An optimized Protocol for Human M2 Macrophages using M‐CSF and IL‐4/IL‐10/TGF‐β Yields a Dominant Immunosuppressive Phenotype

Monocytes are highly abundant circulatory effector cells and play a vital role in driving or resolving inflammatory processes depending on their activation phenotype. We investigated and compared a panel of polarization protocols of blood‐derived monocytes to achieve a stable, optimal and effective regimen for in vitro induction of immunosuppressive human macrophages, evaluating their surface receptor expression, cytokine profile, scavenging function and ability to suppress T‐cell proliferation. Importantly, we assessed the effect of copolarization or secondary pro‐inflammatory stimulation of a primary anti‐inflammatory activation phenotype. A combination of IL‐4/IL‐10/TGF‐β yielded a relatively stable and dominant immunosuppressive phenotype characterized by higher IL‐10 production and down‐regulated TNF‐α, IL‐6, CD86, CD274 and MHC II expression. Functionally, IL‐4/IL‐10/TGF‐β‐stimulated macrophages (M2) had a potent deactivating effect on a subsequent pro‐inflammatory LPS/IFNγ‐activated macrophage (M1) stimulation and significantly suppressed T‐cell proliferation. Monocytes derived from patients with chronic inflammatory diseases could be induced to be anti‐inflammatory using this protocol. Pre‐differentiation with GM‐CSF or M‐CSF was further demonstrated to enhance final M1/M2 activation status. Our findings indicate a robust polarization protocol for generation of specific immunosuppressive human monocyte‐derived macrophages.     

Inhibition of the NLRP3 inflammasome by HSP90 inhibitors

Excessive and dysregulated inflammation is known to contribute to disease progression. HSP90 is an intracellular chaperone known to regulate inflammatory processes including the NLRP3 inflammasome and secretion of the pro‐inflammatory cytokine interleukin(IL)‐1β. Here, primarily using an in vitro inflammasome ASC speck assay, and an in vivo model of murine peritonitis, we tested the utility of HSP90 inhibitors as anti‐inflammatory molecules. We report that the HSP90 inhibitor EC144 effectively inhibited inflammatory processes including priming and activation of NLRP3 in vitro and in vivo. A specific inhibitor of the β HSP90 isoform was ineffective suggesting the importance of the α isoform in inflammatory signalling. EC144 inhibited IL‐1β and IL‐6 in vivo when administered orally, and was brain‐penetrant. These data suggest that HSP90 inhibitors may be useful for targeting inflammation in diverse diseases that are worsened by the presence of inflammation.     

Caspase‐11 non‐canonical inflammasome: a critical sensor of intracellular lipopolysaccharide in macrophage‐mediated inflammatory responses

Inflammatory responses mediated by macrophages are part of the innate immune system, whose role is to protect against invading pathogens. Lipopolysaccharide (LPS) found in the outer membrane of Gram‐negative bacteria stimulates an inflammatory response by macrophages. During the inflammatory response, extracellular LPS is recognized by Toll‐like receptor 4, one of the pattern recognition receptors that activates inflammatory signalling pathways and leads to the production of inflammatory mediators. The innate immune response is also triggered by intracellular inflammasomes, and inflammasome activation induces pyroptosis and the secretion of pro‐inflammatory cytokines such as interleukin‐1β (IL‐1β) and IL‐18 by macrophages. Cysteine‐aspartic protease (caspase)‐11 and the human orthologues caspase‐4/caspase‐5 were recently identified as components of the ‘non‐canonical inflammasome’ that senses intracellular LPS derived from Gram‐negative bacteria during macrophage‐mediated inflammatory responses. Direct recognition of intracellular LPS facilitates the rapid oligomerization of caspase‐11/4/5, which results in pyroptosis and the secretion of IL‐1β and IL‐18. LPS is released into the cytoplasm from Gram‐negative bacterium‐containing vacuoles by small interferon‐inducible guanylate‐binding proteins encoded on chromosome 3 (GBP^chr3)‐mediated lysis of the vacuoles. In vivo studies have clearly shown that caspase‐11^−/− mice are more resistant to endotoxic septic shock by excessive LPS challenge. Given the evidence, activation of caspase‐11 non‐canonical inflammasomes by intracellular LPS is distinct from canonical inflammasome activation and provides a new paradigm in macrophage‐mediated inflammatory responses.     

IL‐1: Discoveries,controversies and future directions

Although there has been a great amount of progress in the 25 years since the first reporting of the cDNA for IL‐1α and IL‐1β, the history of IL‐1 goes back to the early 1940s. In fact, the entire field of inflammatory cytokines, TLR and the innate immune response can be found in the story of IL‐1. This Viewpoint follows the steps from the identification of the fever‐inducing activities of “soluble factors” produced by endotoxin‐stimulated leukocytes through to the discovery of cryopyrin and the caspase‐1 inflammasome and on to the clinical benefits of anti‐IL‐1β‐based therapeutics. It also discusses some of the current controversies regarding the activation of the inflammasome. The future of novel anti‐inflammatory agents to combat chronic inflammation is based, in part, on the diseases that are uniquely responsive to anti‐IL‐1β, which is surely a reason to celebrate the 25th anniversary of the cloning of IL‐1α and IL‐1β.     

Secreted aspartic proteases of Candida albicans activate the NLRP3 inflammasome

In a recent report, we demonstrated that distinct members of the secreted aspartic protease (Sap) family of Candida albicans are able to induce secretion of proinflammatory cytokines by human monocytes, independently of their proteolytic activity and specific pH optima. In particular, C. albicans Sap2 and Sap6 potently induced IL‐1β, TNF‐α, and IL‐6 production. Here, we demonstrate that Sap2 and Sap6 proteins trigger IL‐1β and IL‐18 production through inflammasome activation. This occurs via NLRP3 and caspase‐1 activation, which cleaves pro‐IL‐1β into secreted bioactive IL‐1β, a cytokine that was induced by Saps in monocytes, in monocyte‐derived macrophages and in dendritic cells. Downregulation of NLRP3 by RNA interference strongly reduced the secretion of bioactive IL‐1β. Inflammasome activation required Sap internalization via a clathrin‐dependent mechanism, intracellular induction of K⁺ efflux, and ROS production. Inflammasome activation of monocytes induced by Sap2 and Sap6 differed from that induced by LPS‐ATP in several aspects. Our data reveal novel immunoregulatory mechanisms of C. albicans and suggest that Saps contribute to the pathogenesis of candidiasis by fostering rather than evading host immunity.     

Listeria monocytogenes is sensed by the NLRP3 and AIM2 inflammasome

The inflammasome pathway functions to regulate caspase‐1 activation in response to a broad range of stimuli. Caspase‐1 activation is required for the maturation of the pivotal pro‐inflammatory cytokines of the pro‐IL‐1β family. In addition, caspase‐1 activation leads to a certain type of cell death known as pyroptosis. Activation of the inflammasome has been shown to play a critical role in the recognition and containment of various microbial pathogens, including the intracellularly replicating Listeria monocytogenes; however, the inflammasome pathways activated during L. monocytogenes infection are only poorly defined. Here, we demonstrate that L. monocytogenes activates both the NLRP3 and the AIM2 inflammasome, with a predominant involvement of the AIM2 inflammasome. In addition, L. monocytogenes‐triggered cell death was diminished in the absence of both AIM2 and NLRP3, and is concomitant with increased intracellular replication of L. monocytogenes. Altogether, these data establish a role for DNA sensing through the AIM2 inflammasome in the detection of intracellularly replicating bacteria.     

Human cathelicidin LL‐37 and its derivative IG‐19 regulate interleukin‐32‐induced inflammation

Human cathelicidin LL‐37 protects against infections and endotoxin‐induced inflammation. In a recent study we have shown that IG‐19, an LL‐37‐derived peptide, protects in a murine model of arthritis. Cytokine interleukin‐32 (IL‐32) is elevated and directly associated with the disease severity of inflammatory arthritis. Therefore, in this study we examined the effects of LL‐37 and IG‐19 on IL‐32‐induced responses in human peripheral blood‐derived mononuclear cells (PBMC) and macrophages. We showed that CD14⁺ monocytes are the primary cells that produce pro‐inflammatory tumour necrosis factor‐α (TNF‐α) following stimulation of PBMC with IL‐32. We demonstrated that LL‐37 and IG‐19 significantly suppress IL‐32‐induced production of pro‐inflammatory cytokines, e.g. TNF‐α and IL‐1β, without altering chemokine production. In contrast, LL‐37 and IG‐19 enhance the production of the anti‐inflammatory cytokine IL‐1RA. Further mechanistic studies revealed that LL‐37 and IG‐19 suppress IL‐32‐mediated phosphorylation of Fyn (Y420) Src kinase. In contrast, IL‐32‐mediated phosphorylation of AKT‐1 (T308) and MKP‐1 (S359) is not suppressed by the peptides. LL‐37 and IG‐19 alone induce the phosphorylation of MKP‐1 (S359), which is a known negative regulator of inflammation. Furthermore, the peptides induce the activity of p44/42 mitogen‐activated protein kinase, which is known to phosphorylate MKP‐1 (S359). This is the first study to demonstrate the regulation of IL‐32‐induced inflammation by LL‐37 and its derivative peptide IG‐19. The mechanistic results from this study suggest that regulation of immune‐mediated inflammation by these peptides may be controlled by the dual phosphatase MKP‐1. We speculate that LL‐37 and its derivatives may contribute to the control of immune‐mediated inflammatory diseases.     

RGS16 Restricts the Pro‐Inflammatory Response of Monocytes

Immune cells express powerful and harmful effectors that require tight regulation. Heterotrimeric G proteins are critical mediators in translating extracellular signals into cell responses, which need a fine‐tuned regulation for the control of cell activation. Regulator of G‐protein signalling 16 (RGS16) has been identified as a key factor of G protein‐mediated activation in lymphocytes, modulating inflammatory and survival responses of various cell types. However, data about the expression of this regulatory protein in monocytes are scarce, and it has remained unclear whether activation and migration of these cells are regulated by RGS16. In this study, the impact of RGS16 on the production of inflammatory cytokines by activated human monocytes was investigated in vitro using the human promonocytic cell line THP‐1 as a model. Gain and loss of function experiments showed that RGS16 overexpression reduces the expression of pro‐inflammatory cytokines IL‐1β, IL‐6, IL‐8 and TNFα, while RGS16 knockdown by RNAi upregulates IL‐1β, IL‐6 and TNFα but not IL‐8. RGS16 knockdown was also shown to enhance Pam3‐mediated induction of the anti‐inflammatory cytokine IL‐10. Our results indicate that RGS16 restricts the activation‐induced pro‐inflammatory profile in myeloid cells.     

The NLRP3 inflammasome contributes to host protection during Sporothrix schenckii infection

Sporotrichosis is a mycosis caused by fungi from the Sporothrix schenckii species complex, whose prototypical member is Sporothrix schenckii sensu stricto. Pattern recognition receptors (PRRs) recognize and respond to pathogen‐associated molecular patterns (PAMPs) and shape the following adaptive immune response. A family of PRRs most frequently associated with fungal recognition is the nucleotide‐binding oligomerization domain‐like receptor (NLR). After PAMP recognition, NLR family pyrin domain‐containing 3 (NLRP3) binds to apoptosis‐associated speck‐like protein containing a caspase recruitment domain (ASC) and caspase‐1 to form the NLRP3 inflammasome. When activated, this complex promotes the maturation of the pro‐inflammatory cytokines interleukin‐1β (IL‐1β) and IL‐18 and cell death through pyroptosis. In this study, we aimed to evaluate the importance of the NLRP3 inflammasome in the outcome of S. schenckii infection using the following three different knockout (KO) mice: NLRP3^−/−, ASC^−/− and caspase‐1^−/−. All KO mice were more susceptible to infection than the wild‐type, suggesting that NLRP3‐triggered responses contribute to host protection during S. schenckii infection. Furthermore, the NLRP3 inflammasome appeared to be critical for the ex vivo release of IL‐1β, IL‐18 and IL‐17 but not interferon‐γ. Additionally, a role for the inflammasome in shaping the adaptive immune response was suggested by the lower frequencies of type 17 helper T (Th17) cells and Th1/Th17 but not Th1 cells in S. schenckii‐infected KO mice. Overall, our results indicate that the NLRP3 inflammasome links the innate recognition of S. schenckii to the adaptive immune response, so contributing to protection against this infection.     

The activation of the inflammatory cytokines in overweight patients with mild obstructive sleep apnoea

It is widely accepted that obstructive sleep apnoea (OSA) is linked with cardiovascular diseases. The relationship is complex and remains still poorly understood. The presence of chronic systemic inflammation has been connected with pathogenesis of both OSA and cardiovascular diseases. While atherogenesis is believed to be a process of many years, little is known about the potential impact of the largest OSA subgroup, mild OSA, on the development of cardiovascular diseases. The aim of the present study was to assess whether untreated mild OSA is associated with an activation of inflammatory cytokine system. The adult study population consisted of two groups: 84 patients with mild OSA [apnoea–hypopnoea index (AHI) 5–15 h^?1] and 40 controls (AHI <5 h^?1). Serum concentrations of pro‐ and anti‐inflammatory cytokines were measured before any interventions. After adjustments for age, sex, body mass index, fat percentage, most important cardiometabolic and inflammatory diseases, and non‐steroidal anti‐inflammatory medication, the mean level of tumour necrosis factor‐α was significantly elevated (1.54 versus 1.17 pg mL^?1, P = 0.004), whereas the level of interleukin‐1β (IL‐1β) was reduced (0.19 versus 0.23 pg mL^?1, P = 0.004) in patients with mild OSA compared with controls. The concentrations of the protective anti‐inflammatory cytokines, interleukin‐10 (1.28 versus 0.70 pg mL^?1, P < 0.001) and interleukin‐1 receptor antagonist (478 versus 330 pg mL^?1, P = 0.003) were elevated in the OSA group. The concentrations of C‐reactive protein increased, but IL‐1β decreased along with the increase of AHI. Mild OSA was found to be associated not only with the activation of the pro‐inflammatory, but also with the anti‐inflammatory systems.     

The purinergic 2X7 receptor participates in renal inflammation and injury induced by high‐fat diet: possible role of NLRP3 inflammasome activation

Renal disease associated with type 2 diabetes and the metabolic syndrome is characterized by a distinct inflammatory phenotype. The purinergic 2X₇ receptor (P2X₇R) and the nucleotide‐binding and oligomerization domain‐like receptor containing a pyrin domain 3 (NLRP3) inflammasome have been separately shown to play a role in two models of non‐metabolic chronic kidney disease. Moreover, the NLRP3 inflammasome has been implicated in chronic low‐grade sterile inflammation characterizing metabolic disorders, though the mechanism(s) involved in inflammasome activation under these conditions are still unknown. We investigated the role of P2X₇R (through activation of the NLRP3 inflammasome) in renal inflammation and injury induced by a high‐fat diet, an established model of the metabolic syndrome. On a high‐fat diet, mice lacking P2X₇R developed attenuated renal functional and structural alterations as well as reduced inflammation, fibrosis, and oxidative/carbonyl stress, as compared with wild‐type animals, in the absence of significant differences in metabolic parameters. This was associated with blunted up‐regulation of the NLRP3 inflammasome components NLRP3, apoptosis‐associated speck‐like protein containing a caspase recruitment domain (ASC), pro‐caspase 1, pro‐interleukin (IL)‐1β, and pro‐IL‐18, as well as reduced inflammasome activation, as evidenced by decreased formation of mature caspase 1, whereas mature IL‐1β and IL‐18 were not detected. Up‐regulated expression of NLRP3 and pro‐caspase 1, post‐translational processing of pro‐caspase‐1, and release of IL‐18 in response to lipopolysaccharide + 2′(3′)‐O‐(4‐benzoylbenzoyl)ATP were attenuated by P2X₇R silencing in cultured mouse podocytes. Protein and mRNA expression of P2X₇R, NLRP3, and ASC were also increased in kidneys from subjects with type 2 diabetes and the metabolic syndrome, showing histologically documented renal disease. These data provide evidence of a major role for the purinergic system, at least in part through activation of the NLRP3 inflammasome, in the process driving ‘metabolic’ renal inflammation and injury and identify P2X₇R and NLRP3 as novel therapeutic targets. Copyright © 2013 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

Lipopolysaccharide induces calcitonin gene-related peptide in the RAW264.7 macrophage cell line

Calcitonin gene‐related peptide (CGRP) is widely distributed and plays important roles in a wide array of biological functions. It is enriched in primary sensory neurons and hence involved in nociception and neurogenic inflammation. Recent studies have shown that CGRP can be produced by immune cells such as monocytes/macrophages following inflammatory stimulation, suggesting a role in innate immunity. However, it is unclear how CGRP is up‐regulated in macrophages and if it plays a role in macrophage functions such as the production of cytokines and chemokines. Using enzyme‐linked immunosorbent assay (ELISA) and multiplex ELISA, lipopolysaccharide (LPS) was found to induce CGRP in the RAW 264.7 macrophage cell line. LPS‐induced inflammatory mediators such as nerve growth factor (NGF), interleukin‐1β (IL‐1β), IL‐6, prostaglandin E₂ (PGE₂) and nuclear factor‐κB (NF‐κB) signalling are involved in inducing CGRP, whereas the NGF receptor trkA and CGRP receptor signalling pathways are unexpectedly involved in suppressing LPS‐induced CGRP, which leads to the fine‐tune regulation of CGRP release. Exogenous CGRP and CGRP receptor antagonists, in a concentration‐dependent manner, stimulated, inhibited or had no effect on basal or LPS‐induced release of monocyte chemoattractant protein‐1, IL‐1β, IL‐6, tumour necrosis factor‐α and IL‐10 in RAW macrophages. The ligand‐concentration‐dependent regulation of the production of inflammatory mediators by CGRP receptor signalling is a novel mechanism underlying the stimulating and suppressing role of CGRP in immune and inflammatory responses. Together, our data suggest that monocytes/macrophages are an important source of CGRP. Inflammation‐induced CGRP has a positive or negative reciprocal effect on the production of other pro‐ and anti‐inflammatory mediators. Thereby CGRP plays both facilitating and suppressing roles in immune and inflammatory responses.     

Novel function of histamine signaling in hyperlipidemia‐induced atherosclerosis: Histamine H1 receptors protect and H2 receptors accelerate atherosclerosis

Histamine is not only essential for acute inflammatory reactions, but it also participates in a chronic inflammatory disorder. We generated apolipoprotein E (apoE) and histamine receptors (HHRs), including the major H1 and H2 receptors (HH1R, HH2R) double knockout mice (DKO) to clarify the role of HHRs in hyperlipidemia‐induced atherosclerosis, in which apoE‐KO and DKO mice were fed a high cholesterol diet. We found that pronounced hyperlipidemia‐induced atherosclerotic progression occurred in HH1R/apoE‐DKO mice, but in HH2R/apoE‐DKO mice less atherosclerosis, despite pro‐atherogenic serum cholesterol levels compared with apoE‐KO mice. Furthermore, the increased expressions of scavenger receptors (SRs), such as SR‐A, CD36 and lectin‐like oxidized low‐density lipoprotein receptor 1 (LOX‐1), nuclear factor‐kappa B (NFκB), monocyte chemoattractant protein (MCP‐1), matrix metalloproteinases (MMPs) or liver X receptor (LXR)‐related inflammatory signaling factors, were consistent with the pro‐atherogenic phenotype of HH2R/apoE‐DKO mice. We hypothesize that histamine/HH1R and HH2R signaling has conflicting innate functions, inflammatory/atherogenic and anti‐inflammatory/anti‐atherogenic actions, and that there are innate links between histamine signaling and hyperlipidemia‐induced atherosclerosis, independently of serum cholesterol metabolism. Specific histamine signaling blockers, in particular, HH2R blockers, are a possible novel therapeutic target for hyperlipidemia‐induced atherosclerosis.     

The Inflammation Induced by Lipopolysaccharide can be Mitigated by Short‐chain Fatty Acid,Butyrate, through Upregulation of IL‐10 in Septic Shock

Short‐chain fatty acids (SCFAs) with the anti‐inflammatory capacity are produced by intestinal bacteria; however, their effect on the acute systematical inflammation remains unclear. This study aimed to investigate the effects of SCFAs, acetate, propionate and butyrate, on septic shock and the underlying mechanism. The LPS‐induced septic model was used to evaluate the function of SCFAs by survival rate observation. Only butyrate, but not acetate or propionate, significantly decrease the mortality of septic mice. At 2 h and 6 h of LPS administration, the levels of TNF‐α, IL‐6 and IL‐1β in plasma were measured by ELISA to estimate the effects of butyrate pretreatment on excessive inflammation. And the anti‐inflammatory mediators including TGF‐β, IL‐10 and LXT4 in plasma were detected for further mechanism study in septic mice. Moreover, the murine macrophage‐like RAW 264.7 cells were stimulated by LPS to further confirm the finding in vivo. Pretreatment with butyrate led to significant attenuation of the LPS‐induced elevation of TNF‐α, IL‐6 and IL‐1β levels. However, when detecting the anti‐inflammatory factors, a significant increase in IL‐10, but not TGF‐β or LXT4, was shown in butyrate‐pretreated group. Pretreatment of RAW 264.7 cells with butyrate led to downregulation of LPS‐induced pro‐inflammatory mediators, IL‐6 and IL‐1β, but did not affect the level of TNF‐α, and increased IL‐10 (P < 0.01). In conclusion, SCFA butyrate significantly attenuated the inflammation against sepsis through upregulation of anti‐inflammatory IL‐10.     

The Nlrp3 inflammasome,IL‐1β, and neutrophil recruitment are required for susceptibility to a nonhealing strain of Leishmania major in C57BL/6 mice

Infection of C57BL/6 mice with most Leishmania major strains results in a healing lesion and clearance of parasites from the skin. Infection of C57BL/6 mice with the L. major Seidman strain (LmSd), isolated from a patient with chronic lesions, despite eliciting a strong Th1 response, results in a nonhealing lesion, poor parasite clearance, and complete destruction of the ear dermis. We show here that in comparison to a healing strain, LmSd elicited early upregulation of IL‐1β mRNA and IL‐1β‐producing dermal cells and prominent neutrophil recruitment to the infected skin. Mice deficient in Nlrp3, apoptosis‐associated speck‐like protein containing a caspase recruitment domain, or caspase‐1/11, or lacking IL‐1β or IL‐1 receptor signaling, developed healing lesions and cleared LmSd from the infection site. Mice resistant to LmSd had a stronger antigen‐specific Th1 response. The possibility that IL‐1β might act through neutrophil recruitment to locally suppress immunity was supported by the healing observed in neutropenic Genista mice. Secretion of mature IL‐1β by LmSd‐infected macrophages in vitro was dependent on activation of the Nlrp3 inflammasome and caspase‐1. These data reveal that Nlrp3 inflammasome‐dependent IL‐1β, associated with localized neutrophil recruitment, plays a crucial role in the development of a nonhealing form of cutaneous leishmaniasis in conventionally resistant mice.     

Damage control: Management of cellular stress by the NLRP3 inflammasome

The NLRP3 inflammasome plays a critical role in regulating inflammatory and cell death pathways in response to a diverse array of stimuli. Activation of the NLRP3 inflammasome results in activation of the cysteine protease caspase‐1 and the subsequent processing and secretion of the proinflammatory cytokines IL‐1β and IL‐18. In this issue of the European Journal of Immunology, Licandro et al. [Eur. J. Immunol. 2013. 43, 2126–2137] show that the NLRP3 inflammasome contributes to oxidative DNA damage. In addition, activation of the NLRP3 inflammasome modulates a number of pathways involved in DNA damage repair, cell cycle, and apoptosis, suggesting a novel role for the NLRP3 inflammasome in DNA damage responses following cellular stress.     

Propionibacterium acnes activates caspase‐1 in human neutrophils

Propionibacterium acnes is a Gram‐positive, slow‐growing, anaerobic bacillus, predominantly found as a commensal on the skin and mucous membranes of adults. It is, however, also considered an opportunistic pathogen; mostly associated with acne vulgaris, but rarely also with severe infections such as infective endocarditis, prosthetic joint infections, and deep sternal wound infections following cardiothoracic surgery. In addition, P. acnes has recently been found in high frequency in prostate tissue from patients with prostatitis and prostate cancer. The NOD‐like receptors (NLR) act as intracellular sensors of microbial components, and a number of various bacteria have been found to induce assembling and activation of NLR‐inflammasomes; leading to a pro‐inflammatory response. The inflammasome‐mediated formation of the pro‐inflammatory cytokines interleukin‐1β (IL‐1β) and IL‐18 involves the auto‐proteolytic maturation of caspase‐1. This study investigated if P. acnes activates inflammasomes. Propionibacterium acnes isolates (n = 29) with diverse origin were used as stimuli for peripheral leukocytes obtained from blood donors (BDs). The activity of inflammasomes was determined by measuring caspase‐1 by flow cytometry and cytokine production by ELISA. A significant amount of caspase‐1 was found in neutrophils upon P. acnes stimulation, whereas only a modest activation was seen in monocytes. The activation was mainly produced by components of the bacterial cell and no exo‐products, because heat‐killed and live bacteria caused high activation of caspase‐1 as well as cytokine production, whereas the bacterial supernatant elicited minor effect. The response among different BDs varied significantly, almost fivefold. In addition, P. acnes of various origins showed considerable variation, however, the commensal isolates showed a stronger response compared with the invasive. In conclusion, although regarded as a harmless commensal of the skin, P. acnes strongly activates the inflammasome of human peripheral neutrophils.