Immunological functions of the proteasome

West Nile virus and dengue virus capsid protein negates the antiviral activity of human Sec3 protein through the proteasome pathway

Flavivirus capsid (C) protein is a key structural component of virus particles. The non-structural role of C protein in the pathogenesis of arthropod-borne flaviviruses is not clearly deciphered. This study showed that West Nile virus (WNV) and dengue virus (DENV) utilized C protein to reduce human Sec3p (hSec3p) levels at post-transcriptional level through activation of chymotrypsin-like proteolytic function of 20S proteasome. Mutagenesis studies confirmed amino acids 14, 109-114 of WNV C protein and 13, 102-107 of DENV C protein played an important role in activating the proteolytic function of 20S proteasome. Amino acid residues at 14 (WNV) and 13 (DENV) of C protein were important for C protein-hSec3p binding and physical interaction between C protein and hSec3p was essential to execute hSec3p degradation. Degradation motif required to degrade hSec3p resided between amino acid residues 109-114 of WNV C protein and 102-107 of DENV C protein. Proteasomes, hSec3p binding motif and degradation motif on C protein must be intact for efficient flavivirus production. Clinical isolates of DENV showed more pronounced effect in manipulating the proteasomes and reducing hSec3p levels. This study portrayed the non-structural function of C protein that helped the flavivirus to nullify the antiviral activity of hSec3p by accelerating its degradation and facilitating efficient binding of elongation factor 1α with flaviviral RNA genome.

Rescue of MHC-1 antigen processing machinery by down-regulation in expression of IGF-1 in human glioblastoma cells

Escape from immune recognition has been hypothesized to be a factor in carcinogenesis. It may be mediated for many cancers through down-regulation in the MHC class 1 antigen processing and presentation pathway. TAP-1, TAP-2, tightly linked to LMP-2 and LMP-7 are multiple components of the endogenous, antigen presentation pathway machinery. We addressed the question of alterations in this pathway in human Glioblastoma (HGB) and of its relationship to modulation in expression of IGF-1 that is highly expressed in this cancer. Deficiencies in expression of TAP-1 were demonstrated by RT-PCR and/or by immuno-flow cytometry in the HGB cell line T98G obtained from ATCC, and in 3 of 4 human cell lines established from patients with Glioblastoma Multiforme. Deficiencies in expression of TAP-2 were observed in 3 of 4, deficiencies in expression of LMP-2 in 4 of 4 and deficiencies in LMP-7 in 3 of 4 HGB cell lines examined by RT-PCR and Western blot. Following down-regulation of IGF-1 by transfection with the pAnti IGF-1 vector that expresses IGF-1 RNA in antisense orientation, or by the exogenous addition of IGF-1 receptor monoclonal antibody to cell culture media, the deficiencies in components of the MHC-1 antigen presentation pathway were up-regulated and/or rescued in all HGB cell lines tested. Moreover, this up-regulation in expression was aborted by addition of 100 ng/ml of IGF-1 to the culture media. Unlike in the case of IFN-γ, the restoration of TAP-1 and LMP-2 by down-regulation of IGF-1 in Glioblastoma cells was not correlated to the tyrosine phosphorylation of STAT 1. In summary, the simultaneous reversion in expression of the multiple constituents of MHC-1 antigen processing path and up-regulation in expression of MHC-1 occurring with down-regulation in IGF-1 may have a role in reinforcement of immunity against tumor antigen(s) in some animal cancers and in humans with Glioblastoma Multiforme.

Overview of the immune response

The immune system has evolved to protect the host from a universe of pathogenic microbes that are themselves constantly evolving. The immune system also helps the host eliminate toxic or allergenic substances that enter through mucosal surfaces. Central to the immune system's ability to mobilize a response to an invading pathogen, toxin, or allergen is its ability to distinguish self from nonself. The host uses both innate and adaptive mechanisms to detect and eliminate pathogenic microbes, and both of these mechanisms include self-nonself discrimination. This overview identifies key mechanisms used by the immune system to respond to invading microbes and other exogenous threats and identifies settings in which disturbed immune function exacerbates tissue injury.

Conservation and diversity of influenza A H1N1 HLA-restricted T cell epitope candidates for epitope-based vaccines

BACKGROUND

The immune-related evolution of influenza viruses is exceedingly complex and current vaccines against influenza must be reformulated for each influenza season because of the high degree of antigenic drift among circulating influenza strains. Delay in vaccine production is a serious problem in responding to a pandemic situation, such as that of the current H1N1 strain. Immune escape is generally attributed to reduced antibody recognition of the viral hemagglutinin and neuraminidase proteins whose rate of mutation is much greater than that of the internal non-structural proteins. As a possible alternative, vaccines directed at T cell epitope domains of internal influenza proteins, that are less susceptible to antigenic variation, have been investigated.

METHODOLOGY/PRINCIPAL FINDINGS

HLA transgenic mouse strains expressing HLA class I A*0201, A*2402, and B*0702, and class II DRB1*1501, DRB1*0301 and DRB1*0401 were immunized with 196 influenza H1N1 peptides that contained residues of highly conserved proteome sequences of the human H1N1, H3N2, H1N2, H5N1, and avian influenza A strains. Fifty-four (54) peptides that elicited 63 HLA-restricted peptide-specific T cell epitope responses were identified by IFN-gamma ELISpot assay. The 54 peptides were compared to the 2007-2009 human H1N1 sequences for selection of sequences in the design of a new candidate H1N1 vaccine, specifically targeted to highly-conserved HLA-restricted T cell epitopes.

CONCLUSIONS/SIGNIFICANCE

Seventeen (17) T cell epitopes in PB1, PB2, and M1 were selected as vaccine targets based on sequence conservation over the past 30 years, high functional avidity, non-identity to human peptides, clustered localization, and promiscuity to multiple HLA alleles. These candidate vaccine antigen sequences may be applicable to any avian or human influenza A virus.

Expression pattern of immunoproteasome subunits in human thymus

The expression pattern of immunoproteasomes in human thymus has not been analyzed but may have important consequences during thymic selection. Here we examined the expression patterns of immunoproteasome subunits in fetal and adult thymic tissues by immunohistochemistry and found that all three subunits are expressed in both cortical and medullary stromal cells. These data suggest that thymic selection in human can be affected by peptide repertoires generated by immunoproteasomes.

Insights into the molecular architecture of the 26S proteasome

Cryo-electron microscopy in conjunction with advanced image analysis was used to analyze the structure of the 26S proteasome and to elucidate its variable features. We have been able to outline the boundaries of the ATPase module in the "base" part of the regulatory complex that can vary in its position and orientation relative to the 20S core particle. This variation is consistent with the "wobbling" model that was previously proposed to explain the role of the regulatory complex in opening the gate in the alpha-rings of the core particle. In addition, a variable mass near the mouth of the ATPase ring has been identified as Rpn10, a multiubiquitin receptor, by correlating the electron microscopy data with quantitative mass spectrometry.

Myelin oligodendrocyte glycoprotein peptide-induced experimental allergic encephalomyelitis and T cell responses are unaffected by immunoproteasome deficiency

The inoculation of MOG peptides into C57BL/6 mice induces CD4(+) and CD8(+) T cells, and recent work has shown that adoptive transfer of the latter population, after extensive in vitro stimulation, can cause EAE in naïve recipient mice. Herein, we have evaluated the incidence and severity of EAE, and the induction of CD4(+) and CD8(+) T cells, following MOG peptide inoculation of wt mice and of LMP-2KO mice that lack an intact immunoproteasome, a cytoplasmic organelle that is induced by chronic inflammation and that may be important for the presentation of MHC class I epitopes to CD8(+) T cells. We report that EAE, evaluated by both clinical and histological criteria, is similar in LMP-2KO mice and wildtype C57B/6 mice (wt) in response to immunization with MOG peptides MOG(35-55) and MOG(40-54), suggesting that the immunoproteasome does not play a key role in the development of demyelinating disease. Furthermore, and consistent with previous reports, peptide-specific CD8(+) T cells were barely detectable in the CNS of peptide-immunized mice, although peptide-specific CD4(+) T cells were abundant. Therefore, we used a new technique to look for autoreactive CD8(+) T cells in MOG peptide-immunized mice, and we report the identification of CD4(+) and CD8(+) T cells that, as late as 19 days after peptide injection, are actively producing IFNgamma in vivo, in response to in vivo antigen contact.

Polymorphisms of transporter associated with antigen processing type 1 (TAP1), proteasome subunit beta type 9 (PSMB9) and their common promoter in African children with different manifestations of malaria

The distribution of gene variants of the antigen processing proteins transporter associated with antigen processing type 1 (TAP1) and proteasome subunit beta type 9 (PSMB9) and of their shared bidirectional promoter was assessed in children with either mild or severe malaria. The genetic study was performed on samples collected during a longitudinal study on malariometric indices in an area hyperendemic for Plasmodium falciparum malaria in Gabon. The allele frequencies of the genes did not differ between the mild and the severe malaria groups. The distributions of alleles among children with distinct phenotypes of severe malaria were similar. A negative association of hypoglycaemia with the PSMB9 promoter variant PSMB9-R was found (odds ratio 0.01; chi2=12.1; P<0.0005; Pc<0.03). The promoter allele TAP1-446G was associated with hyperparasitaemia and absence of hypoglycaemia. TAP1, PSMB9, and TAP1/PSMB9 promoter alleles were in strong linkage disequilibrium. DNA sequencing of the TAP1/PSMB9 promoter region revealed a previously unrecognized single nucleotide polymorphism 455 bp upstream of the TAP1 transcription start site.

Escape mutations alter proteasome processing of major histocompatibility complex class I-restricted epitopes in persistent hepatitis C virus infection

Mutations in hepatitis C virus (HCV) genomes facilitate escape from virus-specific CD8+ T lymphocytes in persistently infected chimpanzees. Our previous studies demonstrated that many of the amino acid substitutions in HCV epitopes prevented T-cell receptor recognition or binding to class I major histocompatibility complex molecules. Here we report that mutations within HCV epitopes also cause their destruction by changing the pattern of proteasome digestion. This mechanism of immune evasion provides further evidence of the potency of CD8+ T-cell selection pressure against HCV and should be considered when evaluating the significance of mutations in viral genomes from persistently infected chimpanzees and humans.

Phosphorylation of 20S proteasome alpha subunit C8 (alpha7) stabilizes the 26S proteasome and plays a role in the regulation of proteasome complexes by gamma-interferon

In animal cells there are several regulatory complexes which interact with 20S proteasomes and give rise to functionally distinct proteasome complexes. gamma-Interferon upregulates three immuno beta catalytic subunits of the 20S proteasome and the PA28 regulator, and decreases the level of 26S proteasomes. It also decreases the level of phosphorylation of two proteasome alpha subunits, C8 (alpha7) and C9 (alpha3). In the present study we have investigated the role of phosphorylation of C8 by protein kinase CK2 in the formation and stability of 26S proteasomes. An epitope-tagged C8 subunit expressed in mammalian cells was efficiently incorporated into both 20S proteasomes and 26S proteasomes. Investigation of mutants of C8 at the two known CK2 phosphorylation sites demonstrated that these are the two phosphorylation sites of C8 in animal cells. Although phosphorylation of C8 was not absolutely essential for the formation of 26S proteasomes, it did have a substantial effect on their stability. Also, when cells were treated with gamma-interferon, there was a marked decrease in phosphorylation of C8, a decrease in the level of 26S proteasomes, and an increase in immunoproteasomes and PA28 complexes. These results suggest that the down-regulation of 26S proteasomes after gamma-interferon treatment results from the destabilization that occurs after dephosphorylation of the C8 subunit.

Differential proteasomal processing of hydrophobic and hydrophilic protein regions: contribution to cytotoxic T lymphocyte epitope clustering in HIV-1-Nef

HIV proteins contain a multitude of naturally processed cytotoxic T lymphocyte (CTL) epitopes that concentrate in clusters. The molecular basis of epitope clustering is of interest for understanding HIV immunogenicity and for vaccine design. We show that the CTL epitope clusters of HIV proteins predominantly coincide with hydrophobic regions, whereas the noncluster regions are predominantly hydrophilic. Analysis of the proteasomal degradation products of full-length HIV-Nef revealed a differential sensitivity of cluster and noncluster regions to proteasomal processing. Compared with the epitope-scarce noncluster regions, cluster regions are digested by proteasomes more intensively and with greater preference for hydrophobic P1 residues, resulting in substantially greater numbers of fragments with the sizes and COOH termini typical of epitopes and their precursors. Indeed, many of these fragments correspond to endogenously processed Nef epitopes and/or their potential precursors. The results suggest that differential proteasomal processing contributes importantly to the clustering of CTL epitopes in hydrophobic regions.

Concomitant differentiation and partial proteasome inhibition trigger apoptosis in neuroblastoma cells

Proteasome activity is essential during cAMP-induced terminal differentiation of a murine neuroblastoma cell line (NBP2). However, the mechanisms through which proteasome affects NBP2 differentiation have not been characterized. We hypothesized that proteasome is required to implement the differentiation-mediated effects on cell cycle, and its partial inhibition during differentiation may have adverse consequences. Here we show that partial inhibition of proteasome during cAMP-induced differentiation of NBP2 cells causes apoptosis. Whereas differentiation induced growth arrest at G1 phase, partial proteasome inhibition during differentiation resulted in the accumulation of cells at G2M phase. Cell cycle data correlated with the level of cyclin-dependent kinase inhibitors p21WAF and p27Kip1, and cyclin A. While the level of p21 and p27 increased, the level of cyclin A decreased upon differentiation. In contrast, cells treated with proteasome inhibitor in the presence of cAMP-inducing agents showed increased levels of p21 and cyclin A early in the course of differentiation. However, the level of p21 and p27, but not cyclin A, decreased later during concomitant differentiation and partial proteasome inhibition when cells were undergoing apoptosis. Our data suggest that differentiation-mediated growth arrest is dependent on the temporal activity of cell cycle proteins. Partial inhibition of proteasome interferes with differentiation events partly by stabilizing cell cycle proteins and this triggers apoptosis. Thus, differentiating drugs combined with partial proteasome inhibition may impart higher therapeutic efficacy than differentiating agents alone for the treatment of neuroblastoma tumors.

1. Overview of the immune response

Host defense against pathogenic microbes requires dramatically different responses, depending on the character of the pathogen and on the tissue under attack. Central to the immune system's ability to mobilize a response to an invading pathogen is its ability to distinguish self from nonself. The host has evolved both innate and adaptive mechanisms to respond to and eliminate pathogenic microbes. Both of these mechanisms include self-nonself discrimination. This overview describes key mechanisms used by the immune system to respond to invading microbes and identifies settings in which disturbed immune function exacerbates tissue injury.

Recombinant antibodies with MHC-restricted, peptide-specific, T-cell receptor-like specificity: new tools to study antigen presentation and TCR-peptide-MHC interactions

The advent in recent years of the application of tetrameric arrays of class I peptide-MHC complexes now enables us to detect and study rare populations of antigen-specific CD8+ T cells. However, available methods cannot visualize or determine the number and distribution of these TCR ligands on individual cells or detect antigen-presenting cells (APCs) in tissues. Here we describe a new approach that enables study of human class I peptide-MHC ligand-presentation as well as TCR-peptide-MHC interactions. Such studies are facilitated by applying novel tools in the form of peptide-specific, HLA-A2-restricted human recombinant antibodies directed toward a large variety of tumor-associated as well as viral T-cell epitope peptides. Using a large human antibody phage display library, a large panel of recombinant antibodies that are specific for a particular peptide-MHC class I complex in a peptide-dependent, MHC-restricted manner was isolated. These antibodies were used to directly visualize the specific MHC-peptide complex on tumor cells, antigen-presenting cells or virus-infected cells by flow cytometry. They enabled direct quantitation of the number of MHC-peptide complexes as well as in situ detection of the complex on the surface of APCs after naturally occurring active intracellular processing of the cognate antigen. These studies will enable also the development of a new class of targeting molecules to deliver drugs or toxins to tumor or virus-infected cells. Thus, we demonstrate our ability to transform the unique fine specificity but low intrinsic affinity of TCRs into high-affinity soluble antibody molecules endowed with a TCR-like specificity toward human tumor or viral epitopes. These molecules may prove to be crucial useful tools for studying MHC class I antigen presentation in health and disease as well as for therapeutic purposes in cancer, infectious diseases and autoimmune disorders.