Testing the Role of gp96 as Peptide Chaperone in Antigen Processing

Cancer neoepitopes viewed through negative selection and peripheral tolerance: a new path to cancer vaccines

A proportion of somatic mutations in tumors create neoepitopes that can prime T cell responses that target the MHC I-neoepitope complexes on tumor cells, mediating tumor control or rejection. Despite the compelling centrality of neoepitopes to cancer immunity, we know remarkably little about what constitutes a neoepitope that can mediate tumor control in vivo and what distinguishes such a neoepitope from the vast majority of similar candidate neoepitopes that are inefficacious in vivo. Studies in mice as well as clinical trials have begun to reveal the unexpected paradoxes in this area. Because cancer neoepitopes straddle that ambiguous ground between self and non-self, some rules that are fundamental to immunology of frankly non-self antigens, such as viral or model antigens, do not appear to apply to neoepitopes. Because neoepitopes are so similar to self-epitopes, with only small changes that render them non-self, immune response to them is regulated at least partially the way immune response to self is regulated. Therefore, neoepitopes are viewed and understood here through the clarifying lens of negative thymic selection. Here, the emergent questions in the biology and clinical applications of neoepitopes are discussed critically and a mechanistic and testable framework that explains the complexity and translational potential of these wonderful antigens is proposed.

Expression of CLAVATA3 fusions indicates rapid intracellular processing and a role of ERAD

The 12 amino acid peptide derived from the Arabidopsis soluble secretory protein CLAVATA3 (CLV3) acts at the cell surface in a signalling system that regulates the size of apical meristems. The subcellular pathway involved in releasing the peptide from its precursor is unknown. We show that a CLV3-GFP fusion expressed in transfected tobacco protoplasts or transgenic tobacco plants has very short intracellular half-life that cannot be extended by the secretory traffic inhibitors brefeldin A and wortmannin. The fusion is biologically active, since the incubation medium of protoplasts from CLV3-GFP-expressing tobacco contains the CLV3 peptide and inhibits root growth. The rapid disappearance of intact CLV3-GFP requires the signal peptide and is inhibited by the proteasome inhibitor MG132 or coexpression with a mutated CDC48 that inhibits endoplasmic reticulum-associated protein degradation (ERAD). The synthesis of CLV3-GFP is specifically supported by the endoplasmic reticulum chaperone endoplasmin in an in vivo assay. Our results indicate that processing of CLV3 starts intracellularly in an early compartment of the secretory pathway and that ERAD could play a regulatory or direct role in the active peptide synthesis.

Immunosurveillance of cancer and the heat shock protein-CD91 pathway

The intracellular functions of heat shock proteins (HSPs) as chaperones of macromolecules are well known. Current observations point to a role of these chaperones in initiating and modulating immune responses to tumors via receptor(s) on dendritic cells. In this article we provide an insight into, and a basis for, the importance of these HSP-mediated immune responses in rejecting nascent and emerging tumors.

Biotechnology approaches to produce potent, self-adjuvanting antigen-adjuvant fusion protein subunit vaccines

Traditional vaccination approaches (e.g. live attenuated or killed microorganisms) are among the most effective means to prevent the spread of infectious diseases. These approaches, nevertheless, have failed to yield successful vaccines against many important pathogens. To overcome this problem, methods have been developed to identify microbial components, against which protective immune responses can be elicited. Subunit antigens identified by these approaches enable the production of defined vaccines, with improved safety profiles. However, they are generally poorly immunogenic, necessitating their administration with potent immunostimulatory adjuvants. Since few safe and effective adjuvants are currently used in vaccines approved for human use, with those available displaying poor potency, or an inability to stimulate the types of immune responses required for vaccines against specific diseases (e.g. cytotoxic lymphocytes (CTLs) to treat cancers), the development of new vaccines will be aided by the availability of characterized platforms of new adjuvants, improving our capacity to rationally select adjuvants for different applications. One such approach, involves the addition of microbial components (pathogen-associated molecular patterns; PAMPs), that can stimulate strong immune responses, into subunit vaccine formulations. The conjugation of PAMPs to subunit antigens provides a means to greatly increase vaccine potency, by targeting immunostimulation and antigen to the same antigen presenting cell. Thus, methods that enable the efficient, and inexpensive production of antigen-adjuvant fusions represent an exciting mean to improve immunity towards subunit antigens. Herein we review four protein-based adjuvants (flagellin, bacterial lipoproteins, the extra domain A of fibronectin (EDA), and heat shock proteins (Hsps)), which can be genetically fused to antigens to enable recombinant production of antigen-adjuvant fusion proteins, with a focus on their mechanisms of action, structural or sequence requirements for activity, sequence modifications to enhance their activity or simplify production, adverse effects, and examples of vaccines in preclinical or human clinical trials.

Diverse HLA-I Peptide Repertoires of the APC Lines MUTZ3-Derived Immature and Mature Dendritic Cells and THP1-Derived Macrophages

Dendritic cells (DCs) and macrophages are specialized APCs that process and present self-Ags for induction of tolerance and foreign Ags to initiate T cell-mediated immunity. Related to differentiation states they have specific phenotypes and functions. However, the impact of these differentiations on Ag processing and presentation remains poorly defined. To gain insight into this, we analyzed and compared the HLA-I peptidomes of MUTZ3-derived human immature and mature DC lines and THP1-derived macrophages by liquid chromatography tandem mass spectrometry. We found that the HLA-I peptidomes were heterogeneous and individualized and were dominated by nonapeptides with similar HLA-I binding affinities and anchor residues. MUTZ3-derived DCs and THP1-derived macrophages were able to sample peptides from source proteins of almost all subcellular locations and were involved in various cellular functions in similar proportion, with preference to proteins involved in cell communication, signal transduction, protein metabolism, and transcription factor/regulator activity.

HSP90 and Immune Modulation in Cancer

Heat-shock protein 90 (HSP90) is a highly conserved molecular chaperone that plays prominent functional roles in nearly all aspects of cell biology. As a chaperone, it interacts with literally hundreds of "clients," many of which are important drivers, regulators, and promoters of cancer. Thus, HSP90 is a high-value target in the development of anticancer therapeutics. Despite its popularity, our overall knowledge of HSP90 in immune function has lagged behind its well-recognized tumor-supportive roles. The use of inhibitors of HSP90 as chemical biological probes has been invaluable in revealing important roles for the chaperone in multiple aspects of immune function. Given this critical link, we must now consider the question of how immune outcomes may be affected by the HSP90 inhibitors currently in clinical development for the treatment of cancer. This chapter will review some of the immunological aspects of HSP90 function in terms of its intracellular and extracellular roles in antigen presentation, immune effector cell tasks, and regulation of inflammatory processes. This review will further examine the value of HSP90 inhibitors within the context of cancer immunotherapy and will discuss how these drugs might be optimally utilized in combination with immune stimulatory approaches against cancer.

Endotoxin-minimized HIV-1 p24 fused to murine hsp70 activates dendritic cells, facilitates endocytosis and p24-specific Th1 response in mice

Heat shock proteins hsp70 and gp96 have been confirmed as adjuvants enabling induction of cell- and antibody-mediated immunity specific to associated protein or peptide antigens due to the activation of naive dendritic cells and supporting cross-presentation of associated antigen. An efficacious vaccine preventing HIV-1 infection should induce (1) antibodies neutralizing HIV-1 Env protein, preventing virus spreading and (2) CD4(+) Th1 and CD8(+) T cells specific to viral proteins, especially gag p24, important for elimination of HIV-1 infected cells. As p24 is relatively poorly recognized by dendritic cells, its targeting to DC is important for enhancement of vaccine efficacy. In this study, a p24 protein fused to the C- or N-terminus of murine hsp70 was produced as a recombinant protein and administered without any adjuvant to experimental BALB/c mice. Consequently, p24-specific cellular and humoral immune responses were measured. To minimize the effect of bacterial endotoxin, each protein was subjected to a repeated endotoxin phase extraction until each preparation contained less than 2.5 endotoxin unit (EU) per mg of antigen. In addition, endocytosis of p24 fused to hsp70 by dendritic cells and their activation were characterized. The fusion to hsp70 protein enhanced endocytosis of p24 as well as activation of dendritic cells in vitro. After immunization of mice, hsp70-p24 fusion protein induced the strongest p24-specific CD4(+) and CD8(+) T cells (IFN-γ production) and humoral (IgG2b) responses corresponding to Th1 type dominance, whereas p24-hsp70 or p24 itself induced weaker responses.

Functions of heat shock proteins in pathways of the innate and adaptive immune system

For more than 50 years, heat shock proteins (HSPs) have been studied for their role in protecting cells from elevated temperature and other forms of stress. More recently, several roles have been ascribed to HSPs in the immune system. These include intracellular roles in Ag presentation and expression of innate receptors, as well as extracellular roles in tumor immunosurveillance and autoimmunity. Exogenously administered HSPs can elicit a variety of immune responses that have been used in immunotherapy of cancer, infectious diseases, and autoimmune disease.

Modulated electro-hyperthermia enhances dendritic cell therapy through an abscopal effect in mice

The aim of this study was to assess whether modulated electro-hyperthermia (mEHT) can induce an abscopal effect and thereby enhance the antitumor effects of immunotherapy. We used an intratumoral dendritic cell (DC) injection and mEHT to treat C3H/He mice inoculated with squamous cell carcinoma SCCVII cells in the left leg, and we assessed the whole body antitumor effects. Tumors were examined every two or three days in order to assess growth inhibition. The tumor-draining lymph nodes were removed to enable flow cytometric analysis of CD3+ and CD8+ cells, whereas immunohistochemistry was used to assess CD8, S100 and Foxp3 expression in the tumors. Additionally, GP96 expression in the tumors from the different treatment groups was measured. In the control group, the mean tumor volume was larger than that in other groups. These results indicated that the combination therapy of an intratumoral DC injection and mEHT evoked systemic antitumor activity. A larger number of CD3+ and CD8+ cells were detected by flow cytometric analysis in the DC plus mEHT treatment group. Tumor tissue immunostaining showed that CD8 and S100 were more strongly expressed in the DC plus mEHT treatment group, although Foxp3 expression was much higher in the control group. The GP96 gene expression level in the mEHT group was significantly different from the expression level in the control group. An abscopal effect may be induced by mEHT, and the effect of immunotherapy with DCs was strongly enhanced by the overexpression of GP96. GP96 is thought to be one of the molecules explaining the abscopal effect. Direct intratumoral administration of DCs and mEHT may be a feasible future treatment strategy.

Profiling antibody responses to infections by Chlamydia abortus enables identification of potential virulence factors and candidates for serodiagnosis

Enzootic abortion of ewes (EAE) due to infection with the obligate intracellular pathogen Chlamydia (C.) abortus is an important zoonosis leading to considerable economic loss to agriculture worldwide. The pathogen can be transmitted to humans and may lead to serious infection in pregnant women. Knowledge about epidemiology, clinical course and transmission to humans is hampered by the lack of reliable diagnostic tools. Immunoreactive proteins, which are expressed in infected animals and humans, may serve as novel candidates for diagnostic marker proteins and represent putative virulence factors. In order to broaden the spectrum of immunogenic C. abortus proteins we applied 2D immunoblot analysis and screening of an expression library using human and animal sera. We have identified 48 immunoreactive proteins representing potential diagnostic markers and also putative virulence factors, such as CAB080 (homologue of the “macrophage infectivity potentiator”, MIP), CAB167 (homologue of the “translocated actin recruitment protein”, TARP), CAB712 (homologue of the “chlamydial protease-like activity factor”, CPAF), CAB776 (homologue of the “Polymorphic membrane protein D”, PmpD), and the “hypothetical proteins” CAB063, CAB408 and CAB821, which are predicted to be type III secreted. We selected two putative virulence factors for further characterization, i.e. CAB080 (cMIP) and CAB063, and studied their expression profiles at transcript and protein levels. Analysis of the subcellular localization of both proteins throughout the developmental cycle revealed CAB063 being the first C. abortus protein shown to be translocated to the host cell nucleus.

The 'peptidome' of tumour-derived chaperone-rich cell lysate anti-cancer vaccines reveals potential tumour antigens that stimulate tumour immunity

Tumour-derived chaperone-rich cell lysate (CRCL) when isolated from tumour tissue or when embedded with peptide antigens is a potent anti-cancer vaccine consisting of numerous chaperone/heat shock proteins, including the highly immunogenic Hsp70, Hsp90, glucose regulated protein 94, and calreticulin. We have previously documented that CRCL provides both a source of tumour antigens and danger signals triggering antigen presenting cell activation. In this report we describe the 'peptidome' of potential antigens extracted from CRCL prepared from a murine tumour. Using mass spectrometry techniques we identify almost 60 different proteins of origin for the CRCL peptides; we determine that the parental proteins come from essentially all parts of the cell, and are involved in a broad range of functions. Further in silico analysis demonstrates that the parental proteins are components of major signalling networks of vital importance for cancer cell survival, proliferation, and migration. In many instances the peptides identified possess amino acid sequences that would allow their putative binding and display by murine major histocompatibility complex class I and II molecules, and there are also predicted binding motifs for Hsp70-type chaperones. By mixing fractionated pools of peptides with antigen-free (normal liver) CRCL, we were able to reconstitute effective anti-tumour activity of the vaccine, showing that the peptides are indeed the major purveyors of CRCL vaccines' efficacy.

TAT-mediated gp96 transduction to APCs enhances gp96-induced antiviral and antitumor T cell responses

The heat shock protein gp96 is an adjuvant that can elicit T cell responses against cancer and infectious diseases, via antigen presentation, in both rodent models and clinical trials. Its uptake and internalization into antigen presenting cells (APCs) is a critical step in gp96-mediated immune responses. This study examined strategies to improve the cell internalization and T cell activation of gp96. It was found that recombinant fusion with the cell-penetrating peptide TAT (trans-activator of transcription) slightly decreased the aggregation level of gp96 and significantly increased its internalization into macrophages. Furthermore, immunization with the TAT-gp96 fusion dramatically enhanced gp96-mediated hepatitis B virus (HBV)-specific T cell responses and its antiviral efficiency in HBV transgenic mice compared to rgp96. In addition, the inclusion of TAT significantly improved the antitumor T cell immune response to a gp96 vaccine in the B16 melanoma model. These results provide evidence that the efficient transduction of gp96 into APCs can significantly enhance the outcome of gp96-based immunotherapy, and therefore provide a basis for more efficient approaches to improving the immunoregulatory and adjuvant functions of this unique T cell adjuvant.

High immunogenicity of the human leukocyte antigen peptidomes of melanoma tumor cells

Human leukocyte antigens (HLA) bind peptides generated by limited proteolysis in cells and present them at the cell surfaces for recognition by T cells. Through this antigen presentation function they control the specificity of T cell responses and thereby adaptive immune responses. Knowledge of HLA-bound peptides is thus key to understanding adaptive immunity and to the development of vaccines and other specific immune intervention strategies. To gain insight into the antigenicity of melanomas, peptides were extracted from HLA isolated from the tumor cells, separated by two-dimensional HPLC, and sequenced by mass spectrometry. The spectra were analyzed by database-dependent MASCOT searches and database-independent de novo sequencing and, where required, confirmed with synthetic peptides, which were also used to determine their immunogenicity. Comparing four different melanoma cell lines, little overlap of the HLA-bound peptides was found, suggesting a high degree of individualization of the HLA peptidomes. This notwithstanding, the peptidomes were highly immunogenic in the patients from whom the tumor cells had been established and in unrelated patients. This broad cross-patient immunogenicity was only exceptionally related to individual peptides. The majority of the identified epitopes were derived from low to medium abundance proteins, mostly involved in sensitive cellular processes such as cell cycle control, DNA replication, control of gene expression, tumor suppressor function, and protein metabolism. The peptidomes thus provide insights into processes potentially related to tumorigenesis. Furthermore, analyses of the peptide sequences yield information on the specificity of peptide selection by HLA applicable to the developing prediction algorithms for T cell epitopes.

Active-specific immunotherapy of human cancers with the heat shock protein Gp96-revisited

The passive administration of specific antibodies that selectively target tumors is a well-known strategy in cancer treatment. Active immunotherapy using peptide vaccines, in contrast, is expected to induce specific, cytolytic T cells in the patient, which react against tumor antigens and destroy malignant cells. Although several concepts exist, the identification and low immunogenicity of tumor-specific peptides remain a serious problem. Heat shock proteins (HSPs), notably glycoprotein (Gp) 96, are of special interest, because they are able to take molecular peptide-fingerprints of the protein array characteristic for a particular cell. Association of Gp96 with peptides has been shown to be essential for crosspresentation and activation of T cells. Consequently, Gp96-peptide complexes extracted from cancer cells harbor the tumor-specific peptides and are immunogenic, thus offering a tool for active immunization against the tumor. Already, several immunotherapy studies of human cancers have been carried out, showing no severe adverse effects but unfortunately only limited improvement in the clinical outcome. Vitespen, a commercial HSP-peptide complex vaccine based on tumor-derived Gp96, seems to induce an improved overall survival for subsets of early stage melanoma and kidney cancer patients. The limited access to vaccine material derived from the autologous tumor requires the development of alternative protocols. Moreover, counteracting immunosuppressive mechanisms induced by the malignancy might further improve the efficacy of vaccinations. This review critically analyzes the current state of clinical immunotherapy with Gp96, with special attention to Vitespen.

Redox-regulated peptide transfer from the transporter associated with antigen processing to major histocompatibility complex class I molecules by protein disulfide isomerase

Most antigenic peptides are generated by proteasomes in the cytosol and are transported by the transporter associated with antigen processing (TAP) into the endoplasmic reticulum, where they bind with nascent major histocompatibilitiy complex class I molecule (MHC-I). Although the overall process of peptide-MHC-I complex assembly is well studied, the mechanism by which free peptides are delivered from TAP to MHC-I is unknown. In this study, we investigated the possible role of protein disulfide isomerase (PDI) as a peptide carrier between TAP and MHC-I. Analysis of PDI-peptide complexes reconstituted in vitro showed that PDI exhibits some degree of specificity for peptides corresponding to antigenic ligands of various human leukocyte antigen (HLA) alleles. Mutations of either anchor residues of the peptide ligand or the peptide-binding site of PDI inhibited the PDI-peptide interaction. The PDI-peptide interaction increased under reducing conditions, whereas binding of the peptide to PDI decreased under oxidizing conditions. TAP-associated PDI was predominantly present in the reduced form, whereas the MHC-I-associated PDI was present in the oxidized form. Further, upon binding of optimal peptides, PDI was released from TAP and sequentially associated with HLA-A2.1. Our data revealed a redox-regulated chaperone function of PDI in delivering antigenic peptides from TAP to MHC-I.

Re-examination of CD91 function in GRP94 (glycoprotein 96) surface binding, uptake, and peptide cross-presentation

GRP94 (gp96)-peptide complexes can be internalized by APCs and their associated peptides cross-presented to yield activation of CD8(+) T cells. Investigations into the identity (or identities) of GRP94 surface receptors have yielded conflicting results, particularly with respect to CD91 (LRP1), which has been proposed to be essential for GRP94 recognition and uptake. To assess CD91 function in GRP94 surface binding and endocytosis, these parameters were examined in mouse embryonic fibroblast (MEF) cell lines whose expression of CD91 was either reduced via RNA interference or eliminated by genetic disruption of the CD91 locus. Reduction or loss of CD91 expression abrogated the binding and uptake of receptor-associated protein, an established CD91 ligand. Surface binding and uptake of an N-terminal domain of GRP94 (GRP94.NTD) was unaffected. GRP94.NTD surface binding was markedly suppressed after treatment of MEF cell lines with heparin, sodium chlorate, or heparinase II, demonstrating that heparin sulfate proteoglycans can function in GRP94.NTD surface binding. The role of CD91 in the cross-presentation of GRP94-associated peptides was examined in the DC2.4 dendritic cell line. In DC2.4 cells, which express CD91, GRP94.NTD-peptide cross-presentation was insensitive to the CD91 ligands receptor-associated protein or activated α(2)-macroglobulin and occurred primarily via a fluid-phase, rather than receptor-mediated, uptake pathway. These data clarify conflicting data on CD91 function in GRP94 surface binding, endocytosis, and peptide cross-presentation and identify a role for heparin sulfate proteoglycans in GRP94 surface binding.

Caught with their PAMPs down? The extracellular signalling actions of molecular chaperones are not due to microbial contaminants

In recent years, it has been hypothesised that a new signalling system may exist in vertebrates in which secreted molecular chaperones form a dynamic continuum between the cellular stress response and corresponding homeostatic physiological mechanisms. This hypothesis seems to be supported by the finding that many molecular chaperones are released from cells and act as extracellular signals for a range of cells. However, this nascent field of biological research seems to suffer from an excessive criticism that the biological activities of molecular chaperones are due to undefined components of the microbial expression hosts used to generate recombinant versions of these proteins. In this article, a number of the proponents of the cell signalling actions of molecular chaperones take this criticism head-on. They show that sufficient evidence exists to support fully the hypothesis that molecular chaperones have cell-cell signalling actions that are likely to be part of the homeostatic mechanism of the vertebrate.

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.

Efficient cross-priming of antiviral CD8+ T cells by antigen donor cells is GRP94 independent

Cross-priming, the activation of naive CD8+ T cells by dendritic cells presenting Ags synthesized by other cells, is believed to play an important role in the generation of antiviral and antitumor responses. The molecular mechanism(s) underlying cross-priming remain poorly defined and highly controversial. GRP94 (gp96), an abundant endoplasmic reticulum chaperone with innate immune-activating capacity, has been widely reported to play a major role in cross-priming. In this study, we show that cells whose expression of GRP94 is silenced via transient or stable transfection with GRP94-directed small interfering RNAs demonstrate no reduction in their abilities to generate class I peptide complexes in cultured cells or to prime antiviral CD8+ T cell responses in vivo. In demonstrating the dispensability of GRP94, our finding points to the importance of alternative mechanisms for generation of class I peptide complexes from endogenous and exogenous Ags and immunogens.

Identification and evaluation of a combination of chlamydial antigens to support the diagnosis of severe and invasive Chlamydia trachomatis infections

Chlamydia trachomatis is the most common sexually transmitted organism in industrialized countries. Nucleic acid amplification testing, using non-invasively collected specimens, is considered to be the method of choice for diagnosis of chlamydial infections of the urethra and the lower genital tract. Serological testing has the potential to circumvent the problem of specimen sampling in invasive C. trachomatis infections of the upper genital tract. However, only a few defined chlamydial antigens have been used in a standardized diagnostic assay format. In this study, we used serological two-dimensional proteomic analysis to broaden the spectrum of diagnostically relevant C. trachomatis proteins. The genes encoding an assortment of already known chlamydial antigens, as well as immunogenic proteins that have not been described before, were cloned, and the recombinant proteins were purified in order to compare their diagnostic usefulness in parallel with a newly developed line immunoassay. With 189 sera collected from patients with and without C. trachomatis infection, recombinant major outer membrane protein (MOMP), chlamydial protease-like activity factor (CPAF), outer membrane protein 2 (OMP2), translocated actin-recruiting protein, and polymorphic membrane protein D (PmpD) showed the highest level of diagnostic sensitivity and specificity. In patients suffering from ascending and invasive C. trachomatis infections, such as pelvic inflammatory disease and lymphogranuloma venereum, the sensitivity reached with these proteins ranged between 71% (PmpD) and 94% (OMP2), and the specificity ranged between 82% (PmpD) and 100% (MOMP and OMP2). Recombinant thio-specific antioxidant peroxidase, ribosomal protein S1 (RpsA) and hypothetical protein 17 showed lower sensitivity but comparably high specificity, ranging from 94% to 100%. The novel line immunoassay based on defined recombinant antigens has promise for improved serodiagnosis in severe and invasive C. trachomatis infections.

Heat Shock Protein Vaccines: From Bench to Bedside

Heat shock proteins (HSPs) are immunogenic, with the specificity of the immune response provided by the peptides that they chaperone. Binding of cell surface receptors by HSPs is central to the elicitation of the innate and adaptive immune responses obtained after vaccination and also plays a physiologic role in cross-priming. These effects of HSPs have been exploited in prophylaxis and therapy of cancer and infectious disease. The data obtained from murine studies have been translated into ongoing clinical trials of cancer of which the most recent results are provided here.

Tumor-induced suppression of CTL expansion and subjugation by gp96-Ig vaccination

Established tumors suppress antitumor immune responses and induce tolerance by incompletely characterized mechanisms, and this phenomenon is an important barrier to tumor immunotherapy. Single vaccination with tumor cells expressing gp96-Ig stimulates robust expansion of tumor-specific CTLs in tumor-naïve mice and this expansion is inhibited by established tumors. Interestingly, frequent vaccinations restore antitumor immune responses in the presence of established tumors. Syngeneic EG7 tumor-bearing mice have heterogeneous responses to frequent vaccination with EG7-gp96-Ig, with 32% complete responders and 68% partial responders. Comparison of responders to nonresponders revealed an inverse correlation between tumor-specific CTL expansion in the peripheral blood and tumor size. To identify immune cells and molecules associated with effective antitumor immune responses, reverse transcription-PCR arrays were performed using cells isolated from the vaccination site. ELISAs, cellular phenotyping, and tumor immunohistochemistry were also performed comparing vaccine responders to nonresponders. These data show that up-regulation of T-bet, RORgammat, IFNgamma, CCL8, CXCL9, and CXCL10 at the vaccination site are associated with vaccine-induced antitumor immunity. These data correlate with increased CTL expansion in the peripheral blood of responders, increased infiltration of responder tumors by CD8+ cells and interleukin-17+ cells, and decreased infiltration of responder tumors by CD11b+Gr-1+ cells and FoxP3+ cells. Furthermore, serum ELISAs revealed a significant elevation of transforming growth factor-beta in nonresponders as compared with responders. Interestingly, CD8+ T cells isolated from responders and nonresponders have equivalent cytotoxic activity in vitro. Taken together, our data suggest that established tumors may escape immunosurveillance by preventing clonal expansion of tumor-specific CTL without inducing anergy.

Heat-shock proteins as powerful weapons in vaccine development

Heat-shock proteins (HSPs) have been known as multifunctional proteins. They facilitate the folding and unfolding of proteins, participate in vesicular transport processes, prevent protein aggregation in the densely packed cytosol and are involved in signaling processes. HSPs have been involved in different fields, including autoimmunity, immunity to infections and tumor immunology. Although there are many different kinds of HSPs, only some HSPs, including HSP70 and Gp96, have immunological properties. HSP molecules have been applied into DNA- or protein (peptide)-based vaccines as antigens, chaperones or adjuvants. HSP-based vaccines have been shown to immunize against cancer and infectious diseases in both prophylactic and therapeutic protocols. The immunogenicity of HSPs results from two different properties: a peptide-dependent capacity to chaperone and elicit adaptive cytotoxic T-lymphocyte responses against antigenic peptides and a peptide-independent immunomodulatory capacity. Furthermore, HSPs could be immunoregulatory agents with potent and widely applicable therapeutic uses. Accordingly, certain HSPs, such as HSP70 and Gp96, are highly effective carrier molecules for cross-presentation. Their ability in eliciting immune responses against different pathogens (parasite and virus) and their role in cancer immunity will be discussed in this review.

Phylogenetic conservation of glycoprotein 96 ability to interact with CD91 and facilitate antigen cross-presentation

Although the ability of gp96 to activate APCs and generate CD8 CTLs against peptides they chaperone through interaction with the endocytic receptors CD91 is supported by solid evidence, its biological relevance in immune surveillance is debated. We have used an evolutionary approach to determine whether gp96 interacts with receptors expressed on APCs and promotes MHC class I cross-presentation of minor histocompatibility Ags (H-Ags) to CTLs in the frog Xenopus. We show that in Xenopus gp96 binds the CD91 homolog at the surface of peritoneal leukocytes, and that this binding is inhibited by molar excess of unlabeled gp96 or the CD91 ligand alpha2-macroglobulin, by anti-CD91 Ab and by the specific CD91 antagonist receptor-associated protein. Surface binding followed by internalization of gp96 was confirmed by fluorescent microscopy. Furthermore, adoptive transfer of peritoneal leukocytes pulsed with as little as 800 ng of gp96 chaperoning minor H-Ags, but not minor H-Ag-free gp96, induces potent CD8 T cell infiltration and Ag-specific accelerated rejection of minor H-locus disparate skin grafts. Inhibition of gp96-CD91 interaction by pretreatment with anti-CD91 Ab and receptor-associated protein impairs both CD8 T cell infiltration and acute skin graft rejection. These data provide evidence of the conserved ability of gp96 to facilitate cross-presentation of chaperoned Ags by interacting with CD91. The persistence of this biological process for >350 million years that separate mammals and amphibians from a common ancestor strongly supports the proposition that gp96 and CD91 are critically involved in immune surveillance.

The exception that reinforces the rule: crosspriming by cytosolic peptides that escape degradation

The nature of crosspriming immunogens for CD8(+) T cell responses is highly controversial. By using a panel of T cell receptor-like antibodies specific for viral peptides bound to mouse D(b) major histocompatibility complex class I molecules, we show that an exceptional peptide (PA(224-233)) expressed as a viral minigene product formed a sizeable cytosolic pool continuously presented for hours after protein synthesis was inhibited. PA(224-233) pool formation required active cytosolic heat-shock protein 90 but not ER g96 and uniquely enabled crosspriming by this peptide. These findings demonstrate that exceptional class I binding oligopeptides that escape proteolytic degradation are potent crosspriming agents. Thus, the feeble immunogenicity of natural proteasome products in crosspriming can be attributed to their evanescence in donor cells and not an absolute inability of cytosolic oligopeptides to be transferred to and presented by professional antigen-presenting cells.

Secreted heat shock protein gp96-Ig: an innovative vaccine approach

Heat shock proteins (HSPs) are a large family of proteins with different molecular weights and different intracellular localizations. These proteins undertake crucial functions in maintaining cell homeostasis, and therefore they have been conserved during evolution. HSP gp96 also known as glucose-regulated protein grp94, is the primary chaperone of the endoplasmatic reticulum. Gp96/grp94, because of its peptide chaperone capacity and its ability to interact actively with professional antigen-presenting cells (APCs), is also endowed with crucial immunological functions such as natural adjuvant for priming innate and adaptive immunity. To make gp96 accessible to the immune system without biochemical purification and without cell lysis, we generated a secreted form of gp96. The immunological properties of secreted gp96 and its implications for vaccine in human cancer and infectious diseases will be discussed.

Enhanced immunogenicity of HPV16E7 accompanied by Gp96 as an adjuvant in two vaccination strategies

Human papillomavirus, particularly type 16 (HPV16) is present in more than 99% of cervical cancers. E7 is the major oncogenic protein produced in cervical cancer-associated HPV16. An efficient vaccine against viral infection requires induction of strong humoral and cellular responses against viral proteins. Heat shock proteins (HSPs) like Gp96 have been described as potent tumor vaccines in animal models and are currently studied in human clinical trials. In this study, we investigated the utility of HPV16 E7 along with Gp96 as an adjuvant in C57BL/6 mice model. We compared the level of humoral and cellular immune responses by E7+Gp96 co-injection as DNA/DNA and prime-boost (DNA/protein) immunization strategies. In prime-boost immunization strategies, we first immunized C57BL/6 mice with the complete open-reading frame of E7 and Gp96 (pcDNA-E7 and pcDNA-Gp96) and then boosted with rE7, rNT-gp96 (N-terminal extension of Gp96) and rCT-gp96 (C-terminal extension of Gp96) mixed with Montanide 720 in different formulations. The humoral immune responses against rE7 and the different truncated forms of rGp96 suggested a mixed Th1/Th2 response with high intensity toward Th2. Assessment of lymphoproliferative and cytokine responses against rE7 and the different fragments of Gp96, showed that DNA vaccination including E7 and Gp96 induced Th1 response. We concluded that co-delivery of naked DNA E7+Gp96 plasmid was immunologically more effective than E7 alone. Our study demonstrated that co-delivery of E7+Gp96 as DNA/DNA and E7+CT-gp96 as DNA/protein could be an effective approach to induce E7-specific immune responses as a potential vaccine candidate for cervical cancer.

Heat-shock protein 90 associates with N-terminal extended peptides and is required for direct and indirect antigen presentation

CD8(+) T cells recognize peptide fragments of endogenously synthesized antigens of cancers or viruses, presented by MHC I molecules. Such antigen presentation requires the generation of peptides in the cytosol, their passage to the endoplasmic reticulum, loading of MHC I with peptides, and transport of MHC I-peptide complexes to the cell surface. Heat-shock protein (hsp) 90 is a cytosolic chaperone known to associate with peptide and peptide precursors of MHC I epitopes. We report here that treatment of cells with hsp90 inhibitors leads to generation of "empty" MHC I caused by inhibited loading of MHC I with peptides. Inhibition of hsp90 does not inhibit synthesis of MHC I, nor does it affect the activity of proteasomes. Hsp90-inhibited cells, such as proteasome-inhibited cells, are poor stimulators of T lymphocytes. The role of hsp90 in presentation of an ovalbumin epitope is shown to be at a postproteasomal step: hsp90 associates with N-terminally extended precursors of the SIINFEHL epitope, and such peptides are depleted from hsp90 preparations in hsp90-inhibited cells. Inhibition of hsp90 in the antigen donor cell compromises their ability to cross-prime. Conversely, stressed cells expressing elevated hsp90 levels show a heat-shock factor-dependent, enhanced ability to cross-prime. These results demonstrate a substantial role for hsp90 in chaperoning of antigenic peptides in direct and indirect presentation. The introduction of a stress-inducible component in these pathways has significant implications for their modulation during fever and infection.

Allogeneic tumor-cell-based vaccines secreting endoplasmic reticulum chaperone gp96

Heat-shock proteins are chaperones for proteins including tumor antigens. Heat-shock protein gp96, also known as glucose-regulated protein grp94, is the primary chaperone of the endoplasmic reticulum and a natural adjuvant for priming the innate and adaptive immune system. By transfecting tumor cells with a genetically modified secretory form of gp96, the tumor cells are transformed into vaccine cells. Gp96 vaccines in murine studies trigger robust innate and antigen-specific cellular immune responses and cause tumor rejection followed by long-lasting tumor immunity. The authors briefly review here the generation of cytotoxic T lymphocyte responses by gp96 and the most up to date clinical data in the use of gp96-based cancer vaccines.

Heat-shock protein–peptide complex-96 for the treatment of cancer

Heat-shock proteins (HSPs) are the most abundant and ubiquitous soluble intracellular proteins. Members of the HSP family bind peptides, including antigenic peptides generated within cells. HSPs also interact with antigen-presenting cells (APCs) through CD91 and other receptors, eliciting a cascade of events that includes representation of HSP-chaperoned peptides MHC, translocation of NF-kappaB into the nuclei, and maturation of dendritic cells. These consequences point to a key role of HSPs in fundamental immunologic phenomena such as activation of APCs, indirect presentation (or crosspriming) of antigenic peptides, and chaperoning of peptides during antigen presentation. The properties of HSPs also allow them to be used for immunotherapy of cancers and infections in novel ways. This paper reviews the development and clinical trial progress of vitespen, an HSP peptide complex vaccine based on tumor-derived glycoprotein 96.

Therapeutic vaccination for the treatment of malignant melanoma

With increasing knowledge of tumor-associated antigens and T cell epitopes, and the mechanisms of induction and regulation of T-cellular immune responses, therapeutic vaccination is increasingly being explored as a treatment option for cancer. Several clinical cancer vaccination trials, the majority of them with melanoma patients, have demonstrated efficient induction of tumor-specific cellular immune responses in patients. However, these immune responses, in most cases, do not translate into clinical responses. The clinical response rates in these trials are relatively low. The most likely causes for the lack of correlation of immunological and clinical responsiveness are loss of antigenicity and immune suppression. Nonetheless, many patients in the vaccination trials have experienced extended survival compared to clinical experience. Therapeutic vaccination thus appears suited for maintenance therapy where cure is not possible and is an interesting option for adjuvant therapy after surgical tumor resection. While the clinical efficacy of vaccination is expected to be better for early-stage cancer, advancement of the treatment of advanced-stage disease will require combination with other therapeutic principles.

Autologous renal cell cancer vaccines using heat shock protein-peptide complexes

Investigations into the role of heat shock proteins (HSPs) in immune response have progressed well into a third decade, and indications of their use for the treatment of renal cell carcinoma (RCC) in the adjuvant setting will be revealed in the near future when a randomized phase III clinical trial is completed. Additional ongoing and planned randomized clinical trials will test the efficacy of HSP-based vaccines in more advanced stages of RCC. This review describes the compelling scientific rationale behind testing HSPs in RCC against the backdrop of other immunotherapeutic approaches in this indication.

The N-terminal fragment of GRP94 is sufficient for peptide presentation via professional antigen-presenting cells

The chaperone glucose-regulated protein 94 (GRP94) has long been used to augment peptide presentation to T cells. This chaperone binds antigenic peptides, binds to receptors on professional antigen-presenting cells (APCs), activates these cells and after internalization, transfers the peptides to MHC class I for activation of T cells. Here we show that all these activities reside within amino acids 1-355 of GRP94. This small fragment is sufficient to bind peptides, to bind and be taken up by the receptors CD91 and scavenger receptor type A on either dendritic cells or macrophages. The minimal construct can augment peptide presentation in culture and induce antigen-specific CTL in naive mice only because it loads APCs with the relevant peptide. Thus, the sequence 1-355 is the immunologically sufficient module of GRP94 and we propose that this 'mini-chaperone' can be used in immunotherapy of tumors and vaccine development.

Analysis of purified gp96 preparations from rat and mouse livers using 2-D gel electrophoresis and tandem mass spectrometry

The stress protein gp96 exhibits a number of immunological activities, the majority of studies into which have used gp96 purified from a variety of tissues. On the basis of 1-D gel electrophoresis, the purity of these preparations has been reported to range between 70% and 99%. This study analyzed gp96 preparations from rat and mouse livers using 2-D gel electrophoresis and liquid chromatography electrospray ionization tandem mass spectrometry (MS-MS). The procedure for purifying gp96 was reproducible, as similar protein profiles were observed in replicate gels of gp96 preparations. The purity of the preparations was typically around 70%, with minor co-purified proteins of varying molecular weights and mobilities being present. Dominant bands at 95-100 kDa in preparations from Wistar rats and C57BL/6 mice were identified as gp96 by ECL Western blotting. Multiple bands having similar, yet distinct molecular weights and differing pI mobility on ECL Western blots were confirmed as being gp96 in preparations from Wistar rats using MS-MS. The most striking feature of the 2-D gel analysis was the presence of additional dominant bands at 55 kDa in preparations from Wistar rats, and at 75-90 kDa in preparations from C57BL/6 mice. These were identified as gp96 by ECL Western blotting and, in the case of preparations from Wistar rats, by MS-MS. Although the lower molecular weight, gp96-related molecules might be partially degraded gp96, their reproducible presence, definition and characteristics suggest that they are alternative, species-specific isoforms of the molecule. A 55 kDa protein which exhibited a lower pI value than gp96 was present in all preparations and this was identified as calreticulin, another putative immunoregulatory molecule. This study confirms the reproducibility of the gp96 purification protocol and reveals the presence of multiple gp96 isoforms, some of which likely result from post-translational modifications such as differential glycosylation and phosphorylation.

Molecular chaperones and cancer immunotherapy

As one of the most abundant and evolutionally conserved intracellular proteins, heat shock proteins, also known as stress proteins or molecular chaperones, perform critical functions in maintaining cell homeostasis under physiological as well as stress conditions. Certain chaperones in extracellular milieu are also capable of modulating innate and adaptive immunity due to their ability to chaperone polypeptides and to interact with the host's immune system, particularly professional antigen-presenting cells. The immunomodulating properties of chaperones have been exploited for cancer immunotherapy. Clinical trials using chaperone-based vaccines to treat various malignancies are ongoing.

Binding of the viral immunogenic octapeptide VSV8 to native glucose-regulated protein Grp94 (gp96) and its inhibition by the physiological ligands ATP and Ca2+

The molecular chaperone Grp94 (gp96) of the endoplasmic reticulum (ER) lumen plays an essential role in the structural maturation and/or secretion of proteins destined for transport to the cell surface. Its proposed role in binding and transferring peptides for immune recognition is, however, controversial. Using SPR spectroscopy, we studied the interaction of native glycosylated Grp94 at neutral pH and 25 and 37 degrees C with the viral immunogenic octapeptide RGYVYQGL (VSV8), derived from vesicular stomatitis virus nucleoprotein (52-59). The peptide binds reversibly with low affinity ([A]0.5 approximately 640 microM) and a hyperbolic binding isotherm, and the binding is partially inhibited by ATP and Ca2+ at concentrations that are present in the ER lumen, and the effects are explained by conformational changes in the native chaperone induced by these ligands. Our data present experimental support for the recent proposal that, under native conditions, VSV8 binds to Grp94 by an adsorptive, rather than a bioselective, mechanism, and thus further challenge the proposed in vivo peptide acceptor-donor function of the chaperone in the context of antigen-presenting cell activation.

The seven dirty little secrets of major histocompatibility complex class I antigen processing

Every field has its dirty little secrets (DLSs): assumptions based on flimsy evidence, findings that directly contradict prevailing models or so beg comprehension that they cannot even seed reasonable alternative hypotheses. Although our natural tendency is to hug these DLSs, they should be exposed, for it is these gaps in our understanding that point to the path to enlightenment. Here, I discuss some of the DLSs of major histocompatibility complex class I antigen processing.