Trained immunity: Target for prophylaxis and therapy

Type I interferon signaling controls the early hematopoietic expansion in response to β-glucan

Rapid hematopoietic adaptations are important for building and sustaining the biological response to β-glucan. The signals involved in these early events have not yet been fully explored. Given that type I interferons are produced in response to β-glucan and can profoundly impact hematopoietic stem cell (HSC) function, we hypothesized that this pathway may be involved in the early bone marrow response to β-glucan. In vivo administration of β-glucan led to local interferon-α production in the peritoneal cavity and bone marrow, upregulation of its receptor, IFNAR1, specifically on long-term hematopoietic stem cells (LT-HSCs), and broad expansion of downstream progenitor subpopulations. We demonstrate that intact type I interferon signaling is critical for β-glucan-mediated LT-HSC proliferation, mitochondrial activity, and glycolytic commitment. By determining that type I interferon signaling is important for LT-HSCs, which sit at the apex of the hematopoietic hierarchy, we uncover an important component of the early inflammatory response to β-glucan.

Hematopoietic stem cell state and fate in trained immunity

Trained immunity serves as a de facto memory for innate immune responses, resulting in long-term functional reprogramming of innate immune cells. It enhances resistance to pathogens and augments immunosurveillance under physiological conditions. Given that innate immune cells typically have a short lifespan and do not divide, persistent innate immune memory may be mediated by epigenetic and metabolic changes in long-lived hematopoietic stem cells (HSCs) in the bone marrow. HSCs fine-tune their state and fate in various training conditions, thereby generating functionally adapted progeny cells that orchestrate innate immune plasticity. Notably, both beneficial and maladaptive trained immunity processes can comprehensively influence HSC state and fate, leading to divergent hematopoiesis and immune outcomes. However, the underlying mechanisms are still not fully understood. In this review, we summarize recent advances regarding HSC state and fate in the context of trained immunity. By elucidating the stem cell-intrinsic and extrinsic regulatory network, we aim to refine current models of innate immune memory and provide actionable insights for developing targeted therapies against infectious diseases and chronic inflammation. Furthermore, we propose a conceptual framework for engineering precision-trained immunity through HSC-targeted interventions.

Immune training enhances anti-viral responses and improves outcomes in Pax5-/+ mice susceptible to chronic infection

Viral infections pose a significant global burden. Host susceptibility to pathogens is determined by many factors including genetic variation that can lead to immunodeficient or dysregulated antiviral immune responses. Pax5 heterozygosity (Pax5-/+), resulting in reduced PAX5 levels in mice, mimics germline or somatic PAX5 dysregulation contributing to diseases such as childhood B-cell precursor acute lymphoblastic leukemia (B-ALL). In contrast to the well-characterized roles of PAX5 during early B-cell development, little is known about how Pax5 heterozygosity impacts antiviral responses. We infected Pax5-/+ mice with the noncytopathic Lymphocytic Choriomeningitis Virus (LCMV) and found that infection with the chronic Docile strain resulted in decreased survival of Pax5-/+ mice. While early adaptive CD8+ T-cell (CTL) immunity was robust in Pax5-/+ mice, LCMV-specific neutralizing antibody production was compromised leading to impaired long-term viral clearance and a pro-inflammatory milieu in the bone marrow (BM). Here we show that survival outcomes were improved upon prophylactic treatment with the β-glucan immune trainer through induction of heterologous protection against chronic infection. β-Glucan enhanced viral clearance, CTL immunity, neutralizing antibody production and reduced monocyte immunosuppression in multiple LCMV-resident host organs. New insight from this study will help design effective prophylactic treatment strategies against chronic viral infections, particularly in genetically predisposed susceptible hosts.

Trained immunity-based mucosal immunotherapies for the prevention of respiratory infections

The devastating impact of respiratory infections demonstrates the critical need for novel prophylactic vaccines. In this opinion article, we advocate for bacterial immunotherapies as a complementary tool in our fight against respiratory infections. These immunotherapies can activate a wide spectrum of immunological mechanisms, with trained immunity (TI) being particularly significant. This phenomenon has led to the concept of trained immunity-based vaccines (TIbVs), which represent a novel approach in vaccinology. We discuss examples of TIbVs, including the tuberculosis vaccine Bacille Calmette-Guérin (BCG) and the polybacterial immunotherapy MV130. From our viewpoint, illustrating the mode of action and clinical evidence supports the proposal that TIbVs should be considered as next-generation vaccines to confer protection against a wide range of respiratory infections.

Antigen-presenting innate lymphoid cells induced by BCG vaccination promote a respiratory antiviral immune response through the skin‒lung axis

The route of vaccine administration is associated with various immune outcomes, and the relationship between the route of administration and broad protection against heterologous pathogens remains unclear. Here, we found that subcutaneous vaccination with Bacillus Calmette-Guérin (BCG) promotes respiratory influenza clearance and T-cell responses. Group 1 innate lymphoid cells (ILC1s) express MHCII molecules and engage in antigen processing and presentation after BCG vaccination. During influenza virus infection, ILC1s in the lungs of BCG-vaccinated mice can present influenza virus antigens and prime Th1 cells. After subcutaneous vaccination with BCG, MHCII+ ILC1s migrate from the skin to the lungs and play an antigen-presenting role in influenza infection. Both the BCG and the BCG component lipomannan can induce MHCII expression and skin-to-lung migration of ILC1s via TLR2 signaling. Our study revealed an important regulatory mechanism by which subcutaneous vaccination with BCG promotes respiratory antiviral immune responses via the skin‒lung axis.

IL-25-induced memory ILC2s mediate long-term small intestinal adaptation

The adaptation of intestinal helminths to vertebrates evolved strategies to attenuate host tissue damage to support reproductive needs of parasites necessary to disseminate offspring to the environment. Helminths initiate the IL-25-mediated tuft cell-ILC2 circuit that enhances barrier protection of the host although viable parasites can target and limit the pathway. We used IL-25 to create small intestinal adaptation marked by anatomic, cell compositional and immunologic changes that persisted months after induction. Small intestinal adaptation was associated with heightened resistance to barrier pathogens, including in the lung, and sustained by transcriptionally and epigenetically modified, tissue-resident, memory-effector ILC2s distinct from those described by innate 'training'; epithelial stem cells remained unaltered. Despite requiring IL-25 for induction, memory ILC2s maintained an activated state in the absence of multiple alarmins and supported mucosal resilience while avoiding adverse sensitization to chronic inflammation, revealing a pathway for deploying innate immune cells to coordinate a distributed mucosal defense.

Interleukin-10 inhibits important components of trained immunity in human monocytes

Trained immunity induces antigen-agnostic enhancement of host defense and protection against secondary infections, but inappropriate activation can contribute to the pathophysiology of inflammatory diseases. Tight regulation of trained immunity is therefore needed to avoid pathology, but little is known about the endogenous processes that modulate it. Here, we investigated the potential of interleukin (IL)-10, a prototypical anti-inflammatory cytokine, to inhibit trained immunity. IL-10 induced tolerance and inhibited trained immunity in primary human monocytes at both functional and transcriptional levels. Inhibition of STAT3, a signaling route that mediates IL-10 signals, induced trained immunity. IL-10 downregulated glycolytic and oxidative metabolism in monocytes but did not impact the metabolic effects of β-glucan-induced trained immunity. Furthermore, IL-10 prevented increased reactive oxygen species production in Bacillus Calmette-Guérin (BCG)-induced training but did not influence phagocytosis upregulation. In a cohort study of healthy volunteers vaccinated with BCG, genetic variants that influenced IL-10 or its receptor modulated BCG-induced trained immunity. Furthermore, circulating IL-10 concentrations were negatively correlated with induction of trained immunity after BCG vaccination in a sex-specific manner. In conclusion, IL-10 inhibited several, albeit not all, immunological functions amplified after induction of trained immunity. Follow-up studies should explore the precise molecular mechanism that mediates the effects of IL-10 on trained immunity. Addressing these knowledge gaps is an important step toward optimizing IL-10's potential as a therapeutic target in diseases characterized by inappropriate induction of trained immunity.

Trained immunity in chronic inflammatory diseases and cancer

A decade after the term 'trained immunity' (TRIM) was coined to reflect the long-lasting hyper-responsiveness of innate immune cells with an epigenetically imprinted 'memory' of earlier stimuli, our understanding has broadened to include the potential implications of TRIM in health and disease. Here, after summarizing the well-documented beneficial effects of TRIM against infections, we discuss emerging evidence that TRIM is also a major underlying mechanism in chronic inflammation-related disorders such as periodontitis, rheumatoid arthritis and cardiovascular disease. Furthermore, mounting evidence indicates that the induction of TRIM by certain agonists confers protective antitumour responses. Although the mechanisms underlying TRIM require further study, the current knowledge enables the experimental development of innovative therapeutic approaches to stimulate or inhibit TRIM in a context-appropriate manner, such as the stimulation of TRIM in cancer or its inhibition in inflammatory disorders.

Friends to remember: innate immune memory regulation by the microbiota

Innate immune memory (IIM) is the process by which, upon a primary challenge, innate immune cells alter their epigenetic, transcriptional, and immunometabolic profiles, resulting in modified secondary responses. Unlike infections or other immune-system-related diseases, the role of IIM in nonpathogenic contexts is less understood. An increasing body of research has shown that normal microbiota members or their metabolic byproducts induce alternative memory phenotypes, suggesting that memory cells contribute to homeostasis in mucosal areas. In this review, we discuss the newest insights in the emerging field of IIM to the microbiota and the potential of manipulating these long-term responses to promote better mucosal health.

Transcriptional profile of Mycobacterium tuberculosis infection in people living with HIV

In people with HIV-1 (PWH), Mycobacterium tuberculosis (MTB) infection poses a significant threat. While active tuberculosis (TB) accelerates immunodeficiency, the interaction between MTB and HIV-1 during asymptomatic phases remains unclear. Analysis of peripheral blood mononuclear cells (PBMC) transcriptomic profiles in PWH, with and without controlled viral loads, revealed distinct clustering in MTB-infected individuals. Functional annotation identified alterations in IL-6, TNF, and KRAS pathways. Notably, MTB-related genes displayed an inverse correlation with HIV-1 viremia, at both individual and signature score levels. These findings suggest that MTB infection in PWH induces a shift in immune system activation, inversely related to HIV-1 viral load. These results may explain the observed enhanced antiretroviral control in MTB-infected PWH. This study highlights the complex interplay between MTB and HIV-1, emphasizing the importance of understanding their interaction for managing co-infections in this population.

Trained immunity of synovial macrophages is associated with exacerbated joint inflammation and damage after Staphylococcus aureus infection

OBJECTIVES

Investigate whether and which synoviocytes would acquire trained immunity characteristics that could exacerbate joint inflammation following a secondary Staphylococcus aureus infection.

METHODS

Lipopolysaccharide (LPS) and S. aureus were separately or double injected (21 days of interval) into the tibiofemoral joint cavity of male C57BL/6 mice. At different time points after these stimulations, mechanical nociception was analyzed followed by the analysis of signs of inflammation and damage in the affected joints. The trained immunity markers, including the glycolytic and mTOR pathway, were analyzed in whole tissue or isolated synoviocytes. A group of mice was treated with Rapamycin, an mTOR inhibitor before LPS or S. aureus stimulation.

RESULTS

The double LPS - S. aureus hit promoted intense joint inflammation and damage compared to single joint stimulation, including markers in synoviocyte activation, production of proinflammatory cytokines, persistent nociception, and bone damage, despite not reducing the bacterial clearance. The double LPS - S. aureus hit joints increased the synovial macrophage population expressing CX3CR1 alongside triggering established epigenetic modifications associated with trained immunity events in these cells, such as the upregulation of the mTOR signaling pathway (p-mTOR and HIF1α) and the trimethylation of histone H3. Mice treated with Rapamycin presented reduced CX3CR1+ macrophage activation, joint inflammation, and bone damage.

CONCLUSIONS

There is a trained immunity phenotype in CX3CR1+ synovial macrophages that contributes to the exacerbation of joint inflammation and damage during septic arthritis caused by S. aureus.

Genetic and epigenetic dysregulation of innate immune mechanisms in autoinflammatory diseases

Dysregulation and hyperactivation of innate immune responses can lead to the onset of systemic autoinflammatory diseases. Monogenic autoinflammatory diseases are caused by inborn genetic errors and based on molecular mechanisms at play, can be divided into inflammasomopathies, interferonopathies, relopathies, protein misfolding, and endogenous antagonist deficiencies. On the other hand, more common autoinflammatory diseases are multifactorial, with both genetic and non-genetic factors playing an important role. During the last decade, long-term memory characteristics of innate immune responses have been described (also called trained immunity) that in physiological conditions provide enhanced host protection from pathogenic re-infection. However, if dysregulated, induction of trained immunity can become maladaptive, perpetuating chronic inflammatory activation. Here, we describe the mechanisms of genetic and epigenetic dysregulation of the innate immune system and maladaptive trained immunity that leads to the onset and perpetuation of the most common and recently described systemic autoinflammatory diseases.

Myeloid C-type lectin receptors in host-pathogen interactions and glycan-based targeting

Lectin-glycan interactions play a crucial role in the immune system. An important class of lectins in the innate immune system is myeloid C-type lectin receptors (CLRs). Myeloid CLRs act as pattern recognition receptors and are predominantly expressed by myeloid cells, such as macrophages, dendritic cells, and neutrophils. In innate immunity, CLRs contribute to self/non-self discrimination. While the recognition of pathogen-associated molecular patterns (PAMPs) by CLRs may contribute to a protective immune response, CLR engagement can also be exploited by pathogens for immune evasion. Since various CLRs act as endocytic receptors and trigger distinct signaling pathways in myeloid cells, CLR targeting has proven useful for drug/antigen delivery into antigen-presenting cells and the modulation of immune responses. This review covers recent discoveries of pathogen/CLR interactions and novel approaches for CLR targeting within the period of the past two years.

Non-Coding RNAs and Innate Immune Responses in Cancer

Non-coding RNAs (ncRNAs) and the innate immune system are closely related, acting as defense mechanisms and regulating gene expression and innate immunity. Both are modulators in the initiation, development and progression of cancer. We aimed to review the major types of ncRNAs, including small interfering RNAs (siRNAs), microRNAs (miRNAs), piwi-interacting RNAs (piRNAs), and long non-coding RNAs (lncRNAs), with a focus on cancer, innate immunity, and inflammation. We found that ncRNAs are closely related to innate immunity, epigenetics, chronic inflammation, and cancer and share properties such as inducibility, specificity, memory, and transfer. These similarities and interrelationships suggest that ncRNAs and modulators of trained immunity, together with the control of chronic inflammation, can be combined to develop novel therapeutic approaches for personalized cancer treatment. In conclusion, the close relationship between ncRNAs, the innate immune system, and inflammation highlights their importance in cancer pathways and their potential as targets for novel therapeutic strategies.

High-throughput screen identifies non inflammatory small molecule inducers of trained immunity

Trained immunity is characterized by epigenetic and metabolic reprogramming in response to specific stimuli. This rewiring can result in increased cytokine and effector responses to pathogenic challenges, providing nonspecific protection against disease. It may also improve immune responses to established immunotherapeutics and vaccines. Despite its promise for next-generation therapeutic design, most current understanding and experimentation is conducted with complex and heterogeneous biologically derived molecules, such as β-glucan or the Bacillus Calmette-Guérin (BCG) vaccine. This limited collection of training compounds also limits the study of the genes most involved in training responses as each molecule has both training and nontraining effects. Small molecules with tunable pharmacokinetics and delivery modalities would both assist in the study of trained immunity and its future applications. To identify small molecule inducers of trained immunity, we screened a library of 2,000 drugs and drug-like compounds. Identification of well-defined compounds can improve our understanding of innate immune memory and broaden the scope of its clinical applications. We identified over two dozen small molecules in several chemical classes that induce a training phenotype in the absence of initial immune activation-a current limitation of reported inducers of training. A surprising result was the identification of glucocorticoids, traditionally considered immunosuppressive, providing an unprecedented link between glucocorticoids and trained innate immunity. We chose seven of these top candidates to characterize and establish training activity in vivo. In this work, we expand the number of compounds known to induce trained immunity, creating alternative avenues for studying and applying innate immune training.

Bridging the gap with multispecific immune cell engagers in cancer and infectious diseases

By binding to multiple antigens simultaneously, multispecific antibodies are expected to substantially improve both the activity and long-term efficacy of antibody-based immunotherapy. Immune cell engagers, a subclass of antibody-based constructs, consist of engineered structures designed to bridge immune effector cells to their target, thereby redirecting the immune response toward the tumor cells or infected cells. The increasing number of recent clinical trials evaluating immune cell engagers reflects the important role of these molecules in new therapeutic approaches for cancer and infections. In this review, we discuss how different immune cell types (T and natural killer lymphocytes, as well as myeloid cells) can be bound by immune cell engagers in immunotherapy for cancer and infectious diseases. Furthermore, we explore the preclinical and clinical advancements of these constructs, and we discuss the challenges in translating the current knowledge from cancer to the virology field. Finally, we speculate on the promising future directions that immune cell engagers may take in cancer treatment and antiviral therapy.

Sarcoidosis immunopathogenesis - a new concept of maladaptive trained immunity

Sarcoidosis is a chronic immune disease of unknown origin for which we still lack an immunological framework unifying causal agents, host factors, and natural history of disease. Here, we discuss the initial triggers of disease, and how myeloid cells drive granuloma formation and contribute to immunopathogenesis. We highlight recent advances in our understanding of innate immune memory and propose the hypothesis that maladaptive innate immune training connects previous environmental exposure to granuloma maintenance and expansion. Lastly, we consider how this hypothesis may open novel therapeutic avenues, while corticosteroids remain the front-line treatment.

Endotoxin tolerance and trained immunity: breaking down immunological memory barriers

For decades, innate immune cells were considered unsophisticated first responders, lacking the adaptive memory of their T and B cell counterparts. However, mounting evidence demonstrates the surprising complexity of innate immunity. Beyond quickly deploying specialized cells and initiating inflammation, two fascinating phenomena - endotoxin tolerance (ET) and trained immunity (TI) - have emerged. ET, characterized by reduced inflammatory response upon repeated exposure, protects against excessive inflammation. Conversely, TI leads to an enhanced response after initial priming, allowing the innate system to mount stronger defences against subsequent challenges. Although seemingly distinct, these phenomena may share underlying mechanisms and functional implications, blurring the lines between them. This review will delve into ET and TI, dissecting their similarities, differences, and the remaining questions that warrant further investigation.

Symbiosis takes a front and center role in biology

All animals and plants likely require interactions with microbes, often in strong, persistent symbiotic associations. While the recognition of this phenomenon has been slow in coming, it will impact most, if not all, subdisciplines of biology.

Integrated organ immunity: a path to a universal vaccine

Here, I introduce the concept of ‘integrated organ immunity’ to explain how the innate and adaptive immune systems and non-haematopoietic cells can interact in tissues to generate enduring protective immunity against diverse pathogens in an antigen-agnostic manner. Considering immune responses through this framework could enable the design of a new class of vaccines termed ‘universal vaccines’ that are not pathogen specific.

Mobilization of the innate immune response by a specific immunostimulant β-glucan confers resistance to chronic stress-induced depression-like behavior by preventing neuroinflammatory responses

Pre-stimulation of the innate immune response is an effective strategy to prevent depression-like phenotypes in animals. However, the use of conventional immunostimulants may cause adverse effects. Therefore, the search for agents that stimulate the innate immune response but do not induce a pro-inflammatory response could be a new research direction for the prevention of depression. β-glucan is a polysaccharide from Saccharomyces cerevisiae with unique immunomodulatory activity in microglia without eliciting a pro-inflammatory response that could lead to tissue damage. This suggests that β-glucan may be a suitable drug that can be used to prevent depression-like phenotypes. Our results showed that a single injection of β-glucan 1 day before stress exposure at a dose of 10 or 20 mg/kg, but notat a dose of 5 mg/kg, prevented depression-like behavior in mice treated with chronic unpredictable stress (CUS). This effect of β-glucan disappeared when the time interval between β-glucan and stress was extended from 1 day or 5 days to 10 days, which was rescued by a second injection 10 days after the first injection or by a repeated injection (4×, once daily) 10 days before stress exposure. A single β-glucan injection (20 mg/kg) 1 day before stress exposure prevented the CUS-induced increase in brain pro-inflammatory cytokines, and inhibition of the innate immune response by minocycline (40 mg/kg) abolished the preventive effect of β-glucan on CUS-induced depression-like behaviors and neuroinflammatory responses. These results suggest that β-glucan may prevent chronic stress-induced depression-like phenotypes and neuroinflammatory responses by stimulating the innate immune response.

The immunomodulatory and antioxidant effects of β-glucans in invertebrates

β-glucans (βGs) are carbohydrate polymers linked by β-1,3, 1,4 or 1,6 bonds, they have been used to protect against potential pathogens and prevent lethal diseases. The immune system possesses several receptors that identify a wide range of structures and trigger cellular and humoral mechanisms. However, the mechanisms by which βGs activate the immune system of invertebrate organisms have not been fully clarified. This review is focused on evaluating the effect of βGs on innate immune system in invertebrates. βGs stimulate different cellular and humoral mechanisms, such as phagocytosis, oxygen species production, extracellular trap formation, proPO system, and antimicrobial peptide synthesis, moreover, βGs increase survival rate and decrease pathogen load in several species.