Interferon-gamma, the functional plasticity of the ubiquitin-proteasome system, and MHC class I antigen processing

PRDM1/BLIMP-1 modulates IFN-gamma-dependent control of the MHC class I antigen-processing and peptide-loading pathway

A diverse spectrum of unique peptide-MHC class I complexes guides CD8 T cell responses toward viral or stress-induced Ags. Multiple components are required to process Ag and facilitate peptide loading in the endoplasmic reticulum. IFN-gamma, a potent proinflammatory cytokine, markedly up-regulates transcription of genes involved in MHC class I assembly. Physiological mechanisms which counteract this response are poorly defined. We demonstrate that promoters of functionally linked genes on this pathway contain conserved regulatory elements that allow antagonistic regulation by IFN-gamma and the transcription factor B lymphocyte-induced maturation protein-1 (also known as PR domain-containing 1, with ZNF domain (PRDM1)). Repression of ERAP1, TAPASIN, MECL1, and LMP7 by PRDM1 results in failure to up-regulate surface MHC class I in response to IFN-gamma in human cell lines. Using the sea urchin prdm1 ortholog, we demonstrate that the capacity of PRDM1 to repress the IFN response of such promoters is evolutionarily ancient and that dependence on the precise IFN regulatory factor element sequence is highly conserved. This indicates that the functional interaction between PRDM1 and IFN-regulated pathways antedates the evolution of the adaptive immune system and the MHC, and identifies a unique role for PRDM1 as a key regulator of Ag presentation by MHC class I.

LMP2-specific inhibitors: chemical genetic tools for proteasome biology

The immunoproteasome, having been linked to neurodegenerative diseases and hematological cancers, has been shown to play an important role in MHC class I antigen presentation. However, its other pathophysiological functions are still not very well understood. This can be attributed mainly to a lack of appropriate molecular probes that can selectively modulate the immunoproteasome catalytic subunits. Herein, we report the development of molecular probes that selectively inhibit the major catalytic subunit, LMP2, of the immunoproteasome. We show that these compounds irreversibly modify the LMP2 subunit with high specificity. Importantly, LMP2-rich cancer cells compared to LMP2-deficient cancer cells are more sensitive to growth inhibition by the LMP2-specific inhibitor, implicating an important role of LMP2 in regulating cell growth of malignant tumors that highly express LMP2.

Interferons at age 50: past, current and future impact on biomedicine

The family of interferon (IFN) proteins has now more than reached the potential envisioned by early discovering virologists: IFNs are not only antivirals with a spectrum of clinical effectiveness against both RNA and DNA viruses, but are also the prototypic biological response modifiers for oncology, and show effectiveness in suppressing manifestations of multiple sclerosis. Studies of IFNs have resulted in fundamental insights into cellular signalling mechanisms, gene transcription and innate and acquired immunity. Further elucidation of the multitude of IFN-induced genes, as well as drug development strategies targeting IFN production via the activation of the Toll-like receptors (TLRs), will almost certainly lead to newer and more efficacious therapeutics. Our goal is to offer a molecular and clinical perspective that will enable IFNs or their TLR agonist inducers to reach their full clinical potential.

Role of proteasomes in disease

A functional ubiquitin proteasome system is essential for all eukaryotic cells and therefore any alteration to its components has potential pathological consequences. Though the exact underlying mechanism is unclear, an age-related decrease in proteasome activity weakens cellular capacity to remove oxidatively modified proteins and favours the development of neurodegenerative and cardiac diseases. Up-regulation of proteasome activity is characteristic of muscle wasting conditions including sepsis, cachexia and uraemia, but may not be rate limiting. Meanwhile, enhanced presence of immunoproteasomes in aging brain and muscle tissue could reflect a persistent inflammatory defence and anti-stress mechanism, whereas in cancer cells, their down-regulation reflects a means by which to escape immune surveillance. Hence, induction of apoptosis by synthetic proteasome inhibitors is a potential treatment strategy for cancer, whereas for other diseases such as neurodegeneration, the use of proteasome-activating or -modulating compounds could be more effective. Publication history: Republished from Current BioData's Targeted Proteins database (TPdb; http://www.targetedproteinsdb.com).

Cell cycle-dependent caspase-like activity that cleaves p27KIP1 is the β1 subunit of the 20S proteasome

We previously described a caspase-like activity, which we termed KIPase that is implicated in the turnover of the mammalian cell cycle regulator p27(KIP1). KIPase cleaves a tetra-peptide substrate, Ac-DPSD-AMC, which mimics the target site in p27(KIP1), and inhibitors based on this tetra-peptide are ineffective against other known caspases. Here we describe the purification and characterization of KIPase, and trace its activity to the beta(1) subunit of the 20S proteasome. Further analyses revealed that the activity of the beta(1) subunit is up-regulated as cells enter the cell cycle without concomitant change in the levels of the proteasome beta(1), beta(2) or beta(5) subunits. To our knowledge, this is the first description of cell cycle regulation of the caspase-like activity of the 20S proteasome.

Distinct regulation of MHC molecule expression on astrocytes and microglia during viral encephalomyelitis

The potential interplay of glial cells with T cells during viral induced inflammation was assessed by comparing major histocompatibility complex molecule upregulation and retention on astrocytes and microglia. Transgenic mice expressing green fluorescent protein under control of the astrocyte‐specific glial fibrillary acidic protein promoter were infected with a neurotropic coronavirus to facilitate phenotypic characterization of astrocytes and microglia using flow cytometry. Astrocytes in the adult central nervous system up‐regulated class I surface expression, albeit delayed compared with microglia. Class II was barely detectable on astrocytes, in contrast to potent up‐regulation on microglia. Maximal MHC expression in both glial cell types correlated with IFN‐γ levels and lymphocyte accumulation. Despite a decline of IFN‐γ concomitant to virus clearance, MHC molecule expression on glia was sustained. These data demonstrate distinct regulation of both class I and class II expression by microglia and astrocytes in vivo following viral induced inflammation. Furthermore, prolonged MHC expression subsequent to viral clearance implies a potential for ongoing presentation. © 2007 Wiley‐Liss, Inc.

Role of Direct Effects of IFN-γ on T Cells in the Regulation of CD8 T Cell Homeostasis

It is well recognized that IFN-gamma plays a critical role in the control of CD8 T cell expansion and contraction during immune responses to several intracellular pathogens. However, our understanding of the mechanisms underlying the regulation of T cell fate by IFN-gamma is sorely incomplete. Specifically, it is unclear whether regulation of CD8 T cell homeostasis occurs by a T cell intrinsic IFN-gamma pathway. In this study, we have determined the role of the direct effects of IFN-gamma on T cells in regulating the expansion, contraction, and memory phases of the polyclonal CD8 T cell response to an acute viral infection. Using two complementary approaches we demonstrate that the direct effects of IFN-gamma suppress IL-7R expression on Ag-specific effector CD8 T cells, but clonal expansion or deletion of activated CD8 T cells in vivo can occur in the apparent absence of IFN-gammaR signaling in T cells. These findings have clarified fundamental features of control of T cell homeostasis by IFN-gamma in the context of CD8 T cell memory and protective immunity.

Identification of differentially expressed proteins in experimental autoimmune encephalomyelitis (EAE) by proteomic analysis of the spinal cord

The present study used isobaric tags for relative and absolute quantitation (iTRAQ) to identify novel targets in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. The expression of 41 proteins was significantly altered in the inflamed spinal cord. Twenty of these are implicated in EAE for the first time and many have previously been shown to play a role in antigen processing, inflammation, neuroprotection, or neurodegeneration.

Interferon-gamma induces chronic active myocarditis and cardiomyopathy in transgenic mice

Chronic heart failure is associated with an activation of the immune system characterized among other factors by the cardiac synthesis and serum expression of proinflammatory cytokines. There is unequivocal clinical and experimental evidence that the cytokine tumor necrosis factor-alpha is involved in the development of chronic heart failure, but a putative cardiotoxic potential of the proinflammatory cytokine interferon (IFN)-gamma remains primarily unknown. To investigate this issue we analyzed the cardiac phenotype of SAP-IFN-gamma transgenic mice, which constitutively express IFN-gamma in their livers and hence exhibit high circulating serum levels of this cytokine. SAP-IFN-gamma mice spontaneously developed chronic active myocarditis, characterized by the infiltration of not only CD4(+) and CD8(+) T cells but also Mac2(+) (galectin 3(+)) macrophages and CD11c(+) dendritic cells, eventually culminating in cardiomyopathy. Echocardiographic analyses exhibited a left ventricular dilation and impaired systolic function induced by IFN-gamma overexpression. IFN-gamma-mediated cardiotoxicity was associated with high-level cardiac transcription of the proinflammatory cytokines tumor necrosis factor-alpha and interleukin-12 and the macrophage-attracting chemokines MCP1 and MIP1-alpha. Myotoxic IFN-gamma effects could not be detected in smooth or striated muscle tissue, suggesting cardiomyocellular specificity of the toxic IFN-gamma effect. The precise mechanism of IFN-gamma cardiotoxicity remains to be elucidated.

Crosstalk among Jak-STAT, Toll-like receptor, and ITAM-dependent pathways in macrophage activation

Macrophage phenotype and activation are regulated by cytokines that use the Jak-STAT signaling pathway, microbial recognition receptors that include TLRs, and immunoreceptors that signal via ITAM motifs. The amplitude and qualitative nature of macrophage activation are determined by crosstalk among these signaling pathways. Basal ITAM signaling restrains macrophage responses to TLRs and other activating ligands, whereas strong ITAM signals synergize with the same ligands to activate cells strongly. Similarly, basal ITAM signaling augments IFN signaling and function of receptor activator of NF-kappaB, but extensive ITAM activation inhibits Jak-STAT signaling. Thus, intensity and duration of ITAM signaling determine whether ITAM-coupled receptors augment or attenuate TLR and Jak-STAT responses. IFN-gamma synergizes with TLRs in part by suppressing TLR-induced feedback inhibition, mediated by IL-10 and Stat3, by a mechanism that depends on glycogen synthase kinase (GSK)3 regulation of AP-1 and CREB. IFN-gamma suppresses TLR2 and TLR4 induction/activation of AP-1 by overlapping mechanisms that include regulation of MAPKs, GSK3-dependent suppression of DNA binding, and decreased Fos and Jun protein expression and stability. IFN-gamma suppression of TLR-induced activation of AP-1 and downstream target genes challenges current concepts about the inflammatory role of AP-1 proteins in macrophage activation and is consistent with a role for AP-1 in the generation of noninflammatory osteoclasts. Jak-STAT, TLR, and ITAM pathways are basally active in macrophages and strongly induced during innate responses. Thus, signal transduction crosstalk is regulated in a dynamic manner, which differs under homeostatic and pathologic conditions, and dysregulation of signal transduction crosstalk may contribute to pathogenesis of chronic inflammatory diseases.

Lysis of human chondrosarcoma cells by cytolytic T lymphocytes recognizing a MAGE-A3 antigen presented by HLA-A1 molecules

Treatment of chondrosarcomas is limited to resection because these tumors are unresponsive to standard adjuvant treatments, such as chemotherapy and radiation. We have previously shown that high-grade chondrosarcomas express unspecified members of the Melanoma Antigen (MAGE) gene family. We show here that FS human chondrosarcoma (FS) cells express MAGE-A3 gene and HLA-A1 molecules. In vitro assays show that a cytolytic T-lymphocyte clone (CTL) specific for a MAGE-A3 peptide presented by HLA-A1 specifically lysed FS chondrosarcoma cells. Addition of antigenic peptide did not increase the susceptibility of FS cells to CTL mediated lysis, suggesting that HLA-A1 expression by the chondrosarcoma cells limited their susceptibility to lysis by the anti-MAGE-A3 CTL clone. Incubation of FS cells with 50 U/mL interferon-gamma increased surface expression of HLA class-I molecules, increased their susceptibility to lysis, and had no effect on MAGE-A3 gene expression. These results suggest that immunotherapy targeted against chondrosarcoma cells is possible.

Poxvirus-encoded gamma interferon binding protein dampens the host immune response to infection

Ectromelia virus (ECTV), a natural mouse pathogen and the causative agent of mousepox, is closely related to variola virus (VARV), which causes smallpox in humans. Mousepox is an excellent surrogate small-animal model for smallpox. Both ECTV and VARV encode a multitude of host response modifiers that target components of the immune system and that are thought to contribute to the high mortality rates associated with infection. Like VARV, ECTV encodes a protein homologous to the ectodomain of the host gamma interferon (IFN-gamma) receptor 1. We generated an IFN-gamma binding protein (IFN-gammabp) deletion mutant of ECTV to study the role of viral IFN-gammabp (vIFN-gammabp) in host-virus interaction and also to elucidate the contribution of this molecule to the outcome of infection. Our data show that the absence of vIFN-gammabp does not affect virus replication per se but does have a profound effect on virus replication and pathogenesis in mice. BALB/c mice, which are normally susceptible to infection with ECTV, were able to control replication of the mutant virus and survive infection. Absence of vIFN-gammabp from ECTV allowed the generation of an effective host immune response that was otherwise diminished by this viral protein. Mice infected with a vIFN-gammabp deletion mutant virus, designated ECTV-IFN-gammabp(Delta), produced increased levels of IFN-gamma and generated robust cell-mediated and antibody responses. Using several strains of mice that exhibit differential degrees of resistance to mousepox, we show that recovery or death from ECTV infection is determined by a balance between the host's ability to produce IFN-gamma and the virus' ability to dampen its effects.

Regulation of immunity by a novel population of Qa-1-restricted CD8alphaalpha+TCRalphabeta+ T cells

Regulatory mechanisms involving CD8+ T cells (CD8 regulatory T cells (Tregs)) are important in the maintenance of immune homeostasis. However, the inability to generate functional CD8 Treg clones with defined Ag specificity has precluded a direct demonstration of CD8 Treg-mediated regulation. In the present study, we describe the isolation of functional lines and clones representing a novel population of TCRalphabeta+ Tregs that control activated Vbeta8.2+ CD4 T cells mediating experimental autoimmune encephalomyelitis. They express exclusively the CD8alphaalpha homodimer and recognize a peptide from a conserved region of the TCR Vbeta8.2 chain in the context of the Qa-1a (CD8alphaalpha Tregs). They secrete type 1 cytokines but not IL-2. CD8alphaalpha Tregs kill activated Vbeta8.2+ but not Vbeta8.2- or naive T cells. The CD8alphaalpha Tregs prevent autoimmunity upon adoptive transfer or following in vivo activation. These findings reveal an important negative feedback regulatory mechanism targeting activated T cells and have implications in the development of therapeutic strategies for autoimmune diseases and transplantation.

Isolation of germ cells from leukemia and lymphoma cells in a human in vitro model: potential clinical application for restoring human fertility after anticancer therapy

More than 70% of patients survive childhood cancer, but chemotherapy and radiation therapy may cause irreversible impairment of spermatogenesis. To treat infertility secondary to anticancer treatment for childhood cancer, we have developed a procedure to isolate germ cells from leukemic mice by fluorescence-activated cell sorting with two surface markers, and transplantation of isolated germ cells successfully restored fertility without inducing leukemia. In the present study, we analyzed human germ cells and human malignant cells, including five leukemia cell lines and three lymphoma cell lines, by fluorescence-activated cell sorting with antibodies against MHC class I and CD45. Testicular specimens were obtained from a patient who underwent surgery for testicular rupture. In the high forward scatter and low side scatter region, no malignant cells were found in the MHC class I-negative and CD45-negative fraction (the germ cell fraction), with the exception of K562 cells. A total of 39.2% of the germ cells were found in the germ cell fraction. A total of 1.45% of K562 cells were found in the germ cell fraction. Treatment with IFNgamma induced the expression of MHC class I on K562 cells but not on germ cells and made it possible to isolate germ cells from K562 cells. In conclusion, we isolated human germ cells from malignant cells with two surface markers after treatment with IFNgamma. Immunophenotyping for each patient will be necessary before isolation and induction of surface marker will be clinically applicable.

MHC Class I Antigens and Immune Surveillance in Transformed Cells

MHC class I antigens play a crucial role in the interaction of tumor cells with the host immune system, in particular, in the presentation of peptides as tumor-associated antigens to cytotoxic lymphocytes (CTLs) and in the regulation of cytolytic activity of natural killer (NK) cells. In this review we discuss the role of MHC class I antigens in the recognition and elimination of transformed cells and in the generation of tumor immune escape routes when MHC class I losses occur in tumors. The different altered MHC class I phenotypes and their distribution in different human tumors are the main topic of this review. In addition, molecular defects that underlie MHC alterations in transformed cells are also described in detail. Future research directions in this field are also discussed, including the laboratory analysis of tumor MHC class I-negative variants and the possible restoration of MHC class I expression.

Confronting complexity: real-world immunodominance in antiviral CD8+ T cell responses

Antiviral CD8(+) T cells respond to only a minute fraction of the potential peptide determinants encoded by viral genomes. Immunogenic determinants can be ordered into highly reproducible hierarchies based on the magnitude of cognate CD8(+) T cell responses. Until recently, this phenomenon, termed immunodominance, was largely defined and characterized in model systems utilizing a few strains of inbred mice infected with a handful of viruses with limited coding capacity. Here, I review work that has extended immunodominance studies to viruses of greater complexity and to the real world of human antiviral immunity.

Murine polyomavirus-like particles induce maturation of bone marrow-derived dendritic cells and proliferation of T cells

We analysed the effects of murine polyomavirus-like particles (PLPs) on bone marrow-derived dendritic cells (BMDCs) and T cells in vitro. BMDCs activated with PLPs up-regulated CD40, CD80, CD86 and major histocompatibility complex (MHC) class II surface markers and produced proinflammatory cytokines. Chimeric PLPs [expressing the ovalbumin (OVA)-peptides OVA(257-264) or OVA(323-339)], but not wildtype PLPs, activated OVA-specific CD8 T cells and OVA-specific CD4 T cells, respectively, indicating both MHC class I and II presentation of the peptides by antigen-presenting cells. Our results suggest that PLPs may be used as vaccine adjuvants priming dendritic cells to induce potent T cell responses.

Immunoproteasome subunit LMP2 expression is deregulated in Sjogren's syndrome but not in other autoimmune disorders

BACKGROUND

The proteasome system has a pivotal role in the control of the immune response, which suggests that it might be involved in the pathogenesis of autoimmune disorders.

OBJECTIVE

To investigate the expression profile of selected proteasomal genes in human peripheral blood mononuclear cells in patients with a variety of autoimmune diseases compared with healthy subjects.

METHODS

Real time quantitative RT-PCR was used to analyse the mRNA expression pattern of the proteasome activator subunits PA28alpha and PA28beta and of constitutive proteasome and interferon-gamma-inducible immunoproteasome subunits in peripheral blood mononuclear cells. Simultaneously, protein expression of selected proteasome subunits was quantified by immunoblotting.

RESULTS

Under systemic inflammatory conditions the proteasome subunits LMP2 (beta1i), LMP7 (beta5i), MECL1 (beta2i), and PA28alpha were expressed abundantly at the protein level in the vast majority of systemic autoimmune disorders. However, simultaneous mRNA and protein quantification showed a characteristic proteasome expression signature in primary Sjögren's syndrome. At the transcript level, the interferon-gamma-responsive subunits LMP2 (beta1i), MECL1 (beta2i), and the proteasome activator subunit PA28alpha were markedly up regulated. In contrast, LMP2 (beta1i) deficiency was evident at the protein level, indicating deregulation of proteasome expression in Sjögren's syndrome.

CONCLUSIONS

These data provide evidence for a regulatory defect in the proteasome system in human autoimmune disorders, pointing to a unique role for LMP2 (beta1i) in the pathogenesis of primary Sjögren's syndrome.

Processing of tumor-associated antigen by the proteasomes of dendritic cells controls in vivo T-cell responses

Dendritic cells are unique in their capacity to process antigens and prime naive CD8(+) T cells. Contrary to most cells, which express the standard proteasomes, dendritic cells express immunoproteasomes constitutively. The melanoma-associated protein Melan-A(MART1) contains an HLA-A2-restricted peptide that is poorly processed by melanoma cells expressing immunoproteasomes in vitro. Here, we show that the expression of Melan-A in dendritic cells fails to elicit T-cell responses in vitro and in vivo because it is not processed by the proteasomes of dendritic cells. In contrast, dendritic cells lacking immunoproteasomes induce strong anti-Melan-A T-cell responses in vitro and in vivo. These results suggest that the inefficient processing of self-antigens, such as Melan-A, by the immunoproteasomes of professional antigen-presenting cells prevents the induction of antitumor T-cell responses in vivo.

Comparative Selectivity and Specificity of the Proteasome Inhibitors BzLLLCOCHO, PS-341, and MG-132

The 26S proteasome is a multicatalytic protease responsible for regulated intracellular protein degradation. Its function is mediated by three main catalytic activities: (a) chymotrypsin-like (CT-L), (b) trypsin-like, and (c) peptidylglutamyl peptide hydrolysing (PGPH). Proteasome inhibition is an emerging therapy for many cancers and is a novel treatment for multiple myeloma. Here, we profile the contributions of the three catalytic activities in multiple myeloma cell lines and compare the specificity and cytotoxicity of the novel proteasome inhibitor BzLLLCOCHO and inhibitors PS-341 (Velcade, bortezomib) and MG-132. Using fluorogenic substrates and an active site-directed probe specific for proteasome catalytic subunits, we show differential subunit specificity for each of the inhibitors. Addition of BzLLLCOCHO strongly inhibited all three catalytic activities, treatment with PS-341 completely inhibited CT-L and PGPH activities, and treatment with MG-132 resulted in weak inhibition of the CT-L and PGPH activities. Multiple myeloma cells were more sensitive to induction of apoptosis by PS-341 and MG-132 than BzLLLCOCHO. This study emphasizes the need for further investigation of the effects of these compounds on gene and protein expression in the cell to allow for the development of more specific and targeted inhibitors.

HIV Tat-mediated transcriptional regulation of proteasome protein cleavage specificity

The major antigen-adapted immune response protecting a vertebrate against virus infection is that mediated by CTLs (cytotoxic T-lymphocytes). CTLs destroy virus-infected cells, thereby containing the infection. They are activated by recognition of peptide antigens or epitopes, presented to them in the context of MHC I proteins. These epitopes are derived from proteolytic degradation of endogenously synthesized proteins, which is mediated by the proteasome. Augmentation of epitope presentation by MHC I is thought to be effected by the immunoproteasome, induced in response to IFN-gamma (interferon-gamma) in some cells, and constitutively expressed in others. In this issue of the Biochemical Journal, Remoli and colleagues describe the manipulation of the immunoproteasome by the Tat (transcriptional activation) protein of HIV. The authors show that Tat deregulates the balance of the three proteins, LMP2 (low-molecular-mass polypeptide 2), LMP7 and MECL1 (multicatalytic endopeptidase complex-like 1), which distinguish the immunoproteasome from the proteasome, and they provide a molecular explanation. Intracellular Tat sequesters IRF-1 (interferon-regulatory factor-1) from its cognate promoter element, where normally it associates with STAT1 (signal transducer and activator of transcription 1) to activate basal transcription of the LMP2 gene. LMP2 expression is inhibited as a consequence, skewing the stoichiometry of the immunoproteasome and changing its enzymatic activity. These findings provide a molecular account of an immunomodulatory activity of HIV: changing the peptide antigen profile of cells expressing or exposed to Tat. They may also provide an avenue for manipulating vaccine efficacy and specificity with Tat-based adjuvants.

PRED(TAP): a system for prediction of peptide binding to the human transporter associated with antigen processing

Background

The transporter associated with antigen processing (TAP) is a critical component of the major histocompatibility complex (MHC) class I antigen processing and presentation pathway. TAP transports antigenic peptides into the endoplasmic reticulum where it loads them into the binding groove of MHC class I molecules. Because peptides must first be transported by TAP in order to be presented on MHC class I, TAP binding preferences should impact significantly on T-cell epitope selection.

Description

PRED^TAP is a computational system that predicts peptide binding to human TAP. It uses artificial neural networks and hidden Markov models as predictive engines. Extensive testing was performed to valid the prediction models. The results showed that PRED^TAP was both sensitive and specific and had good predictive ability (area under the receiver operating characteristic curve Aroc>0.85).

Conclusion

PRED^TAP can be integrated with prediction systems for MHC class I binding peptides for improved performance of in silico prediction of T-cell epitopes. PRED^TAP is available for public use at [1].

Regulation of gene expression in RAW 264.7 macrophage cell line by interferon-γ

Macrophages play an important role in immune responses and in inflammatory disease states such as atherosclerosis. Interferon-gamma (IFN-gamma) is a major cytokine involved in the activation of macrophages. To elucidate the primary response of various genes and biological pathways regulated by IFN-gamma in macrophage, we analyzed the gene expression profile in RAW 264.7 macrophage cells treated with IFN-gamma for 4h. Microarray analysis revealed that about 400 genes were differentially expressed, of which about 250 genes were up-regulated and 150 were down-regulated. Functional organization of the transcriptome revealed that induced genes are involved in antimicrobial and antiviral responses, antigen presentation, chemokine and cytokine signaling, and inhibition of cell growth. We also found that expression of genes involved in cell-cycle control, DNA repair, and lipid metabolism was suppressed by IFN-gamma. We also identified induction of multiple transcription factors by IFN-gamma in RAW 264.7 cells. Functional annotation of genes regulated by IFN-gamma in RAW 264.7 cells may provide novel insights into the role of macrophages in immunity and in inflammatory disease.

Inactivation of the proteasome by 4-hydroxy-2-nonenal is site specific and dependant on 20S proteasome subtypes

The proteasome represents a major intracellular proteolytic system responsible for the degradation of oxidized and ubiquitinated proteins in both the nucleus and cytoplasm. We have previously reported that proteasome undergoes modification by the lipid peroxidation product 4-hydroxy-2-nonenal (HNE) and exhibits declines in peptidase activities during cardiac ischemia/reperfusion. This study was undertaken to characterize the effects of HNE on the structure and function of the 20S proteasome. To assess potential tissue-specific differences in the response to HNE, we utilized purified 20S proteasome from heart and liver, tissues that express different proteasome subtypes. Following incubation of heart and liver 20S proteasome with HNE, changes in the 2D gel electrophoresis patterns and peptidase activities of the proteasome were evaluated. Proteasome subunits were identified by mass spectrometry prior to and following treatment with HNE. Our results demonstrate that specific subunits of the 20S proteasome are targeted for modification by HNE and that modified proteasome exhibits selective alterations in peptidase activities. The results provide evidence for a likely mechanism of proteasome inactivation in response to oxidative stress particularly during cardiac ischemia/reperfusion.

Innate immune cells contribute to the IFN-gamma-dependent regulation of antigen-specific CD8+ T cell homeostasis

IFN-gamma has a dual function in the regulation of T cell homeostasis. It promotes the expansion of effector T cells and simultaneously programs their contraction. The cellular mechanisms leading to this functional dichotomy of IFN-gamma have not been identified to date. In this study we show: 1) that expansion of wild-type CD8+ T cells is defective in IFN-gamma-deficient mice but increased in IFN-gammaR-deficient mice; and 2) that contraction of the effector CD8+ T cell pool is impaired in both mouse strains. Furthermore, we show that CD11b+ cells responding to IFN-gamma are sufficient to limit CD8+ T cell expansion and promote contraction. The data presented here reveal that IFN-gamma directly promotes CD8+ T cell expansion and simultaneously induces suppressive functions in CD11b+ cells that counter-regulate CD8+ T cell expansion, promote contraction, and limit memory formation. Thus, innate immune cells contribute to the IFN-gamma-dependent regulation of Ag-specific CD8+ T cell homeostasis.

Antigen presentation and the ubiquitin-proteasome system in host-pathogen interactions

Relatively small genomes and high replication rates allow viruses and bacteria to accumulate mutations. This continuously presents the host immune system with new challenges. On the other side of the trenches, an increasingly well-adjusted host immune response, shaped by coevolutionary history, makes a pathogen's life a rather complicated endeavor. It is, therefore, no surprise that pathogens either escape detection or modulate the host immune response, often by redirecting normal cellular pathways to their advantage. For the purpose of this chapter, we focus mainly on the manipulation of the class I and class II major histocompatibility complex (MHC) antigen presentation pathways and the ubiquitin (Ub)-proteasome system by both viral and bacterial pathogens. First, we describe the general features of antigen presentation pathways and the Ub-proteasome system and then address how they are manipulated by pathogens. We discuss the many human cytomegalovirus (HCMV)-encoded immunomodulatory genes that interfere with antigen presentation (immunoevasins) and focus on the HCMV immunoevasins US2 and US11, which induce the degradation of class I MHC heavy chains by the proteasome by catalyzing their export from the endoplasmic reticulum (ER)-membrane into the cytosol, a process termed ER dislocation. US2- and US11-mediated subversion of ER dislocation ensures proteasomal degradation of class I MHC molecules and presumably allows HCMV to avoid recognition by cytotoxic T cells, whilst providing insight into general aspects of ER-associated degradation (ERAD) which is used by eukaryotic cells to purge their ER of defective proteins. We discuss the similarities and differences between the distinct pathways co-opted by US2 and US11 for dislocation and degradation of human class I MHC molecules and also a putatively distinct pathway utilized by the murine herpes virus (MHV)-68 mK3 immunoevasin for ER dislocation of murine class I MHC. We speculate on the implications of the three pathogen-exploited dislocation pathways to cellular ER quality control. Moreover, we discuss the ubiquitin (Ub)-proteasome system and its position at the core of antigen presentation as proteolysis and intracellular trafficking rely heavily on Ub-dependent processes. We add a few examples of manipulation of the Ub-proteasome system by pathogens in the context of the immune system and such diverse aspects of the host-pathogen relationship as virus budding, bacterial chromosome integration, and programmed cell death, to name a few. Finally, we speculate on newly found pathogen-encoded deubiquitinating enzymes (DUBs) and their putative roles in modulation of host-pathogen interactions.