Varying functions of specific major histocompatibility class II transactivator promoter III and IV elements in melanoma cell lines.

Melanoma cells commonly express MHC class II molecules constitutively. This is a rare, or possibly unique, phenotype for a nonprofessional antigen-presenting cell, where MHC class II expression ordinarily occurs only after IFN-gamma treatment. Despite the fact that constitutive expression of MHC class II on melanoma cells has been observed for decades and that the regulation of the MHC class II genes is well understood for many different cell types, there is no data regarding the basis for constitutive MHC class II expression in melanoma cells. Here we report that MHC class II expression in melanoma cells can be traced to constitutive expression of the class II transactivator protein (CIITA), which mediates both IFN-gamma-inducible and -constitutive MHC class II expression in all other cell types. In addition, we determined that constitutive CIITA expression is the result of the activation of both the B cell-specific CIITA promoter III and the IFN-gamma-inducible CIITA promoter IV, the latter of which previously has never been known to function as a constitutive promoter in any cell type. The recently described B cell-related ARE-1 activity is important for promoter III activation in the melanoma cells. Constitutive promoter IV activation involves the IFN regulatory factor element (IRF-E), which binds members of the IRF family of proteins, although the major, IFN-gamma inducible member of this family, IRF-1, is not constitutively expressed in these cells. In cells with constitutively active promoter IV, the promoter IV IRF-E is most likely activated by IRF-2. The relevance of these results to the pathway of melanoma development is discussed.     

Dissection of the interferon gamma-MHC class II signal transduction pathway reveals that type I and type II interferon systems share common signalling component(s).

We have used a herpes virus thymidine kinase (HSV-TK) based metabolic selection system to isolate mutants defective in the interferon gamma mediated induction of the MHC class II promoter. All the mutations act in trans and result in no detectable induction of MHC and invariant chain (Ii) gene expression. Scatchard analysis indicates that the mutants have a normal number of surface IFN gamma receptors with the same affinity constant. The mutants fall into two broad categories. One class of mutants is still able to induce MHC class I, IRF-1, 9-27, 1-8 and GBP genes by IFN gamma. A second class of mutants is defective for the IFN gamma induction of all the genes tested; surprisingly, the IFN alpha/beta induction of MHC class I, 9-27, ISG54 and ISG15 genes is also defective in these mutants, although different members of this class can be discriminated by the response of the GBP and IRF-1 genes to type I interferons. These data demonstrate that the signalling pathways of both type I and type II interferon systems share common signal transduction component(s). These mutants will be useful for the study of IFN gamma regulation of class II genes and Ii chain, and to elucidate molecular components of type I and type II interferon signal transduction.     

MHC II类分子表达调控的研究进展

MHCII类分子提呈经过加工的抗原给CD4 T淋巴细胞 ,在诱发免疫反应中起重要作用。MHCII类分子不正常表达会引起严重的免疫缺陷疾病 ,如裸淋巴细胞综合征 (BLS)等。目前已识别出四种不同的MHCII调控基因。这些基因分别编码RFXANK、RFX5、RFXAP和CIITA。其中 ,前三个是RFX复合物的亚基 ,RFX是一种结合于所有MHCII类基因启动子上的泛式表达的因子。CIITA是MHCII类分子表达的主要调控因子 ,其严密调控的表达模式决定了MHCII类分子表达的细胞特异性 ,及能否被诱导且在何种水平上表达。本文着重介绍近年来国内外对MHCII类分子表达及其调控研究的新进展     

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.     

ESAT6 differentially inhibits IFN-γ-inducible class II transactivator isoforms in both a TLR2-dependent and -independent manner

Mycobacterium tuberculosis (Mtb) downregulates the surface expression of major histocompatibility class II (MHC II) molecules on macrophages via modulating class II transactivator (CIITA) protein of the host cell. This results in decreased effector function of CD4(+) T cells. In macrophages, CIITA is transcribed by the promoters I (pI) and IV (pIV) and the corresponding gene products are referred to as type I and type IV CIITA, respectively. Earlier studies have mainly focused on CIITA transcribed by pIV; however, these studies also showed that type IV CIITA expression was transient and dispensable for MHC II expression. In the present study, we observed that the Mtb 6-kDa, early secreted antigen (ESAT6) inhibited interferon (IFN)-γ-induced type I as well as type IV CIITA, but, interestingly, inhibition of type I CIITA was found to be independent of Toll-like receptor-2 (TLR2), whereas that of type IV was TLR2 dependent. Moreover, we also present evidence to show that ESAT6-mediated inhibition was regulated via remodeling of the chromatin. We found that ESAT6 caused a decrease in the IFN-γ-stimulated methylation of the histone H3K4, as well as in the levels of histone acetylation at the CIITA pI locus in macrophages. We also found the involvement of mitogen-activated protein kinases ERK1/2 and p38 in the regulation of CIITA by ESAT6. In conclusion, our studies suggest that ESAT6 could inhibit the expression of type I and type IV CIITA through different pathways. Furthermore, ESAT6 could signal through putative receptors other than TLR2, and that the inhibition of IFN-γ-stimulated CIITA by ESAT6 was regulated at the chromatin level.     

Modulation of gene expression by the MHC class II transactivator

The class II transactivator (CIITA) is a master regulator of MHC class II expression. CIITA also modulates the expression of MHC class I genes, suggesting that it may have a more global role in gene expression. To determine whether CIITA regulates genes other than the MHC class II and I family, DNA microarray analysis was used to compare the expression profiles of the CIITA expressing B cell line Raji and its CIITA-negative counterpart RJ2.2.5. The comparison identified a wide variety of genes whose expression was modulated by CIITA. Real time RT-PCR from Raji, RJ2.2.5, an RJ2.2.5 cell line complemented with CIITA, was performed to confirm the results and to further identify CIITA-regulated genes. CIITA-regulated genes were found to have diverse functions, which could impact Ag processing, signaling, and proliferation. Of note was the identification of a set of genes localized to chromosome 1p34-35. The global modulation of genes in a local region suggests that this region may share some regulatory control with the MHC.