression profile is controlled almost exclusively by a single master regulatory factor, which is known as the class II transactivator. As we discuss here, differential activation of the three independent promoters that drive expression of the gene encoding the class II transactivator ultimately determines the exquisitely regulated pattern of MHC class II gene expression.

Expression of the MHC class II transactivator (CIITA) type IV promoter in B lymphocytes and regulation by IFN-gamma

The MHC class II transactivator (CIITA), the master regulator of MHC class II (MHC II) expression, is a co-activator that controls MHC II transcription. Human B lymphocytes express MHC II constitutively due to persistent activity of CIITA promoter III (pIII), one of the four potential promoters (pI-pIV) of this gene. Although increases in MHC II expression in B cells in response to cytokines have been observed and induction of MHC II and CIITA by IFN-gamma has been studied in a number of different cell types, the specific effects of IFN-gamma on CIITA expression in B cells have not been studied. To investigate the regulation of CIITA expression by IFN-gamma in B cells, RT-PCR, in vivo and in vitro protein/DNA binding studies, and functional promoter analyses were performed. Both MHC II and CIITA type IV-specific RNAs increased in human B lymphocytes in response to IFN-gamma treatment. CIITA promoter analysis confirmed that pIV is IFN-gamma inducible in B cells and that the GAS and IRF-E sites are necessary for full induction. DNA binding of IRF-1 and IRF-2, members of the IFN regulatory factor family, was up-regulated in B cells in response to IFN-gamma and increased the activity of CIITA pIV. In vivo genomic footprint analysis demonstrated proteins binding at the GAS, IRF-E and E box sites of CIITA pIV. Although CIITA pIII is considered to be the hematopoietic-specific promoter of CIITA, these findings demonstrate that pIV is active in B lymphocytes and potentially contributes to the expression of CIITA and MHC II in these cells.

Differential activation of astrocytes by innate and adaptive immune stimuli

The immunologic privilege of the central nervous system (CNS) makes it crucial that CNS resident cells be capable of responding rapidly to infection. Astrocytes have been reported to express Toll-like receptors (TLRs), hallmark pattern recognition receptors of the innate immune system, and respond to their ligation with cytokine production. Astrocytes have also been reported to respond to cytokines of the adaptive immune system with the induction of antigen presentation functions. Here we have compared the ability of TLR stimuli and the adaptive immune cytokines interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha) to induce a variety of immunologic functions of astrocytes. We show that innate signals LPS- and poly I:C lead to stronger upregulation of TLRs and production of the cytokines IL-6 and TNF-alpha as well as innate immune effector molecules IFN-alpha4, IFN-beta, and iNOS compared with cytokine-stimulated astrocytes. Both innate stimulation and adaptive stimulation induce similar expression of the chemokines CCL2, CCL3, and CCL5, as well as similar enhancement of adhesion molecule ICAM-1 and VCAM-1 expression by astrocytes. Stimulation with adaptive immune cytokines, however, was unique in its ability to induce upregulation of MHC II and the functional ability of astrocytes to activate CD4(+) T cells. These results indicate potentially important and changing roles for astrocytes during the progression of CNS infection.

Regulation of myeloid cell function and major histocompatibility complex class II expression by tumor necrosis factor

OBJECTIVE

Tumor necrosis factor (TNF)-neutralizing agents are the most successful means of ameliorating systemic autoimmune inflammation. Neutralization of TNF, however, is often associated with the development of autoantibodies, particularly to nuclear antigens, and the mechanisms of this are unknown. We undertook this study to analyze the effect of TNF and its neutralization on the expression of major histocompatibility complex class II molecules and on the function of antigen-presenting myeloid cells in rheumatoid arthritis (RA).

METHODS

Monocytes were isolated from the peripheral blood of RA patients before and after anti-TNF monoclonal antibody (mAb) treatment and from the peripheral blood of controls by negative selection, differentiated in vitro to macrophages, and analyzed by flow cytometry for HLA-DR expression. T cell responses to activation by myeloid cells were assessed in proliferation assays, and messenger RNA (mRNA) levels of the class II transactivator (CIITA) were determined by semiquantitative reverse transcriptase-polymerase chain reaction.

RESULTS

HLA-DR expression was significantly reduced on myeloid cells from RA patients with active disease, but was increased to normal levels after anti-TNF mAb treatment. Concordantly, in vitro application of TNF to monocytes from healthy individuals reduced their ability to up-regulate HLA-DR during differentiation to macrophages and, importantly, inhibited their ability to stimulate T cells in mixed lymphocyte reactions. Molecular analysis revealed that the effect of TNF on HLA-DR expression was mediated via suppression of the transcription factor CIITA.

CONCLUSION

The data indicate that TNF decreases HLA-DR expression by reducing CIITA mRNA levels in myeloid cells, functionally resulting in a decreased capacity of myeloid cells to stimulate T cells. Concordantly, ameliorating disease activity in chronic inflammatory diseases by neutralizing TNF restores expression of HLA-DR on myeloid cells as well as the ability of myeloid cells to stimulate T cells. Thus, anti-TNF treatment might lead to augmented T cell activation by myeloid cells, thereby promoting immune responses to (auto)antigens and the development of antinuclear antibodies that are frequently associated with anti-TNF therapy.

Age and diet act through distinct isoforms of the class II transactivator gene in mouse intestinal epithelium

BACKGROUND & AIMS

Normal weaning induces class II major histocompatibility complex (Ia) and invariant chain (Ii) expression in the mouse intestinal epithelium. Because the class II transactivator protein (CIITA) induces Ia and Ii in most cell types, we hypothesized that diet-induced expression of these genes was through CIITA.

METHODS

Mouse litters were split and weaned onto an elemental diet or a normal (complex) chow diet. On days 24, 31, and 45, epithelial cells were isolated from small intestine with EDTA, and the RNA was extracted from both wild-type and interferon (IFN)-gamma receptor knockout mice. Messenger RNA (mRNA) was measured by Northern blotting, RNase protection assay, and real-time polymerase chain reaction and Ia localized by immunohistochemistry.

RESULTS

By day 31, CIITA mRNA was induced in the intestinal epithelium of normally weaned wild-type mice, and this mirrored the expression of Ii chain mRNA. Mice weaned onto an elemental diet did not exhibit Ii mRNA or increased CIITA mRNA in the intestinal epithelium by day 31, but low levels of Ii mRNA were detectable by day 45. Of the 3 isoforms of CIITA, weaning onto a complex diet induced only CIITA IV by day 31. Mice deficient in the IFN-gamma receptor expressed Ia in the epithelium and they also accumulated Ii mRNA (at low levels) by day 45, irrespective of diet. CIITA III mRNA accumulation mirrored the dietary-independent changes of Ii mRNA.

CONCLUSIONS

Two mechanisms regulate Ii in the mouse intestinal epithelium: a rapid one, which is diet-induced acting through CIITA IV; and a slower, dietary-independent pathway, acting through CIITA III.

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.

Inhibition of IFN-gamma-induced class II transactivator expression by a 19-kDa lipoprotein from Mycobacterium tuberculosis: a potential mechanism for immune evasion

Mycobacterium tuberculosis (MTB) persists inside macrophages despite vigorous immune responses. MTB and MTB 19-kDa lipoprotein inhibit class II MHC (MHC-II) expression and Ag processing by a Toll-like receptor 2-dependent mechanism that is shown in this study to involve a defect in IFN-gamma induction of class II transactivator (CIITA). Exposure of macrophages to MTB or MTB 19-kDa lipoprotein inhibited IFN-gamma-induced MHC-II expression, but not IL-4-induced MHC-II expression, by preventing induction of mRNA for CIITA (total, type I, and type IV), IFN regulatory factor-1, and MHC-II. MTB 19-kDa lipoprotein induced mRNA for suppressor of cytokine signaling (SOCS)1 but did not inhibit IFN-gamma-induced Stat1 phosphorylation. Furthermore, the lipoprotein inhibited MHC-II Ag processing in SOCS1(-/-) macrophages. MTB 19-kDa lipoprotein did not inhibit translocation of phosphorylated Stat1 to the nucleus or Stat1 binding to and transactivation of IFN-gamma-sensitive promoter constructs. Thus, MTB 19-kDa lipoprotein inhibited IFN-gamma signaling independent of SOCS1 and without interfering with the activation of Stat1. Inhibition of IFN-gamma-induced CIITA by MTB 19-kDa lipoprotein may allow MTB to evade detection by CD4(+) T cells.

Protection from autoimmune brain inflammation in mice lacking IFN-regulatory factor-1 is associated with Th2-type cytokines

IFN-regulatory factor-1 (IRF-1) is a transcription factor that regulates the expression of IFN-induced genes and type I IFN. It has previously been demonstrated that IRF-1-deficient mice show reduced susceptibility to experimental autoimmune encephalomyelitis (EAE) induced by a peptide from myelin basic protein. To further study the role of IRF-1 in brain inflammation, we analyzed EAE induced by immunization with a myelin oligodendrocyte glycoprotein-derived peptide in 129/Sv mice lacking IRF-1. We found that these mice were almost completely resistant to EAE induction and that this unresponsiveness was intrinsically related to the IRF-1 deficiency of the T cells, but not with any other cell type. Furthermore, we show that the amelioration of EAE was associated with increased production of T(h)2-type and decreased production of T(h)1-type cytokines. These results demonstrate that absence of IRF-1 in myelin-specific T cells results in protection from severe EAE and is associated with a skewing of the T cell response towards T(h)2.

Normal human kidney HLA-DR-expressing renal microvascular endothelial cells: characterization, isolation, and regulation of MHC class II expression

Human, but not murine, renal peritubular and glomerular capillaries constitutively express class II major histocompatibility (MHC) proteins at high levels in normal human kidney. Expression of class II proteins on renal microvascular endothelial cells (RMEC) makes it available to circulating lymphocytes and imparts a surveillance capacity to RMEC for controlling inflammatory responses. In this report, the co-expression of HLA-DR and the endothelial marker CD31 are used to identify RMEC as a distinct population of cells within a standard renal biopsy using flow cytometry. A three-laser, multicolor flow cytometry analysis using Alexa dyes, developed for characterizing the expression of cell surface antigens, identifies RMEC as a population separate from HLA-DR-expressing leukocytes. HLA-DR RMEC co-express HLA-DP and HLA-DQ. RMEC also express the T cell costimulatory factor CD58 but not CD80, CD86, or CD40. On the basis of high HLA-DR expression, RMEC are isolated for culture using fluorescence-activated cell sorting and magnetic beads. Cultured RMEC require normal basal physiologic concentrations of gamma interferon (gammaIFN) to maintain HLA protein expression. This expression is regulated by CIITA, the MHC class II-specific transcription factor. Four tissue-specific promoters have been described for CIITA. In freshly isolated RMEC, RT-PCR and hybridization using specific oligonucleotide probes to CIITA promoter sequences identify only the statin-sensitive gammaIFN-induced promoter IV of CIITA. Therefore, the constitutive expression of HLA-DR on RMEC in normal human kidney is located in a position for immune surveillance, depends on basal physiologic concentrations of gammaIFN, and may be amenable to regulation with statins.

Toxoplasma gondii inhibits MHC class II expression in neural antigen-presenting cells by down-regulating the class II transactivator CIITA

Major histocompatibility complex (MHC) class II expression by microglia and astrocytes is critical for CD4+-mediated immune responses within the central nervous system. Here, we demonstrate that the obligate intracellular parasite, Toxoplasma gondii, down-regulates activation-induced MHC class II expression in human-derived glioblastoma cells as well as in primary astrocytes and microglia from cortices of rat fetuses. Down-regulation of MHC class II proteins was predominantly observed in parasite-positive, but not parasite-negative, host cells of T. gondii-infected cell cultures. MHC class II transcript levels induced by IFN-gamma alone or in combination with TNF-alpha were also clearly diminished after parasitic infection. Furthermore, T. gondii dose-dependently down-regulated the transcript levels of the class II transactivator CIITA. These results suggest that T. gondii partially evade CD4+-mediated intracerebral immune responses, a mechanism which may contribute to long-term persistence of the parasite within the CNS.

The role of the MHC class II transactivator in class II expression and antigen presentation by astrocytes and in susceptibility to central nervous system autoimmune disease

The role of the MHC class II transactivator (CIITA) in Ag presentation by astrocytes and susceptibility to experimental autoimmune encephalomyelitis (EAE) was examined using CIITA-deficient mice and newly created transgenic mice that used the glial fibrillary acidic protein promoter to target CIITA expression in astrocytes. CIITA was required for class II expression on astrocytes. Like class II-deficient mice, CIITA-deficient mice were resistant to EAE by immunization with CNS autoantigen, although T cells from immunized CIITA-deficient, but not class II-deficient, mice proliferated and secreted Th1 cytokines. CIITA-deficient splenic APC presented encephalitogenic peptide to purified wild-type encephalitogenic CD4(+) T cells, indicating that CIITA-independent mechanisms can be used for class II-restricted Ag presentation in lymphoid tissue. CIITA-deficient mice were also resistant to EAE by adoptive transfer of encephalitogenic class II-restricted CD4(+) Th1 cells, indicating that CIITA-dependent class II expression was required for CNS Ag presentation. Despite constitutive CIITA-driven class II expression on astrocytes in vivo, glial fibrillary acidic protein-CIITA transgenic mice were no more susceptible to EAE than controls. CIITA-transfected astrocytes presented peptide Ag, but in contrast to IFN-gamma-activated astrocytes, they could not process and present native Ag. CIITA-transfected astrocytes did not express cathepsin S without IFN-gamma activation, indicating that CIITA does not regulate other elements that may be required for Ag processing by astrocytes. Although our results demonstrate that CIITA-directed class II expression is required for EAE induction, CIITA-directed class II expression by astrocytes does not appear to increase EAE susceptibility. These results do not support the role of astrocytes as APC for class II-restricted Ag presentation during the induction phase of EAE.

Regulation of class II MHC expression in APCs: roles of types I, III, and IV class II transactivator

Class II transactivator (CIITA) is necessary for expression of class II MHC (MHC-II) molecules. In mice, CIITA expression is regulated by three promoters (pI, pIII, and pIV), producing types I, III, and IV CIITA. The relative roles of different CIITA types remain unclear. Unstimulated bone marrow-derived macrophages expressed low levels of CIITA mRNA; type I CIITA was nine times more abundant than type IV (type III CIITA was barely detected). Exposure to IFN-gamma (6 h) dramatically increased types I and IV CIITA mRNA to similar absolute levels. Type IV CIITA declined over time, but type I was stable for over 72 h. Thus, the dominant form of CIITA evolved with time during activation by IFN-gamma, and type I CIITA explained prolonged expression of MHC-II by macrophages. mRNA half-life was shorter for type I than type IV CIITA, suggesting that sustained transcription contributed to stable expression of type I CIITA induced by IFN-gamma. Splenic B cells expressed mRNA for type III CIITA but very little for types I or IV. Treatment with IL-4 increased surface expression of MHC-II protein, but mRNA for MHC-II and CIITA (total, I, III, and IV) remained unchanged, suggesting posttranslational regulation. Splenic dendritic cells expressed type I CIITA but little type III or IV; CpG DNA induced their maturation and decreased types I and III CIITA, consistent with decreased MHC-II protein synthesis. CIITA types differ in regulation in various APCs under different stimuli, and the predominant type of CIITA varies at different stages of APC activation.

Malignant glioma cells use MHC class II transactivator (CIITA) promoters III and IV to direct IFN-gamma-inducible CIITA expression and can function as nonprofessional antigen presenting cells in endocytic processing and CD4(+) T-cell activation

Malignant gliomas (MGs), lethal human central nervous system (CNS) neoplasms, contain tumor infiltrating lymphocytes (TIL). Although MHC class II molecules are frequently detected on MG cells, suggesting that they may be capable of antigen (Ag) presentation to CD4(+) T cells, deficiencies in CD4(+) T-cell activation are associated with these nonimmunogenic tumors. We evaluated regulation of the MHC class II transactivator (CIITA), the key intermediate that controls class II expression, in MG cells and tested whether MG cells could process native Ag. After interferon-gamma (IFN-gamma) stimulation, MG cells upregulated CIITA and class II molecules. IFN-gamma-inducible CIITA expression in MG cells, as well as primary human astrocytes, was directed by two CIITA promoters, pIV, the promoter for IFN-gamma-inducible CIITA expression in nonprofessional antigen-presenting cells (APC), and pIII, the promoter that directs constitutive CIITA expression in B cells. Both pIII and pIV directed CIITA transcription in vivo in MGs and ex vivo in IFN-gamma-activated primary MG cultures. We also demonstrate for the first time that MG cells can process native Ag for presentation to CD4(+) MHC class II-restricted Th1 cells, indicating that MG cells can serve as nonprofessional APC. CIITA may be a key target to modulate MHC class II expression, which could augment immunogenicity, Ag presentation, and CD4(+) T-cell activation in MG therapy.

Increased binding of IFN regulating factor 1 mediates the synergistic induction of CIITA by IFN-gamma and tumor necrosis factor-alpha in human thyroid carcinoma cells

Expression of MHC class II molecules is restricted to professional antigen-presenting immune cells, but it can be induced by IFN-gamma in other cell types. Thyroid cells have been shown to induce class II expression (mainly HLA-DR) following stimulation with IFN-gamma and addition of tumor necrosis factor (TNF)-alpha synergistically enhanced this expression. Class II transactivator (CIITA) has been implicated as the master regulator of MHC class II molecules and its transcription has been shown to be regulated from four different promoters, one of which is responsible for its induction by IFN-gamma. The aim of this study was to find whether CIITA is synergistically induced by IFN-gamma and TNF-alpha in the human thyroid MRO-87-1 cell line, and to investigate the molecular mechanisms responsible for this synergism. We have demonstrated that IFN-gamma and TNF-alpha synergistically induce HLA-DRalpha and CIITA mRNAs, but prolonged incubation resulted in the inhibition of CIITA mRNA accumulation. Several potential mechanisms that could explain the synergistic effect were explored. NF-kappaB did not bind the CIITA inducible promoter and addition of SN50, which inhibits NF-kappaB translocation to the nucleus, did not change the synergistic effect. Furthermore, IFN-gamma did not induce IkappaBalpha degradation. Synergistic activation of signal transducer and activator of transcription (STAT)-1 or IFN regulating factor (IRF)-1 was not observed, and STAT-1 did not bind the CIITA inducible promoter. IRF-1, although not synergistically induced or activated, bound synergistically to its specific cis element on the CIITA type IV promoter. Thus we propose that IRF-1 binding mediates the synergistic induction of HLA-DRalpha and CIITA in thyroid cells.

Immune function of astrocytes

Astrocytes are the major glial cell within the central nervous system (CNS) and have a number of important physiological properties related to CNS homeostasis. The aspect of astrocyte biology addressed in this review article is the astrocyte as an immunocompetent cell within the brain. The capacity of astrocytes to express class II major histocompatibility complex (MHC) antigens and costimulatory molecules (B7 and CD40) that are critical for antigen presentation and T-cell activation are discussed. The functional role of astrocytes as immune effector cells and how this may influence aspects of inflammation and immune reactivity within the brain follows, emphasizing the involvement of astrocytes in promoting Th2 responses. The ability of astrocytes to produce a wide array of chemokines and cytokines is discussed, with an emphasis on the immunological properties of these mediators. The significance of astrocytic antigen presentation and chemokine/cytokine production to neurological diseases with an immunological component is described.

Upregulation of transcription factors controlling MHC expression in multiple sclerosis lesions

The expression of major histocompatibility complex (MHC) class I and class II in the CNS has received considerable interest because of its importance in neurodegenerative or inflammatory diseases, such as multiple sclerosis (MS). However, at the moment nothing is known about the expression patterns of transcription factors controlling MHC expression in MS lesions. Here, we performed an extensive immunohistochemical analysis on MS affected postmortem brain tissue to determine the cellular localization and distribution of different MHC-controlling transcription factors. We show that phagocytic macrophages in active demyelinating MS lesions displayed a moderate to strong immunostaining of the MHC-specific transcription factors RFX and CIITA, as well as the general transcription factors NF-kappaB, IRF1, STAT1, USF, and CREB, which was congruent with a strongly enhanced expression of HLA-DR, HLA-DQ, HLA-DP, and HLA class I. In the normal-appearing white matter (NAWM), clusters of activated microglial cells forming preactive lesions displayed an overall stronger expression level of these transcription factors, combined with a strong to intense level of MHC class I and class II immunostaining. In general, astrocytes and oligodendrocytes either did not express, or weakly expressed, these transcription factors, correlating with a lack of MHC class II and weak MHC class I expression. Together, the elevated expression level of transcription factors governing expression of MHC class I and class II molecules in activated microglial cells and phagocytic macrophages strongly suggests a general state of microglial cell activation in MS lesions.

Maturation of dendritic cells is accompanied by rapid transcriptional silencing of class II transactivator (CIITA) expression

Cell surface expression of major histocompatibility complex class II (MHCII) molecules is increased during the maturation of dendritic cells (DCs). This enhances their ability to present antigen and activate naive CD4(+) T cells. In contrast to increased cell surface MHCII expression, de novo biosynthesis of MHCII mRNA is turned off during DC maturation. We show here that this is due to a remarkably rapid reduction in the synthesis of class II transactivator (CIITA) mRNA and protein. This reduction in CIITA expression occurs in human monocyte-derived DCs and mouse bone marrow-derived DCs, and is triggered by a variety of different maturation stimuli, including lipopolysaccharide, tumor necrosis factor alpha, CD40 ligand, interferon alpha, and infection with Salmonella typhimurium or Sendai virus. It is also observed in vivo in splenic DCs in acute myelin oligodendrocyte glycoprotein induced experimental autoimmune encephalitis. The arrest in CIITA expression is the result of a transcriptional inactivation of the MHC2TA gene. This is mediated by a global repression mechanism implicating histone deacetylation over a large domain spanning the entire MHC2TA regulatory region.

Selective abrogation of major histocompatibility complex class II expression on extrahematopoietic cells in mice lacking promoter IV of the class II transactivator gene

MHC class II (MHCII) molecules play a pivotal role in the induction and regulation of immune responses. The transcriptional coactivator class II transactivator (CIITA) controls MHCII expression. The CIITA gene is regulated by three independent promoters (pI, pIII, pIV). We have generated pIV knockout mice. These mice exhibit selective abrogation of interferon (IFN)-gamma-induced MHCII expression on a wide variety of non-bone marrow-derived cells, including endothelia, epithelia, astrocytes, and fibroblasts. Constitutive MHCII expression on cortical thymic epithelial cells, and thus positive selection of CD4(+) T cells, is also abolished. In contrast, constitutive and inducible MHCII expression is unaffected on professional antigen-presenting cells, including B cells, dendritic cells, and IFN-gamma-activated cells of the macrophage lineage. pIV(-/-) mice have thus allowed precise definition of CIITA pIV usage in vivo. Moreover, they represent a unique animal model for studying the significance and contribution of MHCII-mediated antigen presentation by nonprofessional antigen-presenting cells in health and disease.

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.

Impaired class II transactivator expression in mice lacking interferon regulatory factor-2

Class II transactivator (CIITA) is required for both constitutive and inducible expression of MHC class II genes. IFN-gamma induced expression of CIITA in various cell types is directed by CIITA type IV promoter. The two transactivators, STAT1 and IRF-1, mediate the IFN-gamma activation of the type IV promoter by binding to the GAS and IRF-E of the promoter, respectively. In addition to IRF-1, IRF-2, another member of the IRF family, also activates the human CIITA type IV promoter, and IRF-2 cooperates with IRF-1 to activate the promoter in transient transfection assays. IRF-1 and IRF-2 can co-occupy the IRF-E of the human CIITA type IV promoter. To understand the effect of loss of IRF-2 on the endogenous CIITA expression, we assayed for CIITA expression in IRF-2 knock-out mice. Both basal and IFN-gamma induced CIITA expression were reduced in IRF-2 knock-out mice. At least half of the amount of inducible CIITA mRNA depends on IRF-2. The reduction of IFN-gamma induced CIITA mRNA in IRF-2 knock-out mice was due to the reduction of the type IV CIITA mRNA induction. The reduction of basal CIITA mRNA was apparently due to the reduction of CIITA mRNA originating from other promoters. These data indicate that IRF-2, like IRF-1, plays a critical role in the regulation of the endogenous CIITA gene. The implications in understanding the previously described phenotypes of IRF-2 defective mice are discussed.

The Smad3 protein is involved in TGF-beta inhibition of class II transactivator and class II MHC expression

TGF-beta is a immunoregulatory cytokine that inhibits class II MHC expression in a variety of cell types. Previous studies have shown that the class II MHC transactivator (CIITA), a master regulator that controls class II MHC expression, is targeted by TGF-beta for repression of IFN-gamma-induced class II MHC expression in astrocytes. The mechanism(s) underlying the TGF-beta inhibitory effect is not understood. In this study, we demonstrate that TGF-beta inhibition of CIITA expression occurs at the transcriptional level, and that both constitutive and IFN-gamma-induced human CIITA type IV promoter activity is inhibited by TGF-beta. TGF-beta does not affect the signaling events that mediate IFN-gamma activation of CIITA expression; i.e, TGF-beta does not inhibit IFN-gamma-induced STAT-1alpha phosphorylation and/or DNA binding ability, nor is IFN-gamma induction of IFN regulatory factor affected. The inhibitory effect of TGF-beta on the type IV CIITA promoter is mediated through a promoter region within 80 bp from the transcription start site. Elimination of TGF-beta inhibition of class II MHC and CIITA expression in Smad3-deficient astrocytes, as well as restoration of the inhibitory effect by overexpression of the Smad3 protein, demonstrates that Smad3 is essential in mediating TGF-beta inhibition of CIITA and class II MHC expression.

IFN-gamma regulation of class II transactivator promoter IV in macrophages and microglia: involvement of the suppressors of cytokine signaling-1 protein

The discovery of the class II transactivator (CIITA) transcription factor, and its IFN-gamma-activated promoter (promoter IV), have provided new opportunities to understand the molecular mechanisms of IFN-gamma-induced class II MHC expression. Here, we investigated the molecular regulation of IFN-gamma-induced murine CIITA promoter IV activity in microglia/macrophages. In the macrophage cell line RAW264.7, IFN-gamma inducibility of CIITA promoter IV is dependent on an IFN-gamma activation sequence (GAS) element and adjacent E-Box, and an IFN response factor (IRF) element, all within 196 bp of the transcription start site. In both RAW cells and the microglia cell line EOC20, two IFN-gamma-activated transcription factors, STAT-1alpha and IRF-1, bind the GAS and IRF elements, respectively. The E-Box binds upstream stimulating factor-1 (USF-1), a constitutively expressed transcription factor. Functionally, the GAS, E-Box, and IRF elements are each essential for IFN-gamma-induced CIITA promoter IV activity. The effects of the suppressors of cytokine signaling-1 (SOCS-1) protein on IFN-gamma-induced CIITA and class II MHC expression were examined. Ectopic expression of SOCS-1 inhibits IFN-gamma-induced activation of CIITA promoter IV and subsequent class II MHC protein expression. Interestingly, SOCS-1 inhibits the constitutive expression of STAT-1alpha and its IFN-gamma-induced tyrosine phosphorylation and binding to the GAS element in CIITA promoter IV. As well, IFN-gamma-induced expression of IRF-1 and its binding to the IRF element is inhibited. These results indicate that SOCS-1 may be responsible for attenuating IFN-gamma-induced CIITA and class II MHC expression in macrophages.

BLIMP-I mediates extinction of major histocompatibility class II transactivator expression in plasma cells

Class II transactivator (CIITA), a coactivator required for class II major histocompatibility complex (MHC) transcription, is expressed in B cells but extinguished in plasma cells. This report identifies B lymphocyte-induced maturation protein I (BLIMP-I), a transcriptional repressor that is capable of triggering plasma cell differentiation, as a developmentally regulated repressor of CIITA transcription. BLIMP-I represses the B cell-specific promoter of the human gene that encodes CIITA (MHC2TA) in a binding site-dependent manner. Decreased CIITA correlates with increased BLIMP-I during plasma cell differentiation in cultured cells. Ectopic expression of BLIMP-I represses endogenous mRNA for CIITA and the CIITA targets, class II MHC, invariant chain and H2-DM (the murine equivalent of HLA-DM) in primary splenic B cells as well as 18-81 pre-B cells. Thus, the BLIMP-I program of B cell differentiation includes loss of antigen presentation via extinction of CIITA expression.

Cyclosporine inhibits class II major histocompatibility antigen presentation by xenogeneic endothelial cells to human T lymphocytes by altering expression of the class II transcriptional activator gene

BACKGROUND

Cyclosporine (CsA) is currently given to recipients of vascularized xenografts as part of the immunosuppressive regimen required to prevent the hyperacute rejection phase. The effects of CsA on non-lymphoid immune cells, such as endothelial cells (ECs), have not been well characterized and sometimes seem contradictory, because both protective and adverse effects have been reported. In the present study, we investigated in vitro whether CsA could alter the antigenicity of activated porcine aortic endothelial cells (PAECs) by reducing class I and class II MHC antigen expression.

METHODS

The effect of CsA on MHC antigen expression during tumor necrosis factor (TNF)-alpha- or lymphocyte-mediated PAEC activation was evaluated in vitro by flow cytometry and correlated to the ability of porcine ECs to promote human T lymphocyte proliferation. The effect of CsA on class II MHC antigen mRNA expression was also analyzed and related to class II transcriptional activator (CIITA) mRNA expression.

RESULTS

Flow cytometry analysis showed that TNF-alpha-mediated induction of class II MHC antigen expression on PAECs was completely inhibited by CsA, whereas expression of class I MHC was reduced by 50%. The inhibition was dose dependent (at drug concentrations ranging from 2.5 microg/ml to 20.0 microg/ml) and was consistently observed at all time points (24-72 hr) during the activation period. Decreased MHC antigen expression dramatically reduced the ability of PAECs to further promote human T-cell proliferation. Similar levels of inhibition were achieved using an anti-porcine class II MHC blocking monoclonal antibody. Pretreatment of PAECs with CsA for 4 hr before coculture with human peripheral blood leukocytes efficiently blocked the induction on PAECs of E-selectin and class II MHC antigens and inhibited overexpression of class I antigens. Semiquantitative reverse transcriptase-polymerase chain reaction experiments showed that CsA markedly reduced the steady-state level of porcine class II (SLA-DRA and SLA-DQA) mRNA at 16 hr, compared with PAECs stimulated with TNF-alpha alone. The reduced level of class II MHC mRNA was associated with a lack of CIITA expression at this time point, suggesting that CsA could alter transcription or promote the rapid decay of CIITA mRNA.

CONCLUSION

Our study indicates that CsA could play a role in preventing porcine MHC antigens being directly presented to human T lymphocytes by xenogeneic ECs.