IFN-gamma-induced MHC class II expression: transactivation of class II transactivator promoter IV by IFN regulatory factor-1 is regulated by protein kinase C-alpha

Leishmania donovani amastigotes impair gamma interferon-induced STAT1alpha nuclear translocation by blocking the interaction between STAT1alpha and importin-alpha5

The protozoan parasite Leishmania donovani, the etiological agent of visceral leishmaniasis, is renowned for its capacity to sabotage macrophage functions and signaling pathways stimulated by activators such as gamma interferon (IFN-gamma). Our knowledge of the strategies utilized by L. donovani to impair macrophage responsiveness to IFN-gamma remains fragmentary. In the present study, we investigated the impact of an infection by the amastigote stage of L. donovani on IFN-gamma responses and signaling via the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) pathway in mouse bone marrow-derived macrophages. The levels of IFN-gamma-induced expression of major histocompatibility complex class II and inducible nitric oxide synthase (iNOS) were strongly reduced in L. donovani amastigote-infected macrophages. As the expression of those genes is mediated by the transcription factors STAT1alpha and IFN regulatory factor 1 (IRF-1), we investigated their activation in amastigote-infected macrophages treated with IFN-gamma. We found that whereas STAT1alpha protein levels and the levels of phosphorylation on Tyr701 and Ser727 were normal, IRF-1 expression was inhibited in infected macrophages. This inhibition of IRF-1 expression correlated with a defective nuclear translocation of STAT1alpha, and further analyses revealed that the IFN-gamma-induced STAT1alpha association with the nuclear transport adaptor importin-alpha5 was compromised in L. donovani amastigote-infected macrophages. Taken together, our results provide evidence for a novel mechanism used by L. donovani amastigotes to interfere with IFN-gamma-activated macrophage functions and provide a better understanding of the strategies deployed by this parasite to ensure its intracellular survival.

Molecular mechanisms of IFN-gamma to up-regulate MHC class I antigen processing and presentation

IFN-gamma up-regulates MHC class I expression and antigen processing and presentation on cells, since IFN-gamma can induce multiple gene expressions that are related to MHC class I antigen processing and presentation. MHC class I antigen presentation-associated gene expression is initiated by IRF-1. IRF-1 expression is initiated by phosphorylated STAT1. IFN-gamma binds to IFN receptors, and then activates JAK1/JAK2/STAT1 signal transduction via phosphorylation of JAK and STAT1 in cells. IFN-gamma up-regulates MHC class I antigen presentation via activation of JAK/STAT1 signal transduction pathway. Mechanisms of IFN-gamma to enhance MHC class I antigen processing and presentation were summarized in this literature review.

Interferon-triggered transcriptional cascades in the oligodendroglial lineage: a comparison of induction of MHC class II antigen between oligodendroglial progenitor cells and mature oligodendrocytes

Interferon-gamma induces major histocompatibility complex class II (MHC-II) in proliferating oligodendroglial progenitor cells (OPC), but to a much lesser extent in mature oligodendrocytes. Interferon-beta has virtually no effects on MHC-II induction even in OPC. Interferon-gamma-mediated transcriptional induction of CIITA, a critical regulator of MHC-II induction, was 6-fold lower in mature oligodendrocytes than in OPC, and entirely dependent on promoter IV, suggesting that the transcriptional activity of promoter IV is down-regulated after differentiation. The distinct difference in MHC-II induction between interferon-gamma and interferon-beta is attributed to transient interferon-beta-mediated activation of STAT1-IRF1 signaling compared to the sustained interferon-gamma-mediated activation.

Toxoplasma gondii glycosylphosphatidylinositols up-regulate major histocompatibility complex (MHC) molecule expression on primary murine macrophages

Toxoplasma gondii is an obligatory intracellular parasite able to block the IFN-gamma-induced up-regulation of major histocompatibility complex (MHC) class I and class II molecules. This facilitates parasite-mediated evasion of T-cell responses. Glycosylphosphatidylinositols (GPIs) are involved in the pathogenicity of protozoan parasites and we investigated if GPIs are responsible for inhibition of MHC expression on macrophages. In contrast to the blockade observed in cells infected with viable tachyzoites, T. gondii GPIs up-regulated MHC class I and class II molecules on the surface of both unstimulated and IFN-gamma-stimulated primary murine macrophages. This effect was correlated to the ability of GPIs to increase the antigen presentation to CD8(+) lymphocytes. T. gondii GPIs did not activate STAT1, one of the factors involved in the transcription of MHC class I and class II genes. However, the GPI-induced MHC class I up-regulation was abrogated by SN50, a specific NF-KB inhibitor. Up-regulation of surface MHC molecules by GPIs may lead to the elimination of non-infected cells of the host immune system, contributing to the immune escape strategy of T. gondii.

Macrophage oxygen sensing modulates antigen presentation and phagocytic functions involving IFN-gamma production through the HIF-1 alpha transcription factor

Low oxygen tension areas are found in inflamed or diseased tissues where hypoxic cells induce survival pathways by regulating the hypoxia-inducible transcription factor (HIF). Macrophages are essential regulators of inflammation and, therefore, we have analyzed their response to hypoxia. Murine peritoneal elicited macrophages cultured under hypoxia produced higher levels of IFN-gamma and IL-12 mRNA and protein than those cultured under normoxia. A similar IFN-gamma increment was obtained with in vivo models using macrophages from mice exposed to atmospheric hypoxia. Our studies showed that IFN-gamma induction was mediated through HIF-1alpha binding to its promoter on a new functional hypoxia response element. The requirement of HIF-alpha in the IFN-gamma induction was confirmed in RAW264.7 cells, where HIF-1alpha was knocked down, as well as in resident HIF-1alpha null macrophages. Moreover, Ag presentation capacity was enhanced in hypoxia through the up-regulation of costimulatory and Ag-presenting receptor expression. Hypoxic macrophages generated productive immune synapses with CD8 T cells that were more efficient for activation of TCR/CD3epsilon, CD3zeta and linker for activation of T cell phosphorylation, and T cell cytokine production. In addition, hypoxic macrophages bound opsonized particles with a higher efficiency, increasing their phagocytic uptake, through the up-regulated expression of phagocytic receptors. These hypoxia-increased immune responses were markedly reduced in HIF-1alpha- and in IFN-gamma-silenced macrophages, indicating a link between HIF-1alpha and IFN-gamma in the functional responses of macrophages to hypoxia. Our data underscore an important role of hypoxia in the activation of macrophage cytokine production, Ag-presenting activity, and phagocytic activity due to an HIF-1alpha-mediated increase in IFN-gamma levels.

Roles of phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase in the regulation of protein kinase C-alpha activation in interferon-gamma-stimulated macrophages

Members of the protein kinase C (PKC) family are activated by interferon-gamma (IFN-gamma) and modulate IFN-gamma-induced cellular responses by regulating the activity of transcription factors. We previously reported that PKC-alpha enhances the ability of IFN regulatory factor-1 to transactivate the class II transactivator (CIITA) promoter IV in IFN-gamma-stimulated macrophages. In addition, we showed that IFN-gamma induces the nuclear translocation of PKC-alpha but the mechanisms for this remain to be elucidated. In this study, we sought to identify signalling pathways involved in IFN-gamma-induced activation of PKC-alpha and to characterize their potential roles in modulating IFN-gamma-induced responses in macrophages. IFN-gamma-mediated nuclear translocation of PKC-alpha was a Janus activated kinase 2 (JAK2)-independent process, which required phosphatidylinositol 3-kinase (PI3K) and p38 mitogen-activated protein kinase (MAPK). However, PKC-alpha phosphorylation was independent of PI3K and p38 MAPK, indicating that IFN-gamma-induced phosphorylation and nuclear translocation of PKC-alpha are mediated by distinct mechanisms. In addition, inhibition of PI3K, but not of p38 MAPK, strongly impaired IFN-gamma-induced CIITA and MHC II gene expression. Finally, PKC-alpha associated with signal transducer and activator of transcription 1 (STAT1) and was required for the phosphorylation of STAT1 on serine 727 in IFN-gamma-stimulated macrophages. Taken together, our data indicate that PI3K and p38 MAPK modulate IFN-gamma-stimulated PKC-alpha nuclear translocation independently of JAK2 activity and that both PI3K and PKC-alpha are required for type IV CIITA and MHC II gene expression in IFN-gamma-stimulated macrophages.

IFN inducibility of major histocompatibility antigens in tumors

Interferons represent a protein family with pleiotropic functions including immunomodulatory, cytostatic, and cytotoxic activities. Based on these effects, interferons are involved in innate as well as adaptive immunity, thereby shaping the tumor host immune responses. These cytokines, alone or in combination, have been successfully implemented for the treatment of some malignancies. However, it has been recently demonstrated that tumor cells could be resistant to interferon treatment, which may be associated with an escape of tumor cells from immune surveillance. Therefore, the aim of this chapter is to summarize the frequency of impaired interferon signal transduction, their underlying molecular mechanisms, and their clinical relevance.

Expression of the retinoblastoma protein RbAp48 in exocrine glands leads to Sjögren's syndrome-like autoimmune exocrinopathy

Although several autoimmune diseases are known to develop in postmenopausal women, the mechanisms by which estrogen deficiency influences autoimmunity remain unclear. Recently, we found that retinoblastoma-associated protein 48 (RbAp48) induces tissue-specific apoptosis in the exocrine glands depending on the level of estrogen deficiency. In this study, we report that transgenic (Tg) expression of RbAp48 resulted in the development of autoimmune exocrinopathy resembling Sjögren's syndrome. CD4(+) T cell-mediated autoimmune lesions were aggravated with age, in association with autoantibody productions. Surprisingly, we obtained evidence that salivary and lacrimal epithelial cells can produce interferon-gamma (IFN-gamma) in addition to interleukin-18, which activates IFN regulatory factor-1 and class II transactivator. Indeed, autoimmune lesions in Rag2(-/-) mice were induced by the adoptive transfer of lymph node T cells from RbAp48-Tg mice. These results indicate a novel immunocompetent role of epithelial cells that can produce IFN-gamma, resulting in loss of local tolerance before developing gender-based autoimmunity.

IFNgamma signaling-does it mean JAK-STAT?

The molecular pathways involved in the cellular response to interferon (IFN)gamma have been the focus of much research effort due to their importance in host defense against infection and disease, as well as its potential as a therapeutic agent. The discovery of the JAK-STAT signaling pathway greatly enhanced our understanding of the mechanism of IFNgamma-mediated gene transcription. However, in recent years it has become apparent that other pathways, including MAP kinase, PI3-K, CaMKII and NF-kappaB, either co-operate with or act in parallel to JAK-STAT signaling to regulate the many facets of IFNgamma biology in a gene- and cell type-specific manner. The complex interactions between JAK/STAT and alternate pathways and the impact of these signaling networks on the biological responses to IFNgamma are beginning to be understood. This review summarizes and appraises current advances in our understanding of these complex interactions, their specificity and proposed biological outcomes.

Mitogen-activated protein kinase ERK1/2 regulates the class II transactivator

The expression of major histocompatibility class II genes is necessary for proper antigen presentation and induction of an immune response. This expression is initiated by the class II transactivator, CIITA. The establishment of the active form of CIITA is controlled by a series of post-translational events, including GTP binding, ubiquitination, and dimerization. However, the role of phosphorylation is less clearly defined as are the consequences of phosphorylation on CIITA activity and the identity of the kinases involved. In this study we show that the extracellular signal-regulated kinases 1 and 2 (ERK1/2) interact directly with CIITA, targeting serine residues in the amino terminus of the protein, including serine 288. Inhibition of this phosphorylation by dominant-negative forms of ERK or by treatment of cells with the ERK inhibitor PD98059 resulted in the increase in CIITA-mediated gene expression from a class II promoter, enhanced the nuclear concentration of CIITA, and impaired its ability to bind to the nuclear export factor, CRM1. In contrast, inhibition of ERK1/2 activity had little effect on serine-to-alanine mutant forms of CIITA. These data suggest a model whereby ERK1/2-mediated phosphorylation of CIITA down-regulates CIITA activity by priming it for nuclear export, thus providing a means for cells to tightly regulate the extent of antigen presentation.

Major histocompatibility class II transactivator expression in smooth muscle cells from A2b adenosine receptor knock-out mice: cross-talk between the adenosine and interferon-gamma signaling

Atherosclerosis characterized by sustained inflammation and aberrant extracellular matrix alterations. Our previous investigation has defined major histocompatibility class II transactivator (CIITA) as a key factor in mediating these two processes in smooth muscle cells. Here, we demonstrate that CIITA and major histocompatibility class II expression are elevated in interferon-gamma (IFN-gamma)-treated smooth muscle cells from A2b adenosine receptor (A2bAR(-/-)) knock-out mice, as compared with wild type cells. An A2-type adenosine receptor agonist suppresses these effects of IFN-gamma in wild type cells, which can be blocked by an A2bAR-specific antagonist. We further identify that increased cellular cAMP levels are responsible for the down-regulation of CIITA expression and, hence, reduced IFN-gamma response as evidenced by the following data: 1) direct activation of adenylyl cyclase activity is both necessary and sufficient to suppress the IFN-gamma response; 2) inhibition of phosphodiesterase activity attenuates IFN-gamma induced transcription events; and 3) direct treatment with cAMP analog abrogates CIITA activation and IFN-gamma response. Therefore, our data establish possible cross-talk between the adenosine signaling through cAMP and IFN-gamma during regulation of CIITA expression.

Stat2-dependent regulation of MHC class II expression

MHC type II (MHC II) expression is tightly regulated in macrophages and potently induced by IFN-gamma (type II IFN). In contrast, type I IFNs (IFN-Is), which are far more widely expressed, fail to induce MHC II expression, even though both classes of IFNs direct target gene expression through Stat1. The unexpected finding that IFN-Is effectively induce MHC II expression in Stat2(-/-) macrophages provided an opportunity to explore this conundrum. The ensuing studies revealed that deletion of Stat2, which uniquely transduces signals for IFN-Is, leads to a loss in the IFN-I-dependent induction of suppressor of cytokine signaling-1. Impairment in the expression of this important negative regulator led to a striking prolongation in IFN-I-dependent Stat1 activation, as well as enhanced expression of the target gene, IFN-regulatory factor-1. The prolonged activity of these two transcription factors synergized to drive the transcription of CIITA, the master regulator of MHC II expression, analogous to the pattern observed in IFN-gamma-treated macrophages. Thus, IFN-I-dependent suppressor of cytokine signaling-1 expression plays an important role in distinguishing the biological response between type I and II IFNs in macrophages.

Minocycline Down-regulates MHC II Expression in Microglia and Macrophages through Inhibition of IRF-1 and Protein Kinase C (PKC)α/βII

Experimental allergic encephalomyelitis, an autoimmune disorder mediated by T cells, results in demyelination, inflammation, and axonal loss in the central nervous system (CNS). Microglia play a critical role in major histocompatibility complex class II (MHC II)-dependent antigen presentation and in reactivation of CNS-infiltrated encephalitogenic T cells. Minocycline, a tetracycline anti-biotic, has profound anti-inflammatory properties and is experimentally used for treatment of many CNS disorders; however, the mechanisms involved in minocycline effects remain unknown. We show that administration of minocycline for 2 weeks ameliorated clinical severity of experimental allergic encephalomyelitis, an effect that partially involves the down-regulation of MHC II proteins in the spinal cord. Therefore, we sought to elucidate the molecular mechanisms of minocycline inhibitory effects on MHC II expression in microglia. Although complex, the co-activator class II transactivator (CIITA) is a key regulator of MHC II expression. Here we show that minocycline inhibited interferongamma (IFNgamma)-induced CIITA and MHC II mRNA. Interestingly, however, it was without effect on STAT1 phosphorylation or IRF-1 expression, transcription factors that are activated by IFNgamma and necessary for CIITA expression. Further experiments revealed that MHC II expression is down-regulated in the presence of the PKC(alpha) inhibitor Gö6976. Minocycline inhibited IFNgamma-induced PKC(alpha/betaII) phosphorylation and the nuclear translocation of both PKC(alpha/betaII) and IRF-1 that subsequently inhibits CIITA expression. Our present data delineate a molecular pathway of minocycline action that includes inhibitory effects on PKC(alpha/betaII) and transcription factors that regulate the expression of critical inflammatory genes such as MHC II. Such a fundamental mechanism may underlie the pleiotropic effects of minocycline in CNS inflammatory disorders.

Role of PKCdelta in IFN-gamma-inducible CIITA gene expression

The class II transactivator (CIITA) is a key regulatory factor for MHC class II expression. Here, we demonstrate that PKCdelta plays an important role in regulating IFN-gamma-inducible CIITA gene expression in macrophages. Inhibition of PKCdelta by either a PKCdelta inhibitor or a dominant negative (DN) mutant form of PKCdelta led to down-regulation of CIITA expression. The decrease in CIITA expression by PKCdelta inhibition was in part due to the reduced recruitment of serine 727-phosphorylated Stat1 and histone acetyltransferases to the CIITA promoter. As a result, IFN-gamma induced histone acetylation at the CIITA promoter is also compromised. However, inhibition of PKCdelta did not affect IRF-1 expression or IRF-1 binding to the CIITA promoter. Therefore, we report, for the first time, that PKCdelta is an essential signaling molecule to achieve the maximal expression of CIITA in response to IFN-gamma in macrophages. In addition, although IRF-1 is a key transcription factor to activate the IFN-gamma inducible CIITA promoter, the effect of PKCdelta on CIITA expression is mediated primarily by serine phosphorylation of Stat 1.

Sequential Modifications in Class II Transactivator Isoform 1 Induced by Lipopolysaccharide Stimulate Major Histocompatibility Complex Class II Transcription in Macrophages

By presenting antigenic peptides on major histocompatibility complex class (MHC) II determinants to CD4(+) T cells, macrophages help to direct the establishment of adaptive immunity. We found that in these cells, lipopolysaccharide stimulates the expression of MHC II genes via the activation of Erk1/2, which is mediated by Toll-like receptor 4. Erk1/2 then phosphorylates the serine at position 357, which is located in a degron of CIITA isoform 1 that leads to its monoubiquitylation. Thus modified, CIITA isoform 1 binds P-TEFb, which mediates the elongation of RNA polymerase II and co-transcriptional processing of nascent transcripts. This induction leads to the expression of MHC II genes. Subsequent polyubiquitylation results in the degradation of CIITA isoform 1. Thus, the signaling cascade from Toll-like receptor 4 to CIITA isoform 1 represents one connection between innate and adaptive immunity in macrophages.

A novel c-Jun-dependent signal transduction pathway necessary for the transcriptional activation of interferon gamma response genes

The biological effects of interferon gamma (IFNgamma) are mediated by interferon-stimulated genes (ISGs), many of which are activated downstream of Janus kinase (JAK)/signal transducer and activator of transcription 1 (STAT1) signaling. Herein we have shown that IFNgamma rapidly activated AP-1 DNA binding that required c-Jun but was independent of JAK1 and STAT1. IFNgamma-induced c-Jun phosphorylation and AP-1 DNA binding required the MEK1/2 and ERK1/2 signaling pathways, whereas the JNK1/2 and p38 mitogen-activated protein kinase pathways were dispensable. The induction of several ISGs, including ifi-205 and iNOS, was impaired in IFNgamma-treated c-Jun-/- cells, but others, such as IP-10 and SOCS3, were unaffected, and chromatin immunoprecipitation demonstrated that c-Jun binds to the iNOS promoter following treatment with IFNgamma. Thus, IFNgamma induced JAK1- and STAT1-independent activation of the ERK mitogen-activated protein kinase pathway, phosphorylation of c-Jun, and activation of AP-1 DNA binding, which are important for the induction of a subset of ISGs. This represents a novel signal transduction pathway induced by IFNgamma that proceeds in parallel with conventional JAK/STAT signaling to activate ISGs.

Protein kinase C delta is essential to maintain CIITA gene expression in B cells

Expression of MHC class II genes requires CIITA. Although the transactivation function of CIITA is well characterized, the signaling events that regulate CIITA expression are less understood. In this study, we report that CIITA expression in B cells depends on protein kinase Cdelta (PKCdelta). PKCdelta controls CIITA gene transcription mainly via modulating CREB recruitment to the CIITA promoter without affecting CIITA mRNA stability. Inhibition of PKCdelta by a pharmacological inhibitor or knocking down of endogenous PKCdelta expression by small interfering RNA reduced CREB binding to the CIITA promoter. The decrease of CIITA gene expression in the presence of the PKCdelta inhibitor was prevented by ectopically expressing a constitutively active form of CREB. In addition, histone acetylation of the CIITA promoter is regulated by PKCdelta since the PKCdelta inhibitor treatment or PKCdelta small interfering RNA resulted in decreased histone acetylation. Taken together, our study reveals that PKCdelta is an important signaling molecule necessary to maintain CIITA and MHC class II expression in B cells.

The nucleoporin Nup96 is required for proper expression of interferon-regulated proteins and functions

Nup98 and Nup96 are components of the nuclear transport machinery and are induced by interferons (IFN). Nup98 is a constituent of an mRNA export pathway that is targeted by viruses and regulated by IFN. However, the role of Nup96 in IFN-related mechanisms has not been established. To investigate the function of Nup96 in vivo, we generated Nup96(+/-) mice that express low levels of Nup96, as Nup96(-/-) mice are lethal. The Nup96(+/-) mice presented selective alterations of the immune system, which resulted in downregulation and impaired IFN alpha- and gamma-mediated induction of MHC I and IFNgamma induction of MHC II, ICAM-1, and other proteins. Frequency of TCRalphabeta+ and CD4+ T cells, which depends on MHC function, is reduced in NUP96(+/-) mice. Upon immunization, Nup96(+/-) mice showed impaired antigen presentation and T cell proliferation. Nup96(+/-) cells and mice were highly susceptible to viral infection, demonstrating a role for Nup96 in innate and adaptive immunity.

PI 3 kinase-dependent Akt kinase and PKCε independently regulate interferon-γ-induced STAT1α serine phosphorylation to induce monocyte chemotactic protein-1 expression

Monocyte chemotactic protein-1 (MCP-1) recruits activated phagocytes to the site of tissue injury. Interferon-gamma (IFN-gamma) present in the microenvironment of glomerulus acts on mesangial cells to induce local production of MCP-1. The mechanism by which IFN-gamma stimulates expression of MCP-1 is not clear. We therefore examined the role of PI 3 kinase signaling in regulating the IFN-gamma-induced MCP-1 expression in mesangial cells. Blocking PI 3 kinase activity with Ly294002 attenuated IFN-gamma-induced MCP-1 protein and mRNA expression. IFN-gamma increased Akt kinase activity in a PI 3 kinase-dependent manner. Expression of dominant negative Akt kinase inhibited serine phosphorylation of STAT1alpha, without any effect on its tyrosine phosphorylation, and decreased IFN-gamma-induced expression of MCP-1. These data for the first time indicate a role for PI 3 kinase-dependent Akt kinase in MCP-1 expression. We have recently shown that along with Akt, PKCepsilon is a downstream target of PI 3 kinase in IFN-gamma signaling. Similar to dominant negative Akt kinase, dominant negative PKCepsilon also inhibited serine phosphorylation of STAT1alpha without any effect on tyrosine phosphorylation. Dominant negative PKCepsilon also abrogated MAPK activity, resulting in decrease in IFN-gamma-induced MCP-1 expression. Furthermore, Akt and PKCepsilon are present together in a signaling complex. IFN-gamma had no effect on this complex formation, but did increase PKCepsilon-associated Akt kinase activity. PKCepsilon did not regulate IFN-gamma-induced Akt kinase. Finally, expression of dominant negative Akt kinase blocked IFN-gamma-stimulated MAPK activation. These data provide the first evidence that PI 3 kinase-dependent Akt and PKCepsilon activation independently regulate MAPK activity and serine phosphorylation of STAT1alpha to increase expression of MCP-1.

RNA interference reveals a role for TLR2 and TLR3 in the recognition ofLeishmania donovani promastigotes by interferon–γ-primed macrophages

Leishmania donovani promastigotes evade the induction of a proinflammatory response during their invasion of naive macrophages. However, their entry into IFN-gamma-primed macrophages is accompanied by the secretion of nitric oxide (NO) and proinflammatory cytokines. In the present study, we addressed the hypothesis that priming with IFN-gamma induces the expression of a receptor that enables mouse macrophages to recognize L. donovani promastigotes. We observed that in IFN-gamma-primed macrophages, L. donovani promastigotes stimulated Interleukin-1 receptor-associated kinase-1 (IRAK-1) activity. We next showed that Toll-like receptor (TLR)3 is barely detectable in naive macrophages but is expressed in IFN-gamma-treated macrophages. Silencing of TLR3, TLR2, IRAK-1 and myeloid differentiation factor 88 (MyD88) expression by RNA interference revealed that both TLR are involved in the secretion of NO and TNF-alpha induced by L. donovani promastigotes. Using L. donovani mutants, we showed that TLR2-mediated responses are dependent on Galbeta1,4Manalpha-PO(4)-containing phosphoglycans, whereas TLR3-mediated responses are independent of these glycoconjugates. Furthermore, our data indicate a participation of TLR2 and TLR3 in the phagocytosis of L. donovani promastigotes and a role for TLR3 in the leishmanicidal activity of the IFN-gamma-primed macrophages. Collectively, our data are consistent with a model where recognition of L. donovani promastigotes depends on the macrophage activation status and requires the expression of TLR3.

PKC alpha depletion in RAW264.7 macrophages following microbial/IFNgamma stimulation is PC-PLC-mediated

Under chronic inflammatory conditions, monocytes/macrophages often exhibit a desensitized phenotype, which is characterized by attenuated reactive oxygen species (ROS) production in close association with depletion of protein kinase C alpha (PKC alpha). This behavior has been observed in monocytes derived from septic blood although the stimulus responsible for initiating these alterations remained obscure. Using RAW264.7 macrophages, we provide evidence that components of neither gram-negative nor gram-positive bacteria deplete PKC alpha, whereas the T(H)1 cytokine interferon-gamma (IFNgamma) does. As shown by western blot analysis, lipopolysaccharide, as well as lipoteichoic acid, did not alter PKC alpha expression, but IFNgamma dose-dependently decreased PKC alpha protein level. Taking into consideration that diacylglycerol and Ca2+ as established PKC alpha activators are released in response to phospholipase C activation, we pretreated cells with the phosphatidylcholine-specific phospholipase C (PC-PLC) inhibitor tricyclodecan-9-yl potassium xanthate (D609) and the phosphatidylinositol-specific phospholipase C inhibitor 1-(6-(17beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5-dione (U73122). In cells preincubated with D609, IFNgamma-mediated PKC alpha depletion was attenuated, whereas U73122 did not impair this process. Moreover, phorbol 12-myristate 13-acetate-initiated ROS formation, which was attenuated in macrophages pretreated with IFNgamma, was restored in the presence of the PC-PLC inhibitor. These results suggest that IFNgamma causes PC-PLC stimulation, diacylglycerol release, Ca2+ influx, and concomitant PKC alpha activation, which subsequently depletes PKC alpha. Strategies to antagonize IFNgamma might be helpful to prevent monocyte/macrophage desensitization.

Activated mouse T-cells synthesize MHC class II, process, and present morbillivirus nucleocapsid protein to primed T-cells

A pivotal step in the initiation of T-cell immunity is the presentation of antigenic peptides by major histocompatibility complex (MHC) expressed on antigen presenting cells. The expression of MHC class II molecules by mouse T-cells has not been shown unequivocally. In the present work, we demonstrate that activated mouse T-cells synthesize MHC class II molecules de novo and express them on their surface. Further, we have demonstrated that in vitro activated T-cells take up extra-cellular soluble nucleocapsid protein of a morbillivirus. The internalized antigen goes to antigen processing compartment as shown by co-localization of antigen and LAMP-1 using confocal microscopy. We show that activated T-cells express H2M, a chaperone molecule known to have a role in antigen presentation. Further, we demonstrate that activated T-cells process and present internalized extra-cellular antigen to primed T-cells as shown by IL-2 secretion and in vitro proliferation. The presentation of antigen by T-cells may have implications in immuno-regulation, control of infection by lymphotropic viruses and maintenance of immunological memory.

Serine Residues 286, 288, and 293 within the CIITA: A Mechanism for Down-Regulating CIITA Activity through Phosphorylation

CIITA is the primary factor activating the expression of the class II MHC genes necessary for the exogenous pathway of Ag processing and presentation. Strict control of CIITA is necessary to regulate MHC class II gene expression and induction of an immune response. We show in this study that the nuclear localized form of CIITA is a predominantly phosphorylated form of the protein, whereas cytoplasmic CIITA is predominantly unphosphorylated. Novel phosphorylation sites were determined to be located within a region that contains serine residues 286, 288, and 293. Double mutations of these residues increased nuclear CIITA, indicating that these sites are not required for nuclear import. CIITA-bearing mutations of these serine residues significantly increased endogenous MHC class II expression, but did not significantly enhance trans-activation from a MHC class II promoter, indicating that these phosphorylation sites may be important for gene activation from intact chromatin rather than artificial plasmid-based promoters. These data suggest a model for CIITA function in which phosphorylation of these specific sites in CIITA in the nucleus serves to down-regulate CIITA activity.