Endotoxin-free heat-shock protein 70 fails to induce APC activation

Diversity of extracellular HSP70 in cancer: advancing from a molecular biomarker to a novel therapeutic target

Heat shock protein 70 (HSP70) is a highly conserved protein functioning as a "molecular chaperone", which is integral to protein folding and maturation. In addition to its high expression within cells upon stressful challenges, HSP70 can be translocated to the cell membrane or released from cells in free form or within extracellular vesicles (EVs). Such trafficking of HSP70 is also present in cancer cells, as HSP70 is overexpressed in various types of patient samples across a range of common malignancies, signifying that extracellular HSP70 (eHSP70) can serve as a tumor biomarker. eHSP70 is involved in a broad range of cancer-related events, including cell proliferation and apoptosis, extracellular matrix (ECM) remodeling, epithelial-mesenchymal transition (EMT), angiogenesis, and immune response. eHSP70 can also induce cancer cell resistance to various treatments, such as chemotherapy, radiotherapy, and anti-programmed death-1 (PD-1) immunotherapy. Though the role of eHSP70 in tumors is contradictory, characterized by both pro-tumor and anti-tumor effects, eHSP70 serves as a promising target in cancer treatment. In this review, we comprehensively summarized the current knowledge about the role of eHSP70 in cancer progression and treatment resistance and discussed the feasibility of eHSP70 as a cancer biomarker and therapeutic target.

The dangerous liaisons in innate immunity involving recombinant proteins and endotoxins: Examples from the literature and the Leptospira field

Endotoxins, also known as lipopolysaccharides (LPS), are essential components of cell walls of diderm bacteria such as Escherichia coli. LPS are microbe-associated molecular patterns that can activate pattern recognition receptors. While trying to investigate the interactions between proteins and host innate immunity, some studies using recombinant proteins expressed in E. coli reported interaction and activation of immune cells. Here, we set out to provide information on endotoxins that are highly toxic to humans and bind to numerous molecules, including recombinant proteins. We begin by outlining the history of the discovery of endotoxins, their receptors and the associated signaling pathways that confer extreme sensitivity to immune cells, acting alone or in synergy with other microbe-associated molecular patterns. We list the various places where endotoxins have been found. Additionally, we warn against the risk of data misinterpretation due to endotoxin contamination in recombinant proteins, which is difficult to estimate with the Limulus amebocyte lysate assay, and cannot be completely neutralized (e.g., treatment with polymyxin B or heating). We further illustrate our point with examples of recombinant heat-shock proteins and viral proteins from severe acute respiratory syndrome coronavirus 2, dengue and HIV, for which endotoxin contamination has eventually been shown to be responsible for the inflammatory roles previously ascribed. We also critically appraised studies on recombinant Leptospira proteins regarding their putative inflammatory roles. Finally, to avoid these issues, we propose alternatives to express recombinant proteins in nonmicrobial systems. Microbiologists wishing to undertake innate immunity studies with their favorite pathogens should be aware of these difficulties.

The role of molecular chaperones in the mechanisms of epileptogenesis

Epilepsy is a group of neurological diseases which requires significant economic costs for the treatment and care of patients. The central point of epileptogenesis stems from the failure of synaptic signal transmission mechanisms, leading to excessive synchronous excitation of neurons and characteristic epileptic electroencephalogram activity, in typical cases being manifested as seizures and loss of consciousness. The causes of epilepsy are extremely diverse, which is one of the reasons for the complexity of selecting a treatment regimen for each individual case and the high frequency of pharmacoresistant cases. Therefore, the search for new drugs and methods of epilepsy treatment requires an advanced study of the molecular mechanisms of epileptogenesis. In this regard, the investigation of molecular chaperones as potential mediators of epileptogenesis seems promising because the chaperones are involved in the processing and regulation of the activity of many key proteins directly responsible for the generation of abnormal neuronal excitation in epilepsy. In this review, we try to systematize current data on the role of molecular chaperones in epileptogenesis and discuss the prospects for the use of chemical modulators of various chaperone groups' activity as promising antiepileptic drugs.

MyD88-dependent Toll-like receptor 2 signaling modulates macrophage activation on lysate-adsorbed Teflon™ AF surfaces in an in vitro biomaterial host response model

The adsorbed protein layer on an implanted biomaterial surface is known to mediate downstream cell-material interactions that drive the host response. While the adsorption of plasma-derived proteins has been studied extensively, the adsorption of damage-associated molecular patterns (DAMPs) derived from damaged cells and matrix surrounding the implant remains poorly understood. Previously, our group developed a DAMP-adsorption model in which 3T3 fibroblast lysates were used as a complex source of cell-derived DAMPs and we demonstrated that biomaterials with adsorbed lysate potently activated RAW-Blue macrophages via Toll-like receptor 2 (TLR2). In the present study, we characterized the response of mouse bone marrow derived macrophages (BMDM) from wildtype (WT), TLR2-/- and MyD88-/- mice on Teflon™ AF surfaces pre-adsorbed with 10% plasma or lysate-spiked plasma (10% w/w total protein from 3T3 fibroblast lysate) for 24 hours. WT BMDM cultured on adsorbates derived from 10% lysate in plasma had significantly higher gene and protein expression of IL-1β, IL-6, TNF-α, IL-10, RANTES/CCL5 and CXCL1/KC, compared to 10% plasma-adsorbed surfaces. Furthermore, the upregulation of pro-inflammatory cytokine and chemokine expression in the 10% lysate in plasma condition was attenuated in TLR2-/- and MyD88-/- BMDM. Proteomic analysis of the adsorbed protein layers showed that even this relatively small addition of lysate-derived proteins within plasma (10% w/w) caused a significant change to the adsorbed protein profile. The 10% plasma condition had fibrinogen, albumin, apolipoproteins, complement, and fibronectin among the top 25 most abundant proteins. While proteins layers generated from 10% lysate in plasma retained fibrinogen and fibronectin among the top 25 proteins, there was a disproportionate increase in intracellular proteins, including histones, tubulins, actins, and vimentin. Furthermore, we identified 7 DAMPs or DAMP-related proteins enriched in the 10% plasma condition (fibrinogen, apolipoproteins), compared to 39 DAMPs enriched in the 10% lysate in plasma condition, including high mobility group box 1 and histones. Together, these findings indicate that DAMPs and other intracellular proteins readily adsorb to biomaterial surfaces in competition with plasma proteins, and that adsorbed DAMPs induce an inflammatory response in adherent macrophages that is mediated by the MyD88-dependent TLR2 signaling pathway.

γδ T cell costimulatory ligands in antitumor immunity

Antitumor immunity relies on the ability of T cells to recognize and kill tumor targets. γδ T cells are a specialized subset of T cells that predominantly localizes to non-lymphoid tissue such as the skin, gut, and lung where they are actively involved in tumor immunosurveillance. γδ T cells respond to self-stress ligands that are increased on many tumor cells, and these interactions provide costimulatory signals that promote their activation and cytotoxicity. This review will cover costimulatory molecules that are known to be critical for the function of γδ T cells with a specific focus on mouse dendritic epidermal T cells (DETC). DETC are a prototypic tissue-resident γδ T cell population with known roles in antitumor immunity and are therefore useful for identifying mechanisms that may control activation of other γδ T cell subsets within non-lymphoid tissues. This review concludes with a brief discussion on how γδ T cell costimulatory molecules can be targeted for improved cancer immunotherapy.

A Highly Sensitive Cell-Based TLR Reporter Platform for the Specific Detection of Bacterial TLR Ligands

Toll-like receptors (TLRs) are primary pattern recognition receptors (PRRs), which recognize conserved microbial components. They play important roles in innate immunity but also in the initiation of adaptive immune responses. Impurities containing TLR ligands are a frequent problem in research but also for the production of therapeutics since TLR ligands can exert strong immunomodulatory properties even in minute amounts. Consequently, there is a need for sensitive tools to detect TLR ligands with high sensitivity and specificity. Here we describe the development of a platform based on a highly sensitive NF-κB::eGFP reporter Jurkat JE6-1 T cell line for the detection of TLR ligands. Ectopic expression of TLRs and their coreceptors and CRISPR/Cas9-mediated deletion of endogenously expressed TLRs was deployed to generate reporter cell lines selectively expressing functional human TLR2/1, TLR2/6, TLR4 or TLR5 complexes. Using well-defined agonists for the respective TLR complexes we could demonstrate high specificity and sensitivity of the individual reporter lines. The limit of detection for LPS was below 1 pg/mL and ligands for TLR2/1 (Pam3CSK4), TLR2/6 (Fsl-1) and TLR5 (flagellin) were detected at concentrations as low as 1.0 ng/mL, 0.2 ng/mL and 10 pg/mL, respectively. We showed that the JE6-1 TLR reporter cells have the utility to characterize different commercially available TLR ligands as well as more complex samples like bacterially expressed proteins or allergen extracts. Impurities in preparations of microbial compounds as well as the lack of specificity of detection systems can lead to erroneous results and currently there is no consensus regarding the involvement of TLRs in the recognition of several molecules with proposed immunostimulatory functions. This reporter system represents a highly suitable tool for the definition of structural requirements for agonists of distinct TLR complexes.

Hspa8 and ICAM-1 as damage-induced mediators of γδ T cell activation

Tissue-resident γδ T cells form the first line of defense at barrier surfaces where they survey host tissue for signs of stress or damage. Following recognition of injury, γδ T cells play a crucial role in the wound-healing response through the production of growth factors and cytokines that promote proliferation in surrounding epithelial cells. To initiate this response, γδ T cells require interactions with a variety of epithelial-expressed costimulatory molecules in addition to primary signaling through their TCR. In the epidermis these signals include the coxsackie and adenovirus receptor (CAR), histocompatibility antigen 60c (H60c), and plexin B2, which interact with γδ T cell-expressed junctional adhesion molecule-like protein (JAML), NKG2D, and CD100, respectively. Here we identify heat shock protein family A member 8 (Hspa8) and ICAM-1 as two additional keratinocyte-expressed costimulatory molecules for epidermal resident γδ T cells (termed DETC). These molecules were rapidly up-regulated in the epidermis following wounding in both mouse and human tissue. Both Hspa8 and ICAM-1 had a costimulatory effect on DETC, inducing proliferation, CD25 up-regulation, and IL-2 production. We also provide evidence that DETC can be activated through the potential ICAM-1 and Hspa8 receptors LFA-1 and CD316. Finally, knockdown of Hspa8 in keratinocytes reduced their ability to activate DETC in culture and ICAM-1-/- mice exhibited impaired rates of healing in skin-organ culture suggesting a role for these proteins in the DETC-mediated damage response. Together with previous work on CAR, H60c, and plexin B2, these results add to a picture of a complex keratinocyte wound signature that is required for efficient DETC activation.

The Role of Extracellular HSP70 in the Function of Tumor-Associated Immune Cells

Simple Summary

The intracellular heat shock protein 70 (HSP70) is essential for cells to respond to stress, for instance, by refolding damaged proteins or inhibiting apoptosis. However, in cancer, HSP70 is overexpressed and can translocate to the extracellular milieu, where it emerged as an important modulator of tumor-associated immune cells. By targeting the tumor microenvironment (TME) through different mechanisms, extracellular HSP70 can trigger pro- or anti-tumorigenic responses. Therefore, understanding the pathways and their consequences is crucial for therapeutically targeting cancer and its surrounding microenvironment. In this review, we summarize current knowledge on the translocation of extracellular HSP70. We further elucidate its functions within the TME and provide an overview of potential therapeutic options.

Abstract

Extracellular vesicles released by tumor cells (T-EVs) are known to contain danger-associated molecular patterns (DAMPs), which are released in response to cellular stress to alert the immune system to the dangerous cell. Part of this defense mechanism is the heat shock protein 70 (HSP70), and HSP70-positive T-EVs are known to trigger anti-tumor immune responses. Moreover, extracellular HSP70 acts as an immunogen that contributes to the cross-presentation of major histocompatibility complex (MHC) class I molecules. However, the release of DAMPs, including HSP70, may also induce chronic inflammation or suppress immune cell activity, promoting tumor growth. Here, we summarize the current knowledge on soluble, membrane-bound, and EV-associated HSP70 regarding their functions in regulating tumor-associated immune cells in the tumor microenvironment. The molecular mechanisms involved in the translocation of HSP70 to the plasma membrane of tumor cells and its release via exosomes or soluble proteins are summarized. Furthermore, perspectives for immunotherapies aimed to target HSP70 and its receptors for cancer treatment are discussed and presented.

Role of a Heat Shock Transcription Factor and the Major Heat Shock Protein Hsp70 in Memory Formation and Neuroprotection

Heat shock proteins (Hsps) represent the most evolutionarily ancient, conserved, and universal system for protecting cells and the whole body from various types of stress. Among Hsps, the group of proteins with a molecular weight of 70 kDa (Hsp70) plays a particularly important role. These proteins are molecular chaperones that restore the native conformation of partially denatured proteins after exposure to proteotoxic forms of stress and are critical for the folding and intracellular trafficking of de novo synthesized proteins under normal conditions. Hsp70s are expressed at high levels in the central nervous system (CNS) of various animals and protect neurons from various types of stress, including heat shock, hypoxia, and toxins. Numerous molecular and behavioral studies have indicated that Hsp70s expressed in the CNS are important for memory formation. These proteins contribute to the folding and transport of synaptic proteins, modulate signaling cascades associated with synaptic activation, and participate in mechanisms of neurotransmitter release. In addition, HSF1, a transcription factor that is activated under stress conditions and mediates Hsps transcription, is also involved in the transcription of genes encoding many synaptic proteins, whose levels are increased in neurons under stress and during memory formation. Thus, stress activates the molecular mechanisms of memory formation, thereby allowing animals to better remember and later avoid potentially dangerous stimuli. Finally, Hsp70 has significant protective potential in neurodegenerative diseases. Increasing the level of endogenous Hsp70 synthesis or injecting exogenous Hsp70 reduces neurodegeneration, stimulates neurogenesis, and restores memory in animal models of ischemia and Alzheimer’s disease. These findings allow us to consider recombinant Hsp70 and/or Hsp70 pharmacological inducers as potential drugs for use in the treatment of ischemic injury and neurodegenerative disorders.

The Effect of a Unique Region of Parvovirus B19 Capsid Protein VP1 on Endothelial Cells

Parvovirus B19 (B19V) is a widespread human pathogen possessing a high tropism for erythroid precursor cells. However, the persistence or active replication of B19V in endothelial cells (EC) has been detected in diverse human pathologies. The VP1 unique region (VP1u) of the viral capsid has been reported to act as a major determinant of viral tropism for erythroid precursor cells. Nevertheless, the interaction of VP1u with EC has not been studied. We demonstrate that recombinant VP1u is efficiently internalized by rats’ pulmonary trunk blood vessel-derived EC in vitro compared to the human umbilical vein EC line. The exposure to VP1u was not acutely cytotoxic to either human- or rat-derived ECs, but led to the upregulation of cellular stress signaling-related pathways. Our data suggest that high levels of circulating B19V during acute infection can cause endothelial damage, even without active replication or direct internalization into the cells.

Transcriptome analysis reveals temperature-dependent early immune response in flounder (Paralichthys olivaceus) after Hirame novirhabdovirus (HIRRV) infection

Hirame novirhabdovirus (HIRRV), as a highly pathogenic fish virus, is frequently prevalent in a variety of aquaculture fish in recent years, which seriously threatens the healthy development of aquaculture industry. Epidemiological studies show that the outbreak of HIRRV is obviously temperature dependent. Virus proliferation experiments in vitro and in vivo at different temperatures indicate the antiviral response of flounder is a main reason affect the replication of HIRRV. The RNA-Seq was used to analyze the different antiviral response in flounder which infected with HIRRV at different temperatures, the experiment set two temperatures of 10 °C and 20 °C. The flounder infected with HIRRV by artificial injection, and the spleens were collected at 24 h after infection. Meanwhile, the fish injected with EPC supernatant at different temperatures were set as control groups. It can obtain four pairwise comparison groups if determine a single variable, and the differentially expressed genes were further selected. The results showed that after infection with HIRRV at 10 °C and 20 °C, the differentially expressed genes in spleen were significantly enriched in inflammatory and immune-related pathways like Arachidonic acid metabolism, Cytokine-cytokine receptor interaction, Toll-like receptor (TLR) signaling pathway, RIG-I-like receptor (RLRs) signaling pathway, NOD-like receptor (NLR) signaling pathway and Cytosolic DNA-sensing pathway etc. In addition, the expression of phagocytes, lysosomes, endocytosis related genes were significantly upregulated at high temperature whether HIRRV positive or not. But compared to the infected flounder at 10 °C, some genes of RLRs signaling pathway were significantly upregulated at 20 °C, it can be speculated that RLRs pathway may be related to the anti-HIRRV response of flounder. Therefore, key genes of RLRs signaling pathway including mda5, lgp2, mita, mavs, irf3, irf7, ifn I-3 and ifn-γ were selected, and the temporal expression patterns of these genes in infected flounder at different temperatures were further detected by qRT-PCR. The results showed that HIRRV infection can significantly stimulate and activate the RLRs pathway of flounder, and the response level of this pathway was significantly higher at 20 °C than 10 °C. In general, this study provides important data for the further study about the pathogenesis of HIRRV infection in flounder.

Hsp70 in cancer: A double agent in the battle between survival and death

The heat shock protein (Hsps) superfamily, also known as molecular chaperones, are highly conserved and present in all living organisms and play vital roles in protein fate. The HspA1A (Hsp70-1), called Hsp70 in this review, is expressed at low or undetectable levels in most unstressed normal cells, but numerous studies have shown that diverse types of tumor cells express Hsp70 at the plasma membrane that leads to resistance to programmed cell death and tumor progression. Hsp70 is released into the extracellular milieu in three forms including free soluble, complexed with cancer antigenic peptides, and exosome forms. Therefore, it seems to be a promising therapeutic target in human malignancies. However, a great number of studies have indicated that both intracellular and extracellular Hsp70 have a dual function. A line of evidence presented that intracellular Hsp70 has a cytoprotective function via suppression of apoptosis and lysosomal cell death (LCD) as well as that extracellular Hsp70 can promote tumorigenesis and angiogenesis. Other evidence showed intracellular Hsp70 can promote apoptosis and membrane-associated/extracellular Hsp70 can elicit antitumor innate and adaptive immune responses. Given the contradictory functions, as a "double agent," could Hsp70 be a promising tool in the future of targeted cancer therapies? To answer this question, in this review, we will discuss the functions of Hsp70 in cancers besides inhibition and stimulation strategies for targeting Hsp70 along with their challenges.

Advances in the Development of Anticancer HSP-based Vaccines

Current advances in cancer treatment are based on the recent discoveries of molecular mechanisms of tumour maintenance. It was shown that heat shock proteins (HSPs) play a crucial role in the development of immune response against tumours. Thus, HSPs represent multifunctional agents not only with chaperone functions, but also possessing immunomodulatory properties. These properties are exploited for the development of HSP-based anticancer vaccines aimed to induce cytotoxic responses against tumours. To date, a number of strategies have been suggested to facilitate HSP-based vaccine production and to increase its effectiveness. The present review focuses on the current trend for the development of HSPbased vaccines aimed at inducing strong immunological tumour-specific responses against cancer cells of distinct etiology and localization.

Heat Shock Proteins Can Be Surrogate Autoantigens for Induction of Antigen Specific Therapeutic Tolerance in Rheumatoid Arthritis

Technologies that enable induction of therapeutic tolerance may revolutionize the treatment of autoimmune diseases by their supposed potential to induce drug-free and lasting disease remission. In combination with diagnostic tests that screen for individuals at risk, these approaches may offer chances to halt disease before serious damage in the tissues can occur. In fact, for healthy individuals at risk, this could lead to a preventive form of vaccination. For therapeutic tolerance to re-instate natural self-tolerance it seems essential to induce tolerance for the critical autoantigens involved in disease. However, for most autoimmune diseases such antigens are poorly defined. This is the case for both disease inciting autoantigens and antigens that become involved through epitope spreading. A possible source of surrogate auto-antigens expressed in tissues during inflammation are heat shock proteins (HSP) or stress proteins. In this mini-review we discuss unique characteristics of HSP which provide them with the capacity to inhibit inflammatory processes. Various studies have shown that epitopes of HSP60 and HSP70 molecules can function as vaccines to downregulate a variety of autoimmune inflammatory diseases. Currently, several research groups are developing cell therapies with the intention to reach therapeutic tolerance. In this review, in which we are proposing to ex vivo load tolerant dendritic cells with a Treg inducing HSP70 derived peptide called B29, we are discussing the chances to develop this as an autologous tolDC therapeutic tolerance therapy for rheumatoid arthritis.

The Biologics Revolution and Endotoxin Test Concerns

The advent of “at will” production of biologics in lieu of harvesting animal proteins (i.e. insulin) or human cadaver proteins (i.e. growth hormone) has revolutionized the treatment of disease. While the fruits of the biotechnology revolution are widely acknowledged, the realization of the differences in the means of production and changes in the manner of control of potential impurities and contaminants in regard to the new versus the old are less widely appreciated. This chapter is an overview of the biologics revolution in terms of the rigors of manufacturing required to produce them, their mechanism of action, and caveats of endotoxin control. It is a continulation of the previous chapter that established a basic background knowledge of adaptive immune principles necessary to understand the mode of action of both disease causation and biologics therapeutic treatment via immune modulation.

Heat Shock Proteins as Immunomodulants

Heat shock proteins (Hsps) are conserved molecules whose main role is to facilitate folding of other proteins. Most Hsps are generally stress-inducible as they play a particularly important cytoprotective role in cells exposed to stressful conditions. Initially, Hsps were generally thought to occur intracellulary. However, recent work has shown that some Hsps are secreted to the cell exterior particularly in response to stress. For this reason, they are generally regarded as danger signaling biomarkers. In this way, they prompt the immune system to react to prevailing adverse cellular conditions. For example, their enhanced secretion by cancer cells facilitate targeting of these cells by natural killer cells. Notably, Hsps are implicated in both pro-inflammatory and anti-inflammatory responses. Their effects on immune cells depends on a number of aspects such as concentration of the respective Hsp species. In addition, various Hsp species exert unique effects on immune cells. Because of their conservation, Hsps are implicated in auto-immune diseases. Here we discuss the various metabolic pathways in which various Hsps manifest immune modulation. In addition, we discuss possible experimental variations that may account for contradictory reports on the immunomodulatory function of some Hsps.

Heat shock proteins and cardiovascular disease

The development of stress drives a host of biological responses that include the overproduction of a family of proteins named heat shock proteins (HSPs), because they were initially studied after heat exposure. HSPs are evolutionarily preserved proteins with a high degree of interspecies homology. HSPs are intracellular proteins that also have extracellular expression. The primary role of HSPs is to protect cell function by preventing irreversible protein damage and facilitating molecular traffic through intracellular pathways. However, in addition to their chaperone role, HSPs are immunodominant molecules that stimulate natural as well as disease-related immune reactivity. The latter may be a consequence of molecular mimicry, generating cross-reactivity between human HSPs and the HSPs of infectious agents. Autoimmune reactivity driven by HSPs could also be the result of enhancement of the immune response to peptides generated during cellular injury and of their role in the delivery of peptides to the major histocompatibility complex in antigen-presenting cells. In humans, HSPs have been found to participate in the pathogenesis of a large number of diseases. This review is focused on the role of HSPs in atherosclerosis and essential hypertension.

The role of autoimmune reactivity induced by heat shock protein 70 in the pathogenesis of essential hypertension

Autoimmunity is increasingly recognized as having a central role in essential hypertension. Heat shock proteins (HSPs) are immunodominant molecules with high interspecies homology and autoimmune reactivity directed against HSP70 may play a role in the pathogenesis of hypertension. Autoimmunity to HSP70 may result from molecular mimicry between human HSP and bacterial HSP or, alternatively, as a response to HSP70-peptide complexes generated during cellular stress and delivered to the major histocompatibility complex by antigen-presenting cells. HSP70 is increased in the circulation and kidney of hypertensive patients, and genetic polymorphisms of HSP70 are associated with essential hypertension. Depending on the route and conditions of administration, HSP70 may induce or suppress immune-related inflammation. Renal inflammation induced by immunity to HSP70 causes hypertension in laboratory animals, and administration of specific peptide sequences of HSP70 results in a protective anti-inflammatory response that prevents and corrects salt-induced hypertension. Potential therapeutic uses of HSP70 in essential hypertension deserve to be investigated. LINKED ARTICLES: This article is part of a themed section on Immune Targets in Hypertension. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.12/issuetoc.

The Enigma of Heat Shock Proteins in Immune Tolerance

The fundamental problem of autoimmune diseases is the failure of the immune system to downregulate its own potentially dangerous cells, which leads to destruction of tissue expressing the relevant autoantigens. Current immunosuppressive therapies offer relief but fail to restore the basic condition of self-tolerance. They do not induce long-term physiological regulation resulting in medication-free disease remissions. Heat shock proteins (HSPs) have shown to possess the capacity of inducing lasting protective immune responses in models of experimental autoimmune diseases. Especially mycobacterial HSP60 and HSP70 were shown to induce disease inhibitory IL-10-producing regulatory T cells in many different models. This in itself may seem enigmatic, since based on earlier studies, HSPs were also coined sometimes as pro-inflammatory damage-associated molecular patterns. First clinical trials with HSPs in rheumatoid arthritis and type I diabetes have also indicated their potential to restore tolerance in autoimmune diseases. Data obtained from the models have suggested three aspects of HSP as being critical for this tolerance promoting potential: 1. evolutionary conservation, 2. most frequent cytosolic/nuclear MHC class II natural ligand source, and 3. upregulation under (inflammatory) stress. The combination of these three aspects, which are each relatively unique for HSP, may provide an explanation for the enigmatic immune tolerance promoting potential of HSP.

T Cell-Mediated Chronic Inflammatory Diseases Are Candidates for Therapeutic Tolerance Induction with Heat Shock Proteins

Failing immunological tolerance for critical self-antigens is the problem underlying most chronic inflammatory diseases of humans. Despite the success of novel immunosuppressive biological drugs, the so-called biologics, in the treatment of diseases such rheumatoid arthritis (RA) and type 1 diabetes, none of these approaches does lead to a permanent state of medicine free disease remission. Therefore, there is a need for therapies that restore physiological mechanisms of self-tolerance. Heat shock proteins (HSPs) have shown disease suppressive activities in many models of experimental autoimmune diseases through the induction of regulatory T cells (Tregs). Also in first clinical trials with HSP-based peptides in RA and diabetes, the induction of Tregs was noted. Due to their exceptionally high degree of evolutionary conservation, HSP protein sequences (peptides) are shared between the microbiota-associated bacterial species and the self-HSP in the tissues. Therefore, Treg mechanisms, such as those induced and maintained by gut mucosal tolerance for the microbiota, can play a role by targeting the more conserved HSP peptide sequences in the inflamed tissues. In addition, the stress upregulated presence of HSP in these tissues may well assist the targeting of the HSP induced Treg specifically to the sites of inflammation.

Unconventional Secretion of Heat Shock Proteins in Cancer

Heat shock proteins (HSPs) are abundant cellular proteins involved with protein homeostasis. They have both constitutive and inducible isoforms, whose expression levels are further increased by stress conditions, such as temperature elevation, reduced oxygen levels, infection, inflammation and exposure to toxic substances. In these situations, HSPs exert a pivotal role in offering protection, preventing cell death and promoting cell recovery. Although the majority of HSPs functions are exerted in the cytoplasm and organelles, several lines of evidence reveal that HSPs are able to induce cell responses in the extracellular milieu. HSPs do not possess secretion signal peptides, and their secretion was subject to widespread skepticism until the demonstration of the role of unconventional secretion forms such as exosomes. Secretion of HSPs may confer immune system modulation and be a cell-to-cell mediated form of increasing stress resistance. Thus, there is a wide potential for secreted HSPs in resistance of cancer therapy and in the development new therapeutic strategies.

Sterile inflammation and pregnancy complications: a review

Inflammation is essential for successful embryo implantation, pregnancy maintenance and delivery. In the last decade, important advances have been made in regard to endogenous, and therefore non-infectious, initiators of inflammation, which can act through the same receptors as pathogens. These molecules are referred to as damage-associated molecular patterns (DAMPs), and their involvement in reproduction has only recently been unraveled. Even though inflammation is necessary for successful reproduction, untimely activation of inflammatory processes can have devastating effect on pregnancy outcomes. Many DAMPs, such as uric acid, high-mobility group box 1 (HMGB1), interleukin (IL)-1 and cell-free fetal DNA, have been associated with pregnancy complications, such as miscarriages, preeclampsia and preterm birth in preclinical models and in humans. However, the specific contribution of alarmins to these conditions is still under debate, as currently there is lack of information on their mechanism of action. In this review, we discuss the role of sterile inflammation in reproduction, including early implantation and pregnancy complications. Particularly, we focus on major alarmins vastly implicated in numerous sterile inflammatory processes, such as uric acid, HMGB1, IL-1α and cell-free DNA (especially that of fetal origin) while giving an overview of the potential role of other candidate alarmins.

Review: Pathogen-associated molecular pattern contamination as putative endogenous ligands of Toll-like receptors

Extensive work in recent years has suggested that a number of endogenous molecules, their derivatives or degradation products may be potent activators of the innate immune system capable of inducing pro-inflammatory cytokine production by the monocyte—macrophage system and the activation and maturation of dendritic cells. The cytokine-like effects of these endogenous molecules are mediated via Toll-like receptor (TLR) signal transduction pathways in a manner similar to pathogen-associated molecular patterns (PAMPs). However, recent evidence suggests that the reported cytokine effects of some of these putative endogenous ligands are in fact due to contaminating PAMPs. The reasons for the failure to recognize PAMP contaminants being responsible for the putative TLR ligands of these endogenous molecules include: (i) failure to use highly purified preparations free of PAMP contamination; (ii) failure to recognize the heat sensitivity of lipopolysaccharide (LPS); and (iii) failure to consider contaminant(s) other than LPS. Strategies are proposed to avoid future designation of PAMP contamination as putative endogenous ligands of TLRs.

Heat shock proteins and their immunomodulatory role in inflammatory arthritis

Autoimmune diseases, including inflammatory arthritis, are characterized by a loss of self-tolerance, leading to an excessive immune responses and subsequent ongoing inflammation. Current therapies are focused on dampening this inflammation, but a permanent state of tolerance is seldom achieved. Therefore, novel therapies that restore and maintain tolerance are needed. Tregs could be a potential target to achieve permanent immunotolerance. Activation of Tregs can be accomplished when they recognize and bind their specific antigens. HSPs are proteins present in all cells and are upregulated during inflammation. These proteins are immunogenic and can be recognized by Tregs. Several studies in animal models and in human clinical trials have shown the immunoregulatory effects of HSPs and their protective effects in inflammatory arthritis. In this review, an overview is presented of the immunomodulatory effects of several members of the HSP family in general and in inflammatory arthritis. These effects can be attributed to the activation of Tregs through cellular interactions within the immune system. The effect of HSP-specific therapies in patients with inflammatory arthritis should be explored further, especially with regard to long-term efficacy and safety and their use in combination with current therapeutic approaches.

Cloning HSP70 and HSP90 genes of kaluga (Huso dauricus) and the effects of temperature and salinity stress on their gene expression

The genes encoding HSP70 and HSP90 proteins were isolated from kaluga by homologous cloning and rapid amplification of complementary DNA (cDNA) ends (RACE). HSP70 (GenBank accession no. KP050541) and HSP90 (GenBank accession no. KP050542) cDNAs were composed of 2275 and 2718 bp and encoded polypeptides of 650 and 725 amino acids, respectively. Basic Local Alignment Search Tool (BLAST) analysis showed that HSP70 and HSP90 of kaluga shared high identities with those of Acipenser ruthenus, Acipenser schrenckii, and Acipenser baerii (98-99 %). Fluorescent real-time RT-PCR under unstressed conditions revealed that HSP70 and HSP90 were expressed in 11 different tissues of kaluga. Messenger RNA (mRNA) expressions of both HSP70 and HSP90 were highest in the intestine and lowest in the muscle. In addition, the patterns of mRNA expression of HSP70 and HSP90 were similar, although the level of expression was more in HSP90 than in HSP70 (P < 0.05).We also analyzed patterns of HSP70 and HSP90 expression in the muscle, gill, and liver of kaluga under different combinations of temperature and salinity stress, including temperatures of 4,10, 25, and 28 °C at 0 ppt salinity, and salinities of 10, 20, 30, and 40 ppt at 16 °C, where 16 °C at 0 ppt (parts per thousand) served as the control. We found that levels of mRNA expression of both HSP70 and HSP90 were highest at 4 °C in the muscle, gill, and liver and changed little with salinity stress. These results increase understanding of the mechanisms of stress response of cold freshwater fish.

Bordetella pertussis filamentous hemagglutinin itself does not trigger anti-inflammatory interleukin-10 production by human dendritic cells

Filamentous hemagglutinin (FHA) is an important adhesin of the whooping cough agent Bordetella pertussis and is contained in most acellular pertussis vaccines. Recently, FHA was proposed to exert an immunomodulatory activity through induction of tolerogenic IL-10 secretion from dendritic cells. We have re-evaluated the cytokine-inducing activity of FHA, placing specific emphasis on the role of the residual endotoxin contamination of FHA preparations. We show that endotoxin depletion did not affect the capacity of FHA to bind primary human monocyte-derived dendritic cells, while it abrogated the capacity of FHA to elicit TNF-α and IL-10 secretion and strongly reduced its capacity to trigger IL-6 production. The levels of cytokines induced by the different FHA preparations correlated with their residual contents of B. pertussis endotoxin. Moreover, FHA failed to trigger cytokine secretion in the presence of antibodies that block TLR2 and/or TLR4 signaling. The TLR2 signaling capacity appeared to be linked to the presence of endotoxin-associated components in FHA preparations and not to the FHA protein itself. These results show that the endotoxin-depleted FHA protein does not induce cytokine release from human dendritic cells.

Innate immune perturbations, accumulating DAMPs and inflammasome dysregulation: A ticking time bomb in ageing

Ageing has pronounced effects on the immune system, including on innate immune cells. Whilst most studies suggest that total numbers of different innate immune cell populations do not change dramatically during ageing, many of their functions such as phagocytosis, antigen presentation and inflammatory molecule secretion decline. In contrast, many endogenous damage-associated molecular patterns (DAMPs) accumulate during ageing. These include reactive oxygen species (ROS) released from damaged mitochondria, extracellular nucleotides like ATP, high mobility group box (HMGB) 1 protein, oxidized low density lipoprotein, amyloid-beta (Aβ), islet amyloid polypeptide and particulates like monosodium urate (MSU) crystals and cholesterol crystals. Some of these DAMPs trigger the activation of inflammasomes, cytosolic danger sensing signalling platforms that drive both the maturation of specific pro-inflammatory mediators such as IL-1β, as well as the initiation of pro-inflammatory pyroptotic cell death. Herein, we review the evidence that dysregulated inflammasome activation, via altered innate immune cell functions and elevated levels of DAMPs, contributes to the establishment of chronic, low-grade inflammation (characterized by elevated levels of IL-6 and C-reactive protein) and the development of age-related pathological processes.

The danger model: questioning an unconvincing theory

Janeway's pattern recognition theory holds that the immune system detects infection through a limited number of the so-called pattern recognition receptors (PRRs). These receptors bind specific chemical compounds expressed by entire groups of related pathogens, but not by host cells (pathogen-associated molecular patterns (PAMPs). In contrast, Matzinger's danger hypothesis postulates that products released from stressed or damaged cells have a more important role in the activation of immune system than the recognition of nonself. These products, named by analogy to PAMPs as danger-associated molecular patterns (DAMPs), are proposed to act through the same receptors (PRRs) as PAMPs and, consequently, to stimulate largely similar responses. Herein, I review direct and indirect evidence that contradict the widely accepted danger theory, and suggest that it may be false.

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.

Update in pathogenesis and prospective in treatment of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is among the most common and devastating diseases in neonates and, despite the significant advances in neonatal clinical and basic science investigations, its etiology is largely understood, specific treatment strategies are lacking, and morbidity and mortality remain high. Improvements in the understanding of pathogenesis of NEC may have therapeutic consequences. Pharmacologic inhibition of toll-like receptor signaling, the use of novel nutritional strategies, and microflora modulation may represent novel promising approaches to the prevention and treatment of NEC. This review, starting from the recent acquisitions in the pathogenic mechanisms of NEC, focuses on current and possible therapeutic perspectives.

Mycobacterial Hsp65 potentially cross-reacts with autoantibodies of diabetes sera and also induces (in vitro) cytokine responses relevant to diabetes mellitus

Diabetes mellitus is a multifactorial disease and its incidence is increasing worldwide. Among the two types of diabetes, type-2 accounts for about 90% of all diabetic cases, whereas type-1 or juvenile diabetes is less prevalent and presents with humoral immune responses against some of the autoantigens. We attempted to test whether the sera of type-1 diabetes patients cross-react with mycobacterial heat shock protein 65 (Hsp65) due to postulated epitope homologies between mycobacterial Hsp65 and an important autoantigen of type-1 diabetes, glutamic acid decarboxylase-65 (GAD65). In our study, we used either recombinant mycobacterial Hsp65 protein or synthetic peptides corresponding to some of the potential epitopes of mycobacterial Hsp65 that are shared with GAD65 or human Hsp60, and a control peptide sourced from mycobacterial Hsp65 which is not shared with GAD65, Hsp60 and other autoantigens of type-1 diabetes. The indirect ELISA results indicated that both type-1 diabetes and type-2 diabetes sera cross-react with conserved mycobacterial Hsp65 peptides and recombinant mycobacterial Hsp65 protein but do not do so with the control peptide. Our results suggest that cross-reactivity of mycobacterial Hsp65 with autoantibodies of diabetes sera could be due to the presence of significantly conserved peptides between mycobacterial Hsp65 and human Hsp60 rather than between mycobacterial Hsp65 and GAD65. The treatment of human peripheral blood mononuclear cells (PBMCs) with recombinant mycobacterial Hsp65 protein or the synthetic peptides resulted in a significant increase in the secretion of cytokines such as IL-1β, IL-8, IL-6, TNF-α and IL-10. Taken together, these findings point towards a dual role for mycobacterial Hsp65: in inducing autoimmunity and in inflammation, the two cardinal features of diabetes mellitus.

Rice bran feruloylated oligosaccharides activate dendritic cells via Toll-like receptor 2 and 4 signaling

This work presents the effects of feruloylated oligosaccharides (FOs) of rice bran on murine bone marrow-derived dendritic cells (BMDCs) and the potential pathway through which the effects are mediated. We found that FOs induced phenotypic maturation of DCs, as shown by the increased expression of CD40, CD80/CD86 and MHC-I/II molecules. FOs efficiently induced maturation of DCs generated from C3H/HeN or C57BL/6 mice with normal toll-like receptor 4 (TLR-4) or TLR-2 but not DCs from mice with mutated TLR4 or TLR2. The mechanism of action of FOs may be mediated by increased phosphorylation of ERK, p38 and JNK mitogen-activated protein kinase (MAPKs) and increased NF-κB activity, which are important signaling molecules downstream of TLR-4 and TLR-2. These data suggest that FOs induce DCs maturation through TLR-4 and/or TLR-2 and that FOs might have potential efficacy against tumor or virus infection or represent a candidate-adjuvant approach for application in immunotherapy and vaccination.

Innate immunity in systemic sclerosis pathogenesis

The innate immune system is a critical part of the response to pathogens and overall immunity. Compared with the adaptive immune response, these innate responses are not antigen-specific and recognize patterns in bacteria, viruses and fungi. Chief among these are TLRs (Toll-like receptors). TLRs are PRRs (pattern recognition receptors) that are germ-line-encoded and are also able to recognize endogenous molecules that are released upon cell damage or stress and have been demonstrated to have a key role in numerous autoimmune diseases, including RA (rheumatoid arthritis) and SSc (systemic sclerosis). SSc is an autoimmune disorder in which vascular injury occurs and there is a chronic low-grade inflammation followed by excessive ECM (extracellular matrix) deposition and ultimately fibrosis. The fibrosis ultimately leads to organ dysfunction and death. The preceding vascular damage and activation of the innate immune system leads to mobilization of the innate lymphoid cells and the up-regulation of multiple genes and pro-fibrotic cytokines. These locally released cytokines activate resident fibroblasts to differentiate into myofibroblasts. The aim of the present review is to explore the role of the innate immune system in SSc and TLRs and how these interact with stromal cells to produce fibrosis. Targeting the innate immune system or specific components of the TLR signalling cascade may be a novel therapeutic option in what is an incurable disease.

HSPA8/HSC70 chaperone protein: structure, function, and chemical targeting

HSPA8/HSC70 protein is a fascinating chaperone protein. It represents a constitutively expressed, cognate protein of the HSP70 family, which is central in many cellular processes. In particular, its regulatory role in autophagy is decisive. We focused this review on HSC70 structure-function considerations and based on this, we put a particular emphasis on HSC70 targeting by small molecules and peptides in order to develop intervention strategies that deviate some of HSC70 properties for therapeutic purposes. Generating active biomolecules regulating autophagy via its effect on HSC70 can effectively be designed only if we understand the fine relationships between HSC70 structure and functions.

Effects of pathogenic bacterial challenge after acute sublethal ammonia-N exposure on heat shock protein 70 expression in Botia reevesae

The objective of this study was to investigate the effects of pathogenic bacterial challenge after acute sublethal ammonia-N exposure on heat shock protein 70 expression in Botia reevesae. After ammonia-N exposure at a constant concentration of 7.21 ± 0.10 mg L(-1) for 96 h, B. reevesae was challenged with Aeromonas hydrophila. Quantitative PCR analysis showed predominant and significant expression of HSP70 in liver, gill, skin, spleen and kidney (P < 0.05), with significantly upregulated expression of the mRNA transcript in these tissues after sublethal ammonia-N exposure and A. hydrophila challenge. Furthermore, following A. hydrophila challenge after ammonia-N exposure, HSP70 mRNA expression was significantly upregulated in kidney and gill tissues, although its expression levels were significantly lower than those detected following A. hydrophila challenge or ammonia-N exposure individually. These results indicate that B. reevesae HSP70 is involved in resistance to pathogenic bacteria. It is hypothesized that ammonia-N results in the downregulation of HSP70 mRNA in immune organs after an A. hydrophila challenge, thus lowering their resistance to pathogenic stress.

Mycobacterial and mouse HSP70 have immuno-modulatory effects on dendritic cells

Previously, it has been shown that heat shock protein 70 (HSP70) can prevent inflammatory damage in experimental autoimmune disease models. Various possible underlying working mechanisms have been proposed. One possibility is that HSP70 induces a tolerogenic phenotype in dendritic cells (DCs) as a result of the direct interaction of the antigen with the DC. Tolerogenic DCs can induce antigen-specific regulatory T cells and dampen pathogenic T cell responses. We show that treatment of murine DCs with either mycobacterial (Mt) or mouse HSP70 and pulsed with the disease-inducing antigen induced suppression of proteoglycan-induced arthritis (PGIA), although mouse HSP70-treated DCs could ameliorate PGIA to a greater extent. In addition, while murine DCs treated with Mt- or mouse HSP70 had no significantly altered phenotype as compared to untreated DCs, HSP70-treated DCs pulsed with pOVA (ovalbumin peptide 323-339) induced a significantly increased production of IL-10 in pOVA-specific T cells. IL-10-producing T cells were earlier shown to be involved in Mt HSP70-induced suppression of PGIA. In conclusion, this study indicates that Mt- and mouse HSP70-treated BMDC can suppress PGIA via an IL-10-producing T cell-dependent manner.

Innate immune signaling in the pathogenesis of necrotizing enterocolitis

Necrotizing enterocolitis (NEC) is a challenging disease to treat, and caring for patients afflicted by it remains both frustrating and difficult. While NEC may develop quickly and without warning, it may also develop slowly, insidiously, and appear to take the caregiver by surprise. In seeking to understand the molecular and cellular processes that lead to NEC development, we have identified a critical role for the receptor for bacterial lipopolysaccharide (LPS) toll like receptor 4 (TLR4) in the pathogenesis of NEC, as its activation within the intestinal epithelium of the premature infant leads to mucosal injury and reduced epithelial repair. The expression and function of TLR4 were found to be particularly elevated within the intestinal mucosa of the premature as compared with the full-term infant, predisposing to NEC development. Importantly, factors within both the enterocyte itself, such as heat shock protein 70 (Hsp70), and in the extracellular environment, such as amniotic fluid, can curtail the extent of TLR4 signaling and reduce the propensity for NEC development. This review will highlight the critical TLR4-mediated steps that lead to NEC development, with a focus on the proinflammatory responses of TLR4 signaling that have such devastating consequences in the premature host.

The immunosuppressive activity of heat shock protein 70

Heat shock protein 70 (HSP70) has previously been described as a potent antitumour vaccine. The mechanism relied on the ability of tumour derived HSP70 to associate with antigenic peptides, which, when cross presented, elicited a T cell mediated antitumour response. Subsequently, HSP70 was incorrectly described as a potent adjuvant of innate immunity, and although mistakes in the experimental approaches were exposed and associated with endotoxin contamination in the recombinant HSP70 specimen, questions still remain regarding this matter. Here we review only publications that have cautiously addressed the endotoxin contamination problem in HSP70 in order to reveal the real immunological function of the protein. Accordingly, “endotoxin free” HSP70 stimulates macrophages and delivers antigenic peptides to APCs, which effectively prime T cells mediating an antitumour reaction. Conversely, HSP70 has potent anti-inflammatory functions as follows: regulating T cell responses, reducing stimulatory capacity of DCs, and inducing development of immunosuppressive regulatory T cells. These activities were further associated with the immune evasive mechanism of tumours and implicated in the modulation of immune reactivity in autoimmune diseases and transplant-related clinical conditions. Consequently, the role of HSP70 in immune regulation is newly emerging and contrary to what was previously anticipated.

Heat shock protein 70: roles in multiple sclerosis

Heat shock proteins (HSP) have long been considered intracellular chaperones that possess housekeeping and cytoprotective functions. Consequently, HSP overexpression was proposed as a potential therapy for neurodegenerative diseases characterized by the accumulation or aggregation of abnormal proteins. Recently, the discovery that cells release HSP with the capacity to trigger proinflammatory as well as immunoregulatory responses has focused attention on investigating the role of HSP in chronic inflammatory autoimmune diseases such as multiple sclerosis (MS). To date, the most relevant HSP is the inducible Hsp70, which exhibits both cytoprotectant and immunoregulatory functions. Several studies have presented contradictory evidence concerning the involvement of Hsp70 in MS or experimental autoimmune encephalomyelitis (EAE), the MS animal model. In this review, we dissect the functions of Hsp70 and discuss the controversial data concerning the role of Hsp70 in MS and EAE.

The anti-inflammatory mechanisms of Hsp70

Immune responses to heat shock proteins (Hsp) develop in virtually all inflammatory diseases; however, the significance of such responses is only now becoming clear. In experimental disease models, Hsp administration can prevent or arrest inflammatory damage, and in initial clinical trials in patients with chronic inflammatory diseases, Hsp peptides have been shown to promote the production of anti-inflammatory cytokines, indicating immunoregulatory potential of Hsp. Therefore, the presence of immune responses to Hsp in inflammatory diseases can be seen as an attempt of the immune system to correct the inflammatory condition. Hsp70 can modulate inflammatory responses in models of arthritis, colitis and graft rejection, and the mechanisms underlying this effect are now being elucidated. Incubation with microbial Hsp70 was seen to induce tolerogenic dendritic cells (DCs) and to promote a suppressive phenotype in myeloid-derived suppressor cells and monocytes. These DC could induce regulatory T cells (Tregs), independently of the antigens they presented. Some Hsp70 family members are associated with autophagy, leading to a preferential uploading of Hsp70 peptides in MHC class II molecules of stressed cells. Henceforth, conserved Hsp70 peptides may be presented in these situations and constitute targets of Tregs, contributing to downregulation of inflammation. Finally, an interfering effect in multiple intracellular inflammatory signaling pathways is also known for Hsp70. Altogether it seems attractive to use Hsp70, or its derivative peptides, for modulation of inflammation. This is a physiological immunotherapy approach, without the immediate necessity of defining disease-specific auto-antigens. In this article, we present the evidence on anti-inflammatory effects of Hsp70 and discuss the need for experiments that will be crucial for the further exploration of the immunosuppressive potential of this protein.

A case of mistaken identity: HSPs are no DAMPs but DAMPERs

Until recently, the immune system was seen solely as a defense system with its primary task being the elimination of unwanted microbial invaders. Currently, however, the functional significance of the immune system has obtained a much wider perspective, to include among others the maintenance and restoration of homeostasis following tissue damage. In this latter aspect, there is a growing interest in the identification of molecules involved, such as the so-called danger or damage-associated molecular patterns (DAMPs), also called alarmins. Since heat shock proteins are archetypical molecules produced under stressful conditions, such as tissue damage or inflammation, they are frequently mentioned as prime examples of DAMPs (Bianchi, J Leukoc Biol 81:1-5, 2007; Kono and Rock, Nat Rev Immunol 8:279-289, 2008; Martin-Murphy et al., Toxicol Lett 192:387-394, 2010). See for instance also a recent review (Chen and Nunez, Science 298:1395-1401, 2010). Contrary to this description, we recently presented some of the arguments against a role of heat shock protein as DAMPs (Broere et al., Nat Rev Immunol 11:565-c1, 2011). With this perspective and reflection article, we hope to elaborate on this debate and provide additional thoughts to further ignite this discussion on this critical and evolving issue.

Intracellular heat shock protein-70 negatively regulates TLR4 signaling in the newborn intestinal epithelium

Necrotizing enterocolitis (NEC) is the leading cause of gastrointestinal-related mortality in premature infants, and it develops under conditions of exaggerated TLR4 signaling in the newborn intestinal epithelium. Because NEC does not develop spontaneously, despite the presence of seemingly tonic stimulation of intestinal TLR4, we hypothesized that mechanisms must exist to constrain TLR4 signaling that become diminished during NEC pathogenesis and focused on the intracellular stress response protein and chaperone heat shock protein-70 (Hsp70). We demonstrate that the induction of intracellular Hsp70 in enterocytes dramatically reduced TLR4 signaling, as assessed by LPS-induced NF-κB translocation, cytokine expression, and apoptosis. These findings were confirmed in vivo, using mice that either globally lacked Hsp70 or overexpressed Hsp70 within the intestinal epithelium. TLR4 activation itself significantly increased Hsp70 expression in enterocytes, which provided a mechanism of autoinhibition of TLR4 signaling in enterocytes. In seeking to define the mechanisms involved, intracellular Hsp70-mediated inhibition of TLR4 signaling required both its substrate-binding EEVD domain and association with the cochaperone CHIP, resulting in ubiquitination and proteasomal degradation of TLR4. The expression of Hsp70 in the intestinal epithelium was significantly decreased in murine and human NEC compared with healthy controls, suggesting that loss of Hsp70 protection from TLR4 could lead to NEC. In support of this, intestinal Hsp70 overexpression in mice and pharmacologic upregulation of Hsp70 reversed TLR4-induced cytokines and enterocyte apoptosis, as well as prevented and treated experimental NEC. Thus, a novel TLR4 regulatory pathway exists within the newborn gut involving Hsp70 that may be pharmacologically activated to limit NEC severity.

Differential MyD88/IRAK4 requirements for cross-priming and tumor rejection induced by heat shock protein 70-model antigen fusion protein

Priming of CD8(+) T cells requires two signals, one produced by T-cell receptor recognition of antigen, and a second that is often provided by the innate immune response. In this context, antigens non-covalently or covalently associated with heat shock proteins (HSP) are internalized and processed in antigen-presenting cells (APC) to be presented by MHC I molecules to CD8(+) T cells, thus, signal 1 has been well characterized in this pathway of cross-presentation. Signal 2 is not fully understood, although there are reports that Toll-like receptors (TLRs) interact with HSP and activate APC. The ability of HSP to activate APC through TLRs is, however, controversial because of the possibility of endotoxin contamination. Using a variety of TLR KO mice, we present evidence that TLRs (TLR2, 3, 4, 7, and 9) and their adaptor molecules MyD88 and IRAK4 are dispensable in cross-priming by a mycobacterial HSP70-antigen (ovalbumin as a model antigen) fusion protein; in contrast, MyD88/IRAK4, but not TLRs, are required for tumor rejection induced by the same reagent. Our results indicate that HSP-mediated cross-priming uses a second signal produced by mechanisms other than TLR cascades. We hypothesize that efficient cross-priming by HSP70 alone is insufficient for tumor rejection and that MyD88/IRAK4-dependent inflammatory stimulation, which might contribute to maintenance of the initially primed effector cells, is required to eradicate tumor burden.

Inducible heat shock protein 70 reduces T cell responses and stimulatory capacity of monocyte-derived dendritic cells

Heat shock protein 70 (Hsp70) has gained a lot of attention in the past decade due to its potential immunoregulatory functions. Some of the described proinflammatory functions of Hsp70 became controversial as they were based on recombinant Hsp70 proteins specimens, which were later shown to be endotoxin-contaminated. In this study we used low endotoxin inducible Hsp70 (also known as Hsp72, HSPA1A), and we observed that after a 24-h incubation of monocyte-derived immature dendritic cells (mo-iDCs) with 20 μg/ml of low endotoxin Hsp70, their ability to stimulate allogenic T cells was reduced. Interestingly, low endotoxin Hsp70 also significantly reduced T cell responses when they were simulated with either IL-2 or phytohemagglutinin, therefore showing that Hsp70 could alter T cell responses independently from its effect on mo-iDCs. We also reported a greater response of Hsp70 treatment when activated versus nonactivated T cells were used. This effect of Hsp70 was similar for all tested populations of T cells that included CD3(+), CD4(+), or CD8(+). Taken together, our observations strongly suggest that Hsp70 might dampen, rather than provoke, T cell-mediated inflammatory reactions in many clinical conditions where up-regulation of Hsp70 is observed.