Toll-like receptor ligand links innate and adaptive immune responses by the production of heat-shock proteins

Rise of the natural red pigment 'prodigiosin' as an immunomodulator in cancer

Cancer is a heterogeneous disease with multifaceted drug resistance mechanisms (e.g., tumour microenvironment [TME], tumour heterogeneity, and immune evasion). Natural products are interesting repository of bioactive molecules, especially those with anticancer activities. Prodigiosin, a red pigment produced by Serratia marcescens, possesses inherent anticancer characteristics, showing interesting antitumour activities in different cancers (e.g., breast, gastric) with low or without harmful effects on normal cells. The present review discusses the potential role of prodigiosin in modulating and reprogramming the metabolism of the various immune cells in the TME, such as T and B lymphocytes, tumour-associated macrophages (TAMs), natural killer (NK) cells, and tumour-associated dendritic cells (TADCs), and myeloid-derived suppressor cells (MDSCs) which in turn might introduce as an immunomodulator in cancer therapy.

Gingko biloba extract EGB761 alleviates heat-stress damage in chicken heart tissue by stimulating Hsp70 expression in vivo in vascular endothelial cells

1. This study aimed to investigate the protective effects of Gingko biloba extract EGB761 on heat-stressed chicken heart in vivo and its underlying relevance to Hsp70.2. A total of 50 one-day-old female chicks were randomly divided into five groups: control (Con), heat-stress (HS), 0.1% EGB761 plus heat-stress (0.1%EGB+HS), 0.3%EGB761 plus heat-stress (0.3%EGB+HS) and 0.6%EGB761 plus heat-stress (0.6%EGB+HS) groups. After administration of EGB761 for 45 days, the chickens in each group were exposed to a single heat-stress event at 38 ± 1°C for 3 h.3. EGB761 attenuated the abnormal symptoms and pathological scores of myocardium of heat-stressed chickens. Despite a reduction in the transcription and translation of the Hsp70 gene in heat-stressed myocardium, EGB761 induced the expression of Hsp70 in endothelial cells of the microarteries and venules into the blood, and reduced heat-stress damage in vascular endothelial cells.4. Supplementation with EGB761 before heat-stress exposure protected chicken myocardium from damage by increasing serum Hsp70 protein from myocardial cells and cardiac microvascular endothelial cells and protected the microvascular system from adverse injury.

Heat shock protein 90 relieves heat stress damage of myocardial cells by regulating Akt and PKM2 signaling in vivo

Heat shock protein 90 (Hsp90) is associated with resisting heat-stress injury to the heart, particularly in myocardial mitochondria. However, the mechanism underlying this effect remains unclear. The present study was based on the high expression of Hsp90 during heat stress (HS) and involved inducing higher expression of Hsp90 using aspirin in mouse hearts. Higher Hsp90 levels inhibited HS-induced myocardial damage and apoptosis, and mitochondrial dysfunction, by stimulating Akt (protein kinase B) activation and PKM2 (pyruvate kinase M2) signaling, and subsequently increasing mitochondrial Bcl-2 (B-cell lymphoma 2) levels and its phosphorylation. Functional inhibition of Hsp90 using geldanamycin verified that reducing the association of Hsp90 with Akt and PKM2 caused the functional decline of phosphorylated (p)-Akt and PKM2 that initiate Bcl-2 to move into mitochondria, where it is phosphorylated. Protection by Hsp90 was weakened by blocking Akt activation using Triciribine, which could not be recovered by normal initiation of the PKM2 pathway. Furthermore, increased Hsp70 levels induced by Akt activation in myocardial cells may flow into the blood to resist heat stress. The results provided in vivo mechanistic evidence that in myocardial cells, Hsp90 resists heat stress via separate activation of the Akt-Bcl-2 and PKM2-Bcl-2 signaling pathways, which contribute toward preserving cardiac function and mitochondrial homeostasis.

Heme as a danger molecule in pathogen recognition

Appropriate control of redox mechanisms are critical for and effective innate immune response, which employs multiple cell types, receptors and molecules that recognize danger signals when they reach the host. Recognition of pathogen-associated pattern molecules (PAMPs) is a fundamental host survival mechanism for efficient elimination of invading pathogens and resolution of the infection and inflammation. In addition to PAMPs, eukaryotic cells contain a plethora of intracellular molecules that are normally secured within the confines of the plasma membrane, but if liberated and encountered in the extracellular milieu can provoke rapid cell activation. These are known as Alarmins or Danger-Associated Molecular Patterns (DAMPs) and can be released actively by cells or passively as a result of sterile cellular injury after trauma, ischemia, or toxin-induced cell rupture. Both PAMPs and DAMPs are recognized by a series of cognate receptors that increase the generation of free radicals and activate specific signaling pathways that result in regulation of a variety of stress response, redox sensitive genes. Multiple mediators released, as cells die include, but are not limited to ATP, hydrogen peroxide, heme, formyl peptides, DNA or mitochondria provide the second signal to amplify immune responses. In this review, we will focus on how sterile and infective stimuli activate the stress response gene heme oxygenase-1 (Hmox1, HO-1), a master gene critical to an appropriate host response that is now recognized as one with enormous therapeutic potential. HO-1 gene expression is regulated in large part by redox-sensitive proteins including but not limited to nrf2. Both PAMPs and DAMPs increase the activation of nrf2 and HO-1. Heme is a powerful pro-oxidant and as such should be qualified as a DAMP. With its degradation by HO-1a molecule of carbon monoxide (CO) is generated that in turn serves as a bioactive signaling molecule. PAMPs such as bacterial endotoxin activate HO-1, and the CO that is generated diffuses into the extracellular milieu where it interacts with bacteria, altering their behavior to increase production of ATP, which then functions as a second signal danger molecule. This two-hit cycle scenario results in efficient and effective activation of host leukocytes to attack and clear bacteria in part via enhanced reactive oxygen species generation. We discuss this intimate communication that occurs between host and bacteria and how these molecules serve as critical regulators of the acute inflammatory response, the overall redox status of the cell, and survival of the host.

Heat shock proteins and regulatory T cells

Heat shock proteins (HSPs) are important molecules required for ideal protein function. Extensive research on the functional properties of HSPs indicates that HSPs may be implicated in a wide range of physiological functions including immune function. In the immune system, HSPs are involved in cell proliferation, differentiation, cytokine release, and apoptosis. Therefore, the ability of the immune system, in particular immune cells, to function optimally and in unison with other physiological systems is in part dependent on signaling transduction processes, including bidirectional communication with HSPs. Regulatory T cells (Tregs) are important T cells with suppressive functions and impairments in their function have been associated with a number of autoimmune disorders. The purpose of this paper is to examine the relationship between HSPs and Tregs. The interrelationship between cells and proteins may be important in cellular functions necessary for cell survival and expansion during diseased state.

Vitamin C Up-regulates Expression of CD80, CD86 and MHC Class II on Dendritic Cell Line, DC-1 Via the Activation of p38 MAPK

Vitamin C is an essential water-soluble nutrient which primarily exerts its effect on host defense mechanisms and immune homeostasis, but the mechanism related to immune-potentiation is poorly understood. Since dendritic cells (DCs) are known as a potent antigen presenting cell (APC) that could enhance the antigen specific immune responses, we investigate the effects of vitamin C on activation of DCs and its related mechanism by using dendritic cell lines, DC-1. First, we found that there was no damage on DC-1 by 2.5 mM of vitamin C. In the presence of vitamin C, the expression of CD80, CD86, and MHC molecules was increased, but it was decreased by the pre-treatment of SB203580, p38 MAPK-specific inhibitor. We confirmed the phosphorylation of p38 MAPK was increased by the treatment of vitamin C. Taken together, these results suggest that vitamin C could enhance the activity of dendritic cells via the up-regulation of the expression of CD80, CD86, and MHC molecules and the activation of p38 MAPK is related to this process.

Mapping of interactions between human macrophages and Staphylococcus aureus reveals an involvement of MAP kinase signaling in the host defense

Staphylococcus aureus is a dangerous opportunistic human pathogen that causes serious invasive diseases when it reaches the bloodstream. Recent studies have shown that S. aureus is highly resistant to killing by professional phagocytes and that such cells even provide a favorable environment for intracellular survival of S. aureus. Importantly, the reciprocal interactions between phagocytes and S. aureus have remained largely elusive. Here we have employed kinase profiling to define the nature and time resolution of the human THP-1 macrophage response toward S. aureus and proteomics to identify the response of S. aureus toward macrophages. The results of these studies reveal major macrophage signaling pathways triggered by S. aureus and proteomic signatures of the responses of S. aureus to macrophages. We also identify human proteins bound to S. aureus that have potential roles in bacterial killing and internalization. Most noticeably, our observations challenge the classical concept that macrophage responses are mainly mediated through Toll-like receptor 2 and NF-κB signaling and highlight the important role of the stress-activated MAP kinase signaling in orchestrating the host defense.

Phosphatidylserine inhibits NFκB and p38 MAPK activation in human monocyte derived dendritic cells

Phosphatidylserine (PS) is an anionic phospholipid restricted to the inner surface of the plasma membrane. PS translocates to the cell surface during early apoptosis where it serves as a marker for rapid uptake by phagocytes. PS is also thought to regulate immune responses. Dendritic cells (DC) are the most potent antigen presenting cells. Previous studies demonstrated that PS inhibits the expression of MHC and co-stimulatory molecules, the secretion of IL-12p70, and the ability to activate T cells by human monocyte derived DCs. However, the cell signaling mechanisms by which PS regulated DCs are not well described. In the current study we tested the effects of PS on signal transduction pathways thought to regulate human myeloid DC maturation and IL-12p70 production. We showed that PS inhibited the activation of nuclear factor-κB (NFκB) in response to LPS by preventing IκBα phosphorylation and degradation. PS also increased the total IκBα levels in immature DCs and inhibited p38 mitogen activated protein kinase (MAPK) phosphorylation and activation. The findings suggest a possible mechanism for regulating the immunostimulatory function of DCs by PS.

Serum heat shock protein 60 can predict remission of flare-up in juvenile idiopathic arthritis

Heat shock protein (Hsp) 60 has been implicated in the pathogenesis of various inflammatory and autoimmune diseases. This study aimed to investigate synovial fluid and serum concentrations of Hsp60 and anti-Hsp60 and their relationship with juvenile idiopathic arthritis (JIA). Forty-eight patients with JIA, including 22 oligo-articular, 19 poly-articular, and 7 systemic diseases, and 33 normal controls were enrolled in this study. Synovial fluid and serum Hsp60 and anti-Hsp60 concentrations were measured via ELISA. Serum concentrations of Hsp60 of active and inactive oligo- and poly-articular JIA were significantly higher than those of normal controls. Serum concentration of anti-Hsp60 in active oligo-articular JIA was higher than that of normal controls (49.25 vs. 35.76 ng/mL, p = 0.059). Similarly, serum concentration of anti-Hsp60 in active poly-articular JIA was significantly higher than that of inactive samples (65.05 vs. 26.54 ng/mL, p = 0.008). In addition, serum concentration of Hsp60 correlated with the time required for remission from flare-ups in patients with JIA. Serum concentration of Hsp60 correlated well with time required for remission from flare-ups in patients with JIA, representing a potential disease marker to monitor disease activity.

Heterologous CD8 T cell immune response to HSV induced by toll like receptor ligands

A memory response is established following primary antigen exposure that stays more or less constant. It appears to adopt a set-point in magnitude but upon re-exposure the response is quicker and better and there is an upward shift in memory frequency that varies with individuals based on the exposure pattern to other microbes or its components. Our investigations were designed to test such differences of non-specific stimulation by PAMPs in lowering the threshold of activation. Neonatal mice were pre-exposed to TLR-ligands intermittently and later analyzed for its resilience to challenge with virus during adult-life. Secondly, adult mice with pre-existing memory to virus were exposed to various TLR-ligands and analyzed for their quality of memory response. The TLR-ligands exposed animals were better responders to a new agent exposure compared to the animals kept in sterile surroundings. Moreover, immune memory recall and the viral specific CD8(+) T cells response with TLR-ligands were comparable to the recall response with the cognate antigen. The results provide insights into the role of hyper-sanitized environment versus PAMPs mediated signaling in adaptive immunity and long-term immune memory.

Established and newly proposed mechanisms of chronic cyclosporine nephropathy

Cyclosporine (CsA) has improved patient and graft survival rates following solid-organ transplantation and has shown significant clinical benefits in the management of autoimmune diseases. However, the clinical use of CsA is often limited by acute or chronic nephropathy, which remains a major problem. Acute nephropathy depends on the dosage of CsA and appears to be caused by a reduction in renal blood flow related to afferent arteriolar vasoconstriction. However, the mechanisms underlying chronic CsA nephropathy are not completely understood. Activation of the intrarenal renin-angiotensin system (RAS), increased release of endothelin-1, dysregulation of nitric oxide (NO) and NO synthase, up-regulation of transforming growth factor-beta1 (TGF-beta1), inappropriate apoptosis, stimulation of inflammatory mediators, enhanced innate immunity, endoplasmic reticulum stress, and autophagy have all been implicated in the pathogenesis of chronic CsA nephropathy. Reducing the CsA dosage or using other renoprotective drugs (angiotensin II receptor antagonist, mycophenolate mofetil, and statins, etc.) may ameliorate chronic CsA-induced renal injury. This review discusses old and new concepts in CsA nephropathy and preventive strategies for this clinical dilemma.

Heat shock protein 70 on Neuro2a cells is a putative receptor for Japanese encephalitis virus

Japanese encephalitis virus (JEV) envelope (E) protein has been shown to play a critical role in attachment to cells. However, the receptor interacting with envelope protein has not been conclusively identified. Using mouse neuroblastoma (Neuro2a) cells and purified JEV-E protein in 'Virus Overlay Protein Binding Assay' followed by MALDI-TOF analysis, we identified 'heat shock protein 70' (Hsp70) as a possible receptor for JEV. Indirect immunofluorescence and flow-cytometry analysis demonstrated localization of Hsp70 on Neuro2a cell surface. Co-immunoprecipitation followed by Western blot analysis reconfirmed the interaction between Hsp70 and JEV-E protein. Further, anti-Hsp70 polyclonal-antibodies were able to block JEV entry into Neuro2a cells. Additionally, using the bioinformatic tool - FTDOCK, docking between the proteins was performed. Amongst six interacting structural poses studied one pose involving RGD motif on JEV-E and leucine(539) on Hsp70 displayed stable interaction. These observations indicate that Hsp70 serves as putative receptor for JEV in Neuro2A cells.

Effects of CpG-B ODN on the protein expression profile of swine PBMC

The CpG motif within bacterial DNA is a potent immuno-stimulatory moiety. Here, using a 2-D electrophoretic approach, we investigated the effect of synthetic oligodeoxynucleotide containing a B type CpG motif (CpG-B ODN) on the protein expression profile of swine peripheral blood mononuclear cells (PBMC). We found that several proteins including spondin 1, N-acetolactate alpha linked acidic dipeptidase; V kappa light chain, T cell receptor variable alpha chain, heat shock protein (Hsp) 60, Hsp70, KIAA0857 protein, and PNAS-146 were up-regulated in PBMC by CpG-B ODN stimulation. Further studies showed that CpG-B ODN-mediated Hsp60, Hsp70 and Hsp90 expressions were closely associated with the TLR9 signalling pathway. Pretreatment with inhibitors of Hsp70, Hsp90 and TLR9 all blocked the CpG-B ODN-mediated anti-apoptotic effect in swine PBMC. These results suggest that CpG-B ODN treatment of swine PBMC may enhance the expression of biologically active proteins, notably spondin 1, V kappa light chain, T cell receptor variable alpha chain and Hsps, which may play an important role in CpG-B ODN-mediated activation of immune responses and enhancement of swine PBMC survival.

Stimulation of cysteinyl leukotriene production in mast cells by heat shock and acetylsalicylic acid

Immunoglobulin (Ig) E-dependent activation of mast cells is central to the allergic response. The engagement of IgE-occupied receptors initiates a series of molecular events that causes the release of preformed, and de novo synthesis of, allergic mediators. Cysteinyl leukotrienes are able to contract airway smooth muscle and increase mucus secretion and vascular permeability and recruit eosinophils. Mast cells have also recently been recognized as active participants in innate immune responses. Heat stress can modulate innate immunity by inducing stress proteins such as heat-shock proteins (HSPs). We previously demonstrated that treatment of mast cells with heat shock or acetylsalicylic acid results in an increase of TNF-alpha and IL-6 release. This effect was paralleled by expression of HSP70. In the current study, we further investigated the effects of heat shock and acetylsalicylic acid on the activation of mast cells and the release of cysteinyl leukotrienes. In mouse mast cells, derived from a culture of bone marrow cells, responsiveness to heat shock, acetylsalicylic acid and exogenous or endogenous HSP70 was monitored by measuring leukotriene C4 release. We show that after heat shock treatment and exposure to acetylsalicylic acid leukotriene production was increased. Moreover, exogenous rHSP70 also induced leukotriene production. Because it has been reported that leukotriene production in mast cells may be mediated by Toll like receptor (TLR) activation, and HSP70 also activates TLRs signaling, we further explored these issues by using mast cells that are not able to produce HSP70, i.e. heat shock factor-1 (HSF-1) knockout cells. We found that in HSF-1 knockout bone marrow derived mast cells, heat shock and acetylsalicylic acid failed to induce release of leukotrienes. Moreover, in wild type cells the surface expression of TLR4 was attenuated, whereas the intracellular expression was up-regulated. We conclude that heat shock and acetylsalicylic acid induce the production and release of heat shock proteins from mast cells, which in turn stimulate leukotriene synthesis through activation of TLR4.

Postulated vasoactive neuropeptide autoimmunity in fatigue-related conditions: a brief review and hypothesis

Disorders such as chronic fatigue syndrome (CFS) and gulf war syndrome (GWS) are characterised by prolonged fatigue and a range of debilitating symptoms of pain, intellectual and emotional impairment, chemical sensitivities and immunological dysfunction. Sudden infant death syndrome (SIDS) surprisingly may have certain features in common with these conditions. Post-infection sequelae may be possible contributing factors although ongoing infection is unproven. Immunological aberration may prove to be associated with certain vasoactive neuropeptides (VN) in the context of molecular mimicry, inappropriate immunological memory and autoimmunity. Adenylate cyclase-activating VNs including pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP) act as hormones, neurotransmitters, neuroregulators, immune modulators and neurotrophic substances. They and their receptors are potentially immunogenic. VNs are widely distributed in the body particularly in the central and peripheral nervous systems and have been identified in the gut, adrenal gland, blood cells, reproductive system, lung, heart and other tissues. They have a vital role in maintaining cardio-respiratory function, thermoregulation, memory, concentration and executive functions such as emotional responses including social cues and appropriate behaviour. They are co-transmitters for a number of neurotransmitters including acetylcholine and gaseous transmitters, are potent immune regulators with primarily anti-inflammatory activity, and have a significant role in protection of the nervous system against toxic assault as well as being important in the maintenance of homeostasis. This paper describes a biologically plausible mechanism for the development of certain fatigue-related syndromes based on loss of immunological tolerance to these VNs or their receptors following infection, other events or de novo resulting in significant pathophysiology possibly mediated via CpG fragments and heat shock (stress) proteins. These conditions extend the public health context of autoimmunity and VN dysregulation and have implications for military medicine where radiological, biological and chemical agents may have a role in pathogenesis. Possible treatment and prevention options are considered.

Immunology of Apoptosis and Necrosis

A complex of reactions regulating the number of cells in organs and tissues under normal and pathologic conditions is one of the most important systems of multicellular organisms. In this system, which controls both cell proliferation and clearance, clearance has been given special attention during the last three decades. Some stages of the clearance are known (the choice of "unwanted" cells, their destruction not affecting the surrounding tissue, and, finally, removal of the corpses), and undeniable progress has been achieved in the understanding of the second stage mechanisms, whereas mechanisms of elimination per se of cells or their fragments still continue to be terra incognita. The clearance of such cells is mainly determined by different components of natural and adaptive immunity: phagocytes, complement, opsonins, antigen-presenting cells, etc. Recently specific "danger signals", such as hydrolases, DNA, heat shock proteins, and other potential immunogens released by cells during their elimination have been discovered. Entering the extracellular space, these signals induce inflammation and injury of the surrounding tissues, i.e., autoimmune reactions. Heat shock proteins, in addition to chaperon activity, act as signaling, costimulating, and antigen-carrying molecules in the interactions of dying cells and the immune system.

Adrenergic receptors mediate stress-induced elevations in extracellular Hsp72

Heat-shock protein concentrations in the blood increase after exposure to a variety of stressors, including trauma and psychological stress. Although the physiological function of extracellular heat shock protein remains controversial, there is evidence that extracellular heat shock protein 72 (Hsp72) can facilitate immunologic responses. The signal(s) that mediate(s) the in vivo elevation of extracellular Hsp72 in the blood after stressor exposure remain(s) unknown. Here we report that Hsp72 increases in the circulation via an α1-adrenergic receptor-mediated signaling pathway. Activation of α1-adrenoceptors results in a rapid increase in circulating Hsp72, and blockade of α1-adrenoceptors prevents the stress-induced rise in circulating Hsp72. Furthermore, our studies exclude a role for β-adrenoceptors, glucocorticoids, and ACTH in mediating stress-induced elevations in circulating extracellular Hsp72. Understanding the signals involved in elevating extracellular Hsp72 could facilitate the use of extracellular Hsp72 to bolster immunity and perhaps prevent exacerbation of inflammatory diseases during stress.

Heat shock proteins as endogenous adjuvants in sterile and septic inflammation

Heat shock proteins (HSPs) have been reported to stimulate the immune system via innate receptors. However, the role of HSPs as endogenous adjuvants has been challenged by reports claiming that pure HSPs are not innate ligands; it is only the bacterial molecules trapped by the HSPs that can signal the innate immune system. In this review, we discuss data suggesting that both views, in essence, are correct; pure HSPs are indeed innate immunostimulators, but HSPs can also function as transducers of pathogen signals. In other words, HSPs perform diverse functions in two alternative modes of inflammation: sterile inflammation, which results from endogenous stimuli and is necessary for body maintenance, and septic inflammation, which protects us from environmental pathogens. Endogenous HSPs are key players in the modulation of these two modes of inflammation, and as such, they are potential targets for new and more efficient therapies for cancer, infections, and autoimmunity.

DNA vaccines coding for heat-shock proteins (HSPs): tools for the activation of HSP-specific regulatory T cells

Heat-shock proteins (HSPs) perform opposing functions in autoimmune arthritis. HSP-specific T cells drive the progression of adjuvant arthritis (AA), an experimental model of autoimmune arthritis. However, HSP-specific T cells can also have a regulatory phenotype, controlling arthritogenic T cells and inhibiting AA progression. This manuscript reviews the use of DNA vaccines coding for HSPs to analyse the role of these proteins in the regulation of arthritis. Recent studies suggest that HSPs participate in the control of pathological autoimmunity. Indeed, DNA vaccines coding for HSPs can be used to activate these HSP-specific built-in regulatory mechanisms. Thus, DNA vaccines coding for HSPs may serve not only as tools for the dissection of immunoregulatory mechanisms, but also as agents for the treatment of autoimmune disorders.

Heat-shock proteins and the host-pathogen interaction during bacterial infection

Heat-shock proteins (HSPs) are expressed at high levels by bacterial pathogens during adaptation to intracellular survival. Both host and pathogen heat-shock proteins contribute to immunity by receptor-mediated activation of the innate immune response and by participation in the presentation of antigens for the adaptive immune response. Manipulation of these interactions presents a potential route to improved control of infection by vaccination or immunotherapy.

Do cytosine guanine dinucleotide (CpG) fragments induce vasoactive neuropeptide mediated fatigue-related autoimmune disorders?

Autoimmune dysfunction of certain vasoactive neuropeptides (e.g., vasoactive intestinal peptide, pituitary adenylate cyclase activating polypeptide) may be implicated in a range of disorders associated with fatigue-like states (chronic fatigue syndrome, Gulf War syndrome) and even sudden infant death syndrome (SIDS). The important roles of these vasoactive neuropeptides make them a vulnerable target for autoimmune dysfunction. They are known to be associated with heat shock proteins for intracellular functioning with which they may form immunostimulating complexes. Cytosine guanine dinucleotide (CpG) fragments are potently immunogenic DNA fragments which serve as friend or foe recognition systems between bacterial (hypomethylated) and mammalian (methylated) DNA and are being assessed for suitability for use in human vaccines as adjuvants. Interactions between CpG fragments, heat shock proteins and vasoactive neuropeptides may be associated with fatigue-related autoimmune conditions.

Pathogen-induced apoptotic neutrophils express heat shock proteins and elicit activation of human macrophages

Ingestion of aged or irradiated apoptotic neutrophils actively suppresses stimulation of macrophages (Mphi). Many bacterial pathogens can also provoke apoptosis in neutrophils, but little is known about how such apoptotic cells influence Mphi activation. We found that neutrophils undergoing apoptosis induced by UV irradiation, Escherichia coli, or Staphylococcus aureus could either stimulate or inhibit Mphi activation. In contrast to Mphi that had ingested irradiated apoptotic neutrophils, Mphi that had phagocytosed bacteria-induced apoptotic neutrophils exhibited markedly increased production of the proinflammatory cytokine TNF-alpha, but not the anti-inflammatory cytokine TGF-beta. Moreover, ingestion of bacteria, but not UV-induced apoptotic neutrophils, caused increased expression of FcgammaRI on Mphi, and this effect was not provoked directly by bacteria associated with the apoptotic neutrophils. Instead, we found that a link between pathogen-induced apoptotic neutrophils and up-regulation of the heat shock proteins HSP60 and HSP70, and we also observed that recombinant HSP60 and HSP70 potentiated LPS-stimulated production of TNF-alpha in Mphi. The opposing macrophage responses to neutrophils undergoing apoptosis induced in different ways may represent a novel mechanism that regulates the extent of the immune response to invading microbes in two steps: first by aiding the functions of Mphi at an early stage of infection, and subsequently by deactivating those cells through removal of uninfected apoptotic neutrophils. HSP induction in neutrophils may provide the danger signals required to generate a more effective macrophage response.