Interferon-gamma-inducible regulation of the human invariant chain gene

Interferon-gamma acts directly on rejecting renal allografts to prevent graft necrosis

In transplant rejection interferon (IFN)-gamma regulates the recipient immune response but also acts directly on IFN-gamma receptors in the graft. We investigated these direct actions by comparing rejecting kidneys from donors lacking IFN-gamma receptors (GRKO mice) or control donors (129Sv/J) in CBA recipients. Beginning day 5, 129Sv/J kidneys displayed high major histocompatibility complex (MHC) expression, progressive infiltration by inflammatory cells, but no thrombosis and little necrosis, even at day 21. GRKO kidneys showed increasing fibrin thrombi in small veins, peritubular capillary congestion, hyaline casts, and patchy parenchymal necrosis, progressing to near total necrosis at day 10. Terminal dUTP nick-end labeling assays were positive only in the interstitial infiltrate, confirming that massive cell death in GRKO transplants was not apoptotic. Paradoxically, GRKO kidneys showed little donor MHC induction and less inflammatory infiltration. Both GRKO and 129Sv/J allografts evoked vigorous host immune responses including alloantibody and mRNA for cytotoxic T cell genes (perforin, granzyme B, Fas ligand), and displayed similar expression of complement inhibitors (CD46, CD55, CD59). GRKO kidneys displayed less mRNA for inducible nitric oxide synthase and monokine inducible by IFN-gamma but increased heme oxygenase-1 mRNA. Thus IFN-gamma acting on IFN-gamma receptors in allografts promotes infiltration and MHC induction but prevents early thrombosis, congestion, and necrosis.

pRB is required for interferon-gamma-induction of the MHC class II abeta gene

pRB is required for IFN-gamma-induction of MHC class II in human tumor cell lines, providing a potential link between tumor suppressors and the immune system. However, other genes, such as cyclin D1, show pRB-dependency only in tumor cells, so by analogy, pRB may not be necessary for cII-regulation in normal cells. Here, we demonstrate that induction of the mouse MHC class II I-A heterodimer is normal in RB+/+ mouse embryonic fibroblasts (MEFs), but deficient in RB-/- MEFs. Inducibility is restored in RB-/- MEFs stably transfected with wild type RB cDNA or infected with an adenovirus expressing pRB. Thus, involvement of pRB in MHC class II expression is conserved in the mouse and is not an aberrant feature of tumorigenic, aneuploid, human tumor cells. Although cII genes are generally induced in a coordinate fashion, suggesting a common mechanism, we found that pRB was specifically required for induction of the Abeta, but not Aalpha or other MHC cII genes including Ebeta, Ii and H2-Malpha. Finally, IFN-gamma-induction of class II transactivator (CIITA), was pRB-independent, suggesting that pRB works downstream of this master-regulator of MHC class II expression.

Discoordinate Expression of Invariant Chain and MHC Class II Genes in Class II Transactivator-Transfected Fibroblasts Defective for RFX5

MHC class II deficiency or bare lymphocyte syndrome is a severe combined immunodeficiency caused by defects in MHC-specific transcription factors. In the present study, we show that fibroblasts derived from a recently identified bare lymphocyte syndrome patient, SSI, were mutated for RFX5, one of the DNA-binding components of the RFX complex. Despite the lack of functional RFX5 and resulting MHC class II-deficient phenotype, transfection of exogenous class II transactivator (CIITA) in these fibroblasts can overcome this defect, resulting in the expression of HLA-DR, but not of DP, DQ, and invariant chain. The lack of invariant chain expression correlated with lack of CIITA-mediated transactivation of the invariant chain promoter in transient transfection assays in SSI fibroblast cells. Consequently, these CIITA transfectants lacked Ag-presenting functions.

Identification of Distinct Regions of 5′ Flanking DNA That Mediate Constitutive, IFN-γ, STAT1, and TGF-β-Regulated Expression of the Class II Transactivator Gene

Class II transactivator (CIITA) is a master regulator required for constitutive and IFN-γ-inducible expression of class II MHC genes. Although the role of CIITA is greatly appreciated, the mechanisms underlying constitutive and IFN-γ-induced expression of CIITA are not understood. The study of CIITA induction is extremely important, but has been fraught with difficulty. This study describes for the first time a large (7-kb) fragment of 5′ flanking sequences that mediates the B cell-specific, IFN-γ-induced, and TGF-β-suppressed expression of CIITA. This pattern of expression matches the authentic expression of the endogenous gene. Within the 7-kb fragment, sequences that lie between nucleotides −545 and −113 relative to the transcriptional start site are critical for constitutive promoter expression in B cells. In contrast, inducible activation of CIITA by IFN-γ requires sequences contained in an additional 4 kb of upstream DNA. This region mediates an IFN-γ response when linked to either the endogenous CIITA promoter or a heterologous promoter. A role for STAT1 in regulation of the CIITA promoter is shown by the rescue of IFN-γ induction by expression of STAT1 in STAT1-defective U3A cells. TGF-β significantly inhibits IFN-γ-mediated induction of the CIITA promoter in 2fTGH fibroblasts, which indicates that the promoter is a target for TGF-β. This inhibition is achieved by suppression of the basal promoter. This study provides a focal point for understanding the mechanism of B cell-specific, IFN-γ-induced, and TGF-β-suppressed expression of CIITA.

IFN-gamma inducibility of class II transactivator is specifically lacking in human tumour lines: relevance to retinoblastoma protein rescue of IFN-gamma inducibility of the HLA class II genes

We have previously reported that HLA class II induction by IFN-gamma is rescuable by reconstitution of functional retinoblastoma protein (RB) in two RB-defective tumour lines: the breast carcinoma line, MDA-468-S4 (S4) and the non-small cell lung carcinoma line, H2009. To determine the range of tumours and tumour types in which RB rescues HLA class II inducibility, we examined another RB-defective tumour line, the retinoblastoma line, WERI-Rb1. As in the case of S4 and H2009, HLA-DRA and -DRB were non-inducible by IFN-gamma in WERI-Rb1. However, neither inducibility of DRA nor DRB mRNA was resulted in an RB-positive stable transformant of WERI-Rb1, WLRB-8. While guanylate-binding protein (GBP) inducibility indicated that the basic IFN-gamma signal transduction pathway remained intact in WERI-Rb1, mRNA for class II transactivator (CIITA), a mediator of the IFN-gamma activation of the HLA class II genes and several other genes related to immune function, was not detectable in IFN-gamma-treated WERI-Rb1, indicating that the lack of CIITA expression was responsible, at least in part, for the inability of RB to rescue HLA class II-inducibility. The HLA class II-associated invariant chain (Ii), the expression of which is also up-regulated by CIITA, was non-inducible in WERI-Rb1, consistent with non-inducible CIITA. Also, IFN-gamma failed to activate the DRA, DRB and Ii promoters in WERI-Rb1. However, exogenous CIITA expression in WERI-Rb1 activated the DRA, DRB and Ii promoter-chloramphinocol acetyltransferase constructs, confirming that CIITA was not induced in WERI-Rb1 and indicating that other proteins required for activation of the class II and Ii promoters were functional in this cell line. Examination of additional cell lines for GBP and CIITA induction revealed that a specific lack of the CIITA IFN-gamma response is common in human tumour lines. The possible role of CIITA defects in tumorigenesis is discussed.

Evidence for the in vivo role of class II transactivator in basal and IFN-gamma induced class II expression in mouse tissue

The class II transactivator (CIITA) is a protein that induces the transcription of MHC class II genes. We studied the expression of CIITA in vivo, comparing steady state levels of CIITA and class II mRNA in various mouse tissues. Many tissues in normal mice contained mRNA for CIITA, correlating with class II mRNA. The basal expression of CIITA and class II mRNA in mice with disrupted IFN-gamma genes (GKO mice) was similar to that in wild-type mice. Injection of rIFN-gamma strongly induced CIITA and class II mRNA: CIITA mRNA increased at 2 hr and declined to baseline by 48 hr, whereas class II mRNA increased at 24 hr and returned to baseline at 7 days. Proinflammatory stimuli that induce IFN-gamma production (allogeneic cells and LPS) induce CIITA and class II expression in wild-type mice, but not in GKO mice. CIITA induction by IFN-gamma was partially sensitive to cycloheximide, suggesting that another protein is required for CIITA induction. The data suggest that CIITA is a major regulator of basal and induced class II expression in vivo.

Major histocompatibility complex class II-associated invariant chain gene expression is up-regulated by cooperative interactions of Sp1 and NF-Y

Expression of the major histocompatibility complex (MHC) class II-associated invariant chain (Ii) is required for efficient and complete presentation of antigens by MHC class II molecules and a normal immune response. The Ii gene is generally co-regulated with the MHC class II molecules at the level of transcription and a shared SXY promoter element has been described. This report defines the proximal promoter region of Ii which may regulate Ii transcription distinct from MHC class II. In vivo genomic footprinting identified an occupied, imperfect CCAAT box and an adjacent GC box in the proximal region. These sites are bound in Ii-ositive cell lines and upon interferon-gamma induction of Ii transcription. In contrast, both sites are unoccupied in Ii-egative cell lines and in inducible cell lines prior to interferon-gamma treatment. Together these two sites synergize to stimulate transcription. Independently, the transcription factor NF-Y binds poorly to the imperfect CCAAT box with a rapid off rate, while Sp1 binds to the GC box. Stabilization of NF-Y binding occurs upon Sp1 binding to DNA. In addition, the half-life of Sp1 binding also increased in the presence of NF-Y binding. These findings suggest a mechanism for the complete functional synergy of the GC and CCAAT elements observed in Ii transcription. Furthermore, this report defines a CCAAT box of imperfect sequence which binds NF-Y and activates transcription only when stabilized by an adjacent factor, Sp1.

An interferon-gamma activation sequence mediates the transcriptional regulation of the IgG Fc receptor type IC gene by interferon-gamma

Expression of the IgG Fc receptor type I (Fc gamma RI) on myeloid cells is dramatically increased by treatment with interferon-gamma (IFN-gamma). We observed that Fc gamma RI transcript levels in monoblast-like U937 cells were elevated within 3 hr and peaked 12 hr after exposure to IFN-gamma. Treatment of U937 with IFN-gamma for 9 hr in the presence of cycloheximide led to super-induction of Fc gamma RI expression. Nuclear run-on analysis revealed that the rate of Fc gamma RI transcription was increased by IFN-gamma. Genomic sequence upstream of the Fc gamma RIC gene was cloned and subjected to primer extension analysis, which demonstrated a single transcription initiation site without a TATA box. Transient transfections of CAT reporter gene constructs containing various Fc gamma RIC promoter sequences into U937 cells revealed that a 20-bp region surrounding the transcription start site (-7 to +13) was capable of mediating transcription initiation and that an IFN-gamma responsive element (GIRE) was present within 74 bp upstream of the transcription initiation site. A 17-bp sequence between positions -51 and -35 conferred IFN-gamma responsiveness on a heterologous promoter. Double-stranded GIRE sequence, but not a scrambled sequence, was specifically bound by nuclear proteins from IFN-gamma treated U937 cells. Gel shift experiments further showed that the STAT1 alpha protein bound to the Fc gamma RIC GIRE in response to IFN-gamma treatment of U937 cells. The Fc gamma RIC GIRE is homologous to the IFN-gamma activation sequence (GAS) of the guanylate binding protein and to X box elements of class II MHC genes. Our results demonstrate that transcriptional regulation of the Fc gamma RIC gene by IFN-gamma involves the binding of STAT1 alpha to a 17-bp GAS homology in the proximal promoter.

Retinoblastoma protein regulation of surface CD74 (invariant chain) expression in breast carcinoma cells

The HLA class II genes encode heterodimeric cell surface proteins which bind peptide antigen recognized by T-cell receptors on CD4+ T-cells. The class II proteins are inducible by IFN-gamma, and this induction requires, or is strongly enhanced, by retinoblastoma protein (RB) in a series of breast carcinoma cell lines. Loading of peptide onto the class II protein appears to be regulated by CD74, which associates with class II during their transition to the endosomal compartment, where class II binds peptide. Class II proteins and CD74 are largely regulated in concert, provoking the question, is CD74 induction by IFN-gamma affected by RB? Results described here indicate that IFN-gamma induction of CD74 surface expression in a series of breast carcinoma lines is enhanced by RB, while RB has no effect on CD74 mRNA induction. Also, neither the class II nor the CD74 promoter regions are activated by RB in cotransfection experiments where RB activates the SV40 promoter.

Molecular analysis of G1B and G3A IFN gamma mutants reveals that defects in CIITA or RFX result in defective class II MHC and Ii gene induction

Class II major histocompatibility complex (MHC) genes and the invariant (Ii) gene are inducible by interferon-gamma (IFN gamma) but not by interferon-alpha and interferon-beta. The promoter regions of these genes contain three regulatory elements that mediate constitutive and IFN gamma-induced expressions; however, none of the DNA-binding proteins that interact with these elements are regulated by IFN gamma. Recently, a gene coding for a transactivator (CIITA) of class II MHC genes that complements a HLA-DR-negative immunodeficiency has been isolated. Using one IFN gamma mutant cell line (G3A) that is selectively defective in HLA-DR and Ii induction, four lines of evidence are presented to show that CIITA mediates the IFN gamma induction of HLA-DR and Ii genes. Analysis of another mutant line, G1B, indicates that the lack of DRA and Ii gene induction by IFN gamma is correlated with the lack of RFX DNA binding activity, thus providing the link between RFX and an IFN gamma response.

Induction of the murine class-II antigen-associated invariant chain by TNF-alpha is controlled by an NF-kappa B-like element

The murine class-II antigen-associated invariant chain (Ii) is a glycoprotein whose synthesis is co-regulated with the major histocompatibility class-II antigens. The Ii mRNA is inducible by tumor necrosis factor-alpha (TNF-alpha). An NF-kappa B protein-binding site (NBS), located in the Ii promoter at bp -121 to -111, confers TNF-alpha inducibility on chimeric reporter constructs expressing the human growth hormone-encoding gene. A distal NF-kappa B-like protein-binding site (NBLS) located at bp -167 to -157 is not required for TNF-alpha inducibility, and deletion of this distal NBLS results in increased constitutive, as well as TNF-alpha-induced, expression of the reporter gene. The NBS is well protected during DNase I footprinting assays and specifically binds proteins in electrophoretic mobility shift assays. A distinct protein-DNA complex is observed when nuclear extracts from TNF-alpha-treated L-cells are used; this complex is not seen in extracts from untreated cells.

The HXY box regulatory element modulates expression of the murine IA antigen-associated invariant chain in L fibroblasts

The murine invariant chain (Ii) gene has been shown to be interferon-gamma (IFN-gamma)-inducible in a number of nonlymphoid cell types. In mouse L cells, steady-state levels of Ii mRNA are barely detectable in untreated cells but increase sharply upon IFN-gamma treatment. In IFN-gamma treated L cells, transcription starts 23, 28, 38, and 40 bases downstream of the TATA box. To identify cis-acting elements regulating expression of the Ii gene, reporter plasmids containing deletions of the Ii promoter have been constructed and transfected into mouse L cells. Deletion of the H box results in a 50-100% increase in basal expression. Deletion of both the H and X boxes increases basal expression by 200-300% above that seen in constructs containing all three elements. A 25% decrease in basal level expression is seen for constructs that lack the Y-box element when compared to constructs containing the Y-box element but not the H- and X-box elements. DNase I footprinting analysis demonstrates protection of the H, X, and Y boxes as well as a nonconserved region between the H and X boxes. Mobility-shift experiments detect a factor specifically interacting with the Y box. Although the H-, X-, and Y-box elements interact with nuclear protein and are regulatory elements in L cells, these elements do not appear to play a role in IFN-gamma induction suggesting that other regulatory mechanisms must account for IFN-gamma's induction of the Ii in L cells.