LPS Induces Opposing Memory-like Inflammatory Responses in Mouse Bone Marrow Neutrophils

STING-dependent trained immunity contributes to host defense against Clostridium perfringens infection via mTOR signaling

Clostridium perfringens (C. perfringens) infection is recognized as one of the most challenging issues threatening food safety and perplexing agricultural development. To date, the molecular mechanisms of the interactions between C. perfringens and the host remain poorly understood. Here, we show that stimulator of interferon genes (STING)-dependent trained immunity protected against C. perfringens infection through mTOR signaling. Heat-killed Candida albicans (HKCA) training elicited elevated TNF-α and IL-6 production after LPS restimulation in mouse peritoneal macrophages (PM). Although HKCA-trained PM produced decreased levels of TNF-α and IL-6, the importance of trained immunity was demonstrated by the fact that HKCA training resulted in enhanced bacterial phagocytic ability and clearance in vivo and in vitro during C. perfringens infection. Interestingly, HKCA training resulted in the activation of STING signaling. We further demonstrate that STING agonist DMXAA is a strong inducer of trained immunity and conferred host resistance to C. perfringens infection in PM. Importantly, corresponding to higher bacterial burden, reduction in cytokine secretion, phagocytosis, and bacterial killing were shown in the absence of STING after HKCA training. Meanwhile, the high expression levels of AKT/mTOR/HIF1α were indeed accompanied by an activated STING signaling under HKCA or DMXAA training. Moreover, inhibiting mTOR signaling with rapamycin dampened the trained response to LPS and C. perfringens challenge in wild-type (WT) PM after HKCA training. Furthermore, STING‑deficient PM presented decreased levels of mTOR signaling-related proteins. Altogether, these results support STING involvement in trained immunity which protects against C. perfringens infection via mTOR signaling.

Trained Innate Immunity in Animal Models of Cardiovascular Diseases

Acquisition of immunological memory is an important evolutionary strategy that evolved to protect the host from repetitive challenges from infectious agents. It was believed for a long time that memory formation exclusively occurs in the adaptive part of the immune system with the formation of highly specific memory T cells and B cells. In the past 10-15 years, it has become clear that innate immune cells, such as monocytes, natural killer cells, or neutrophil granulocytes, also have the ability to generate some kind of memory. After the exposure of innate immune cells to certain stimuli, these cells develop an enhanced secondary response with increased cytokine secretion even after an encounter with an unrelated stimulus. This phenomenon has been termed trained innate immunity (TI) and is associated with epigenetic modifications (histone methylation, acetylation) and metabolic alterations (elevated glycolysis, lactate production). TI has been observed in tissue-resident or circulating immune cells but also in bone marrow progenitors. Risk-factors for cardiovascular diseases (CVDs) which are associated with low-grade inflammation, such as hyperglycemia, obesity, or high salt, can also induce TI with a profound impact on the development and progression of CVDs. In this review, we briefly describe basic mechanisms of TI and summarize animal studies which specifically focus on TI in the context of CVDs.

Subcutaneous Bacillus Calmette-Guérin Administration Induces Innate Training in Monocytes in Preweaned Holstein Calves

The bacillus Calmette-Guérin (BCG) vaccine, administered to prevent tuberculosis, is a well-studied inducer of trained immunity in human and mouse monocytes. We have previously demonstrated that aerosol BCG administration induces innate training in calves. The current study aimed to determine whether s.c. BCG administration could induce innate training, identify the cell type involved, and determine whether innate training promoted resistance to bovine respiratory syncytial virus (BRSV) infection, a major cause of bovine respiratory disease in preweaned calves. A total of 24 calves were enrolled at 1-3 d of age and blocked by age into two treatment groups (BCG, n = 12; control, n = 12). BCG was given s.c. to preweaned calves. The control calves received PBS. We observed a trained phenotype, demonstrated by enhanced cytokine production in response to in vitro stimulation with LPS (TLR-4 agonist) in PBMCs and CD14+ monocytes from the BCG group 2 wk (IL-1β, p = 0.002) and 4 wk (IL-1β, p = 0.005; IL-6, p = 0.013) after BCG administration, respectively. Calves were experimentally infected via aerosol inoculation with BRSV strain 375 at 5 wk after BCG administration and necropsied on day 8 postinfection. There were no differences in disease manifestation between the treatment groups. Restimulation of bronchoalveolar lavage fluid cells isolated on day 8 after BRSV infection revealed enhanced IL-1β (p = 0.014) and IL-6 (p = 0.010) production by the BCG group compared with controls. In conclusion, results from our study show that s.c. administration of the BCG vaccine can induce trained immunity in bovine monocytes and influence cytokine production in the lung environment after BRSV infection.

Exploring the Role of IL-17A in Oral Dysbiosis-Associated Periodontitis and Its Correlation with Systemic Inflammatory Disease

Oral microbiota play a pivotal role in maintaining homeostasis, safeguarding the oral cavity, and preventing the onset of disease. Oral dysbiosis has the potential to trigger pro-inflammatory effects and immune dysregulation, which can have a negative impact on systemic health. It is regarded as a key etiological factor for periodontitis. The emergence and persistence of oral dysbiosis have been demonstrated to mediate inflammatory pathology locally and at distant sites. The heightened inflammation observed in oral dysbiosis is dependent upon the secretion of interleukin-17A (IL-17A) by various innate and adaptive immune cells. IL-17A has been found to play a significant role in host defense mechanisms by inducing antibacterial peptides, recruiting neutrophils, and promoting local inflammation via cytokines and chemokines. This review seeks to present the current knowledge on oral dysbiosis and its prevention, as well as the underlying role of IL-17A in periodontitis induced by oral dysbiosis and its impact on systemic inflammatory disease.

ELMO1 Deficiency Reduces Neutrophil Chemotaxis in Murine Peritonitis

Peritoneal inflammation remains a major cause of treatment failure in patients with kidney failure who receive peritoneal dialysis. Peritoneal inflammation is characterized by an increase in neutrophil infiltration. However, the molecular mechanisms that control neutrophil recruitment in peritonitis are not fully understood. ELMO and DOCK proteins form complexes which function as guanine nucleotide exchange factors to activate the small GTPase Rac to regulate F-actin dynamics during chemotaxis. In the current study, we found that deletion of the Elmo1 gene causes defects in chemotaxis and the adhesion of neutrophils. ELMO1 plays a role in the fMLP-induced activation of Rac1 in parallel with the PI3K and mTORC2 signaling pathways. Importantly, we also reveal that peritoneal inflammation is alleviated in Elmo1 knockout mice in the mouse model of thioglycollate-induced peritonitis. Our results suggest that ELMO1 functions as an evolutionarily conserved regulator for the activation of Rac to control the chemotaxis of neutrophils both in vitro and in vivo. Our results suggest that the targeted inhibition of ELMO1 may pave the way for the design of novel anti-inflammatory therapies for peritonitis.

Training vs. Tolerance: The Yin/Yang of the Innate Immune System

For almost nearly a century, memory functions have been attributed only to acquired immune cells. Lately, this paradigm has been challenged by an increasing number of studies revealing that innate immune cells are capable of exhibiting memory-like features resulting in increased responsiveness to subsequent challenges, a process known as trained immunity (known also as innate memory). In contrast, the refractory state of endotoxin tolerance has been defined as an immunosuppressive state of myeloid cells portrayed by a significant reduction in the inflammatory capacity. Both training as well tolerance as adaptive features are reported to be accompanied by epigenetic and metabolic alterations occurring in cells. While training conveys proper protection against secondary infections, the induction of endotoxin tolerance promotes repairing mechanisms in the cells. Consequently, the inappropriate induction of these adaptive cues may trigger maladaptive effects, promoting an increased susceptibility to secondary infections—tolerance, or contribute to the progression of the inflammatory disorder—trained immunity. This review aims at the discussion of these opposing manners of innate immune and non-immune cells, describing the molecular, metabolic and epigenetic mechanisms involved and interpreting the clinical implications in various inflammatory pathologies.

The role of neutrophils in trained immunity

The principle of trained immunity represents innate immune memory due to sustained, mainly epigenetic, changes triggered by endogenous or exogenous stimuli in bone marrow (BM) progenitors (central trained immunity) and their innate immune cell progeny, thereby triggering elevated responsiveness against secondary stimuli. BM progenitors can respond to microbial and sterile signals, thereby possibly acquiring trained immunity-mediated long-lasting alterations that may shape the fate and function of their progeny, for example, neutrophils. Neutrophils, the most abundant innate immune cell population, are produced in the BM from committed progenitor cells in a process designated granulopoiesis. Neutrophils are the first responders against infectious or inflammatory challenges and have versatile functions in immunity. Together with other innate immune cells, neutrophils are effectors of peripheral trained immunity. However, given the short lifetime of neutrophils, their ability to acquire immunological memory may lie in the central training of their BM progenitors resulting in generation of reprogrammed, that is, "trained", neutrophils. Although trained immunity may have beneficial effects in infection or cancer, it may also mediate detrimental outcomes in chronic inflammation. Here, we review the emerging research area of trained immunity with a particular emphasis on the role of neutrophils and granulopoiesis.

Phenotypic, functional and plasticity features of human PBMCs induced by venom secreted PLA2s

Bothrops venom contains a high amount of secreted phospholipase A₂ (sPLA₂s) enzymes responsible for the inflammatory reaction and activation of leukocytes in cases of envenoming. PLA₂s are proteins that have enzymatic activity and can hydrolyze phospholipids at the sn-2 position, thereby releasing fatty acids and lysophospholipids precursors of eicosanoids, which are significant mediators of inflammatory conditions. Whether these enzymes have a role in the activation and function of peripheral blood mononuclear cells (PBMCs) is not known. Here we show for the first time how two secreted PLA₂s (BthTX-I and BthTX-II) isolated from the venom of Bothrops jararacussu affect the function and polarization of PBMCs. Neither BthTX-I nor BthTX-II exhibited significant cytotoxicity to isolated PBMCs compared with the control at any of the time points studied. RT-qPCR and enzyme-linked immunosorbent assays were used to determine changes in gene expression and the release of pro-inflammatory (TNF-α, IL-6, and IL-12) and anti-inflammatory (TGF-β and IL-10) cytokines, respectively, during the cell differentiation process. Lipid droplets formation and phagocytosis were also investigated. Monocytes/macrophages were labeled with anti-CD14, -CD163, and -CD206 antibodies to assay cell polarization. Both toxins caused a heterogeneous morphology (M1 and M2) on days 1 and 7 based on immunofluorescence analysis, revealing the considerable flexibility of these cells even in the presence of typical polarization stimuli. Thus, these findings indicate that the two sPLA₂s trigger both immune response profiles in PBMCs indicating a significant degree of cell plasticity, which may be crucial for understanding the consequences of snake envenoming.

How are granulocytes for transfusion best used? The past, the present and the future

Granulocyte transfusions continue to be used in clinical practice, predominantly for treatment of refractory infection in the setting of severe neutropenia. There is biological plausibility for effectiveness in these patients with deficiencies of neutrophils, either as a consequence of disease or treatment. However, there is a chequered history of conducting and completing interventional trials to define optimal use, and many uncertainties remain regarding schedule and dose. Practice and clinical studies are severely limited by the short shelf life and viability of current products, which often restricts the timely access to granulocyte transfusions. In the future, methods are needed to optimise donor-derived granulocyte products. Options include use of manufactured neutrophils, expanded and engineered from stem cells. Further possibilities include manipulation of neutrophils to enhance their function and/or longevity. Granulocyte transfusions contain a heterogeneous mix of cells, and there is additional interest in how these transfusions may have immunomodulatory effects, including for potential uses as adjuncts for anti-cancer effects.

Drosophila melanogaster as a model to study innate immune memory

Over the last decades, research regarding innate immune responses has gained increasing importance. A growing body of evidence supports the notion that the innate arm of the immune system could show memory traits. Such traits are thought to be conserved throughout evolution and provide a survival advantage. Several models are available to study these mechanisms. Among them, we find the fruit fly, Drosophila melanogaster. This non-mammalian model has been widely used for innate immune research since it naturally lacks an adaptive response. Here, we aim to review the latest advances in the study of the memory mechanisms of the innate immune response using this animal model.

Does Oral Endotoxin Contribute to Systemic Inflammation?

The oral microbiome, with a unique emphasis on Porphyromonas gingivalis has been associated with a constellation of inflammatory diseases such as cardiovascular disease, rheumatoid arthritis, Alzheimer's disease, type II diabetes, and non-alcoholic associated fatty liver disease. Periodontal disease has also been shown to induce "leaky gut" leading to metabolic endotoxemia. Several recent studies investigating the habitants of the blood microbiome have found the majority of species appear to be derived from oral and skin bacterial communities in otherwise healthy individuals. Many of the same pathologies associated with perturbations of oral health, such as cardiovascular disease, show alterations to the composition of the blood microbiome as well as circulating neutrophil phenotypes. Gingival inflammation is associated with activated blood neutrophil phenotypes that can exacerbate a distal inflammatory insult which may explain the connection between oral and systemic inflammatory conditions. While in the oral cavity, neutrophils encounter oral microbes that are adept in manipulating neutrophil activity which can re-enter the vasculature thereafter. Endotoxin from oral microbes can differ significantly depending on bacterial community and state of oral health to alter cellular LPS tolerance mechanisms which may contribute to the primed neutrophil phenotype seen in periodontitis and provide a mechanism by which the oral-microbes can affect systemic health outcomes. This review synthesizes the studies between inflammatory diseases and oral health with emphasis on microbiome and corresponding lipopolysaccharides in immune tolerance and activation.

Endothelial Dysfunction Induced by Extracellular Neutrophil Traps Plays Important Role in the Occurrence and Treatment of Extracellular Neutrophil Traps-Related Disease

Many articles have demonstrated that extracellular neutrophil traps (NETs) are often described as part of the antibacterial function. However, since the components of NETs are non-specific, excessive NETs usually cause inflammation and tissue damage. Endothelial dysfunction (ED) caused by NETs is the major focus of tissue damage, which is highly related to many inflammatory diseases. Therefore, this review summarizes the latest advances in the primary and secondary mechanisms between NETs and ED regarding inflammation as a mediator. Moreover, the detailed molecular mechanisms with emphasis on the disadvantages from NETs are elaborated: NETs can use its own enzymes, release particles as damage-associated molecular patterns (DAMPs) and activate the complement system to interact with endothelial cells (ECs), drive ECs damage and eventually aggravate inflammation. In view of the role of NETs-induced ED in different diseases, we also discussed possible molecular mechanisms and the treatments of NETs-related diseases.

Trained immunity in the mucosal diseases

Immune memory is well known as a signature of the adaptive immune system. Recently, enhanced responses to subsequent triggers are also observed in innate immune system, termed trained immunity (TI). Awakening of innate immune memory is required for host defense, such as anti-pathogen and anti-tumor responses. However, hyper-reactivation of trained innate immune cells also gives rise to undesirable inflammation. Mucosa immune system serves as the first defense line against pathogens. Trained immunity of mucosal immune system is tightly associated with the outcomes of mucosal diseases. In this review, we discuss the role of trained immunity in mucosal-associated diseases and the underlying mechanisms. We summarize the metabolic and epigenetic changes of trained immune cells and highlight their potential in clinical treatment. This article is categorized under: Infectious Diseases > Molecular and Cellular Physiology.

Gut Microbiota-Derived Small Extracellular Vesicles Endorse Memory-like Inflammatory Responses in Murine Neutrophils

Neutrophils are classically characterized as merely reactive innate effector cells. However, the microbiome is known to shape the education and maturation process of neutrophils, improving their function and immune-plasticity. Recent reports demonstrate that murine neutrophils possess the ability to exert adaptive responses after exposure to bacterial components such as LPS (Gram-negative bacteria) or LTA (Gram-positive bacteria). We now ask whether small extracellular vesicles (EVs) from the gut may directly mediate adaptive responses in neutrophils in vitro. Murine bone marrow-derived neutrophils were primed in vitro by small EVs of high purity collected from colon stool samples, followed by a second hit with LPS. We found that low-dose priming with gut microbiota-derived small EVs enhanced pro-inflammatory sensitivity as indicated by elevated levels of TNF-α, IL-6, ROS and MCP-1 and increased migratory and phagocytic activity. In contrast, high-dose priming resulted in a tolerant phenotype, marked by increased IL-10 and decreased transmigration and phagocytosis. Alterations in TLR2/MyD88 as well as TLR4/MyD88 signaling were correlated with the induction of adaptive cues in neutrophils in vitro. Taken together, our study shows that small EVs from stools can drive adaptive responses in neutrophils in vitro and may represent a missing link in the gut-immune axis.