Multiple transcription factors regulate the inducible expression of the human complement receptor 2 promoter

MYC directly transactivates CR2/CD21, the receptor of the Epstein-Barr virus, enhancing the viral infection of Burkitt lymphoma cells

MYC is an oncogenic transcription factor dysregulated in about half of total human tumors. While transcriptomic studies reveal more than 1000 genes regulated by MYC, a much smaller fraction of genes is directly transactivated by MYC. Virtually all Burkitt lymphoma (BL) carry chromosomal translocations involving MYC oncogene. Most endemic BL and a fraction of sporadic BL are associated with Epstein-Barr virus (EBV) infection. The currently accepted mechanism is that EBV is the BL-causing agent inducing MYC translocation. Herein we show that the EBV receptor, CR2 (also called CD21), is a direct MYC target gene. This is based on several pieces of evidence: MYC induces CR2 expression in both proliferating and arrested cells and in the absence of protein synthesis, binds the CR2 promoter and transactivates CR2 in an E-box-dependent manner. Moreover, using mice with conditional MYC ablation we show that MYC induces CR2 in primary B cells. Importantly, modulation of MYC levels directly correlates with EBV's ability of infection in BL cells. Altogether, in contrast to the widely accepted hypothesis for the correlation between EBV and BL, we propose an alternative hypothesis in which MYC dysregulation could be the first event leading to the subsequent EBV infection.

Time course analysis of immunity-related gene expression in the sea cucumber Apostichopus japonicus during exposure to thermal and hypoxic stress

Temperature and dissolved oxygen concentration are important abiotic factors that can limit the growth and survival of sea cucumbers by affecting their immune systems. As global warming intensifies, sea cucumbers are increasingly exposed to adverse environmental conditions, which can cause severe economic losses and limit the sustainable development of sea cucumber aquaculture. It is therefore important to better understand how sea cucumbers respond to environmental stress, especially with regard to its effects on immunity. In the present study, the time series of immunity-related gene expression in sea cucumbers under thermal and hypoxic stresses were analyzed separately. The expression trends of 17 genes related to the nuclear factor κB (NF-κB) pathway, the protease family, the complement system, heat shock proteins (HSPs) and the transferrin family during exposure to two stresses at eight time points were concluded. These genes have interconnected roles in stress defense. The expression levels of genes relating to the NF-κB pathways and HSPs were strongly affected in the sea cucumber thermal stress response, while melanotransferrin (Mtf), ferritin (Ft) and mannan-binding C-type lectin (MBCL) were affected by hypoxia. In contrast, complement factor B (Bf), myosin V (Mys) and serine protease inhibitor (SPI) were not that sensitive during the initial period of environmental stress. Similar expression patterns under both thermal and hypoxic stress for certain genes, including an increase in Hsp90 and decreases in lysozyme (Lys), major yolk protein (MYP) and cathepsin C (CTLC) were observed in sea cucumbers. Conversely, NF-κB and Hsp70 were differentially affected by the two stress treatments. Lysozyme-induced immune defense was inconstant in sea cucumbers coping with stress. A gene ontology (GO) analysis of the selected genes revealed that the most co-involved terms related to immunity and iron ion. Our analysis suggests that sea cucumbers demonstrate complex and varied immune responses to different types of stresses. This dynamic image of the immune responses and stress tolerance of sea cucumbers provides new insights into the adaptive strategies of holothurians in adverse environments.

PD-1 immunobiology in systemic lupus erythematosus

Programmed death (PD)-1 receptors and their ligands have been identified in the pathogenesis and development of systemic lupus erythematosus (SLE). Two key pathways, toll-like receptor and type I interferon, are significant to SLE pathogenesis and modulate the expression of PD-1 and the ligands (PD-L1, PD-L2) through activation of NF-κB and/or STAT1. These cell signals are regulated by tyrosine kinase (Tyro, Axl, Mer) receptors (TAMs) that are aberrantly activated in SLE. STAT1 and NF-κB also exhibit crosstalk with the aryl hydrocarbon receptor (AHR). Ligands to AHR are identified in SLE etiology and pathogenesis. These ligands also regulate the activity of the Epstein-Barr virus (EBV), which is an identified factor in SLE and PD-1 immunobiology. AHR is important in the maintenance of immune tolerance and the development of distinct immune subsets, highlighting a potential role of AHR in PD-1 immunobiology. Understanding the functions of AHR ligands as well as AHR crosstalk with STAT1, NF-κB, and EBV may provide insight into disease development, the PD-1 axis and immunotherapies that target PD-1 and its ligand, PD-L1.

Focused transcription from the human CR2/CD21 core promoter is regulated by synergistic activity of TATA and Initiator elements in mature B cells

Complement receptor 2 (CR2/CD21) is predominantly expressed on the surface of mature B cells where it forms part of a coreceptor complex that functions, in part, to modulate B-cell receptor signal strength. CR2/CD21 expression is tightly regulated throughout B-cell development such that CR2/CD21 cannot be detected on pre-B or terminally differentiated plasma cells. CR2/CD21 expression is upregulated at B-cell maturation and can be induced by IL-4 and CD40 signaling pathways. We have previously characterized elements in the proximal promoter and first intron of CR2/CD21 that are involved in regulating basal and tissue-specific expression. We now extend these analyses to the CR2/CD21 core promoter. We show that in mature B cells, CR2/CD21 transcription proceeds from a focused TSS regulated by a non-consensus TATA box, an initiator element and a downstream promoter element. Furthermore, occupancy of the general transcriptional machinery in pre-B versus mature B-cell lines correlate with CR2/CD21 expression level and indicate that promoter accessibility must switch from inactive to active during the transitional B-cell window.

Functional polymorphism in the 5'-UTR of CR2 is associated with susceptibility to nasopharyngeal carcinoma

Epstein-Barr virus (EBV)-associated nasopharyngeal carcinoma (NPC) is a squamous cell cancer endemic in Southern China and Southeast Asia. It has been shown that inflammatory and immune responses during EBV infection contribute to the development of NPC. The complement receptor 2 (CR2) gene plays central roles during inflammatory and immune responses and, therefore, is a good candidate susceptibility gene for NPC. We performed PCR-based sequencing to identify multiple single-nucleotide polymorphisms (SNPs) within the exon regions of the CR2 gene in a Cantonese population. Two SNPs were screened in 528 NPC patients and 408 normal individuals to perform a case-control study matched according to age, gender and residence. Furthermore, we cloned the entire 5′-UTR and entire CR2 promoter into a luciferase report system and compared the luciferase activities between the different allelic constructs. A SNP in the 5′-UTR of CR2 (24 T/C, rs3813946) showed a significant association (P<0.01) with NPC in the Cantonese population studied. The subjects were categorized into 2 age groups: group 1, age ≤45 years and group 2, age >45 years. In group 1, the allelic frequencies of 24 T/C in the patients were significantly different from those of the controls (P=0.0034). The odds ratio (OR=1.81) also indicated a higher risk of NPC in individuals who carried the minor allele C. All constructs exerted allelic differences on luciferase activities, but only the susceptible allele +24C construct showed increased activity. Our findings implicate CR2 as a susceptibility gene for NPC and suggest that enhanced CR2 expression may be involved in the oncogenesis and development of NPC.

Transcriptional effects of a lupus-associated polymorphism in the 5' untranslated region (UTR) of human complement receptor 2 (CR2/CD21)

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with a strong genetic component that determines risk. A common three single-nucleotide polymorphism (SNP) haplotype of the complement receptor 2 (CR2) gene has been associated with increased risk of SLE (Wu et al., 2007; Douglas et al., 2009), and a less common haplotype consisting of the major allele at SNP1 and minor alleles at SNP2 and 3 confers protection (Douglas et al., 2009). SNP1 (rs3813946), which is located in the 5' untranslated region (UTR) of the CR2 gene, altered transcriptional activity of a CR2 promoter-luciferase reporter gene construct transiently transfected into a B cell line (Wu et al., 2007) and had an independent effect in the protective haplotype (Douglas et al., 2009). In this study, we show that this SNP alters transcriptional activity in a transiently transfected non B-cell line as well as in stably transfected cell lines, supporting its relevance in vivo. Furthermore, the allele at this SNP affects chromatin accessibility of the surrounding sequence and transcription factor binding. These data confirm the effects of rs3813946 on CR2 transcription, identifying the 5' UTR to be a novel regulatory element for the CR2 gene in which variation may alter gene function and modify the development of lupus.

Cell- and stage-specific chromatin structure across the Complement receptor 2 (CR2/CD21) promoter coincide with CBF1 and C/EBP-beta binding in B cells

Stringent developmental transcription requires multiple transcription factor (TF) binding sites, cell-specific expression of signaling molecules, TFs and co-regulators and appropriate chromatin structure. During B-lymphopoiesis, human Complement receptor 2 (CR2/CD21) is detected on immature and mature B cells but not on B cell precursors and plasma cells. We examined cell- and stage-specific human CR2 gene regulation using cell lines modeling B-lymphopoiesis. Chromatin accessibility assays revealed a region between -409 and -262 with enhanced accessibility in mature B cells and pre-B cells, compared to either non-lymphoid or plasma cell-types, however, accessibility near the transcription start site (TSS) was elevated only in CR2-expressing B cells. A correlation between histone acetylation and CR2 expression was observed, while histone H3K4 dimethylation was enriched near the TSS in both CR2-expressing B cells and non-expressing pre-B cells. Candidate sites within the CR2 promoter were identified which could regulate chromatin, including a matrix attachment region associated with CDP, SATB1/BRIGHT and CEBP-beta sites as well as two CBF1 sites. ChIP assays verified that both CBF1 and C/EBP-beta bind the CR2 promoter in B cells raising the possibility that these factors facilitate or respond to alterations in chromatin structure to control the timing and/or level of CR2 transcription.

Complement receptor 2 polymorphisms associated with systemic lupus erythematosus modulate alternative splicing

Genetic factors influence susceptibility to systemic lupus erythematosus (SLE). A recent family-based analysis in Caucasian and Chinese populations provided evidence for association of single-nucleotide polymorphisms (SNPs) in the complement receptor 2 (CR2/CD21) gene with SLE. Here we confirmed this result in a case-control analysis of an independent European-derived population including 2084 patients with SLE and 2853 healthy controls. A haplotype formed by the minor alleles of three CR2 SNPs (rs1048971, rs17615, rs4308977) showed significant association with decreased risk of SLE (30.4% in cases vs 32.6% in controls, P=0.016, OR=0.90 (0.82-0.98)). Two of these SNPs are in exon 10, directly 5' of an alternatively spliced exon preferentially expressed in follicular dendritic cells (FDC), and the third is in the alternatively spliced exon. Effects of these SNPs and a fourth SNP in exon 11 (rs17616) on alternative splicing were evaluated. We found that the minor alleles of these SNPs decreased splicing efficiency of exon 11 both in vitro and ex vivo. These findings further implicate CR2 in the pathogenesis of SLE and suggest that CR2 variants alter the maintenance of tolerance and autoantibody production in the secondary lymphoid tissues where B cells and FDCs interact.

Transcriptional control of complement receptor gene expression

Immune complement is a critical system in the immune response and protection of host cells from damage by complement is critical during inflammation. The expression of the receptors for the inflammatory anaphylatoxin molecules is also key in immunity. In order to fully appreciate the biology of complement, a basic understanding of the molecular regulation of complement receptor gene expression is critical, yet these kinds of studies are lacking for many genes. Importantly, recent genetic studies have demonstrated that promoter-enhancer polymorphisms can contribute to pathology in diseases such as atypical hemolytic uremic syndrome. This review will focus on what is currently known about the genetic regulation of key protective complement receptors genes including CR1 (CD35), CR2 (CD21), Crry, MCP (CD46), DAF (CD55), and CD59. In addition, the regulation of the anaphylatoxin receptors genes, C3aR and C5aR (CD88) will also be discussed. Since new research continuously uncovers novel functions for these proteins, a greater appreciation of the mechanisms involved in gene regulation will be critical for understanding the biology of these molecules.

Defining In Vivo Transcription Factor Complexes of the Murine CD21 and CD23 Genes

The expression of the CD21 and CD23 genes is coincident with differentiation from transition 1 B cells (T1) to transition 2 B cells (T2). To define constituents controlling CD21 and CD23 expression, we conducted chromatin immunoprecipitation analyses for candidate transcription factors. We found constitutive binding of Oct-1, NFAT species, YY1, NF-kappaB-p52, Pax5, E2A, and RBP-Jkappa to CD21 sequences and NF-kappaB-p52, Pax5, NFAT species, E2A, and RBP-Jkappa to CD23 promoter sequences. Splenic T and B cell subsets displayed constitutive binding of YY1, NF-kappaB-p52, Pax5, and Oct-1 proteins to CD21 sequences in B cells but no specific binding of NFATc3 or Pax5 in T cells. Similarly, CD23 sequences demonstrated constitutive binding of NF-kappaB-p52 in splenic T and B cells but only Pax5 in B cells. Of the various NFAT species, only a subset were found forming constitutive DNA/protein complexes with the CD21, CD23, and IL-2 gene sequences. Maturing B cells in the marrow possess stable Pax5 complexes on CD19, CD21, and CD23 gene promoters in the nuclei of such cells, even though only CD19 is expressed. The similarity of genetic controlling elements between the CD21 and CD23 genes does not suggest a mechanism for alternative regulation of these genes; however, separation of splenic B cell subsets into T1, T2, marginal zone (MZ), and mature follicular B cells, followed by quantitative RT-PCR, demonstrated the lack of appreciable CD23 transcripts in CD21(+) MZ cells. We propose an alternative derivation of MZ cells as maturing directly from T1 cells, leaving CD23 transcriptionally inactive in that lineage of cells.

Association of a common complement receptor 2 haplotype with increased risk of systemic lupus erythematosus

A genomic region on distal mouse chromosome 1 and its syntenic human counterpart 1q23-42 show strong evidence of harboring lupus susceptibility genes. We found evidence of linkage at 1q32.2 in a targeted genome scan of 1q21-43 in 126 lupus multiplex families containing 151 affected sibpairs (nonparametric linkage score 2.52, P = 0.006). A positional candidate gene at 1q32.2, complement receptor 2 (CR2), is also a candidate in the murine Sle1c lupus susceptibility locus. To explore its role in human disease, we analyzed 1,416 individuals from 258 Caucasian and 142 Chinese lupus simplex families and demonstrated that a common three-single-nucleotide polymorphism CR2 haplotype (rs3813946, rs1048971, rs17615) was associated with lupus susceptibility (P = 0.00001) with a 1.54-fold increased risk for the development of disease. Single-nucleotide polymorphism 1 (rs3813946), located in the 5' untranslated region of the CR2 gene, altered transcriptional activity, suggesting a potential mechanism by which CR2 could contribute to the development of lupus. Our findings reveal that CR2 is a likely susceptibility gene for human lupus at 1q32.2, extending previous studies suggesting that CR2 participates in the pathogenesis of systemic lupus erythematosus.

Distribution of the chromatin protein DEK distinguishes active and inactive CD21/CR2 gene in pre- and mature B lymphocytes

DEK is an abundant and ubiquitous chromatin protein that has only recently attracted attention. DEK preferentially binds to cruciform and superhelical DNA and induces positive supercoils into closed circular DNA. It is quite likely therefore that DEK performs an important architectural function in chromatin. However, it is not known how DEK is distributed in chromatin. As the first study of its kind, we investigate the distribution of DEK at the CD21/complement receptor 2 gene regulatory regions in two B lymphocyte lines, namely Ramos, which expresses the CD21 gene, and Nalm-6, which does not. We use a chromatin immunoprecipitation approach and show that DEK appears to be distributed over various regions of the expressed and silent genes, but occurs in 2- to 3-fold higher amounts at a promoter-proximal site of the expressed gene. Moreover, induction of CD21 expression in Nalm-6 cells leads to accumulation of DEK at this site. We propose that the accumulation of DEK is functionally linked to gene expression.

Determination of interleukin-4-responsive region in the human cytochrome P450 2E1 gene promoter

Cytochrome P450 2E1 (CYP2E1) gene expression is known to be induced by interleukin-4 (IL4) and repressed by inflammatory cytokines, such as interleukin-1beta3 (IL1beta3) in human hepatocytes. The mechanisms involved in these transcriptional regulations remain elusive. In order to study these mechanisms, various constructs of the human CYP2E1 promoter were prepared and transfected into the human HepG2 hepatoma cell line. Our findings revealed that an IL4-responsive region of 128bp (-671/-544) was required to mediate induction by IL4. IL1beta caused moderate but significant decrease of the promoter activity, which was abolished when the two cytokines were combined. The IL1beta inhibitory effect is mediated through a regulatory sequence independent of that of IL4. Furthermore, by using specific signaling pathway inhibitors, we demonstrated that IL4 activation required protein kinase C (PKC) activation. In addition, our results suggest that induction by IL4 was not dependent on a single binding site but rather on a complex region which includes putative binding sites for signal transducer and activator of transcription (STAT)6, activator protein (AP)-1, nuclear factor kappa-B (NFkappaB), nuclear factor of activated T cells (NFAT) and CCAAT enhancer binding protein (C/EBP). Electrophoretic mobility shift assays suggest that AP1 and NFAT transcription factors are able to bind to three sites in the IL4-responsive region.

Complement and autoimmunity

The complement system is comprised of a number of serum and membrane-bound proteins that play an important role in the elimination of foreign microorganisms while protecting the host organism from complement-related damage. Complement has also been shown to participate in the generation of normal humoral immune responses to foreign antigens. Recent studies suggest that the functions of complement may be extended to include the maintenance of B cell tolerance. Complement receptor 2 (CR2/CD21) has been implicated in lupus susceptibility in both humans and animal models of disease. Located primarily on B cells and follicular dendritic cells, CR2 binds C3 degradation products that have become covalently bound to antigen or immune complexes in the process of complement activation. The mechanism by which CR2 might regulate B cell reactivity to autoantigens has not been elucidated, but may involve direct effects on B cell tolerance or indirect effects on T cell tolerance.

NF-kappaB regulates the expression of the human complement receptor 2 gene

CR2 is a key regulator of the B cell response to Ag. Here we show that NF-kappaB enhances the expression of the human CR2 gene. Promoter truncation, deletion, and mutagenesis studies indicated a functional role for a consensus NF-kappaB promoter element, as well as a heterogeneous nuclear ribonucleoprotein D element and an overlapping X box/E box. By supershift analysis, the first two elements bound NF-kappaB p50 and p65 and heterogeneous nuclear ribonucleoprotein RNP D, respectively. The X box/E box bound regulatory factor X5 and, surprisingly, NF-kappaB p50 and p65. Overexpression of NF-kappaB p50 enhanced the activity of the CR2 promoter in B cell lines and primary B cells, suggesting a direct role for NF-kappaB in regulating promoter activity. Importantly, mutation of the NF-kappaB element or the X box/E box rendered the promoter unresponsive to NF-kappaB p50. Using chromatin immunoprecipitation in live B cell lines and primary B cells, we found that NF-kappaB proteins p50, p65, and c-Rel bound to the genomic promoter at two locations that overlap with the consensus NF-kappaB element or the X box/E box. Finally, stimuli that activate NF-kappaB enhanced the activity of the CR2 promoter, and LPS rapidly increased the number of CR2 proteins on the surface of primary B cells. We propose that the NF-kappaB signaling pathway enhances the expression of the CR2 gene, as a result of NF-kappaB proteins binding to two CR2 promoter elements. Thus, at the onset of an infection, LPS could sensitize the B cell to Ag by enhancing the level of CR2-costimulatory molecules on the cell surface.

Functional analysis of the human complement receptor 2 (CR2/CD21) promoter: characterization of basal transcriptional mechanisms

Human complement receptor (CR) type 2 (CR2/CD21) is a 145-kDa membrane protein encoded within the regulators of complement activation gene cluster localized on human chromosome 1q32. Understanding the mechanisms that regulate CR2 expression is important because CR2 is expressed during specific stages of B cell development, and several lines of evidence suggest a role for altered CR2 function or expression in a number of autoimmune diseases. Additionally, even modest changes in CR2 expression are likely to affect relative B cell responses. In this study we have delineated the transcriptional requirements of the human CR2 gene. We have studied the human CR2 proximal promoter and identified sites important for controlling the level of transcription in CR2-expressing cells. We have determined that four functionally relevant sites lie within very close proximity to the transcriptional initiation site. These sites bind the transcription factors USF1, an AP-2-like transcription factor, and Sp1.