Acetate moderately attenuates the generation of neutrophil extracellular traps

Metabolic regulation of neutrophil functions in homeostasis and diseases

Neutrophils are the most abundant leukocytes in humans and play a role in the innate immune response by being the first cells attracted to the site of infection. While early studies presented neutrophils as almost exclusively glycolytic cells, recent advances show that these cells use several metabolic pathways other than glycolysis, such as the pentose phosphate pathway, oxidative phosphorylation, fatty acid oxidation, and glutaminolysis, which they modulate to perform their functions. Metabolism shifts from fatty acid oxidation-mediated mitochondrial respiration in immature neutrophils to glycolysis in mature neutrophils. Tissue environments largely influence neutrophil metabolism according to nutrient sources, inflammatory mediators, and oxygen availability. Inhibition of metabolic pathways in neutrophils results in impairment of certain effector functions, such as NETosis, chemotaxis, degranulation, and reactive oxygen species generation. Alteration of these neutrophil functions is implicated in certain human diseases, such as antiphospholipid syndrome, coronavirus disease 2019, and bronchiectasis. Metabolic regulators such as AMPK, HIF-1α, mTOR, and Arf6 are linked to neutrophil metabolism and function and could potentially be targeted for the treatment of diseases associated with neutrophil dysfunction. This review details the effects of alterations in neutrophil metabolism on the effector functions of these cells.

The role and mechanism of action of microbiota-derived short-chain fatty acids in neutrophils: From the activation to becoming potential biomarkers

Short-chain fatty acids (SCFAs), such as acetate, propionate, and butyrate, have emerged as critical mediators in the communication between the human microbiota and its host. As the first responder to the inflammatory site, neutrophils play an important role in protecting the host against bacterial infections. Recent investigations revealed that SCFAs generated from microbiota influence various neutrophil activities, including activation, migration, and generation of mediators of inflammatory processes. SCFAs have also been demonstrated to exhibit potential therapeutic benefits in a variety of disorders related to neutrophil dysfunction, including inflammatory bowel disease, viral infectious disorders, and cancer. This study aims to examine the molecular processes behind the complicated link between SCFAs and neutrophils, as well as their influence on neutrophil-driven inflammatory disorders. In addition, we will also provide an in-depth review of current research on the diagnostic and therapeutic value of SCFAs as possible biomarkers for neutrophil-related diseases.

Short chain fatty acids reduce the respiratory burst of human neutrophils in response to cystic fibrosis isolates of Staphylococcus aureus

Short chain fatty acids (SCFA) are produced by anaerobic bacteria. The most common SCFAs are acetate, propionate and butyrate. SCFAs have been implicated in several inflammatory diseases including cystic fibrosis (CF) where they are present in the airways at millimolar concentrations. Staphylococcus aureus is one of the main respiratory pathogens in CF. Polymorphonuclear neutrophil granulocytes (PMN) represent the most important immune defense the host uses against S. aureus. However, the reason why PMNs are unable to clear S. aureus in CF remains largely unclear. We hypothesized that SCFAs impair effector functions of PMNs in response to S. aureus. To test this, human PMNs were exposed to CF clinical isolates of S. aureus in vitro in the presence or absence of SCFAs and effector functions of PMNs were assessed. Our data show that SCFAs do not affect the viability of PMNs and do not stimulate the release of neutrophil extracellular traps (NET) from human PMNs. Production of reactive oxygen species (ROS), another important antimicrobial function of PMNs, on the other hand, was significantly inhibited by SCFAs in response to the bacterium. SCFAs did not compromise the ability of PMNs to kill CF isolates of S. aureus in vitro. Overall, our results provide new knowledge into the interactions between SCFAs and the immune system, and indicate that SCFAs produced by anaerobic bacteria in the CF lung could interfere with reactive oxidant production of PMNs in response to S. aureus, one of the prominent respiratory pathogens in this disease.

Effects of Hormone, NEFA and SCFA on the Migration of Neutrophils and the Formation of Neutrophil Extracellular Traps in Dairy Cows

Simple Summary

Perinatal dairy cows face the challenge of maintaining the resilience of defense against invading pathogens. During the perinatal period, hormonal or metabolic changes cause the decline of immune function of dairy cows and further lead to varying degrees of immunosuppression. The results of this study indicate that, hormones, nonesterified fatty acids (NEFAs) and short-chain fatty acids (SCFAs) can regulate neutrophil migration and the NETs formation of dairy cows in vitro. These results help to further explain the effects of changes in hormone secretion and metabolites on immunosuppression and the increased risk of disease in perinatal dairy cows.

Abstract

Polymorphonuclear neutrophils (PMN) are the first line of defense against the invasion of foreign pathogenic microorganisms and play an essential role in the immune system of dairy cows. The changes in hormone secretion and metabolites of dairy cows during the perinatal period are the key factors that cause immunosuppression and increased risk of diseases. However, the effects of the hormone, nonesterified fatty acid (NEFA), and short-chain fatty acid (SCFA) on the transmammary epithelial migration of dairy cows and the formation of neutrophil extracellular traps (NETs) have rarely been studied. This study explored the effects of hormones, NEFAs and SCFAs on the neutrophil migration and NETs formation of dairy cows in vitro. It was found that P₄ and Ac can regulate the transepithelial migration of PMN; SA and Pr can regulate the formation of NETs; E₂, OA and Bt can regulate PMN transepithelial migration and NET formation. These results help to further explain the effects of changes in hormone secretion and metabolites on immunosuppression and the increased risk of disease in perinatal dairy cows.

Microbiota metabolite butyrate constrains neutrophil functions and ameliorates mucosal inflammation in inflammatory bowel disease

Host-microbial cross-talk plays a crucial role in maintenance of gut homeostasis. However, how microbiota-derived metabolites, e.g., butyrate, regulate functions of neutrophils in the pathogenesis of inflammatory bowel disease (IBD) remains elusive. We sought to investigate the effects of butyrate on IBD neutrophils and elucidate the therapeutic potential in regulating mucosal inflammation. Peripheral neutrophils were isolated from IBD patients and healthy donors, and profiles of proinflammatory cytokines and chemokines were determined by qRT-PCR and ELISA, respectively. The migration and release of neutrophil extracellular traps (NETs) were studied by a Transwell model and immunofluorescence, respectively. The in vivo role of butyrate in regulating IBD neutrophils was evaluated in a DSS-induced colitis model in mice. We found that butyrate significantly inhibited IBD neutrophils to produce proinflammatory cytokines, chemokines, and calprotectins. Blockade of GPCR signaling with pertussis toxin (PTX) did not interfere the effects whereas pan-histone deacetylase (HDAC) inhibitor, trichostatin A (TSA) effectively mimicked the role of butyrate. Furthermore, in vitro studies confirmed that butyrate suppressed neutrophil migration and formation of NETs from both CD and UC patients. RNA sequencing analysis revealed that the immunomodulatory effects of butyrate on IBD neutrophils were involved in leukocyte activation, regulation of innate immune response and response to oxidative stress. Consistently, oral administration of butyrate markedly ameliorated mucosal inflammation in DSS-induced murine colitis through inhibition of neutrophil-associated immune responses such as proinflammatory mediators and NET formation. Our data thus reveal that butyrate constrains neutrophil functions and may serve as a novel therapeutic potential in the treatment of IBD.

Alcohol Consumption in Rheumatoid Arthritis: A Path through the Immune System

Benefits and harms of different components of human diet have been known for hundreds of years. Alcohol is one the highest consumed, abused, and addictive substances worldwide. Consequences of alcohol abuse are increased risks for diseases of the cardiovascular system, liver, and nervous system, as well as reduced immune system function. Paradoxically, alcohol has also been a consistent protective factor against the development of autoimmune diseases such as type 1 diabetes, multiple sclerosis, systemic lupus erythematosus, and rheumatoid arthritis (RA). Here, we focused on summarizing current findings on the effects of alcohol, as well as of its metabolites, acetaldehyde and acetate, on the immune system and RA. Heavy or moderate alcohol consumption can affect intestinal barrier integrity, as well as the microbiome, possibly contributing to RA. Additionally, systemic increase in acetate negatively affects humoral immune response, diminishing TFH cell as well as professional antigen-presenting cell (APC) function. Hence, alcohol consumption has profound effects on the efficacy of vaccinations, but also elicits protection against autoimmune diseases. The mechanism of alcohol's negative effects on the immune system is multivariate. Future studies addressing alcohol and its metabolite acetate's effect on individual components of the immune system remains crucial for our understanding and development of novel therapeutic pathways.

Physiological concentrations of short-chain fatty acids induce the formation of neutrophil extracellular traps in vitro

Neutrophils represent the first line of host cellular defense against various pathogens. The most recently described microbicidal mechanism of these cells is the release of neutrophil extracellular traps (NET). Currently, a wide range of chemical and biological stimuli are known to induce this response; however, the effect of short-chain fatty acids (SCFAs) on the induction of NET is still unknown. SCFAs are produced mainly by bacterial fermentation of dietary fiber and are found in host tissues and blood. This study aimed to determine whether physiological levels of SCFAs can induce the formation of NET. Previously reported concentrations of SCFAs (as found in the colonic lumen and peripheral blood in postprandial and basal states) were used to stimulate the neutrophils. In order to determine the signaling pathway utilized by SCFAs, we tested the inhibition of the Free Fatty Acid 2 Receptor (FFA2R) expressed in neutrophils using CATPB, the inhibitor of FFA2R, genistein, an inhibitor of the downstream Gα/q11 proteins and DPI, an inhibitor of the NADPH oxidase complex. The SCFAs at colonic intestinal lumen concentrations were able to induce the formation of NET, and when tested at concentrations found in the peripheral blood, only acetic acid at 100 μM (fasting equivalent) and 700 μM (postprandial equivalent) was found to induce the formation of NET. The administration of the competitive inhibitor against the receptor or blockade of relevant G protein signaling and the inhibition of NADPH oxidase complex decreased NET release. SCFAs stimulate NET formation in vitro and this effect is mediated, in part, by the FFA2R.

Current Understanding on the Metabolism of Neutrophils

Neutrophils are innate immune cells that constitute the first line of defense against invading pathogens. Due to this characteristic, they are exposed to diverse immunological environments wherein sources for nutrients are often limited. Recent advances in the field of immunometabolism revealed that neutrophils utilize diverse metabolic pathways in response to immunological challenges. In particular, neutrophils adopt specific metabolic pathways for modulating their effector functions in contrast to other immune cells, which undergo metabolic reprogramming to ensure differentiation into distinct cell subtypes. Therefore, neutrophils utilize different metabolic pathways not only to fulfill their energy requirements, but also to support specialized effector functions, such as neutrophil extracellular trap formation, ROS generation, chemotaxis, and degranulation. In this review, we discuss the basic metabolic pathways used by neutrophils and how these metabolic alterations play a critical role in their effector functions.