Bacteria- and fungus-derived PAMPs induce innate immune memory via similar functional, metabolic, and transcriptional adaptations

PTEN inhibits scavenger receptor-mediated phagocytosis of methicillin-resistant Staphylococcus aureus

Phagocytosis requires the coordination of various classes of receptors and the activation of multiple signaling programs, culminating in actin cytoskeletal rearrangement and ingestion. Given the pleiotropic nature of the events necessary for proper microbial ingestion, identifying molecules that control distinct steps of phagocytosis could reveal potential strategies to enhance microbial clearance. PTEN is a lipid/protein phosphatase traditionally recognized as a tumor suppressor. While PTEN inhibits various arms of the innate immune response, its role during Staphylococcus aureus infection remains unclear. We hypothesize that PTEN inhibits the functions of scavenger receptors (SRs) and the actin cytoskeleton during methicillin-resistant S. aureus (MRSA) infection in macrophages. RNAseq analysis of PTEN KO immortalized bone marrow-derived macrophages (iBMDMs) unveiled increased expression of genes involved in actin polymerization, pathogen recognition, and SRs, which leads to enhanced MRSA phagocytosis in both iBMDMs and primary peritoneal macrophages lacking PTEN. PTEN is physically associated with 2 SRs, MARCO and CD36, and blocking these receptors prevents the increased phagocytosis seen in PTEN KO macrophages. PTEN binds to the actin depolymerizing factor cofilin-1 during infection, inhibiting F-actin (the essential form of actin for phagocytosis) while increasing G-actin pools. Cytometry by time of flight (CyTOF) analysis of human myeloid cell populations from a PTEN-haploinsufficient patient suggests that PTEN is necessary for generating specific monocyte and dendritic subclasses. This study identifies the role of PTEN in macrophage phagocytosis of a gram-positive pathogen and in the development of monocyte subsets. This highlights the spectrum of PTEN importance in host defense mechanisms in both murine and human phagocytes.

Trained immunity enhances host resistance to infection in aged mice

Aging significantly increases the incidence and severity of infections, with individuals aged 65 and above accounting for 65% of sepsis cases. Innate immune training, known as "trained immunity" or "innate immune memory," has emerged as a potential strategy to enhance infection resistance by modulating the aging immune system. We investigated the impact of -glucan-induced trained immunity on aged mice (18 to 20 mo old) compared with young adult mice (10 to 12 wk old). Our findings showed that β-glucan equally augmented the host resistance to infection in both young and aged mice. This enhancement was characterized by augmented bacterial clearance, enhanced leukocyte β, and decreased cytokine production in response to Pseudomonas aeruginosa infection. Furthermore, young and aged trained macrophages displayed heightened metabolic capacity and improved antimicrobial functions, including enhanced phagocytosis and respiratory burst. RNA-seq analysis showed a distinctive gene expression pattern induced by trained immunity in macrophages characterized by activation of pathways regulating inflammation and the host response to infection and suppression of pathways regulating cell division, which was consistently observed in both young and aged groups. As compared with macrophages from young mice, aged macrophages showed increased activation of gene ontology pathways regulating angiogenesis, connective tissue deposition, and wound healing. Our results indicate that immune training can be effectively induced in aging mice, providing valuable insights into potential strategies for enhancing infection resistance in the elderly.

Activating Dectin-1/SOCS1 signaling attenuates pseudomonas aeruginosa-induced lung injury

OBJECTIVE

To investigate the role of Dectin-1 in alleviating Pseudomonas aeruginosa (PA)-induced lung injury and its underlying mechanism.

METHODS

Wild-type and Dectin-1 knockout (KO) C57BL/6 mice were exposed to PAvia intratracheal instillation. PAO1 strains were cultured, inactivated, and quantified. MHS cells were used in vitro. Curdlan was employed to activate Dectin-1 signaling, and SOCS1 expression was modulated through genetic manipulation. Levels of Dectin-1, Syk, p-Syk, SOCS1, p-p65, and p65 were assessed. Lung injury was evaluated using H&E and TUNEL staining, cell counts and protein content in bronchoalveolar lavage fluid (BALF), the lung tissue wet/dry ratio, and seven-day survival rates. Bacterial burden in the lung was assessed by PA colony formation. Inflammatory responses were measured by TNF-α, IL-6, and IL-1β levels in BALF, as well as the p-p65/p65 ratio in lung or cell lysates. Apoptosis rates in cells were determined by flow cytometry.

RESULTS

Dectin-1 expression was downregulated in the lungs and MHS cells following PA infection. Dectin-1 depletion exacerbated PA-induced lung injury. Activation of Dectin-1 by curdlan significantly alleviated PA-induced lung injury. PA infection reduced SOCS1 expression, and SOCS1 interference exacerbated the inflammatory response and apoptosis in MHS cells, nullifying the protective effects of curdlan. Overexpression of SOCS1 significantly reduced inflammation and apoptosis in both MHS cells and Dectin-1 KO mice.

CONCLUSIONS

Activation of Dectin-1 significantly mitigates PA-induced lung injury, with SOCS1 playing a critical role in this process.

Immunoresponsive gene 1 facilitates TLR4 agonist-induced augmentation of innate antimicrobial immunity

Treatment with the toll-like receptor 4 agonist monophosphoryl lipid A conditions innate immunocytes to respond robustly to subsequent infection, a phenotype termed innate immune memory. Our published studies show that metabolic reprogramming of macrophages is a prominent feature of the memory phenotype. We undertook studies to define the functional contributions of tricarboxylic acid cycle reprogramming to innate immune memory. We observed that priming of wild-type mice with monophosphoryl lipid A potently facilitated accumulation of the tricarboxylic acid cycle metabolite itaconate at sites of infection and enhanced microbial clearance. Augmentation of itaconate accumulation and microbial clearance was ablated in Irg1-deficient mice. We further observed that monophosphoryl lipid A potently induces expression of Irg1 and accumulation of itaconate in macrophages. Compared to wild-type macrophages, the ability of Irg1-deficient macrophages to kill Pseudomonas aeruginosa was impaired. We further observed that itaconate is directly antimicrobial against P. aeruginosa at pH 5, which is characteristic of the phagolysosome, and is facilitated by reactive oxygen species. Monophosphoryl lipid A-induced augmentation of glycolysis, oxidative phosphorylation, and accumulation of the tricarboxylic acid cycle metabolites succinate and malate was decreased in Irg1 knockout macrophages compared to wild-type controls. RNA sequencing revealed suppressed transcription of genes associated with phagolysosome function and increased expression of genes associated with cytokine production and chemotaxis in Irg1-deficient macrophages. This study identifies a contribution of itaconate to monophosphoryl lipid A-induced augmentation of innate antimicrobial immunity via facilitation of microbial killing as well as impact on metabolic and transcriptional adaptations.

Role of Trained Immunity in Heath and Disease

PURPOSE OF REVIEW

This review aims to explore the role of immune memory and trained immunity, focusing on how innate immune cells like monocytes, macrophages, and natural killer cells undergo long-term epigenetic and metabolic rewiring. Specifically, it examines the mechanisms by which trained immunity, often triggered by infection or vaccination, could impact cardiac processes and contribute to both protective and pathological responses within the cardiovascular system.

RECENT FINDINGS

Recent research demonstrates that vaccination and infection not only activate immune responses in circulating monocytes and tissue macrophages but also affect immune progenitor cells within the bone marrow environment, conferring lasting protection against heterologous infections. These protective effects are attributed to epigenetic and metabolic reprogramming, which enable a heightened immune response upon subsequent encounters with pathogens. However, while trained immunity is beneficial in combating infections, it has been linked to exacerbated inflammation, which may increase susceptibility to cardiovascular diseases, including atherosclerosis and heart failure. Our review highlights the dual nature of trained immunity: while it offers protective advantages against infections, it also poses potential risks for cardiovascular health by promoting chronic inflammation. Understanding the molecular mechanisms underlying immune memory's impact on cardiac processes could lead to new therapeutic strategies to mitigate cardiovascular diseases, such as atherosclerosis, heart failure, and diabetes. These insights build the grounds for future research to balance the benefits of trained immunity with its potential risks in cardiovascular disease management.

Hypothesis: hematogenous metastatic cancer cells of solid tumors may disguise themselves as memory macrophages for metastasis

German pathologist Otto Aichel suggested, a century ago, that the cancer cell acquired its metastatic property from a leukocyte via cell-cell fusion. Since then, several revised versions of this theory have been proposed. Most of the proposals attribute the generation of the metastatic cancer cell to the fusion between a primary cancer cell and a macrophage. However, these theories have not addressed several issues, such as dormancy and stem cell-like self-renewal, of the metastatic cancer cell. On the other hand, recent studies have found that, like T- and B-/plasma cells, macrophages can also be categorized into naïve, effector, and memory/trained macrophages. As a memory/trained macrophage can enter dormancy/quiescence, be awakened from the dormancy/quiescence by acquainted primers, and re-populate via stem cell-like self-renewal, we, therefore, further specify that the macrophage fusing with the cancer cell and contributing to metastasis, belongs with the memory/trained macrophage, not other subtypes of macrophages. The current theory can explain many puzzling clinical features of cancer, including the paradoxal effects (recurrence vs. regression) of microbes on tumors, "spontaneous" and Coley's toxin-induced tumor regression, anticancer activities of β-blockers and anti-inflammatory/anti-immune/antibiotic drugs, oncotaxis, surgery- and trauma-promoted metastasis, and impact of microbiota on tumors. Potential therapeutic strategies, such as Coley's toxin-like preparations, are proposed. This is the last article of our trilogy on carcinogenesis theories.

Epigenetic regulation of innate immune dynamics during inflammation

Innate immune cells play essential roles in modulating both immune defense and inflammation by expressing a diverse array of cytokines and inflammatory mediators, phagocytizing pathogens to promote immune clearance, and assisting with the adaptive immune processes through antigen presentation. Rudimentary innate immune "memory" states such as training, tolerance, and exhaustion develop based on the nature, strength, and duration of immune challenge, thereby enabling dynamic transcriptional reprogramming to alter present and future cell behavior. Underlying transcriptional reprogramming are broad changes to the epigenome, or chromatin alterations above the level of DNA sequence. These changes include direct modification of DNA through cytosine methylation as well as indirect modifications through alterations to histones that comprise the protein core of nucleosomes. In this review, we will discuss recent advances in our understanding of how these epigenetic changes influence the dynamic behavior of the innate immune system during both acute and chronic inflammation, as well as how stable changes to the epigenome result in long-term alterations of innate cell behavior related to pathophysiology.