A factor that regulates the class II major histocompatibility complex gene DPA is a member of a subfamily of zinc finger proteins that includes a Drosophila developmental control protein

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.

Specific complex formation between the type II bare lymphocyte syndrome-associated transactivators CIITA and RFX5

Two of the genes defective in the five complementation groups identified in the class II-negative bare lymphocyte syndrome or corresponding laboratory mutants have been cloned. One gene encodes a protein, RFX5, that is a member of the RFX family of DNA binding proteins. The other, CIITA, encodes a large protein with a defined acidic transcriptional activation domain; this protein does not interact with DNA. Expression plasmids encoding regions of RFX5 fused to the GAL4 DNA binding domain activated transcription from a reporter construct containing GAL4 sites in a cotransfection assay in the Raji human B cell line. However, these plasmids produced transcriptional activity in HeLa cells only in conjunction with interferon gamma stimulation, a condition in which expression of both CIITA and class II major histocompatibility complex surface proteins are induced. Furthermore, these plasmids were not active in RJ2.2.5, an in vitro mutagenized derivative of Raji in which both copies of CIITA are defective. Transcriptional activation by the RFX5 fusion protein could be restored in RJ2.2.5 by cotransfection with a CIITA expression plasmid. Finally, a direct interaction between RFX5 and CIITA was detected with the yeast two-hybrid and far-Western blot assays. Thus, RFX5 can activate transcription only in cooperation with CIITA. RFX5 and CIITA associate to form a complex capable of activating transcription from class II major histocompatibility complex promoters. In this complex, promoter specificity is determined by the DNA binding domain of RFX5 and the general transcription apparatus is recruited by the acidic activation domain of CIITA.

Transactivation by CIITA, the type II bare lymphocyte syndrome-associated factor, requires participation of multiple regions of the TATA box binding protein

CIITA is a positive regulator of class II major histocompatibility complex gene transcription that has been found to be defective in one of the five complementation groups of class II major histocompatibility complex-negative cell lines. Its N-terminal region is capable of activating transcription from a reporter gene when fused to a DNA binding domain. We have investigated the mechanism of transactivation mediated by the CIITA activation domain by studying its role in the process of transcription initiation and elongation. Specifically the altered specificity TBP (TATA box binding protein) assay has been used to analyze the response of the CIITA activation domain to mutations in TBP known to disrupt its interaction with its associated general factors. Transactivation by CIITA was extremely sensitive to a mutation in TBP that in yeast is known to abolish VP16-mediated transcription but leaves basal transcription unaffected. A TBP mutant defective in interaction with TBP-associated factor TAFII250 also failed to mediate transactivation through the CIITA activation domain. Certain interactions between TBP and general factors that are specifically required for acidic activation domains were also required for CIITA-mediated transactivation to reach its full potential. Finally, like VP16, CIITA was able to stimulate elongation of transcription. Overall the mechanism of transactivation by the human B-cell-specific CIITA is very similar to that mediated by the herpes virus transactivator VP16 in the ways that have been tested.

Bach proteins belong to a novel family of BTB-basic leucine zipper transcription factors that interact with MafK and regulate transcription through the NF-E2 site

Members of the small Maf family (MafK, MafF, and MafG) are basic region leucine zipper (bZip) proteins that can function as transcriptional activators or repressors. The dimer compositions of their DNA binding forms determine whether the small Maf family proteins activate or repress transcription. Using a yeast two-hybrid screen with a GAL4-MafK fusion protein, we have identified two novel bZip transcription factors, Bach1 and Bach2, as heterodimerization partners of MafK. In addition to a Cap'n'collar-type bZip domain, these Bach proteins possess a BTB domain which is a protein interaction motif; Bach1 and Bach2 show significant similarity to each other in these regions but are otherwise divergent. Whereas expression of Bach1 appears ubiquitous, that of Bach2 is restricted to monocytes and neuronal cells. Bach proteins bind in vitro to NF-E2 binding sites, recognition elements for the hematopoietic transcription factor NF-E2, by forming heterodimers with MafK. Furthermore, a DNA binding complex that contained MafK as well as Bach2 or a protein related closely to Bach2 was found to be present in mouse brain cells. Bach1 and Bach2 function as transcription repressors in transfection assays using fibroblast cells, but they function as a transcriptional activator and repressor, respectively, in cultured erythroid cells. The results suggest that members of the Bach family play important roles in coordinating transcription activation and repression by MafK.

The new dysmorphology: application of insights from basic developmental biology to the understanding of human birth defects

Information obtained from studies of developmental and cellular processes in lower organisms is beginning to make significant contributions to the understanding of the pathogenesis of human birth defects, and it is now becoming possible to treat birth defects as inborn errors of development. Mutations in genes for transcription factors, receptors, cell adhesion molecules, intercellular junctions, molecules involved in signal transduction, growth factors, structural proteins, enzymes, and transporters have been identified in genetically caused human malformations and dysplasias. The identification of these mutations and the analysis of their developmental effects have been greatly facilitated by the existence of natural or engineered models in the mouse and even of related mutations in Drosophila, and in some instances a remarkable conservation of function in development has been observed, even between widely separated species.

Cooperativity between the J and S elements of class II major histocompatibility complex genes as enhancers in normal and class II-negative patient and mutant B cell lines

The class II major histocompatibility complex genes all contain in their proximal promoters three cis-elements called S, X, and Y that are conserved in both sequence and position, and a fourth element, J, conserved in sequence but not in position. J, X, and Y and, to some extent, S, have been shown to be functionally important in regulation of expression of these genes. In the present study, a protein factor that binds cooperatively to the S plus J elements of the promoter of the class II major histocompatibility complex gene DPA has been detected. Moreover, functional cooperativity between S and J in activation of the enhancerless -40 interferon-beta (-40 IFN-beta) promoter has been demonstrated. Finally, the latter assay appears to subdivide complementation group A of class II negative human B cell lines that includes both mutants generated in vitro and cells from patients with the bare lymphocyte syndrome (type II). In three of these cell lines, the enhancerless -40 IFN-beta promoter containing the S plus J elements was functionally active, while in the others it was inactive.