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

PU.1 binds to a distal regulatory element that is necessary for B cell-specific expression of CIITA

The transcriptional coactivator CIITA regulates MHC class II genes. In the mouse, CIITA is expressed from three distinct promoters (pI, pIII, and pIV) in a developmental and cell type-specific manner with pIII being responsible for B lymphocyte-specific expression. Although the promoter proximal sequences that regulate CIITA in B cells have been described, nothing is known about additional distal elements that may regulate its expression in B cells. Sequence homology comparisons, DNase I hypersensitivity assays, and histone modification analysis revealed a potential regulatory element located 11 kb upstream of pIII. Deletion of this element, termed hypersensitive site 1 (HSS1), in a bacterial artificial chromosome encoding the entire CIITA locus and surrounding genes, resulted in a complete loss of CIITA expression from the bacterial artificial chromosome following transfection into B cells. HSS1 and pIII displayed open chromatin architecture features in B cell but not in plasma cell lines, which are silenced for CIITA expression. PU.1 was found to bind HSS1 and pIII in B cells but not in plasma cells. Depletion of PU.1 by short hairpin RNA reduced CIITA expression. Chromatin conformation capture assays showed that HSS1 interacted directly with pIII in B cells and that PU.1 was important for this interaction. These results provide evidence that HSS1 is required for B cell-specific expression of CIITA and that HSS1 functions by interacting with pIII, forming a long-distance chromatin loop that is partly mediated through PU.1.

Monocytes and macrophages regulate immunity through dynamic networks of survival and cell death

Monocytes and macrophages are central cells of the innate immune system, responsible for defending against diverse pathogens. While they originate from a common myeloid precursor and share functions in innate immunity, each has a very distinct life span finely tuned by the apoptotic caspases. Normally, circulating monocytes are short-lived and undergo spontaneous apoptosis on a daily basis. Macrophages, however, have a longer life span. In chronic inflammatory diseases and, as recently recognized, in the tumor microenvironment, the inhibition of the apoptotic program promotes monocyte survival contributing to the accumulation of macrophages and the persistence of an inflammatory milieu. A complex network of differentiation factors and inflammatory stimuli determine monocyte/macrophage life span by blocking the apoptotic pathway and activating a myriad of survival pathways. Our understanding of apoptosis has flourished over the last decade, and its relevance in the regulation of the immune system is now indisputable. Nevertheless, how the complicated networks of survival and apoptotic regulators are integrated to determine cellular life span remains elusive. This review summarizes the contribution of the caspases and their regulators in monocyte/macrophage cell fate and discusses how these molecules orchestrate the initiation, maintenance, and resolution of inflammation. More provocatively, we discuss possible strategies to control inflammation by manipulating leukocyte life span.

Differential nerve injury-induced expression of MHC class II in the mouse correlates to genetic variability in the type I promoter of C2ta

Major histocompatibility complex (MHC) class II is of critical importance for the induction of immune responses. Levels of MHC class II in the nervous system are normally low, but expression is up-regulated in many disease conditions. In rat and human, variation in the MHC class II transactivator gene (C2ta) is associated with differential expression of MHC class II and susceptibility to autoimmune disease. Here we have characterized the response to facial nerve transection in 7 inbred mouse strains (C57BL/6J, DBA/2J, 129X1/SvJ, BALB/cJ, SJL/J, CBA/J, and NOD). The results demonstrate differences in expression of C2ta and markers for MHC class I and II expression, glial activation, and T cell infiltration. Expression levels of C2ta and Cd74 followed similar patterns, in contrast to MHC class I and markers of glial activation. The regulatory region of the C2ta gene was subsequently sequenced in the four strains (C57BL/6/J, DBA/2J, SJL/J and 129X1/SvJ) that represented the phenotypical extremes with regard to C2ta/Cd74 expression. We found 3 single nucleotide polymorphisms in the type I (pI) and type III (pIII) promoters of C2ta, respectively. Higher expression of pI in 129X1/SvJ correlated with the pI haplotype specific for this strain. Furthermore, congenic strains carrying the 129X1/SvJ C2ta allele on B6 background displayed significantly higher C2ta and Cd74 expression compared to parental controls. We conclude that genetic polymorphisms in the type I promoter of C2ta regulates differential expression of MHC class II, but not MHC class I, Cd3 and other markers of glial activation.

Post-translational modification of {alpha}-dystroglycan is not critical for lymphocytic choriomeningitis virus receptor function in vivo

alpha-Dystroglycan (alpha-DG) is a ubiquitously expressed molecule that has been identified as a cellular receptor for lymphocytic choriomeningitis virus (LCMV) and other arenaviruses. Recently, it was demonstrated that LCMV receptor function is critically dependent on post-translational modifications, namely glycosylation. In particular, it was shown that O-mannosylation, a rare type of mammalian O-linked glycosylation, is important in determining the binding of LCMV to its cellular receptor. All studies carried out so far showed a dependence on glycosylation in LCMV receptor function in vitro. This work extended these studies to two in vivo models of alpha-DG hypoglycosylation. The results confirm earlier findings on the in vitro dependence of carbohydrate modifications in LCMV receptor function. However, experiments in animal models showed that this dependence was only very weak in vivo. It is likely that alternative receptors or alternative entry pathways may account for this attenuated in vivo phenotype.

Autoantigen-specific interactions with CD4+ thymocytes control mature medullary thymic epithelial cell cellularity

Medullary thymic epithelial cells (mTECs) are specialized for inducing central immunological tolerance to self-antigens. To accomplish this, mTECs must adopt a mature phenotype characterized by expression of the autoimmune regulator Aire, which activates the transcription of numerous genes encoding tissue-restricted self-antigens. The mechanisms that control mature Aire(+) mTEC development in the postnatal thymus remain poorly understood. We demonstrate here that, although either CD4(+) or CD8(+) thymocytes are sufficient to sustain formation of a well-defined medulla, expansion of the mature mTEC population requires autoantigen-specific interactions between positively selected CD4(+) thymocytes bearing autoreactive T cell receptor (TCR) and mTECs displaying cognate self-peptide-MHC class II complexes. These interactions also involve the engagement of CD40 on mTECs by CD40L induced on the positively selected CD4(+) thymocytes. This antigen-specific TCR-MHC class II-mediated crosstalk between CD4(+) thymocytes and mTECs defines a unique checkpoint in thymic stromal development that is pivotal for generating a mature mTEC population competent for ensuring central T cell tolerance.

Distinct regulation of MHC molecule expression on astrocytes and microglia during viral encephalomyelitis

The potential interplay of glial cells with T cells during viral induced inflammation was assessed by comparing major histocompatibility complex molecule upregulation and retention on astrocytes and microglia. Transgenic mice expressing green fluorescent protein under control of the astrocyte‐specific glial fibrillary acidic protein promoter were infected with a neurotropic coronavirus to facilitate phenotypic characterization of astrocytes and microglia using flow cytometry. Astrocytes in the adult central nervous system up‐regulated class I surface expression, albeit delayed compared with microglia. Class II was barely detectable on astrocytes, in contrast to potent up‐regulation on microglia. Maximal MHC expression in both glial cell types correlated with IFN‐γ levels and lymphocyte accumulation. Despite a decline of IFN‐γ concomitant to virus clearance, MHC molecule expression on glia was sustained. These data demonstrate distinct regulation of both class I and class II expression by microglia and astrocytes in vivo following viral induced inflammation. Furthermore, prolonged MHC expression subsequent to viral clearance implies a potential for ongoing presentation. © 2007 Wiley‐Liss, Inc.

Stat2-dependent regulation of MHC class II expression

MHC type II (MHC II) expression is tightly regulated in macrophages and potently induced by IFN-gamma (type II IFN). In contrast, type I IFNs (IFN-Is), which are far more widely expressed, fail to induce MHC II expression, even though both classes of IFNs direct target gene expression through Stat1. The unexpected finding that IFN-Is effectively induce MHC II expression in Stat2(-/-) macrophages provided an opportunity to explore this conundrum. The ensuing studies revealed that deletion of Stat2, which uniquely transduces signals for IFN-Is, leads to a loss in the IFN-I-dependent induction of suppressor of cytokine signaling-1. Impairment in the expression of this important negative regulator led to a striking prolongation in IFN-I-dependent Stat1 activation, as well as enhanced expression of the target gene, IFN-regulatory factor-1. The prolonged activity of these two transcription factors synergized to drive the transcription of CIITA, the master regulator of MHC II expression, analogous to the pattern observed in IFN-gamma-treated macrophages. Thus, IFN-I-dependent suppressor of cytokine signaling-1 expression plays an important role in distinguishing the biological response between type I and II IFNs in macrophages.

Inhibitory effects of anti-CII TA M1-RNA on IFN-γ induced major histocompatibility complex class II antigens expression on cultured human chondrocytes

Major histocompatibility complex class II (MHC-II) trans-activator (CII TA) has been shown to be required for constitutive and IFN-gamma-induced MHC-II transcription. This study investigated the inhibitory effect of anti-CII TA M1-RNA on expression of MHC-II in chondrocytes in response to IFN-gamma. M1-RNAs with different guide sequence (GS) recognizing 452 or 3408 sites in CII TA (M1-452-GS and M1-3408-GS, respectively) were cloned into pUC19 vector. Target mRNA (3176-3560) in CII TA was obtained from Raji cell and inserted into pGEM-7zf(+) plasmid. The recombinant M1-RNAs and their target mRNA were incubated in a cell-free condition. It showed that only M1-3408-GS could cleave the target mRNA exclusively. M1-3408-GS was also cloned into psNAV vector (named pA3408). Chondrocytes was stably transfected with pA3408 and expressions of classical MHC-II (HLA-DR, -DP, -DQ) were analyzed by Flow Cytometry. The level of CII TA mRNA was measured by RT-PCR. Peripheral blood mono-nucleated cells (PBMNCs) were stimulated by pA3408-positive chondrocytes in mixed lymphocyte reaction, and proliferation of PBMNCs and IL-2 mRNA were detected. The expression of HLA-DR and HLA-DP on pA3408-positive chondrocytes in response to IFN-gamma decreased 73.00%+/-5.24%, 88.47%+/-2.02%, respectively (P<0.05); So did the content of CII TA mRNA (70.11%+/-5.79%, P<0.05). Proliferation of PBMNCs and production of IL-2 mRNA were both inhibited by pA3408 in mixed lymphocyte reaction. This is the first description that anti-CII TA M1-RNA could prevent IFN-gamma-induced CII TA transcription and results in a decreased MHC-II expression in chondrocytes.

Pathologic expression of MHC class II is driven by mitogen-activated protein kinases

The class II transactivator (CIITA) is the master regulator of MHC class II molecules (MHC II). In melanoma, the MHC II are constitutively expressed due to an abnormal transcription of CIITA from its promoter III (pIII), and requires the presence of a 1-kb enhancer located upstream from this latter. Since mitogen-activated protein kinases (MAPK) have been shown to be activated in most melanomas, we sought to analyze their possible involvement in CIITA expression. Using chemical inhibitors and dominant-negative constructs of MAPK-ERK kinase (Mek1) and MAPK-JNK, we evidenced the inhibition of MHC II and CIITA expression in melanoma cell lines displaying activated MAPK. Transcriptional regulation by MAPK is known to involve the AP-1 transcription factor family. Sequence analysis revealed an AP-1-responsive motif in the enhancer of CIITA pIII at -5954/-5947 from the site of transcription initiation. Its mutagenesis reduced CIITA expression four- to fivefold in melanoma cell lines and alleviated the effect of dominant-negative constructs of the MAPK pathway. Together, our findings demonstrate that MAPK-ERK and MAPK-JNK are regulators of CIITA transcription in melanoma, and pinpoint an AP-1-responsive site in the CIITA gene pIII. This should have considerable impact on our understanding of the physio-pathologic expression of MHC II.

Hexim1 sequesters positive transcription elongation factor b from the class II transactivator on MHC class II promoters

The class II transactivator (CIITA) is the master integrator of expression of MHC class II genes. It interacts with variety of basal transcription factors to initiate and elongate transcription of these genes. Among others, it recruits positive transcription elongation factor b (P-TEFb) to MHC class II promoters. In cells, P-TEFb is found in small active or large inactive complexes. The large complex is composed of P-TEFb, 7SK small nuclear RNA, and hexamethylene bisacetamide-inducible protein 1 (Hexim1). The present study identifies Hexim1 as a potent inhibitor of CIITA-mediated transcription. Not only the exogenously expressed but also IFN-gamma-induced CIITA was inhibited by Hexim1. This inhibition did not result from an association between Hexim1 and CIITA but depended on the intact Cyclin T1-binding domain in Hexim1. Importantly, Hexim1 sequestered P-TEFb from CIITA, as documented by binding competition and ChIP assays. Conversely, the depletion of Hexim1 from cells by siRNA increased CIITA-mediated transcription. Thus, modulating ratios between active and inactive P-TEFb complexes is an additional mechanism of regulating transcriptional activators such as CIITA.

Circulating, soluble forms of major histocompatability complex antigens are not exosome-associated

In vitro studies have shown that soluble MHC (sMHC) released by cell lines is bound to nano-vesicles termed exosomes. It is thought that exosomes may represent the major reservoir of sMHC class I and II molecules in biological fluids. However, most studies have been confined to in vitro assays performed with cell lines. We show here that sMHC in the serum or plasma differs from exosome-bound sMHC in five ways: In contrast to exosome-associated sMHC, circulating sMHC is of low density, has a low apparent molecular mass (40-300 kDa) and is not detergent-labile. Moreover, the majority of MHC class II isoforms and MHC class I in blood are not physically linked and circulating HLA-DR is accessible to an antibody specific for the HLA-DR alpha-chain intracellular epitope, which is masked by its association with cellular or exosomal membranes. Finally, utilizing transcriptional activator of murine MHC class II (C2ta) promoter-mutant mice, we showed that the release of sMHC class II into the circulation is dependent on the C2ta pI promoter, but not pIII or pIV. This suggests that myeloid dendritic cells and/or macrophages, which preferentially use promoter pI of the C2ta gene, produce most of the sMHC class II found in the circulation.

Repression of IFN-gamma induction of class II transactivator: a role for PRDM1/Blimp-1 in regulation of cytokine signaling

MHC class II is expressed in restricted lineages and is modulated in response to pathogens and inflammatory stimuli. This expression is controlled by MHC CIITA, which is transcribed from multiple promoters. Although factors required for induction of CIITA are well characterized, less is known about the mechanisms leading to repression of this gene. During plasma cell differentiation, B lymphocyte-induced maturation protein-1 (PRDM1/Blimp-1) represses promoter (p)III of CIITA, responsible for constitutive expression in B cells. pIV is inducible by IFN-gamma in epithelia, macrophages and B cells. An IFN regulatory factor-element (IRF-E) in CIITA-pIV, which is bound by IRF-1 and IRF-2, is necessary for this response. This site matches the PRDM1/Blimp-1 consensus binding site, and PRDM1/Blimp-1 is expressed in cell lineages in which this promoter is operative. We, therefore, investigated whether PRDM1 regulates CIITA-pIV and found that PRDM1 bound to CIITA-pIV in vivo and the IRF-E in vitro. PRDM1 repressed IFN-gamma-mediated induction of a CIITA-pIV luciferase reporter in a fashion dependent on an intact consensus sequence and competes with IRF-1/IRF-2 for binding to the IRF-E and promoter activation. In human myeloma cell lines that express IRFs, PRDM1 occupancy of CIITA-pIV was associated with resistance to IFN-gamma stimulation, while short interfering RNA knockdown of PRDM1 led to up-regulation of CIITA. Our data indicate that PRDM1 is a repressor of CIITA-pIV, identifying a target of particular relevance to macrophages and epithelia. These findings support a model in which PRDM1/Blimp-1 can modulate the cellular response to IFN-gamma by competing with IRF-1/IRF-2 dependent activation of target promoters.

Functional CD8+ but not CD4+ T cell responses develop independent of thymic epithelial MHC

The role of nonthymic epithelial (non-TE) MHC in T cell repertoire selection remains controversial. To analyze the relative roles of thymic epithelial (TE) and non-TE MHC in T cell repertoire selection, we have generated tetraparental aggregation chimeras (B6-nude<=>BALB/c and B6<=>BALB/c-nude) harboring T and B cells from both parents, whereas TE cells originated exclusively from the non-nude donor. These chimeras mounted protective virus-specific TE and non-TE MHC-restricted T cell responses. To further evaluate whether non-TE MHC alone was sufficient to generate a functional T cell repertoire, we generated tetraparental aggregation chimeras lacking MHC class II (B6-nude<=>MHCII(-/-)) or both MHC molecules (B6-nude<=>MHCI(-/-)II(-/-)) on TE cells, but not on cells of B6-nude origin. Chimeras with MHC-deficient TE cells mounted functional virus-specific CD8+ but not CD4+ T cell responses. Thus, maturation of functional CD4+ T cell responses required MHC class II on thymic epithelium, whereas CD8+ T cells matured in the absence of TE MHC.

Collagen and major histocompatibility class II expression in mesenchymal cells from CIITA hypomorphic mice

Major histocompatibility class II (MHC II) transactivator (CIITA) is critical for interferon-gamma (IFN-gamma)-induced repression of collagen [Xu, Y., Wang, L., Buttice, G., Sengupta, P.K., Smith, B.D., 2004. Major histocompatibility class II transactivator (CIITA) mediates repression of collagen (COL1A2) transcription by interferon gamma (IFN-gamma). J. Biol. Chem. 279, 41319-41332] and activation of MHC II transcription. To better understand the role of CIITA and IFN-gamma induced repression of collagen, mesenchymal cells (lung fibroblasts, adventitial fibroblasts, and smooth muscle cells) were isolated from a CIITA deficient mouse (C2ta(tm1Ccum)). IFN-gamma induced MHC II expression and repressed collagen type I expression in all three cell types isolated from the wild type background. As expected, IFN-gamma treatment of cells isolated from CIITA deficient mice did not induce MHC II production or activate the MHC II promoter. Interestingly, collagen gene expression and promoter activity was similar to that of wild type. Moreover, IFN-gamma induced CIITA mRNA and a truncated form of CIITA protein in all cells isolated from CIITA deficient mice. Most importantly, truncated CIITA occupied the collagen alpha 2(I) gene (col1a2) transcription start site during IFN-gamma treatment, but it did not occupy the MHC II promoter as judged by chromatin immunoprecipitation assays. Exogenous expression of a similar truncated form of CIITA maintained its ability to repress col1a2 transcription, but lost its ability to activate MHC II gene transcription suggesting a role for the CIITA C-terminal domain in activation, but not repression. IFN-gamma induced primarily types I and IV CIITA isoforms in the mouse cells. All three isoforms of CIITA were capable of repressing col1a2 and activating MHC II gene transcription. These data suggest that the previously described CIITA knockout mouse carries a hypomorphic mutation, rather than a null mutation. The removal of the leucine rich region in CIITA blocks activation of MHC II without altering repression of collagen transcription.

Epigenetic Regulation during B Cell Differentiation Controls CIITA Promoter Accessibility

B cell to plasma cell maturation is marked by the loss of MHC class II expression. This loss is due to the silencing of the MHC class II transcriptional coactivator CIITA. In this study, experiments to identify the molecular mechanism responsible for CIITA silencing were conducted. CIITA is expressed from four promoters in humans, of which promoter III (pIII) controls the majority of B cell-mediated expression. Chromatin immunoprecipitation assays were used to establish the histone code for pIII and determine the differences between B cells and plasma cells. Specific histone modifications associated with accessible promoters and transcriptionally active genes were observed at pIII in B cells but not in plasma cells. A reciprocal exchange of histone H3 lysine 9 acetylation to methylation was also observed between B cells and plasma cells. The lack of histone acetylation correlated with an absence of transcription factor binding to pIII, particularly that of Sp1, PU.1, CREB, and E47. Intriguingly, changes in chromatin architecture of the 13-kb region encompassing all CIITA promoters showed a remarkable deficit in histone H3 and H4 acetylation in plasma cells, suggesting that the mechanism of silencing is global. When primary B cells were differentiated ex vivo, most of the histone marks associated with pIII activation and expression were lost within 24 h. The results demonstrate that CIITA silencing occurs by controlling chromatin accessibility through a multistep mechanism that includes the loss of histone acetylation and transcription factor binding, and the acquisition of repressive histone methylation marks.

Prolonged MHC class II expression and CIITA transcription in human keratinocytes

A transcriptional cofactor, the MHC class II transactivator (CIITA), is a master regulator of MHC class II gene expression. CIITA expression is restricted in MHC class II-positive cells and is regulated by 4 (human) or 3 (mouse) promoters. To clarify the usage of human CIITA promoters in keratinocytes, we analyzed CIITA mRNA levels in IFN-gamma-stimulated normal human keratinocytes (NHK) by real-time PCR using promoter-specific primers. When the amount of total CIITA mRNA in NHK was quantified at 6h after IFN-gamma-stimulation, the amount of CIITA mRNA detected in NHK did not differ from that seen in the B cell line Raji or the IFN-gamma-stimulated macrophage cell line THP-1. Quantitative real-time PCR using promoter-specific primers showed that type IV CIITA mRNA was strongly transcribed and that type III CIITA mRNA was weakly transcribed in stimulated NHK, while no transcripts from pI and pII were detected. Although type IV mRNA in THP-1 was transiently transcribed by IFN-gamma-stimulation, transcription of type IV in IFN-gamma-stimulated keratinocytes was prolonged. This difference subsequently caused significantly higher expression at 72 h of MHC class II on NHK, compared with THP-1. This is the first report to quantitatively analyze each type of CIITA transcript in NHK.

ERK and p38 MAPK Signaling Pathways Negatively Regulate CIITA Gene Expression in Dendritic Cells and Macrophages

The CIITA is a master regulator for MHC class II expression, but the signaling events that control CIITA expression remain poorly understood. In this study, we report that both constitutive and IFN-gamma-inducible expression of CIITA in mouse bone marrow-derived dendritic cells (DC) and macrophages, respectively, are regulated by MAPK signals. In DC, the inhibitory effect of LPS on CIITA expression was prevented by MyD88 deficiency or pharmacological MAPK inhibitors specific for MEK (U0126) and p38 (SB203580), but not JNK (SP600125). In macrophages, LPS inhibited IFN-gamma-inducible CIITA and MHC class II expression without affecting expression of IFN regulatory factor-1 and MHC class I. Blocking ERK and p38 by MAPK inhibitors not only rescued LPS-mediated inhibition, but also augmented IFN-gamma induction of CIITA. Moreover, the induction of CIITA by IFN-gamma was enhanced by overexpressing MAPK phosphatase-1 that inactivates MAPK. Conversely, CIITA expression was attenuated in the absence of MAPK phosphatase-1. The down-regulation of CIITA gene expression by ERK and p38 was at least partly due to decreased histone acetylation of the CIITA promoter. Our study indicates that both MAPK and phosphatase play an important role for CIITA regulation in DC and macrophages.

Positive regulatory domain I-binding factor 1 mediates repression of the MHC class II transactivator (CIITA) type IV promoter

MHC class II transactivator (CIITA), a co-activator that controls MHC class II (MHC II) transcription, functions as the master regulator of MHC II expression. Persistent activity of the CIITA type III promoter (pIII), one of the four potential promoters of this gene, is responsible for constitutive expression of MHC II by B lymphocytes. In addition, IFN-gamma induces expression of CIITA in these cells through the type IV promoter (pIV). Positive regulatory domain 1-binding factor 1 (PRDI-BF1), called B lymphocyte-induced maturation protein 1 (Blimp-1) in mice, represses the expression of CIITA pIII in plasma and multiple myeloma cells. To investigate regulation of CIITA pIV expression by PRDI-BF1 in the B lymphocyte lineage, protein/DNA-binding studies, and functional promoter analyses were performed. PRDI-BF1 bound to the IFN regulatory factor-element (IRF-E) site in CIITA pIV. Ectopic expression of either PRDI-BF1 or Blimp-1 repressed this promoter in B lymphocytes. In vitro binding and functional analyses of CIITA pIV demonstrated that the IRF-E is the target of this repression. In vivo genomic footprint analysis demonstrated protein binding at the IRF-E site of CIITA pIV in U266 myeloma cells, which express PRDI-BF1. PRDI-BF1beta, a truncated form of PRDI-BF1 that is co-expressed in myeloma cells, also bound to the IRF-E site and repressed CIITA pIV. These findings demonstrate for the first time that, in addition to silencing expression of CIITA pIII in B lymphocytes, PRDI-BF1 is capable of binding and suppressing CIITA pIV.

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.

The heat shock protein 90-CDC37 chaperone complex is required for signaling by types I and II interferons

Interferon signaling pathways are critical to both innate and adaptive immunity. We have demonstrated here that the inhibition of heat shock protein 90 (Hsp90) functions by small interfering RNAs or chemical inhibitors blocking interferon-induced gene expression. Hsp90 was required for signal transducers and activators of transcription 1 phosphorylation, and in its absence, Janus kinase (JAK) 1/2 were degraded by the proteosome. JAK1 interacts with Hsp90 and the CDC37 co-chaperone, and both interactions are destabilized by Hsp90 inhibitors. The biological consequences were suggested by experiments showing that T cell activation by interferon-gamma-primed macrophages and the antiviral response of interferons required Hsp90. We conclude that JAK1/2 are client proteins of Hsp90 and that Hsp90 and CDC37 play a critical role in types I and II interferon pathways.

Expression of MHC II genes

Innate and adaptive immunity are connected via antigen processing and presentation (APP), which results in the presentation of antigenic peptides to T cells in the complex with the major histocompatibility (MHC) determinants. MHC class II (MHC II) determinants present antigens to CD4+ T cells, which are the main regulators of the immune response. Their genes are transcribed from compact promoters that form first the MHC II enhanceosome, which contains DNA-bound activators and then the MHC II transcriptosome with the addition of the class II transactivator (CIITA). CIITA is the master regulator of MHC II transcription. It is expressed constitutively in dendritic cells (DC) and mature B cells and is inducible in most other cell types. Three isoforms of CIITA exist, depending on cell type and inducing signals. CIITA is regulated at the levels of transcription and post-translational modifications, which are still not very clear. Inappropriate immune responses are found in several diseases, including cancer and autoimmunity. Since CIITA regulates the expression of MHC II genes, it is involved directly in the regulation of the immune response. The knowledge of CIITA will facilitate the manipulation of the immune response and might contribute to the treatment of these diseases.

Contrasting models of promiscuous gene expression by thymic epithelium

Medullary thymic epithelial cells (mTECs) express a broad spectrum of tissue- restricted self-antigens (TRAs), which are required for the development of central tolerance. A new study suggests that TRA expression is a specialized property of terminally differentiated mTECs. However, as discussed here, an alternative model-whereby TRA expression is regulated by conserved developmental programs active in developing mTECs-may be equally plausible.

MHC immunoevasins: protecting the pathogen reservoir in infection

Alteration of antigen recognition by T cells as result of insufficient major histocompatibility complex (MHC)-dependent antigen-presenting function has been observed in many cases of infections, particularly in in vitro systems. To hide themselves from an efficient immune response, pathogens may act on MHC-related functions at three levels: (i) by limiting the number of potential antigens that can be presented to naïve T cells; (ii) by synthesizing proteins which directly affect MHC cell-surface expression; and (iii) by altering the normal intracellular pathway of peptide loading on MHC. Here, we review examples of pathogens' action on each single step of MHC function and we suggest that the result of these often synergistic actions is both a limitation of the priming of naïve T cells and, more importantly, a protection of the pathogen's reservoir from the attack of primed T cells. The above mechanisms may also generate a skewing effect on immune effector mechanisms, which helps preserving the reservoir of infection from sterilization by the immune system.

Modular usage of the HLA-DRA promoter in extra-hematopoietic and hematopoietic cell types of transgenic mice

Class II MHC genes (for example, the human HLA-DRA gene) are expressed at the cell surface in many professional and nonprofessional antigen-presenting cells in a variety of anatomical locations. Here, we report about 13 mouse transgenic lines (11 of which have not been previously described) generated with four distinct sets of DRA transgenes carrying progressive, informative 5' and 3' deletions. DRA expression was assessed in B lymphocytes, dendritic cells, macrophages, and extra-hematopoietic cells (particularly kidney epithelial cells). A compact transcriptional unit was identified that efficiently directs DRA expression [both constitutive and interferon (IFN)-gamma induced] in extra-hematopoietic tissues and dendritic cells. It extends from position -266 upstream of the transcription initiation site to position +119 downstream of the last DRA exon. The same fragment, however, did not efficiently direct IFN-gamma-induced DRA expression in macrophages, that required additional 5' sequences. Thus, IFN-gamma uses distinct promoter segments and mechanisms to up-regulate class II in different cell lineages. In contrast to previous results in transgenic mice expressing murine class II transgenes, we were unable to generate reproducible patterns of HLA-DRA expression in B cells.

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.

MHC class II biosynthesis by activated rat CD4+ T cells: development of repression in vitro and modulation by APC-derived signals

This study focused on synthesis of MHC class II glycoproteins (MHCII) by rat CD4(+) T-helper cells. During activation in Con A and IL-2, purified rat splenic CD4(+) T cells expressed abundant surface MHCII together with transcripts for I-A alpha/beta, invariant chain, and the type III and type IV MHC class II transactivator (CIITA). Activated thymic CD8(+)CD4(-) and CD8(+)CD4(+) T cells exhibited essentially the same phenotype. MHCII synthesis by CD4(+) T cells enabled presentation of myelin basic protein (MBP) to antigen-specific responders. T cell expression of MHCII was due to direct biosynthesis rather than adsorption from professional APC; indeed, T cell-mediated expression of MHCII was optimal in the absence of professional APC. Despite periodic reactivation with Con A during 3-4 weeks of culture, CD4(+) T cells repressed MHCII synthesis and reverted to a MHCII(-) phenotype. These short-term lines resembled established lines of MBP-specific T cells in that mitogenic activation elicited extensive blastogenesis without MHCII synthesis. Activation-dependent synthesis of MHCII however was partially restored in lines of mitogen-stimulated T cells when the cultures were reconstituted with irradiated splenic APC. These data indicate that most naive rat CD4(+) T cells exhibit activation-dependent synthesis of MHCII whereas continuously propagated T cells require an APC-derived signal to support MHCII synthesis.

Epigenetic inactivation of class II transactivator (CIITA) is associated with the absence of interferon-gamma-induced HLA-DR expression in colorectal and gastric cancer cells

Tightly regulated at the level of transcription, expression of MHC class II molecules varies significantly among gastrointestinal cancers. High levels of MHC class II expression are often associated with a better prognosis, which is indicative of the involvement of CD4+ lymphocytes in tumor suppression, but the molecular mechanism by which MHC class II expression is regulated remains unclear. In the present study, we investigated the expression of one inducible MHC class II molecule, HLA-DR, and its coactivators in a panel of colorectal and gastric cancer cell lines. Interferon-gamma induced expression of HLA-DR in 14 of 20 cell lines tested; the remaining six cell lines did not express HLA-DR. Analysis of the expression of transcription factors and coactivators associated with HLA-DR revealed that the loss of CIITA expression was closely associated with the absence of HLA-DR induction. Moreover, DNA methylation of the 5' CpG island of CIITA-PIV was detected in all cancer cells that lacked CIITA. The methylation and resultant silencing of CIITA-PIV depended on the activities of two DNA methyltransferases, DNMT1 and DNMT3B, and their genetic inactivation restored CIITA-PIV expression. It thus appears that CIITA methylation is a key mechanism that enables some gastrointestinal cancer cells to escape immune surveillance.

Inhibition of MHC II gene transcription by nitric oxide and antioxidants

MHC class II molecules are expressed on the surface of antigen presenting cells and are loaded with peptides processed from the phagosomal compartment of these cells. Such complexes interact with the CD4 positive T lymphocyte receptor for antigen and a strong interaction is followed by T cell activation and proliferation. As class II expression is critical for antigen specific immunity its expression mostly restricted to a few cell types but can be induced on others in response to interferon gamma. This expansion of antigen presenting ability plays a role in increasing the duration and intensity of the immune response. Nitric oxide and antioxidants attenuate this class II induction through negative effects on the induction of class II transactivator protein expression and on the binding of transcription factor NF-Y to the class II promoter.

Altered Th1 Cell Differentiation Programming by CIITA Deficiency

CD4 T cell differentiation is a complex process affected by many transcription factors interacting in a tightly regulated manner. We have previously shown that CIITA-deficient mouse Th1 cells expressed Th2-type cytokines, while IFN-gamma expression was normal. In this study, we show that CIITA-deficient Th1 cells contain three distinct populations: cells secreting IL-4 alone, IFN-gamma alone, and both IL-4 and IFN-gamma together. This novel phenotype is stable over multiple rounds of stimulation in the presence of Th1-inducing factors. CIITA-deficient Th1 cells require TCR-mediated signaling to express Th2 cytokines, and this occurs with similar kinetics as wild-type Th2 cells. Both GATA-3 and IL-4 appear to be required for CIITA-deficient Th1 cells to express Th2-type cytokines. Interestingly, however, CIITA-deficient Th1 cells can produce IL-4 in the absence of exogenous IL-4. Introducing either CIITA or antisense GATA-3 during Th1 differentiation partially reduces Th2-type cytokine expression. With the exception of Th2-type cytokine expression, Th1 differentiation occurs normally in the absence of CIITA, as measured by expression of T-bet, IL-12Rbeta2, IL-18Ralpha, and IFN-gamma. Therefore, CIITA plays a key role to repress Th2-type cytokine expression as naive CD4 T cells differentiate toward the Th1 lineage.

MHC class II expression is differentially regulated in plasmacytoid and conventional dendritic cells

Major histocompatibility complex (MHC) class II-restricted antigen presentation is essential for the function of dendritic cells (DCs). We show here that plasmacytoid DCs (pDCs) differ from all other DC subsets with respect to expression of CIITA, the 'master regulator' of MHC class II genes. The gene encoding CIITA is controlled by three cell type-specific promoters: pI, pIII and pIV. With gene targeting in mice, we demonstrate that pDCs rely strictly on the B cell promoter pIII, whereas macrophages and all other DCs depend on pI. The molecular mechanisms driving MHC class II expression in pDCs are thus akin to those operating in lymphoid rather than myeloid cells.

Inactivation of class II transactivator by DNA methylation and histone deacetylation associated with absence of HLA-DR induction by interferon-gamma in haematopoietic tumour cells

By presenting immunogenic peptides at the cell surface, major histocompatibility complex (MHC) class II molecules play a key role in the control of adaptive immune responses. Whether expressed constitutively or induced by interferon-γ, expression of MHC class II molecules is regulated via coactivator class II transactivator (CIITA); moreover, suppression of their expression is one mechanism by which cancer cells escape host immunity. In this study, we surveyed the relationship between the expression of one MHC class II antigen, HLA–DR, and its coactivators in a group of haematopoietic cell lines, and explored the role of the aberrant DNA methylation in silencing HLA-DR expression. Among 26 cell lines studied, HLA-DR expression was lost from eight T-cell and two myeloid leukaemia cell lines, and this loss was closely associated with suppression of CIITA-PIV expression. Notably, nine of the 10 cell lines that lost CIITA-PIV expression showed methylation of the gene's 5′ CpG island. Thus, DNA methylation is believed to inhibit the expression of MHC class II molecules in haematopoietic tumour cells by silencing its coactivator, CIITA-PIV. Furthermore, methylation of CIITA-PIV was detected in seven of 32 primary acute myeloid leukaemia specimens, indicating that epigenetic alteration is not a cell line-specific phenomenon. Collectively, these data suggest that, by suppressing expression of MHC class II molecules, epigenetic inactivation of CIITA provides a survival advantage to a subset of haematopoietic tumours.

In vitro biological activity of anti-C II TA hammerhead ribozyme--a novel approach for autoimmune diseases

This study investigated the feasibility of using an hammerhead ribozyme against C II TA, a major regulator of MHC II antigens, to repress the expression of MHC II molecules on Hela cells. A hammerhead ribozyme (Rz464) specific to 463-465 GUC triplet of C II TA and its target gene were transcribed, then mixed up and incubated in vitro. The cleavage products were analyzed by PAGE and silver-staining. Rz464 was then inserted into the pIRES2-EGFP vector (pRz464). Stable transfectants of Hela with pRz464 were tested for class II MHC induction by recombinant human interferon-gamma (IFN-gamma). mRNA of C II TA was measured by RT-PCR. Our results showed that Rz464 could exclusively cleave C II TA RNA. When induced with IFN-gamma, the expression of HLA-DR, -DP, -DQ on pRz464+ Hela was induced, and the mRNA content of C II TA decreased too. It is concluded that Rz464 could inhibit C II TA and thus the family of genes was regulated by C II TA:MHC II molecules. These results provided insight into the future application of Rz464 as a new nucleic acid drug against auto-immune diseases.

IFN-gamma-induced MHC class II expression: transactivation of class II transactivator promoter IV by IFN regulatory factor-1 is regulated by protein kinase C-alpha

Previous studies based on pharmacological evidence suggested a requirement for protein kinase C (PKC) activity in the regulation of IFN-gamma-induced MHC class II (MHC-II) expression. In the present study, we investigated the molecular mechanisms by which PKC-alpha modulates IFN-gamma-induced MHC-II expression in the mouse macrophage cell line RAW 264.7. Overexpression of a dominant-negative (DN) mutant of PKC-alpha inhibited the expression of IFN-gamma-induced MHC-II but had no effect on IFN-gamma-induced STAT1 nuclear translocation and DNA binding activity, as well as on the expression of inducible NO synthase, IFN consensus sequence binding protein, MHC class I, IFN regulatory factor (IRF)-1, and IFN-gamma-inducible protein-10. Further analysis showed that IFN-gamma-induced expression of the MHC class II transactivator (CIITA), a transcriptional coactivator essential for MHC-II expression, was inhibited in DN PKC-alpha-overexpressing cells. Studies with reporter constructs containing the promoter IV region of CIITA revealed that overexpression of a constitutively active mutant of PKC-alpha enhanced IRF-1, but not IRF-2, transcriptional activity. Furthermore, characterization of IRF-1 from both normal and DN PKC-alpha-overexpressing cells revealed differences in IRF-1 posttranslational modifications. Collectively, our data suggest a novel regulatory mechanism for IFN-gamma-induced MHC-II expression, whereby PKC regulates CIITA expression by selectively modulating the transcriptional activity of IRF-1.

Expression of CII TA gene in five human malignant hematological cell lines and its significance

The role of MHC class II transactivator (C II TA) in constitutive or IFN-gamma inducible expression of HLA molecules in human malignant hematological cell lines was investigated. The expression of HLA molecules and C II TA protein was detected by Western blot, immunohistochemistry and flow cytometry. The expression of C II TA gene was determined by RT-PCR. The capability of peripheral blood T cell reaction stimulated by tumor cells was monitored by mixed lymphocyte reaction. It was found that the HLAII-positive tumor cells expressed the C II TA quite well, and the expression of HLA I + II was increased in the tumor cells with constitutive or inducible expression of C II TA after induced by IFN-gamma. The tumor cells which did not express C II TA after induced by IFN-gamma were not response to the expression of HLA II promoted by IFN-gamma. It suggests a correlation between the inability of some malignant hematological cell lines in response to IFN-gamma for HLA expression and the deficiency in the inducible expression of C II TA, indicating C II TA might take part in the regulation of HLA I + II expression in the tumor cells, which might play an important role in tumor immunologic escape.

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.

Differential Usage of Class II Transactivator Promoters PI and PIV during Inflammation and Injury in Kidney

ABSTRACT. Expression of class II transactivator (CIITA), the transcriptional regulator that controls all class II expression, is controlled in cell linesin vitroby three promoters: the dendritic cell promoter PI, the B cell promoter PIII, and the interferon-γ (IFN-γ)–inducible promoter, PIV. The authors examined the promoter usagein vivoin mouse kidney in the basal state and in response to IFN-γ, endotoxin, allostimulation, and renal injury. Genetically modified mice were used to examine the dependency of each promoter on IFN-γ and on the transcription factor interferon regulatory factor 1 (IRF-1). Usage of distinct CIITA promoters was monitored by real-time reverse transcriptase polymerase chain reaction (RT-PCR) using the unique sequences in the 5′ end of the transcript from each promoter. Kidneys in both control mice and IFN-γ knockouts expressed chiefly PI- and PIV–related products. Administration of recombinant IFN-γ activated only promoter PIV. Endotoxin or allogeneic stimulation elevated the PIV-related mRNA, dependent on IFN-γ and on IRF-1. Ischemic renal injury, however, increased the PI- and PIV–driven mRNA expression in wild-type but also in IFN-γ–deficient mice. Thus thein vivocontrol of CIITA promoters in kidney is similar to that observedin vitro(i.e., basal-state usage of PI and IFN-γ–dependent usage of PIV during inflammation), but it also shows additional levels of control: IFN-γ–independent basal activity of PIV and IFN-γ–independent induction of PIV during tissue injury. E-mail: phil.halloran@ualberta.ca

Regulation of the class II MHC pathway in primary human monocytes by granulocyte-macrophage colony-stimulating factor

GM-CSF stimulates the growth and differentiation of hematopoietic progenitors and also affects mature cell function. These effects have led to the use of GM-CSF as a vaccine adjuvant with promising results; however, the mechanisms underlying GM-CSF-mediated immune potentiation are incompletely understood. In this study, we investigated the hypothesis that the immune stimulatory role of GM-CSF is in part due to effects on class II MHC Ag presentation. We find that, in primary human monocytes treated for 24-48 h, GM-CSF increases surface class II MHC expression and decreases the relative level of the invariant chain-derived peptide, CLIP, bound to surface class II molecules. GM-CSF also increases expression of the costimulatory molecules CD86 and CD40, but not the differentiation marker CD1a or CD16. Furthermore, GM-CSF-treated monocytes are better stimulators in a mixed leukocyte reaction. Additional analyses of the class II pathway revealed that GM-CSF increases total protein and RNA levels of HLA-DR, DM, and DOalpha. Expression of class II transactivator (CIITA) types I and III, but not IV, transcripts increases in response to GM-CSF. Furthermore, GM-CSF increases the amount of CIITA associated with the DR promoter. Thus, our data argue that the proinflammatory role of GM-CSF is mediated in part through increased expression of key molecules involved in the class II MHC pathway via induction of CIITA.

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.

Repression of allo-cell transplant rejection through CIITA ribonuclease P+ hepatocyte

AIM: Allo-cell transplant rejection and autoimmune responses were associated with the presence of class II major histocompatibility complex (MHC II) molecules on cells. This paper studied the effect of Ribonuclease P (RNase P) against CIITA, which was a major regulator of MHCII molecules, on repressing the expression of MHCII molecules on hepatocyte.

METHODS: M1-RNA is the catalytic RNA subunit of RNase P from Escherichia coli. It were constructed that M1-RNA with guide sequences (GS) recognizing the 452, 3408 site of CIITA by PCR from pTK117 plasmid, then were cloned into the EcoR I/Bgl II or EcoR I/Sal I site of vector psNAV (psNAV-M1-452-GS, psNAV-M1-3408-GS) respectively. The target mould plate (3176-3560) of CIITA was obtained from Raji cell by RT-PCR, and then inserted into the Xho I/EcoR I of pGEM-7zf(+) plasmid (pGEM-3176). These recombinant plasmids were screened out by sequence analysis. psNAV-M1-452-GS, psNAV-M1-3408-GS and its target RNA pGEM-3176 were transcribed and then mixed up and incubated in vitro. It showed that M1-3408-GS could exclusively cleave target RNA that formed a base pair with the GS. Stable transfectants of hepatocyte cell line with psNAV-M1-3408-GS were tested for expression of class II MHC through FCM, for mRNA abundance of MHCII, Ii and CIITA by RT-PCR, for the level of IL-2 mRNA on T cell by mixed lymphocyte reaction.

RESULTS: When induced with recombinant human interferon-gamma (IFN-γ), the expression of HLA-DR, -DP, -DQ on psNAV-M1-3408-GS⁺ hepatocyte was reduced 83.27%, 88.93%, 58.82% respectively, the mRNA contents of CIITA, HLA-DR, -DP, -DQ and Ii decreased significantly. While T cell expressed less IL-2 mRNA in the case of psNAV-M1-3408-GS⁺ hepatocyte.

CONCLUSION: The Ribonuclease P against CIITA-M1-3408-GS could effectively induce antigen-specific tolerance through cleaving CIITA. These results provided insight into the future application of M1-3408-GS as a new nucleic acid drug against allo-transplantation rejection and autoimmune diseases.

Promoter IV of the class II transactivator gene is essential for positive selection of CD4+ T cells

Major histocompatibility complex class II (MHCII) expression is regulated by the transcriptional coactivator CIITA. Positive selection of CD4(+) T cells is abrogated in mice lacking one of the promoters (pIV) of the Mhc2ta gene. This is entirely due to the absence of MHCII expression in thymic epithelia, as demonstrated by bone marrow transfer experiments between wild-type and pIV(-/-) mice. Medullary thymic epithelial cells (mTECs) are also MHCII(-) in pIV(-/-) mice. Bone marrow-derived, professional antigen-presenting cells (APCs) retain normal MHCII expression in pIV(-/-) mice, including those believed to mediate negative selection in the thymic medulla. Endogenous retroviruses thus retain their ability to sustain negative selection of the residual CD4(+) thymocytes in pIV(-/-) mice. Interestingly, the passive acquisition of MHCII molecules by thymocytes is abrogated in pIV(-/-) mice. This identifies thymic epithelial cells as the source of this passive transfer. In peripheral lymphoid organs, the CD4(+) T-cell population of pIV(-/-) mice is quantitatively and qualitatively comparable to that of MHCII-deficient mice. It comprises a high proportion of CD1-restricted natural killer T cells, which results in a bias of the V beta repertoire of the residual CD4(+) T-cell population. We have also addressed the identity of the signal that sustains pIV expression in cortical epithelia. We found that the Jak/STAT pathways activated by the common gamma chain (CD132) or common beta chain (CDw131) cytokine receptors are not required for MHCII expression in thymic cortical epithelia.

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.

Deregulated MHC class II transactivator expression leads to a strong Th2 bias in CD4+ T lymphocytes

The MHC class II (MHC-II) transactivator (CIITA) is the master transcriptional regulator of genes involved in MHC-II-restricted Ag presentation. Fine tuning of CIITA gene expression determines the cell type-specific expression of MHC-II genes. This regulation is achieved by the selective usage of multiple CIITA promoters. It has recently been suggested that CIITA also contributes to Th cell differentiation by suppressing IL-4 expression in Th1 cells. In this study, we show that endogenous CIITA is expressed at low levels in activated mouse T cells. Importantly CIITA is not regulated differentially in murine and human Th1 and Th2 cells. Ectopic expression of a CIITA transgene in multiple mouse cell types including T cells, does not interfere with normal development of CD4(+) T cells. However, upon TCR activation the CIITA transgenic CD4(+) T cells preferentially differentiate into IL-4-secreting Th2-type cells. These results imply that CIITA is not a direct Th1-specific repressor of the IL-4 gene and that tight control over the expression of CIITA and MHC-II is required to maintain the normal balance between Th1 and Th2 responses.

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 and function of class II major histocompatibility complex, CD40, and B7 expression in macrophages and microglia: Implications in neurological diseases

The ability of microglia, the brain's resident macrophage, to present antigen through the class II major histocompatibility complex (MHC) to T cells allows these normally quiescent cells to play a critical role in shaping the outcome of many neurological diseases. The expression of class II MHC antigens and the costimulatory molecules CD40 and B7 on microglia and infiltrating macrophages is regulated through a complex network of cytokines in the inflamed brain. In this review, we describe the molecular mechanisms underlying class II MHC, CD40 and B7 regulation in microglia and macrophages. Our focus is on the cis-elements in the promoters of their genes and the transcription factors activated by cytokines that bind them. The functional implications of aberrant class II MHC, CD40 and B7 expression by microglia and macrophages as related to the diseases of Multiple Sclerosis and Alzheimer's Disease are discussed.

Dual roles for IFN-gamma, but not for IL-4, in spontaneous autoimmune thyroiditis in NOD.H-2h4 mice

Spontaneous autoimmune thyroiditis (SAT) is an organ-specific autoimmune disease characterized by chronic inflammation of the thyroid by T and B lymphocytes. To investigate the roles of Th1 and Th2 cytokines in the pathogenesis of SAT, IFN-gamma(-/-) and IL-4(-/-) NOD.H-2h4 mice were generated. IL-4(-/-) mice developed lymphocytic SAT (L-SAT) comparable to that of wild-type (WT) mice. They produced little anti-mouse thyroglobulin (MTg) IgG1, but had levels of anti-MTg IgG2b comparable to WT mice. Compared with WT mice, IFN-gamma(-/-) mice produced significantly less anti-MTg IgG1 and IgG2b. Absence of IFN-gamma resulted in abnormal proliferation of thyroid epithelial cells with minimal lymphocyte infiltration. Thyroids of IFN-gamma(-/-) mice had markedly reduced B lymphocyte chemoattractant expression, B cell and plasma cell infiltration, and decreased MHC class II expression on thyrocytes compared with WT mice. Adoptive transfer of WT splenocytes to IFN-gamma(-/-) mice restored the capacity to develop typical L-SAT, enhanced anti-MTg IgG1 and IgG2b production, up-regulated MHC class II expression on thyrocytes and decreased thyrocyte proliferation. These results suggest that IFN-gamma plays a dual role in the development of SAT. IFN-gamma is required for development of L-SAT, and it also functions to inhibit thyroid epithelial cell proliferation.