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

Differential Usage of Class II Transactivator Promoters PI and PIV during Inflammation and Injury in Kidney

ABSTRACT. Expression of class II transactivator (CIITA), the transcriptional regulator that controls all class II expression, is controlled in cell linesin vitroby three promoters: the dendritic cell promoter PI, the B cell promoter PIII, and the interferon-γ (IFN-γ)–inducible promoter, PIV. The authors examined the promoter usagein vivoin mouse kidney in the basal state and in response to IFN-γ, endotoxin, allostimulation, and renal injury. Genetically modified mice were used to examine the dependency of each promoter on IFN-γ and on the transcription factor interferon regulatory factor 1 (IRF-1). Usage of distinct CIITA promoters was monitored by real-time reverse transcriptase polymerase chain reaction (RT-PCR) using the unique sequences in the 5′ end of the transcript from each promoter. Kidneys in both control mice and IFN-γ knockouts expressed chiefly PI- and PIV–related products. Administration of recombinant IFN-γ activated only promoter PIV. Endotoxin or allogeneic stimulation elevated the PIV-related mRNA, dependent on IFN-γ and on IRF-1. Ischemic renal injury, however, increased the PI- and PIV–driven mRNA expression in wild-type but also in IFN-γ–deficient mice. Thus thein vivocontrol of CIITA promoters in kidney is similar to that observedin vitro(i.e., basal-state usage of PI and IFN-γ–dependent usage of PIV during inflammation), but it also shows additional levels of control: IFN-γ–independent basal activity of PIV and IFN-γ–independent induction of PIV during tissue injury. E-mail: phil.halloran@ualberta.ca

Tissue selectivity of interferon-stimulated gene expression in mice infected with Dam(+) versus Dam(-) Salmonella enterica serovar Typhimurium strains

The host interferon (IFN) system plays an important role in protection against microbial infections. Salmonella enterica serovar Typhimurium is highly virulent in the mouse model, whereas mutants that lack DNA adenine methylase (Dam(-)) are highly attenuated and elicit fully protective immune responses against murine typhoid fever. We examined the expression of IFN-responsive genes in several mouse tissues following infection with Dam(+) or Dam(-) Salmonella. Infection of mice with Dam(+) Salmonella resulted in the induction of host genes known to be indicators of IFN bioactivity and regulated by either IFN-alpha/beta (Mx1) or IFN-gamma (class II transactivator protein [CIITA] and inducible nitric oxide synthase [iNOS]) or by both IFN-alpha/beta and IFN-gamma (RNA-specific adenosine deaminase [ADAR1] and RNA-dependent protein kinase [PKR]) in a tissue-specific manner compared to uninfected animals. Since the Mx1 promoter is IFN-alpha/beta specific and the Mx1 gene is not inducible directly by IFN-gamma, these data suggest a role of IFN-alpha/beta in the host response to Salmonella infection. Mice infected with Dam(-) Salmonella showed reduced expression of the same set of IFN-stimulated genes (ISGs) as that observed after infection with wild-type Salmonella. The reduced capacity to induce ISGs persisted in Dam(-)-vaccinated mice after challenge with the virulent (Dam(+)) strain. Finally, although no Dam(-) organisms were recovered from the liver or spleen after oral infection of mice, ADAR, PKR, Mx, and CIITA expression levels were elevated in these tissues relative to those in uninfected mice, suggestive of the distant action of a signaling molecule(s) in the activation of ISG expression.

Human cytomegalovirus inhibition of major histocompatibility complex transcription and interferon signal transduction

Pathogens have evolved diverse mechanisms for escaping host innate and adaptive immunity. Viruses that maintain a persistent infection are particularly effective at disabling key arms of the host immune response. For example, the herpesviruses establish a persistent infection in human and animal hosts, in part through critical immunoevasive strategies. Cytomegalovirus, a beta-herpesvirus, impairs major histocompatibility complex (MHC) class I and class II antigen presentation by decreasing MHC expression on the surface of the infected cell, thus enabling infected cells to escape CD8+ and CD4+ T lymphocyte immunosurveillance. Moreover, cytomegalovirus blocks the interferon signal transduction pathway, thereby limiting the direct and indirect antiviral effects of the interferons. In this review, we focus on an emerging paradigm in which the effectiveness of viruses, particularly human cytomegalovirus, to escape antiviral immune responses is significantly enhanced by their ability to inhibit MHC transcription and interferon (IFN)-stimulated (JAK/STAT) signal transduction.

The role of the class II transactivator (CIITA) in MHC class I and II regulation and graft rejection in kidney

Class II transactivator (CIITA) induces transcription of MHC class II genes, and induces class I in some cell lines. We examined the effect of CIITA deficiency on class I and II expression in mouse kidney. In CIITA knockout (CIITAKO) mice, basal class II was absent, but class I levels were mildly but significantly increased. Allogeneic stimulation or ischemic injury increased class I and II expression in kidneys of control (wild-type, WT) mice but induced only class I in CIITAKO mice. Thus, in kidney, all basal and induced class II expression was CIITA-dependent, but neither basal nor induced class I was CIITA-dependent. Rejecting kidney allografts from CIITAKO mice in CBA hosts manifested intense induction of donor class I but no donor class II expression. Rejecting kidneys from both WT and CIITAKO donors showed predominantly CD8 T-cell infiltration at days 7 and 21, with increasing tubulitis and arteritis at day 21. CIITAKO kidneys showed fewer infiltrating cells than WT kidneys at day 21. Thus CIITA-deficient kidneys have no basal and induced class II expression but display intense induction of class I expression, and evoke typical rejection lesions, although some indices of infiltration are reduced at day 21.

Mutation in the class II trans-activator leading to a mild immunodeficiency

The expression of MHC class II molecules is essential for all Ag-dependent immune functions and is regulated at the transcriptional level. Four trans-acting proteins control the coordinate expression of MHC class II molecules: class II trans-activator (CIITA), regulatory factor binding to the X box (RFX)-associated protein; RFX protein containing ankyrin repeats, and RFX5. In humans, defects in these genes result in MHC class II expression deficiency and cause combined immunodeficiency. Most patients with this deficiency suffer from severe recurrent infections that frequently lead to death during early childhood. We investigated three sisters, now ages 21, 22, and 24 years, in whom MHC-II deficiency was detected. Even though the eldest sibling was asymptomatic and the other two had only mild immunodeficiency, none of the three class II isotypes was expressed on T cell blasts, fibroblasts, EBV B cell lines, or epidermal dendritic cells. Residual HLA-II expression was detected in fresh PBMC. Somatic complementation identified the disease as CIITA deficiency. A homozygous T1524C (L469P) substitution was found in the coding region of the CIITA cDNA and was shown to be responsible for the defect in MHC-II expression. This missense mutation prevents the normal functioning of MHC-II but does not lead to the nuclear exclusion of the L469P CIITA. Transfection experiments demonstrated that the CIITA L469P mutant had residual MHC class II trans activation activity, which might explain the unusual clinical course of the patients studied. This study shows that an attenuated clinical phenotype or an asymptomatic clinical course can be observed in patients despite a profound defect in the expression of MHC class II genes. The frequency of the inherited MHC class II deficiency might thus be underestimated.

Role of the peroxisome proliferator-activated receptor-gamma (PPAR-gamma) and its natural ligand 15-deoxy-Delta12, 14-prostaglandin J2 in the regulation of microglial functions

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) is a member of a large group of nuclear receptors controlling the proliferation of peroxisomes that is involved in the downregulation of macrophage functions. Here, we report that PPAR-gamma was constitutively expressed in rat primary microglial cultures and that such expression was downregulated during microglial activation by endotoxin (LPS). The presence of the PPAR-gamma natural ligand 15-deoxy-Delta12,14-prostaglandin J2 (15d-PGJ2) counteracted the repression of PPAR-gamma expression caused by LPS. In microglial cultures stimulated by LPS, interferon-gamma (IFN-gamma) or by their combination, 15d-PGJ2 reduced the production of nitric oxide (NO) and the expression of inducible NO synthase (iNOS). The inhibitory effect was dose-dependent and did not involve an elevation of cyclic AMP, a second messenger known to inhibit NOS expression in microglia. In addition, 15d-PGJ2 down-regulated other microglial functions, such as tumour necrosis factor-alpha (TNF-alpha) synthesis and major histocompatibility complex class II (MHC class II) expression. The effects of 15d-PGJ2 occurred, at least in part, through the repression of two important transcription factors, the signal transducer and activator of transcription 1 and the nuclear factor kappaB, known to mediate IFN-gamma and LPS cell signalling. Our observations suggest that 15d-PGJ2, the synthesis of which is likely to occur within the brain, could play an important role in preventing brain damage associated with excessive microglial activation.

TGF-beta1 down-regulates induced expression of both class II MHC and B7-1 on primary murine renal tubular epithelial cells

BACKGROUND

We examined the immunomodulatory effects of transforming growth factor-beta1 (TGF-beta1) on the regulation of class II MHC and costimulatory molecule expression in a primary renal tubular epithelial cell line, called F1K.

METHODS

Class II major histocompatibility complex (MHC), class II transactivator, B7-1, intercellular adhesion molecule-1 (ICAM-1) and interferon-gamma (IFN-gamma) receptor beta chain were evaluated in untreated and cytokine-treated F1K by Northern hybridization analysis and flow cytometry. T cell activation studies were performed to assess TGF-beta1-mediated effects on antigen presenting cell function of F1K.

RESULTS

Pretreatment of F1K with TGF-beta1 markedly inhibited IFN-gamma-induced class II MHC expression, by both FACS and Northern analysis. Total class II transactivator mRNA levels were also diminished by TGF-beta1, indicating that class II MHC modulation in F1K results from inhibition of this intermediate protein. As previous studies demonstrated that cotreatment of F1K cells with IFN-gamma + lipopolysaccharide (LPS) induces B7-1, we evaluated the potential regulatory effects of TGF-beta1 exposure on B7-1 expression. Our studies revealed that B7-1 mRNA and cell-surface expression in IFN-gamma + LPS-treated F1K were decreased by TGF-beta1 pretreatment. Functional studies evaluating TGF-beta1-mediated effects were performed with IFN-gamma + LPS-treated F1K and MR1.3, a nephritogenic CD4+ Th2 clone derived from kidneys of animals with autoimmune glomerulonephritis. Interleukin (IL)-4 production assays demonstrated activation of MR1. 3 by IFN-gamma + LPS-treated cells, but not by IFN-gamma + LPS-treated cells previously exposed to TGF-beta1, indicating that TGF-beta1-mediated inhibition of class II MHC and B7-1 expression alters the antigen presenting cell function of F1K.

CONCLUSIONS

These studies describe the proscriptive influence of TGF-beta1 on class II MHC and B7-1 expression in renal tubular epithelial cells. Such findings indicate that TGF-beta1 alters the antigen presenting cell function of renal tubular epithelial cells in vitro, and suggest a potential mechanism for immunosuppression of T cell-mediated renal immune responses in vivo.

Viral effects on antigen processing

Viruses have evolved numerous mechanisms that modulate MHC-mediated antigen presentation, which in turn protect infected cells from T-lymphocyte-mediated immunosurveillance. Recent studies of previously identified viral immunomodulatory proteins reveal the allelic specificity of these proteins, their ability to function in xenogeneic systems and the difficulty in translating in vitro data to in vivo models; moreover, new mechanisms of viral modulation of MHC expression have emerged.

Quantitative control of MHC class II expression by the transactivator CIITA

Activation of T lymphocytes is quantitatively controlled by the level of expression of MHC class II molecules. Both constitutive and inducible expression of MHC class II genes is regulated by the transactivator CIITA, which is itself tightly regulated. Since the level of MHC class II molecules expressed is a functionally essential parameter, it was of interest to explore whether MHC class II expression is quantitatively controlled by the level of the transactivator. This report shows that in a variety of experimental conditions one does indeed observe, in both mouse and man, a quantitative control of MHC class II expression by the level of CIITA. This relationship between the regulator gene, which behaves as a rate-limiting factor, and its target genes clarifies our understanding of the quantitative modulation of MHC class II expression, and thus of T lymphocyte activation.

In vivo class II transactivator expression in mice is induced by a non-interferon-gamma mechanism in response to local injury

BACKGROUND

Tissue injury induces MHC class II expression, which could be important in the recognition of that tissue as an allograft. The class II transcriptional activator (CIITA) is the major regulator of basal and induced MHC class II expression and is essential for antigen presentation. The role of CIITA in the induction of class II by tissue injury is unknown. In this study, we examined CIITA induction in the course of acute ischemic or toxic renal injury in mice, including the role of interferon (IFN)-gamma and of the transcription factor, interferon regulatory factor (IRF)-1.

METHODS

Kidneys were injured by ischemia or by gentamicin toxicity and were then studied for changes in gene expression using Northern blot, reverse transcriptase-polymerase chain reaction, radioimmunoassay, and tissue staining. We compared wild-type (WT) mice to IFN-gamma knockout (GKO) or IRF-1 knockout mice.

RESULTS

Ischemic injury induced CIITA and class II expression in the kidney, in WT and GKO mice. Gentamicin injury also induced both CIITA and class II expression, independent of IFN-gamma, in WT and GKO mice. After ischemic injury, the induction of class II protein levels and CIITA and class II mRNA levels were induced, to a lesser degree, in IRF-1 knockout mice.

CONCLUSIONS

These data indicate that CIITA is induced by tissue injury, and probably accounts for class II induction during tissue injury. CIITA induction by injury is largely IFN-gamma independent but requires IRF-1. The similarities of the pattern of CIITA and class II induction in ischemic and toxic injury suggest that this is a stereotyped response of injured tissue and not a consequence of a particular mechanism of injury.