Human MHC class II gene transcription directed by the carboxyl terminus of CIITA, one of the defective genes in type II MHC combined immune deficiency

Structural aspects of the MHC expression control system

The major histocompatibility complex (MHC) spans innate and adaptive immunity by presenting antigenic peptides to CD4⁺ and CD8⁺ T cells. Multiple transcription factors form an enhanceosome complex on the MHC promoter and recruit transcriptional machinery to activate gene transcription. Immune signals such as interferon-γ (IFN-γ) control MHC level by up-regulating components of the enhanceosome complex. As MHC plays crucial roles in immune regulation, alterations in the MHC enhanceosome structure will alter the pace of rapid immune responses at the transcription level and lead to various diseases related to the immune system. In this review, we discuss the current understanding of the MHC enhanceosome, with a focus on the structures of MHC enhanceosome components and the molecular basis of MHC enhanceosome assembly.

The MHC Class II Transactivator CIITA: Not (Quite) the Odd-One-Out Anymore among NLR Proteins

In this review, we discuss the major histocompatibility complex (MHC) class II transactivator (CIITA), which is the master regulator of MHC class II gene expression. CIITA is the founding member of the mammalian nucleotide-binding and leucine-rich-repeat (NLR) protein family but stood apart for a long time as the only transcriptional regulator. More recently, it was found that its closest homolog, NLRC5 (NLR protein caspase activation and recruitment domain (CARD)-containing 5), is a regulator of MHC-I gene expression. Both act as non-DNA-binding activators through multiple protein-protein interactions with an MHC enhanceosome complex that binds cooperatively to a highly conserved combinatorial cis-acting module. Thus, the regulation of MHC-II expression is regulated largely through the differential expression of CIITA. In addition to the well-defined role of CIITA in MHC-II GENE regulation, we will discuss several other aspects of CIITA functions, such as its role in cancer, its role as a viral restriction element contributing to intrinsic immunity, and lastly, its very recently discovered role as an inhibitor of Ebola and SARS-Cov-2 virus replication. We will briefly touch upon the recently discovered role of NLRP3 as a transcriptional regulator, which suggests that transcriptional regulation is, after all, not such an unusual feature for NLR proteins.

Genome-wide CIITA-binding profile identifies sequence preferences that dictate function versus recruitment

The class II transactivator (CIITA) is essential for the expression of major histocompatibility complex class II (MHC-II) genes; however, the role of CIITA in gene regulation outside of MHC-II biology is not fully understood. To comprehensively map CIITA-bound loci, ChIP-seq was performed in the human B lymphoblastoma cell line Raji. CIITA bound 480 sites, and was significantly enriched at active promoters and enhancers. The complexity of CIITA transcriptional regulation of target genes was analyzed using a combination of CIITA-null cells, including a novel cell line created using CRISPR/Cas9 tools. MHC-II genes and a few novel genes were regulated by CIITA; however, most other genes demonstrated either diminished or no changes in the absence of CIITA. Nearly all CIITA-bound sites were within regions containing accessible chromatin, and CIITA's presence at these sites was associated with increased histone H3K27 acetylation, suggesting that CIITA's role at these non-regulated loci may be to poise the region for subsequent regulation. Computational genome-wide modeling of the CIITA bound XY box motifs provided constraints for sequences associated with CIITA-mediated gene regulation versus binding. These data therefore define the CIITA regulome in B cells and establish sequence specificities that predict activity for an essential regulator of the adaptive immune response.

NLRC5 interacts with RIG-I to induce a robust antiviral response against influenza virus infection

The NLR protein, NLRC5 is an important regulator of MHC class I gene expression, however, the role of NLRC5 in other innate immune responses is less well defined. In the present study, we report that NLRC5 binds RIG-I and that this interaction is critical for robust antiviral responses against influenza virus. Overexpression of NLRC5 in the human lung epithelial cell line, A549, and normal human bronchial epithelial cells resulted in impaired replication of influenza virus A/Puerto Rico/8/34 virus (PR8) and enhanced IFN-β expression. Influenza virus leads to induction of IFN-β that drives RIG-I and NLRC5 expression in host cells. Our results suggest that NLRC5 extends and stabilizes influenza virus induced RIG-I expression and delays expression of the viral inhibitor protein NS1. We show that NS1 binds to NLRC5 to suppress its function. Interaction domain mapping revealed that NLRC5 interacts with RIG-I via its N-terminal death domain and that NLRC5 enhanced antiviral activity in an leucine-rich repeat domain independent manner. Taken together, our findings identify a novel role for NLRC5 in RIG-I-mediated antiviral host responses against influenza virus infection, distinguished from the role of NLRC5 in MHC class I gene regulation.

Combined immunodeficiency evolving into predominant CD4+ lymphopenia caused by somatic chimerism in JAK3

Purpose

Idiopathic CD4 lymphopenia constitutes a heterogeneous group of immunodeficiencies with characteristically low CD4+ T-cell counts with largely unknown genetic etiology. We here sought to determine the underlying molecular cause in an index family with two patients suffering from combined immunodeficiency that evolved into predominant CD4+ lymphopenia. The more severely affected index patient also presented with selective antibody deficiency against bacterial polysaccharide antigens.

Methods

For the genetic analysis, we used combined homozygosity mapping and exome sequencing. Functional assays included immunoblot analysis, flow cytometry and TCR Vβ spectratyping.

Results

A novel homozygous missense mutation was revealed in the kinase domain of JAK3 (c.T3196C, p.Cys1066Arg). Further analysis showed revertant chimerism in CD8+ T-cells in both patients. The additional presence of revertant CD4+ T-cells was associated with a milder clinical and immunological phenotype in the second patient, although the role somatic chimerism plays in amelioration of disease phenotype is uncertain, as presence of revertant cells had no effect on residual CD4 cell JAK3 signaling function. Residual activity of JAK3-dependent STAT3 and STAT5 signaling was also found in immortalized B-cell lines indicating a hypomorphic nature of the described mutation which likely contributes to the milder clinical phenotype.

Conclusions

We here present the first case of revertant mosaicism in JAK3 deficiency, manifesting as combined immunodeficiency evolving into predominant CD4+ lymphopenia. Revertant chimerism or hypomorphic mutations in genes typically associated with more severe T-cell deficiency should be considered when assessing patients with milder forms of combined immunodeficiencies.

Electronic supplementary material

The online version of this article (doi:10.1007/s10875-014-0088-2) contains supplementary material, which is available to authorized users.

The genetics of innate immunity sensors and human disease

Since their discovery, innate immunity microbial sensors have been increasingly studied and shown to play a critical role in innate responses to microbes in several experimental in vitro, ex vivo, and animal models. However, their role in the human response to infection in natural conditions has just started to be deciphered, by means of clinical studies of primary immunodeficiencies and epidemiological genetic studies. Here, we summarize the major findings concerning the genetic diversity of the various families of microbial sensors in humans, and of other molecules involved in the signaling pathways they trigger. Specifically, we review the genetic associations, revealed by both clinical and epidemiological genetics studies, of microbial sensors from five different families: Toll-like receptors, C-type lectin receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. In particular, we consider the relationships between variation at the genes encoding these molecules and susceptibility to and the severity of infectious diseases and other clinical conditions associated with immune dysfunction, including autoimmunity, inflammation, allergy, and cancer. Despite the fact that the genetic links between innate immunity sensors and human disorders remain still limited, human genetics studies are increasingly improving our understanding of the genuine functions of microbial sensors and downstream signaling molecules in the natural setting.

Age-dependent variation of genotypes in MHC II transactivator gene (CIITA) in controls and association to type 1 diabetes

The major histocompatibility complex class II transactivator (CIITA) gene (16p13) has been reported to associate with susceptibility to multiple sclerosis, rheumatoid arthritis and myocardial infarction, recently also to celiac disease at genome-wide level. However, attempts to replicate association have been inconclusive. Previously, we have observed linkage to the CIITA region in Scandinavian type 1 diabetes (T1D) families. Here we analyze five Swedish T1D cohorts and a combined control material from previous studies of CIITA. We investigate how the genotype distribution within the CIITA gene varies depending on age, and the association to T1D. Unexpectedly, we find a significant difference in the genotype distribution for markers in CIITA (rs11074932, P=4 × 10(-5) and rs3087456, P=0.05) with respect to age, in the collected control material. This observation is replicated in an independent cohort material of about 2000 individuals (P=0.006, P=0.007). We also detect association to T1D for both markers, rs11074932 (P=0.004) and rs3087456 (P=0.001), after adjusting for age at sampling. The association remains independent of the adjacent T1D risk gene CLEC16A. Our results indicate an age-dependent variation in CIITA allele frequencies, a finding of relevance for the contrasting outcomes of previously published association studies.

NLRC5 controls basal MHC class I gene expression in an MHC enhanceosome-dependent manner

Nucleotide-binding domain and leucine-rich repeat (NLR) proteins play important roles in innate immune responses as pattern-recognition receptors. Although most NLR proteins act in cell autonomous immune pathways, some do not function as classical pattern-recognition receptors. One such NLR protein is the MHC class II transactivator, the master regulator of MHC class II gene transcription. In this article, we report that human NLRC5, which we recently showed to be involved in viral-mediated type I IFN responses, shuttles to the nucleus and activates MHC class I gene expression. Knockdown of NLRC5 in different human cell lines and primary dermal fibroblasts leads to reduced MHC class I expression, whereas introduction of NLRC5 into cell types with very low expression of MHC class I augments MHC class I expression to levels comparable to those found in lymphocytes. Expression of NLRC5 positively correlates with MHC class I expression in human tissues. Functionally, we show that both the N-terminal effector domain of NLRC5 and its C-terminal leucine-rich repeat domain are needed for activation of MHC class I expression. Moreover, nuclear shuttling and function depend on a functional Walker A motif. Finally, we identified a promoter sequence in the MHC class I promoter, the X1 box, to be involved in NLRC5-mediated MHC class I gene activation. Taken together, this suggested that NLRC5 acts in a manner similar to class II transactivator to drive MHC expression and revealed NLRC5 as an important regulator of basal MHC class I expression.

Novel functions for TAF7, a regulator of TAF1-independent transcription

The transcription factor TFIID components TAF7 and TAF1 regulate eukaryotic transcription initiation. TAF7 regulates transcription initiation of TAF1-dependent genes by binding to the acetyltransferase (AT) domain of TAF1 and inhibiting the enzymatic activity that is essential for transcription. TAF7 is released from the TAF1-TFIID complex upon completion of preinitiation complex assembly, allowing transcription to initiate. However, not all transcription is TAF1-dependent, and the role of TAF7 in regulating TAF1-independent transcription has not been defined. The IFNγ-induced transcriptional co-activator CIITA activates MHC class I and II genes, which are vital for immune responses, in a TAF1-independent manner. Activation by CIITA depends on its intrinsic AT activity. We now show that TAF7 binds to CIITA and inhibits its AT activity, thereby repressing activated transcription. Consistent with this TAF7 function, siRNA-mediated depletion of TAF7 resulted in increased CIITA-dependent transcription. A more global role for TAF7 as a regulator of transcription was revealed by expression profiling analysis: expression of 30-40% of genes affected by TAF7 depletion was independent of either TAF1 or CIITA. Surprisingly, although TAF1-dependent transcripts were largely down-regulated by TAF7 depletion, TAF1-independent transcripts were predominantly up-regulated. We conclude that TAF7, until now considered only a TFIID component and regulator of TAF1-dependent transcription, also regulates TAF1-independent transcription.

Association of the 19S proteasomal ATPases with the ATPase-binding domain of CIITA is essential for CIITA stability and MHC class II expression

Major histocompatibility class II (MHC class II) molecules are glycoproteins that present extracellular antigens to CD4(+) T cells and are essential for initiation of adaptive immune responses. MHC class II expression requires recruitment of a master regulator, the class II transactivator (CIITA), to the MHC class II promoter. Others and we have earlier linked CIITA to the ubiquitin-proteasome system by showing that mono-ubiquitination of CIITA increases its transactivity, whereas poly-ubiquitination of CIITA leads to its degradation. We have further shown that the 26S proteasome also has non-proteolytic functions in MHC class II transcription, as 19S ATPase subunits of the 26S proteasome positively regulate MHC class II transcription and are necessary for stable promoter binding of CIITA. Although these basic requirements of the proteasome to initiate MHC class II transcription are known, how CIITA is recruited, stabilized, and degraded remains unclear. Here, we identify a novel N-terminal 19S ATPase-binding domain of CIITA. The ATPase-binding domain lies within the proline/serine/threonine-rich region of CIITA and encompasses a majority of the CIITA degron sequence. Absence of the ATPase-binding domain increases the half-life of CIITA, but blocks MHC class II surface expression, indicating that CIITA requires interaction with the 19S ATPases for both appropriate deployment and destruction.

Mitogen-activated protein kinase ERK1/2 regulates the class II transactivator

The expression of major histocompatibility class II genes is necessary for proper antigen presentation and induction of an immune response. This expression is initiated by the class II transactivator, CIITA. The establishment of the active form of CIITA is controlled by a series of post-translational events, including GTP binding, ubiquitination, and dimerization. However, the role of phosphorylation is less clearly defined as are the consequences of phosphorylation on CIITA activity and the identity of the kinases involved. In this study we show that the extracellular signal-regulated kinases 1 and 2 (ERK1/2) interact directly with CIITA, targeting serine residues in the amino terminus of the protein, including serine 288. Inhibition of this phosphorylation by dominant-negative forms of ERK or by treatment of cells with the ERK inhibitor PD98059 resulted in the increase in CIITA-mediated gene expression from a class II promoter, enhanced the nuclear concentration of CIITA, and impaired its ability to bind to the nuclear export factor, CRM1. In contrast, inhibition of ERK1/2 activity had little effect on serine-to-alanine mutant forms of CIITA. These data suggest a model whereby ERK1/2-mediated phosphorylation of CIITA down-regulates CIITA activity by priming it for nuclear export, thus providing a means for cells to tightly regulate the extent of antigen presentation.

MHC Class II levels and intracellular localization in human dendritic cells are regulated by calmodulin kinase II

Dendritic cells (DC) are professional APC, which activate the adaptive immune response. A Ca2+-calmodulin (CaM)-CaM kinase II (CaMKII) pathway regulates maturation and MHC Class II antigen presentation in human DC. The objective of this study was to characterize the mechanisms by which CaMKII modulates the levels and subcellular distribution of MHC Class II molecules. Inhibition of CaMKII via the highly specific, autoinhibitory peptide derived from the enzyme's regulatory domain resulted in rapid (60 min) and sustained (24 h) reduction of MHC Class II levels in antigen-stimulated, primary, human DC. The initial depletion of intracellular and cell surface MHC Class II was associated with its enhanced lysosomal trafficking and increased activity of specific proteases in the absence of effects on other transmembrane proteins (CD1b and CD34) or a detectable change in lysosomal degradation of exogenous protein. Inhibition of CaMKII also resulted in significant reductions in the level and stability of MHC Class II mRNA and the levels and nucleocytosolic localization of its major transcriptional regulator CIITA. These data support a model in which CaMKII regulates the levels and localization of MHC Class II protein in human DC via transcriptional, post-transcriptional, and post-translational mechanisms. These pathways are likely important to the physiologic regulation of MHC Class II as well as to its dysregulation in disease states associated with altered CaMKII function.

Suppression of major histocompatibility complex class II-associated invariant chain enhances the potency of an HIV gp120 DNA vaccine

Summary One function of the major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is to prevent MHC class II molecules from binding endogenously generated antigenic epitopes. Ii inhibition leads to MHC class II presentation of endogenous antigens by APC without interrupting MHC class I presentation. We present data that in vivo immunization of BALB/c mice with HIV gp120 cDNA plus an Ii suppressive construct significantly enhances the activation of both gp120-specific T helper (Th) cells and cytotoxic T lymphocytes (CTL). Our results support the concept that MHC class II-positive/Ii-negative (class II(+)/Ii(-)) antigen-presenting cells (APC) present endogenously synthesized vaccine antigens simultaneously by MHC class II and class I molecules, activating both CD4(+) and CD8(+) T cells. Activated CD4(+) T cells locally strengthen the response of CD8(+) CTL, thus enhancing the potency of a DNA vaccine.

CATERPILLER: a novel gene family important in immunity, cell death, and diseases

The newly discovered CATERPILLER (CLR) gene family encodes proteins with a variable but limited number of N-terminal domains, followed by a nucleotide-binding domain (NBD) and leucine-rich repeats (LRR). The N-terminal domain consists of transactivation, CARD, Pyrin, or BIR domains, with a minority containing undefined domains. These proteins are remarkably similar in structure to the TIR-NBD-LRR and CC-NBD-LRR disease resistance (R) proteins that mediate immune responses in plants. The NBD-LRR architecture is conserved in plants and vertebrates, but only remnants are found in worms and flies. The CLRs regulate inflammatory and apoptotic responses, and some act as sensors that detect pathogen products. Several CLR genes have been genetically linked to susceptibility to immunologic disorders. We describe prominent family members, including CIITA, CARD4/NOD1, NOD2/CARD15, CIAS1, CARD7/NALP1, and NAIP, in more detail. We also discuss implied roles of these proteins in diversifying immune detection and in providing a check-and-balance during inflammation.

MHC class II gene expression is not induced in HPIV3-infected respiratory epithelial cells

The major histocompatibility complex (MHC) class II transactivator (CIITA) typically is required for both constitutive and inducible expression of MHC class II genes. However, transcription of class II MHC genes has been observed in specific cell types (e.g., thymic epithelial cells) in CIITA-deficient mice as well as in specific situations (e.g., following viral infections or in natural killer [NK]/target cell interaction). These observations have been interpreted by some to indicate that a CIITA-independent pathway of class II gene expression might be germane to processes such as the acquisition of tolerance during thymic selection or in the evasion of immune surveillance by a subset of viruses. One of the most striking examples of CIITA-independent, inducible class II gene expression has involved the de novo expression of class II MHC molecules on respiratory epithelial cells following infection by human parainfluenza virus type 3 (HPIV3). We report here that despite careful analysis using multiple techniques, we have been unable to detect HPIV3-dependent, CIITA-independent (or CIITA-dependent) induction of class II MHC genes. Thus, whereas there may still be an intriguing role for CIITA-independent gene expression in facets of the immune response, this is unlikely to manifest in the analysis of HPIV3 infection of respiratory epithelial cells.

Serine Residues 286, 288, and 293 within the CIITA: A Mechanism for Down-Regulating CIITA Activity through Phosphorylation

CIITA is the primary factor activating the expression of the class II MHC genes necessary for the exogenous pathway of Ag processing and presentation. Strict control of CIITA is necessary to regulate MHC class II gene expression and induction of an immune response. We show in this study that the nuclear localized form of CIITA is a predominantly phosphorylated form of the protein, whereas cytoplasmic CIITA is predominantly unphosphorylated. Novel phosphorylation sites were determined to be located within a region that contains serine residues 286, 288, and 293. Double mutations of these residues increased nuclear CIITA, indicating that these sites are not required for nuclear import. CIITA-bearing mutations of these serine residues significantly increased endogenous MHC class II expression, but did not significantly enhance trans-activation from a MHC class II promoter, indicating that these phosphorylation sites may be important for gene activation from intact chromatin rather than artificial plasmid-based promoters. These data suggest a model for CIITA function in which phosphorylation of these specific sites in CIITA in the nucleus serves to down-regulate CIITA activity.

Update on primary immunodeficiency: defects of lymphocytes

The recent identification of the genes involved in many primary immunodeficiency disorders has led to a significant increase in our understanding of the pathogenesis of these defects. Many of these disorders share a clinical phenotype with common features such as recurrent infections, chronic inflammation, and autoimmunity. Although some of these immune defects have mild presentations and better outcomes, others result in severe infections and significant morbidity and mortality. For these, early diagnosis and treatment are critical. This review provides an overview of the genetic defects and clinical features of primary immune deficiencies due to defects in lymphocytes.

Steroid receptor coactivator 1 links the steroid and interferon gamma response pathways

We show here that steroid receptor coactivator 1 (SRC-1) is a coactivator of MHC class II genes that stimulates their interferon gamma (IFNgamma) and class II transactivator (CIITA)-mediated expression. SRC-1 interacts physically with the N-terminal activation domain of CIITA through two regions: one central [extending from amino acids (aa) 360-839] that contains the nuclear receptors binding region and one C-terminal (aa 1138-1441) that contains the activation domain 2. Using chromatin immunoprecipitation assays we show that SRC-1 recruitment on the class II promoter is enhanced upon IFNgamma stimulation. Most importantly, SRC-1 relieves the inhibitory action of estrogens on the IFNgamma-mediated induction of class II genes in transient transfection assays. We provide evidence that inhibition by estradiol is due to multiple events such as slightly reduced recruitment of CIITA and SRC-1 and severely inhibited assembly of the preinitiation complex.

K-562 cells lack MHC class II expression due to an alternatively spliced CIITA transcript with a truncated coding region

The focus of this study was to determine the functional capacity of class II transactivator (CIITA), a regulatory factor of major histocompatibility complex (MHC) class II genes, in K-562 cells. We show that CIITA mRNA is present in K-562 cells and the interferon-gamma (IFN-gamma)-inducible CIITA promoter-IV exhibits low levels of basal activity, which is greatly enhanced upon treatment with IFN-gamma. Further study revealed that the CIITA cDNA contains an insertion of genomic sequence, which introduces a stop codon. The truncated coding region of the CIITA transcript in K-562 cells provides a possible explanation for the absence of MHC class II molecules.

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.

Structural and functional characteristics of a dominant-negative isoform of porcine MHC class II transactivator

The MHC class II transactivator, CIITA, is critical for MHC class II gene expression in all species studied to date. We isolated an interferon (IFN)-gamma-inducible isoform of porcine CIITA (pCIITA') encoding a protein of 566 amino acids (aa) with significant homology to human CIITA (hCIITA). Analysis indicated that pCIITA' lacks the entire GTP-binding domain that is important for nuclear translocation and activation of target genes by hCIITA. In pCIITA' this region is replaced by a 14-aa motif with homology to several signalling peptide sequences. Expression of pCIITA' in porcine (ST-IOWA) and human (HeLa) cell lines resulted in suppression of IFN-gamma-stimulated MHC class II gene expression, at the protein and mRNA levels. We also identified two IFN-gamma-inducible variants of hCIITA, hCIITAlo and hCIITA' from Hela cells, both exhibiting dominant-negative suppression of MHC class II gene expression. Interestingly, hCIITA' encodes a predicted protein of 546 aa with a strikingly similar organization to pCIITA' including the 14-aa GTP-binding domain-replacement motif in which 10 out of 14 amino acids are identical to the pig sequence. Expression of hCIITA' and hCIITAlo sequences in Hela cells suppressed IFN-gamma-induced MHC class II gene expression. hCIITAlo, a predicted 303-aa protein with deleted GTP-binding and carboxy-terminal domain, displayed a more subtle suppression of IFN-gamma-induced MHC class II expression. These in vitro data indicate that there may be a role in vivo for isoforms of CIITA that can suppress full-length CIITA-mediated MHC class II gene expression. Both humans and now, potentially, pigs are candidate donors for organ and tissue allografts and xenografts, respectively. Regulation of MHC class II gene expression by manipulation of CIITA isoform expression in humans and pigs may provide a useful strategy for attenuation of T-cell-mediated cellular rejection.

Generating MHC Class II+/Ii- phenotype after adenoviral delivery of both an expressible gene for MHC Class II inducer and an antisense Ii-RNA construct in tumor cells

Tumor cells engineered by gene transduction to be MHC Class II+/Ii- are novel APCs capable of presenting endogenous tumor antigen epitopes to activate T helper cells. The MHC Class II+/Ii- tumor cell phenotype is created by transfecting genes for either CIITA or IFN-gamma, and inhibiting induced Ii mRNA by an Ii reverse gene construct (Ii-RGC). Adenoviral vectors are preferred for the delivery of such genes because of high infection efficiency and ubiquity of the adenoviral receptor on many cell types and tumors. Here we show that at 5 MOI (multiplicity of infection), recombinant adenoviruses with CIITA or IFN-gamma genes converted virtually all MC-38 colon adenocarcinoma cells and Renca renal carcinoma cells in culture to MHC Class II+/Ii+ cells. A single recombinant adenovirus with both genes for IFN-gamma and Ii-RGC (rAV/IFN-gamma/Ii-RGC) efficiently induced the MHC Class II+/Ii- phenotype. Injection of tumor nodules with rAV/Ii-RGC and rAV/CIITA/IFN-gamma combined with a suboptimal dose of rAV/IL-2 induced a potent antitumor immune response. The methods are adaptable for producing enhanced genetic vaccines, attenuated virus vaccines (eg, vaccinia), and ex vivo cell-based vaccines (dendritic and tumor cells).

Transcriptional regulation of the MHC class II trans-activator (CIITA) promoter III: identification of a novel regulatory region in the 5'-untranslated region and an important role for cAMP-responsive element binding protein 1 and activating transcription factor-1 in CIITA-promoter III transcriptional activation in B lymphocytes

The class II trans-activator (CIITA), which acts as a master regulator for expression of MHC class II genes, is expressed constitutively in mature B cells. This constitutive expression of CIITA is driven by CIITA promoter III (CIITA-PIII). However, little is known about the factors that control the B cell-mediated trans-activation of CIITA-PIII. In this study using B cells we have identified several cAMP-responsive elements (CREs) in the proximal promoter and in the 5'-untranslated region (5'-UTR) that are involved in the activation of CIITA-PIII. We show that activating transcription factor (ATF)/CRE binding protein (CREB) factors bind to the CREs in vitro and in vivo. Notably, our results also reveal that the 5'-UTR of CIITA-PIII functions as an important regulatory region in B lymphocytes. Furthermore, transient cotransfections of a CIITA-PIII luciferase reporter construct with either CREB-1 or ATF-1 boost CIITA-PIII trans-activation in a dose-dependent manner, which was further enhanced by addition of general coactivator CREB-binding protein. Transient transfections using mutant CIITA-PIII luciferase reporter constructs that either lack the (5'-UTR) or abolish binding of CREB-1 and ATF-1 to the CRE located in activation response element-2, displayed severely reduced promoter activity in B cells. A similar successive deletion of the CREs resulted in a subsequent reduction of CREB-1-induced activity of CIITA-PIII in B cells. Together our results argue for an important role of ATF/CREB factors and the 5'-UTR of CIITA-PIII in the trans-activation of CIITA-PIII in B cells.

Leucine-rich repeats of the class II transactivator control its rate of nuclear accumulation

Activation of class II major histocompatibility complex (MHC) gene expression is regulated by a master regulator, class II transcriptional activator (CIITA). Transactivation by CIITA requires its nuclear import. This study will address a mechanistic role for the leucine-rich repeats (LRR) of CIITA in regulating nuclear translocation by mutating 12 individual consensus-motif "leucine" residues in both its alpha-motifs and beta-motifs. While some leucine mutations in the LRR motif of CIITA cause congruent loss of transactivation function and nuclear import, other alanine substitutions in both the alpha-helices and the beta-sheets have normal transactivation function but a loss of nuclear accumulation (i.e., functional mutants). This seeming paradox is resolved by the observations that nuclear accumulation of these functional mutants does occur but is significantly less than wild-type. This difference is revealed only in the presence of leptomycin B and actinomycin D, which permit examination of nuclear accumulation unencumbered by nuclear export and new CIITA synthesis. Further analysis of these mutants reveals that at limiting concentrations of CIITA, a dramatic difference in transactivation function between mutants and wild-type CIITA is easily detected, in agreement with their lowered nuclear accumulation. These experiments reveal an interesting aspect of LRR in controlling the amount of nuclear accumulation.

Primary cellular immunodeficiencies

Genetic defects in T-cell function lead to susceptibility to infections or to other clinical problems that are more grave than those seen in disorders resulting in antibody deficiency alone. Those affected usually present during infancy with either common or opportunistic infections and rarely survive beyond infancy or childhood. The spectrum of T-cell defects ranges from the syndrome of severe combined immunodeficiency, in which T-cell function is absent, to combined immunodeficiency disorders in which there is some, but not adequate, T-cell function for a normal life span. Recent discoveries of the molecular causes of many of these defects have led to a new understanding of the flawed biology underlying the ever-growing number of defects. Most of these conditions could be diagnosed by means of screening for lymphopenia or for T-cell deficiency in cord blood at birth. Early recognition of those so afflicted is essential to the application of the most appropriate treatments for these conditions at a very early age. The latter treatments include both transplantation and gene therapy in addition to immunoglobulin replacement. Fully defining the molecular defects of such patients is also essential for genetic counseling of family members and prenatal diagnosis.

Interferon regulatory factor 4 (IRF4) interacts with NFATc2 to modulate interleukin 4 gene expression

Proteins of the nuclear factor of activated T cells (NFAT) family of transcription factors are critical for lymphocyte activation in the immune system. In particular, NFATs are important regulators of inducible IL-4 gene expression. Interferon regulatory factor 4 (IRF4) is an immune system-restricted interferon regulatory factor that is required for lymphocyte activation, but its molecular functions in the T lineage remain to be elucidated. We demonstrate that IRF4 potently synergizes with NFATc2 to specifically enhance NFATc2-driven transcriptional activation of the IL-4 promoter. This function is dependent on the physical interaction of IRF4 with NFATc2. IRF4 synergizes with NFATc2 and the IL-4-inducing transcription factor, c-maf, to augment IL-4 promoter activity as well as to elicit significant levels of endogenous IL-4 production. Furthermore, naïve T helper cells from mice lacking IRF4 are compromised severely for the production of IL-4 and other Th2 cytokines. The identification of IRF4 as a partner for NFATc2 in IL-4 gene regulation provides an important molecular function for IRF4 in T helper cell differentiation.

Lack of IFN-gamma-mediated induction of the class II transactivator (CIITA) through promoter methylation is predominantly found in developmental tumor cell lines

Downregulation of major histocompatibility complex (MHC) molecules by tumor cells impairs cellular immune recognition and contributes to inefficient cell-mediated tumor eradication. Low or lack of expression of MHC molecules is frequently observed in early developmental or embryonically derived tumor cells. Considering the central role of the class II transactivator (CIITA) in MHC class II- and class I-mediated antigen presentation, we compared the induction of CIITA by interferon-gamma (IFN-gamma) in a diverse panel of developmental and more differentiated tumor cell lines. In contrast to the more differentiated tumor cell lines, none of the developmental tumor cell lines were capable of expressing CIITA after treatment with IFN-gamma. Remarkably, in transient transfection assays, CIITA promoter IV (CIITA-PIV) was found to be induced by IFN-gamma. Southern blot analysis of genomic DNA obtained from the developmental tumor cell lines indicated that the absence of endogenous CIITA induction was due to methylation of the CIITA-PIV region. Exposure to 5-azacytidine restored induction of CIITA and congruent HLA-DRA expression in these cells. The observation that only developmental tumor cell lines, originating from various tissues, employ methylation to silence CIITA expression may reflect the natural status of CIITA expression during early development rather than oncogenic transformation.

Activated human T cells accomplish MHC class II expression through T cell-specific occupation of class II transactivator promoter III

Activated human T cells express HLA-DR, HLA-DQ, and HLA-DP on their surface, but the regulation and functioning of MHC class II molecules in T lymphocytes are poorly understood. Because the MHC class II transactivator (CIITA) is essential for MHC class II expression, we have investigated transcriptional activation of CIITA in activated T cells. In this study, we show that in human activated CD4(+) T cells, CIITA promoter III (CIITA-PIII) drives the expression of CIITA. The in vivo genomic footprint analysis revealed activated T cell-specific occupation of CIITA-PIII. Subsequent EMSA analysis of several promoter regions showed differences in banding pattern among activated T cells, naive T cells, primary B cells, and Raji B cells. Activating response element (ARE)-1 is shown to interact with the acute myeloid leukemia 2 transcription factor in nuclear extracts derived from both T and B cells. Interestingly, the acute myeloid leukemia 3 transcription factor was bound in nuclear extracts of T cells only. The ARE-2 sequence is able to bind CREB/activating transcription factor family members in both T and B cells. In addition, a yet unidentified Ets family member was found to interact with site C in activated T cells, whereas in B cells site C was bound by PU.1 and Pip/IFN regulatory factor 4/IFN consensus sequence binding protein for activated T cells. In Jurkat T cells, both ARE-1 and ARE-2 are crucial for CIITA-PIII activity, similar to Raji B cells. The differential banding pattern in in vivo genomic footprinting and transcription factor binding at the ARE-1 and site C between T cells and B cells probably reflects differences in CIITA-PIII activation pathways employed by these cell types.

Inhibition of class II trans-activator function by HIV-1 tat in mouse cells is independent of competition for binding to cyclin T1

The Tat trans-activator protein from HIV-1 inhibits the function of the class II trans-activator protein (CIITA), resulting in reduced MHC class II gene transcription in human cells. Tat does so by competing with CIITA for binding to cyclin T1, a component of the transcriptional elongation complex PTEFb. Since Tat does not functionally interact with mouse cyclin T1, we decided to examine the ability of Tat to inhibit CIITA in mouse cells. We found that Tat inhibited CIITA activity in mouse cells though this inhibition was independent of cyclin T1. The inhibition required the transcriptional activation domain of CIITA, but did not involve alterations in MHC class II promoter occupancy. Although Tat blocked the interaction between CIITA protein and human cyclin T1, it had no effect on the binding between CIITA and mouse cyclin T1. Therefore, Tat can inhibit the ability of CIITA to activate transcription of MHC class II genes in mouse cells by a mechanism that appears to be distinct from that proposed for human cells.

Human cytomegalovirus inhibition of major histocompatibility complex transcription and interferon signal transduction

Pathogens have evolved diverse mechanisms for escaping host innate and adaptive immunity. Viruses that maintain a persistent infection are particularly effective at disabling key arms of the host immune response. For example, the herpesviruses establish a persistent infection in human and animal hosts, in part through critical immunoevasive strategies. Cytomegalovirus, a beta-herpesvirus, impairs major histocompatibility complex (MHC) class I and class II antigen presentation by decreasing MHC expression on the surface of the infected cell, thus enabling infected cells to escape CD8+ and CD4+ T lymphocyte immunosurveillance. Moreover, cytomegalovirus blocks the interferon signal transduction pathway, thereby limiting the direct and indirect antiviral effects of the interferons. In this review, we focus on an emerging paradigm in which the effectiveness of viruses, particularly human cytomegalovirus, to escape antiviral immune responses is significantly enhanced by their ability to inhibit MHC transcription and interferon (IFN)-stimulated (JAK/STAT) signal transduction.

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.

Transcriptional control of MHC genes in fetal trophoblast cells

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

The NOD: a signaling module that regulates apoptosis and host defense against pathogens

Nods, a growing family of proteins containing a nucleotide-binding oligomerization domain (NOD), are involved in the regulation of programmed cell death (PCD) and immune responses. Members of the family include Apaf-1, Ced-4, Nod1, Nod2, and the cytosolic products of plant disease resistance genes. The NOD module is homologous to the ATP-binding cassette (ABC) found in a large number of proteins with diverse biological function. The centrally located NOD promotes activation of effector molecules through self-association and induced proximity of binding partners. The C-terminal domain of Nods serves as a sensor for intracellular ligands, whereas the N-terminal domain mediates binding to dowstream effector molecules and activation of diverse signaling pathways. Thus, Nods activate, through the NOD module, diverse signaling pathways involved in the elimination of cells via PCD and the host defense against pathogens.

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.

Mutation in the class II trans-activator leading to a mild immunodeficiency

The expression of MHC class II molecules is essential for all Ag-dependent immune functions and is regulated at the transcriptional level. Four trans-acting proteins control the coordinate expression of MHC class II molecules: class II trans-activator (CIITA), regulatory factor binding to the X box (RFX)-associated protein; RFX protein containing ankyrin repeats, and RFX5. In humans, defects in these genes result in MHC class II expression deficiency and cause combined immunodeficiency. Most patients with this deficiency suffer from severe recurrent infections that frequently lead to death during early childhood. We investigated three sisters, now ages 21, 22, and 24 years, in whom MHC-II deficiency was detected. Even though the eldest sibling was asymptomatic and the other two had only mild immunodeficiency, none of the three class II isotypes was expressed on T cell blasts, fibroblasts, EBV B cell lines, or epidermal dendritic cells. Residual HLA-II expression was detected in fresh PBMC. Somatic complementation identified the disease as CIITA deficiency. A homozygous T1524C (L469P) substitution was found in the coding region of the CIITA cDNA and was shown to be responsible for the defect in MHC-II expression. This missense mutation prevents the normal functioning of MHC-II but does not lead to the nuclear exclusion of the L469P CIITA. Transfection experiments demonstrated that the CIITA L469P mutant had residual MHC class II trans activation activity, which might explain the unusual clinical course of the patients studied. This study shows that an attenuated clinical phenotype or an asymptomatic clinical course can be observed in patients despite a profound defect in the expression of MHC class II genes. The frequency of the inherited MHC class II deficiency might thus be underestimated.

Combinations of dominant-negative class II transactivator, p300 or CDK9 proteins block the expression of MHC II genes

The class II transactivator (CIITA) regulates not only the transcription of HLA-DR, -DQ, -DP, but also invariant chain, DMA and DMB genes. A hybrid mutant CIITA protein, which contained residues from positions 302 to 1130 in CIITA fused to the enhanced green fluorescent protein (EdCIITA), inhibited the function of the wild-type protein. EdCIITA extinguished the inducible and constitutive expression of MHC II genes in epithelial cells treated with IFN-gamma and B lymphoblastoid cells respectively. Also, it blocked T cell activation by superantigen. This inhibition correlated with the localization of EdCIITA but not CIITA in the cytoplasm of cells. However, when EdCIITA was co-expressed with a dominant-negative form of the nucleoporin Nup214/CAN, it also accumulated in the nucleus. These data suggest that EdCIITA not only competes with the wild-type protein for the binding to MHC II promoters but sequesters a critical co-factor of CIITA in the cytoplasm. CIITA also recruits the histone acetyltransferase cAMP responsive element binding protein (CREB) binding protein and positive transcription elongation factor b (p-TEFb) for the transcription of MHC II genes. Dominant-negative p300 (DNp300) or CDK9 (DNCDK9) proteins inhibited the function of CIITA and of the DRA promoter. Thus, combinations of EdCIITA and DNp300 and/or DNCDK9 proteins extinguished the transcription of MHC II genes. They might become useful for future genetic therapeutic approaches in organ transplantation and autoimmune diseases.

Downregulation of CIITA function by protein kinase a (PKA)-mediated phosphorylation: mechanism of prostaglandin E, cyclic AMP, and PKA inhibition of class II major histocompatibility complex expression in monocytic lines

Prostaglandins, pleiotropic immune modulators that induce protein kinase A (PKA), inhibit gamma interferon induction of class II major histocompatibility complex (MHC) genes. We show that phosphorylation of CIITA by PKA accounts for this inhibition. Treatment with prostaglandin E or 8-bromo-cyclic AMP or transfection with PKA inhibits the activity of CIITA in both mouse and human monocytic cell lines. This inhibition is independent of other transcription factors for the class II MHC promoter. These same treatments also greatly reduced the induction of class II MHC mRNA by CIITA. PKA phosphorylation sites were identified using site-directed mutagenesis and phosphoamino acid analysis. Phosphorylation at CIITA serines 834 and 1050 accounts for the inhibitory effects of PKA on CIITA-driven class II MHC transcription. This is the first demonstration that the posttranslational modification of CIITA mediates inhibition of class II MHC transcription.

Dendritic cell-specific MHC class II transactivator contains a caspase recruitment domain that confers potent transactivation activity

The MHC class II transactivator (CIITA) is a critical transcription factor that regulates genes involved in antigen presentation function. At least three functional forms of CIITA gene products are transcribed from three different promoters. The CIITA gene expressed in dendritic cells (DC-CIITA) has a unique first exon encoding an extended N-terminal region of CIITA. Here, we show that the N terminus of DC-CIITA has high homology to a caspase recruitment domain (CARD) found in components of apoptosis and nuclear factor-kappaB signaling pathways. However, DC-CIITA does not regulate cell death, nor does it induce nuclear factor-kappaB activity. Instead, DC-CIITA is transcriptionally a more potent activator of the MHC class II gene than the form expressed in B cells. A single amino acid substitution in the CARD of DC-CIITA, predicted to disrupt CARD-CARD interactions, diminished the transactivation potential of DC-CIITA. These results indicate that the CARD in the context of CIITA serves as a regulatory domain for transcriptional activity and may function to selectively enhance MHC class II gene expression in dendritic cells.

Two distinct domains within CIITA mediate self-association: involvement of the GTP-binding and leucine-rich repeat domains

CIITA is the master regulator of class II major histocompatibility complex gene expression. We present evidence that CIITA can self-associate via two domains: the C terminus (amino acids 700 to 1130) and the GTP-binding domain (amino acids 336 to 702). Heterotypic and homotypic interactions are observed between these two regions. Deletions within the GTP-binding domain that reduce GTP-binding and transactivation function also reduce self-association. In addition, two leucine residues in the C-terminal leucine-rich repeat region are critical for self-association as well as function. This study reveals for the first time a complex pattern of CIITA self-association. These interactions are discussed with regard to the apoptosis signaling proteins, Apaf-1 and Nod1, which share domain arrangements similar to those of CIITA.

Cytomegalovirus and transcriptional down-regulation of major histocompatibility complex class II expression

CD4(+)T lymphocytes are a significant component of the afferent and efferent arms of adaptive immunity and are critical for controlling viral infections. CD4(+)T lymphocytes secrete cytokines that augment CD8(+)T lymphocyte and B lymphocyte responses and directly inhibit viral replication. The interface between the CD4(+)T lymphocyte and virus is the major histocompatibility complex (MHC) class II molecule. Cytomegalovirus, a beta-herpesvirus, has evolved mechanisms for inhibiting MHC class II expression and thus escaping CD4(+)T lymphocyte immunosurveillance. Herein, we review cytomegalovirus-mediated down-regulation of inducible and constitutive MHC class II expression, while focusing on lesions that occur at the level of MHC class II transcription.

Transcriptional coactivator, CIITA, is an acetyltransferase that bypasses a promoter requirement for TAF(II)250

The CIITA coactivator is essential for transcriptional activation of MHC class II genes and mediates enhanced MHC class I transcription. We now report that CIITA contains an intrinsic acetyltransferase (AT) activity that maps to a region within the N-terminal segment of CIITA, between amino acids 94 and 132. The AT activity is regulated by the C-terminal GTP-binding domain and is stimulated by GTP. CIITA-mediated transactivation depends on the AT activity. Further, we report that, although constitutive MHC class I transcription depends on TAF(II)250, CIITA activates the promoter in the absence of functional TAF(II)250.

Acetylation by PCAF enhances CIITA nuclear accumulation and transactivation of major histocompatibility complex class II genes

The class II transactivator (CIITA), the master regulator of the tissue-specific and interferon gamma-inducible expression of major histocompatibility complex class II genes, synergizes with the histone acetylase coactivator CBP to activate gene transcription. Here we demonstrate that in addition to CBP, PCAF binds to CIITA both in vivo and in vitro and enhances CIITA-dependent transcriptional activation of class II promoters. Accordingly, E1A mutants defective for PCAF or CBP interaction show reduced ability in suppressing CIITA activity. Interestingly, CBP and PCAF acetylate CIITA at lysine residues within a nuclear localization signal. We show that CIITA is shuttling between the nucleus and cytoplasm. The shuttling behavior and activity of the protein are regulated by acetylation: overexpression of PCAF or inhibition of cellular deacetylases by trichostatin A increases the nuclear accumulation of CIITA in a manner determined by the presence of the acetylation target lysines. Furthermore, mutagenesis of the acetylated residues reduces the transactivation ability of CIITA. These results support a novel function for acetylation, i.e., to regulate gene expression by stimulating the nuclear accumulation of an activator.

CIITA leucine-rich repeats control nuclear localization, in vivo recruitment to the major histocompatibility complex (MHC) class II enhanceosome, and MHC class II gene transactivation

The major histocompatibility complex (MHC) class II transactivator CIITA plays a pivotal role in the control of the cellular immune response through the quantitative regulation of MHC class II expression. We have analyzed a region of CIITA with similarity to leucine-rich repeats (LRRs). CIITA LRR alanine mutations abolish both the transactivation capacity of full-length CIITA and the dominant-negative phenotype of CIITA mutants with N-terminal deletions. We demonstrate direct interaction of CIITA with the MHC class II promoter binding protein RFX5 and could also detect novel interactions with RFXANK, NF-YB, and -YC. However, none of these interactions is influenced by CIITA LRR mutagenesis. On the other hand, chromatin immunoprecipitation shows that in vivo binding of CIITA to the MHC class II promoter is dependent on LRR integrity. LRR mutations lead to an impaired nuclear localization of CIITA, indicating that a major function of the CIITA LRRs is in nucleocytoplasmic translocation. There is, however, evidence that the CIITA LRRs are also involved more directly in MHC class II gene transactivation. CIITA interacts with a novel protein of 33 kDa in a manner sensitive to LRR mutagenesis. CIITA is therefore imported into the nucleus by an LRR-dependent mechanism, where it activates transcription through multiple protein-protein interactions with the MHC class II promoter binding complex.

Associations and interactions between bare lymphocyte syndrome factors

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

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

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

Identification of class II transcriptional activator-induced genes by representational difference analysis: discoordinate regulation of the DN alpha/DO beta heterodimer

Class II transcriptional activator (CIITA) is a master regulator of MHC class II genes, including DR, DP, and DQ, and MHC class II-associated genes DM and invariant chain. To determine the repertoire of genes that is regulated by CIITA and to identify uncharacterized CIITA-inducible genes, we used representational difference analysis. Representational difference analysis screens for differentially expressed transcripts. All CIITA-induced genes were MHC class II related. We have identified the alpha subunit, DN alpha, of the class II processing factor DO as an additional CIITA-inducible gene. Northern analysis confirmed that DN alpha is induced by IFN-gamma in 2fTGH fibrosarcoma cells, and CIITA is necessary for high-level expression in B cells. The beta subunit, DO beta, is not inducible in fibrosarcoma cells by IFN-gamma or exogenous CIITA expression. Moreover, in contrast to other class II genes, DO beta expression remains high in the absence of CIITA in B cells. The promoters for DN alpha and DO beta contain the highly conserved WXY motifs, and, like other class II genes, expression of both DN alpha and DO beta requires RFX. These findings demonstrate that both DN alpha and DO beta are regulated by RFX. However, DN alpha is defined for the first time as a CIITA-inducible gene, and DO beta as a MHC class II gene whose expression is independent of CIITA.