Transcriptional regulation of MHC class II genes

HLA-DR antigen-negative acute myeloid leukemia

Human leukocyte antigen (HLA) Class II antigens are variably expressed on acute myeloid leukemia (AML) blasts. The biological and clinical significance of HLA Class II antigen expression by AML cells is not known. Therefore, we sought to characterize cases of AML without detectable HLA-DR expression. Samples from 248 consecutive adult AML patients were immunophenotyped by multiparameter flow cytometry at diagnosis. HLA-DR antigens were not detected on AML cells from 43 patients, including 20 with acute promyelocytic leukemia (APL), and 23 with other subtypes of AML. All APL cases had t(15;17), but there were no characteristic chromosome abnormalities in non-APL cases. No direct expression of other antigens was identified in HLA-DR-negative APL and non-APL cases. Interestingly, cells from three HLA-DR-negative non-APL patients had similar morphology to that of the hypogranular variant of APL. This morphology, however, was not present in any HLA-DR-positive AML cases. Treatment response was similar in the 23 HLA-DR-negative non-APL and the 205 HLA-DR-positive patients. Finally, relapse was infrequently associated with changes in HLA-DR antigen expression, as the HLA-DR antigen was lost at relapse in only 4% of HLA-DR-positive cases, and was gained at relapse in only 17% of HLA-DR-negative cases. We conclude that HLA-DR-negative AML includes approximately equal numbers of APL and non-APL cases, and that the morphology of HLA-DR-negative non-APL cases can mimic the hypogranular variant of APL. The diagnosis of APL cannot be based on morphology and lack of HLA-DR antigen expression; rather, it requires cytogenetic or molecular confirmation.

Human choriocarcinoma-derived JEG-3 cells transfected with murine MHC class II Aq expression vectors present antigen to Aq-restricted murine T-cell hybridoma

MHC class II molecules are not normally expressed on the cell surface of placental cells. This absence of class II molecules is assumed to be of importance for mammalian reproduction, since such expression is likely to increase the risk of harmful anti-placental immune responses. The present study was aimed to clarify whether post-transcriptional events prohibit proper cell surface expression of MHC class II molecules in cell lines of placental origin. The murine trophoblast cell line SM9-1 as well as the human choriocarcinoma-derived cell line JEG-3 were transiently co-transfected with MHC class II Aq a and b genes under the control of viral promoter systems. The transfected cells were stained for surface expression of MHC class II and assayed for antigen presentation in vitro. Only a small proportion of the transfected murine SM9-1 cells showed detectable class II cell surface expression, which made functional studies of this cell line difficult. The transfected JEG-3 cells, however, showed a high proportion of cells with distinct surface expression of murine class II Aq molecules and the antigen presentation assays revealed T cell activation upon addition of processed antigen, but not with unprocessed antigen. These results show that ectopic MHC class II gene transcription can result in cell surface expression of immunohistochemically detectable MHC class II on cells of placental origin. The fact that murine class II molecules could be expressed in a functional manner on human JEG-3 cells also strongly suggests that proper accessory gene activities are not essential for obtaining surface expression.

HLA Class II Expression in Uninducible Hepatocarcinoma Cells After Transfection of AIR-1 Gene Product CIITA: Acquisition of Antigen Processing and Presentation Capacity

The AIR-1-encoded CIITA transcriptional activator is crucial for both constitutive and IFN-γ-induced MHC class II gene transcription. We show here that the MHC class II negative phenotype of the human hepatocarcinoma cell lines Alexander and HepG2 remains unmodified after treatment with IFN-γ, although MHC class I expression is up-modulated. This correlates with absence of CIITA mature transcripts. Transfection of an expressible CIITA cDNA in Alexander cells resulted in a very high cell surface expression of all three human class II subsets, HLA-DR, -DP and -DQ, indicating that normally observed induction of CIITA expression by IFN-γ is probably blocked, in the hepatocarcinoma cell lines, at the level of CIITA transcription and not at the level of IFN-γ receptor binding and signal transduction mechanisms. To assess whether MHC class II expression on CIITA-transfected Alexander cells could have functional relevance, we tested their capacity to present antigenic peptides to an HLA-DR-restricted T cell line specific for a peptide of Mycobacterium tuberculosis Ag85 protein. It was found that the transfected cells could not only present the exogenously supplemented peptide but also process Ag85 protein to generate the specific epitope recognized by the HLA-DR-restricted T cell line. Similar results were obtained with CIITA-transfected CFPAC-1 pancreatic adenocarcinoma cells, which differed from Alexander cells in that they were inducible by IFN-γ. These results suggest new strategies to act on CIITA for increasing the potential of a tumor cell to present putative tumor Ags to the immune system.

Assembly of functional regulatory complexes on MHC class II promoters in vivo

Regulatory factors that bind to the X box 1 to 5 (RFX1 to RFX5) and p36 interact with the X box in major histocompatibility class II promoters. RFX1 and RFX5 bind to DNA as a homodimer (RFX1) and heterodimer with p36 (RFX5:p36, the RFX complex), respectively. In this study, we characterized the binding of RFX1 and the RFX complex to the X box in vivo, and evaluated contributions of other proteins that bind to flanking conserved upstream sequences (CUS: S, X, X2, and Y boxes) to these protein-DNA interactions. For this purpose, an intracellular DNA-binding assay was developed. Hybrid protein effectors between RFX1 and RFX5 and the activation domain of VP16 from the herpes simplex virus were co-expressed with plasmid targets, which contained the isolated X box, X box and selected flanking CUS, or the entire DRA promoter. Whereas RFX1 bound better to isolated X boxes, the Y box selected for the binding of the RFX complex and against the binding of RFX1 to the X box. With proper spacing, S and X boxes stabilized the binding of both RFX1 and the RFX complex. The X2 box did not contribute significantly to the binding of either RFX1 or the RFX complex to the X box. Thus, complex protein-protein and protein-DNA interactions dictate the binding of functionally relevant proteins to conserved upstream sequences which regulate class II transcription.

The class II trans-activator CIITA interacts with the TBP-associated factor TAFII32

The class II trans- activator (CIITA) is the main transcriptional co-activator for the expression of MHC class II proteins. Its N-terminal 125 amino acids function as an independent transcriptional activation domain. Analyses of the primary amino acid sequence of the activation domain predict the presence of three alpha-helices, each with a high proportion of acidic residues. Using site-directed mutagenesis, we found that two of these predicted alpha-helices are required for full transcriptional activation by CIITA. Moreover, a CIITA protein in which both functional alpha-helices have been deleted displays a dominant negative phenotype. This activation domain of CIITA interacts with the 32 kDa subunit of the general transcription complex TFIID, TAFII32. Decreased transcriptional activation by N-terminal deletions of CIITA is correlated directly with their reduced binding to TAFII32. We conclude that interactions between TAFII32 and CIITA are responsible for activation of class II genes.

Physical and functional interaction between the human T-cell lymphotropic virus type 1 Tax1 protein and the CCAAT binding protein NF-Y

Tax1, a potent activator of human T-cell lymphotropic virus type 1 (HTLV-1) transcription, has been shown to modulate expression of many cellular genes. Tax1 does not bind DNA directly but regulates transcription through protein-protein interactions with sequence-specific transcription factors. Using the yeast two-hybrid system to screen for proteins which interact with Tax1, we isolated the B subunit of the CCAAT binding protein NF-Y from a HeLa cDNA library. The interaction of Tax1 with NF-YB was specific in that NF-YB did not interact with a variety of other transcription factors, including human immunodeficiency virus Tat, human papillomavirus E6, and Bicoid, or with the M7 (amino acids 29CP-AS) Tax1 mutant. However, NF-YB did interact with the C-terminal Tax1 mutants M22 (130TL-AS) and M47 (319LL-RS). We also show that in vitro-translated NF-YB specifically bound to a glutathione S-transferase-Tax1 fusion protein. Further, Tax1 coimmunoprecipitated with NF-Y from nuclear extracts of HTLV-1-transformed cells, providing evidence for in vivo interaction of Tax1 and NF-YB. We further demonstrate that Tax1 specifically activated the NF-Y-responsive DQbeta promoter, as well as a minimal promoter which contains only the Y-box element. In addition, mutation of the Y-box element alone abrogated Tax1-mediated activation. Taken together, these data indicate that Tax1 interacts with NF-Y through the B subunit and that this interaction results in activation of the major histocompatibility complex class II promoter. Through activation of this and other NF-Y driven promoters, the Tax1-NF-Y interaction may play a critical role in causing cellular transformation and HTLV-1 pathogenesis.

Polymorphic MHC class II promoters exhibit distinct expression pattern in various antigen-presenting cell lines

The promoter regions of MHC class II genes are characterized by the presence of conserved sequence motifs called S,X and Y boxes, which are crucial for regulation of transcription of these genes. In humans, promoter polymorphism is known to result in differential transcriptional activity at both inter-locus and inter-allelic levels, but it is not yet known how this relates to tissue-specific expression of MHC class II molecules. We sequenced the 5' regulatory regions of alpha and beta genes of I-A and I-E molecules from four mouse haplotypes and found allelic polymorphisms which were mainly confined to the X box. The promoter sequences of I-Ea genes were non-polymorphic. Transfection of four antigen-presenting cell types with promoter-reporter gene constructs revealed that the promoter sequence polymorphisms result in distinct allele- and tissue-specific activity patterns. Mutagenesis experiments in which the X2 box was reshuffled between I-A beta alleles demonstrated that this box contributes to regulation of differential MHC class II expression in the four cell types. The possibility is discussed that tissue-specific MHC class II expression may control differentiation of T-cell subsets.

Functional significance of polymorphism among MHC class II gene promoters

The functional significance of polymorphism among MHC class II promoters in man and mouse is here reviewed, mainly in terms of the hypothesis of differential expression. The hypothesis proposes that differences between antigen-presenting cells in MHC class II expression exert a co-dominant effect on the Th1-Th2 cytokine balance, such that class II molecules of one type come to control to a greater extent the production of one group of cytokines, and those of another type the production of the alternative group. The survey deals with the influence of signal strength and antigen-presenting cell type on T-cell subset differentiation; functional differences between MHC class II molecules not obviously related to determinant selection; disease protection mediated by HLA alleles; mechanisms possibly responsible for allotypic and isotypic bias; overdominance (heterozygous advantage) in selection for expression of class II alleles; MHC class II promoter structure and function; inter-locus and inter-allele variability within human MHC class II gene upstream regulatory regions; a comparison of these polymorphisms in mouse and man; read-out of class II promoter function; and a comparison with expression of MHC class I. We conclude that the evidence that this variation is functionally active (i.e. controls expression) is increasing, but is not yet compelling. The crucial test still to come, we suggest, is whether or not the biological effects attributable to this polymorphism will line up with molecular studies on expression.

Active suppression of the class II transactivator-encoding AIR-1 locus is responsible for the lack of major histocompatibility complex class II gene expression observed during differentiation from B cells to plasma cells

In this study the genetic control of major histocompatibility complex (MHC) class II gene expression during the transition from B cell to plasma cell has been analyzed. Class II molecules are not expressed in plasma cells because of an active suppression resulting in the abrogation of class II gene transcription. We show here that the plasma cell-specific repressor function, designated SIR (suppressor of immune response genes), does not act directly on the transcription of class II genes, but instead on the transcription of the AIR-1 gene, whose product, the class II transactivator (CIITA), is fundamental for the regulation of the constitutive and inducible expression of MHC class II genes. This was unambiguously demonstrated by the fact that plasmacytoma x B cell hybrids carrying an AIR-1 locus derived from CIITA-expressing cells do not express CIITA-specific transcripts. Transfection of a cDNA containing the human CIITA coding sequence under the control of an heterologous promoter restores expression of human MHC class II genes in the hybrids and is responsible for de novo expression of mouse MHC class II genes in both the mouse plasmacytoma cell line and the hybrids. These results confirm and extend the notion of the functional conservation of the AIR-1 gene product across species barriers. Interestingly, in CIITA-transfected cell hybrids, cell surface expression of the human HLA-DQ heterodimer was not observed. This result was not attributable to lack of HLA-DQ alpha or -DQ beta transcription, because both transcripts were present in the CIITA-transfected hybrids, although at reduced levels. These findings further support our previous observations on the distinct regulation of expression of the human HLA-DQ class II subset, which may be thus controlled at the posttranscriptional level by a CIITA-independent mechanism.

Binding and cooperative interactions between two B cell-specific transcriptional coactivators

The class II transactivator (CIITA) and B cell octamer-binding protein 1/octamer-binding factor 1/Oct coactivator from B cells (Bob1/OBF-1/OCA-B) represent two B cell-specific transcriptional coactivators. CIITA and Bob1 interact with proteins that bind to conserved upstream sequences in promoters of class II major histocompatibility genes and octamer-binding transcription factors Oct-1 and Oct-2, respectively. Both CIITA and Bob1 increase the expression from the DRA promoter, which is a prototypic class II promoter. Moreover, in the presence of CIITA, interactions between class II promoters and Bob1 are independent of the octamer-binding site. Using in vivo and in vitro binding assays, we confirm that Bob1 binds to CIITA. Thus, CIITA not only activates the expression of class II genes but recruits another B cell-specific coactivator to increase transcriptional activity of class II promoters in B cells.