In response to the entry of any microbes, the innate and adaptive immune systems of the body recognize, destroy, and eliminate the invading organism through phagocytosis, without significant injury to the host (19). In general, innate immunity is the first line of defense that recognizes the pathogen through phagocytic pathogen recognition receptors (PRRs) on its surface, which bind with the surface molecules of pathogens, namely the pathogen-associated molecular patterns (PAMPs), or the host-derived danger-associated molecular patterns (DAMPs). The innate immune cells include monocytes, neutrophils, DCs and macrophages, mast cells, basophils, eosinophils, and NK cells. Among these cells, phagocytes, such as neutrophils and macrophages are mainly seen in infected tissues and are involved in engulfing/phagocytosing, degrading, and clearing the microbial- and host-cell-derived debris (20). Innate immune cells, such as macrophages express PRRs, such as Toll-like receptors (TLRs) C-type lectin receptors (CLRs), and NOD-like receptors (NLRs), which recognize PAMPs derived from pathogens, including, bacterial lipopolysaccharides (LPS), viral nucleic acids, and fungal cell wall components as well as DAMPs, which are endogenous molecules released from damaged or stressed host cells. Although PRRs exhibit broad specificity and recognize a range of both PAMPs and DAMPs, the response of phagocytes to various PAMPs and DAMPs is context-dependent (20–23). For example, TLRs, located on cell surfaces can recognize various PAMPs and initiate immune responses, while NLRs, found in the cytoplasm, form inflammasomes upon detecting PAMPs or DAMPs, leading to cytokine production (21–23). Furthermore, while PAMPs are mostly invariant antigens within a class of microbial agents, they are distinguishable from DAMPs, which are “danger” signaling molecules produced by the host because of inflammation, infection, and/or cell/tissue damage (21). Importantly, the interaction of PRRs with PAMP or DAMP activates phagocytosis and antigen processing within the phagocytes. Activated phagocytes can also migrate to regional draining lymph nodes to “present” the antigen to T cells of the adaptive immune response, through cognitive major histocompatibility complex (MHC) molecules (21). Hence, phagocytes are also termed as antigen-presenting cells (APCs). Upon activation, APCs produce pro-inflammatory cytokines and chemokines, leading to the recruitment and activation of other immune cells, including T and B lymphocytes, which results in the production of antibodies and cytotoxic T-cell activation (21–23). Thus, PRRs serve as a vital signaling link between the innate and adaptive immune cells.

Innate immunity also encompasses a complement system as an efficient defense mechanism against pathogens, contributing to pathogen clearance, inflammation, and modulation of adaptive immune responses through a complex network of soluble proteins like C3b. These proteins aid in the opsonization of microbes by binding to their surface and enhancing the phagocytosis leading to the secretion of inflammatory mediators, formation of the membrane attack complex for pathogen lysis as well as direct microbial killing, and clearing the apoptotic cells and immune complexes, thus regulating the host immune responses (24). Additionally, complement activation enhances adaptive immunity by promoting antigen presentation and antibody production. Overall, the complement system provides a rapid and effective defense mechanism against invading pathogens while regulating immune responses to maintain tissue homeostasis.

In addition to phagocytes, the innate immune system also contains non-phagocytic cells, such as mast cells, basophils, eosinophils, and NK cells. Among these, mast cells, basophils, and eosinophils are granular immune cells, that generate inflammatory responses via histamine release. It should be noted that these responses are part of the body’s defense mechanism that can sometimes contribute to disease pathology, such as in allergic reactions (25). The NK cells are a subset of cytotoxic cells of the innate immune system that mainly recognize infected cells via specific receptors, lysing and enabling them to be eliminated effectively, as recently shown for lethal CMV viremia or transcriptional control of HIV-1 (26–28). Thus, innate immunity has a significant role in protecting the host system from external agents in a non-specific manner; however, innate immune cells, in general, do not effectively function as classical memory cells to actively mount an immune response upon re-encountering the same PAMPs. While innate immunity lacks conventional cellular memory, phenomena such as trained immunity suggest a form of non-specific memory response in innate immune cells.

The adaptive immune response is also effective in eliminating pathogens, similar to innate immunity but in a different perspective (19, 29). Activation of adaptive immunity eliminates pathogens along with their toxic molecules, mostly by generating a memory response to specific PAMPs of the pathogens (30). Adaptive immunity is further categorized into humoral, and cell-mediated immunity. Humoral immunity is mediated by B lymphocytes (B cells), with a unique antigen receptor on its surface, known as the B cell receptor (BCR). The BCR is a membrane-bound immunoglobulin molecule, that recognizes specific epitopes or antigens. Affinity maturation is a process that refines the specificity and affinity of antibodies resulting in the production of antibodies that neutralize the pathogen and its secreted toxins leading to their elimination (31). The antigen-specific response is the hallmark of adaptive immunity, which refers to the ability of lymphocytes to recognize and respond to specific antigens, ensuring targeted immune responses against pathogens. On the other hand, affinity maturation, primarily occurring in B cell responses, involves the refinement of antibody binding affinity to antigens through somatic hypermutation. This process leads to the generation of antibodies with progressively higher affinity, enhancing the effectiveness of the immune response over time. Together, antigen specificity and affinity maturation are fundamental aspects of the adaptive immune system’s ability to mount precise and potent responses tailored to encountered antigens.

In addition to mounting an immediate, antigen-specific effector function, the adaptive immune system develops long-term memory responses through the formation of memory T and B cells during the primary immune response. These memory cells remain quiescent but quickly respond upon re-exposure to the same pathogen, leading to a faster and more potent secondary immune response. This memory provides long-lasting immunity against previously encountered pathogens (31). Thus, adaptive immunity advances to target pathogens more effectively through processes such as antigen presentation, clonal selection, affinity maturation, and memory cell formation. While APCs initiate adaptive immune responses by presenting antigens to T and B cells, which then undergo clonal selection, leading to the expansion of cells targeting the pathogen, the affinity maturation fine-tunes those responses by generating antibodies with higher affinity for the antigen (32). Further differentiation of adaptive immune cells into effector cells aids in coordinating and executing immune responses, and finally, the memory cell formation ensures rapid and potent responses upon re-exposure to the same pathogen. Thus, adaptive immunity continuously evolves to mount faster, more specific, and more effective responses against pathogens.

Trained immunity (a.k.a. Innate memory response) is an emerging branch of host immune response that defines the ability of innate immune cells to generate non-specific immunological memory responses, which can confer long-term protection against infections ( Figure 1A ). Trained immunity and innate immunity provide broad, non-specific protection against a range of pathogens. Innate immunity encompasses the broader array of non-specific defense mechanisms present from birth, whereas trained immunity specifically refers to the enhanced responsiveness of innate immune cells following exposure to certain stimuli. Similarly, while adaptive immunity is marked by a precise, antigen-specific recall response, the hallmark of trained immunity is its non-specific and antigen-independent immune response. Trained immunity is induced by innate immune cells, such as macrophages, monocytes, and NK cells, or in case of reentry of pathogen or vaccination. The development of trained immunity occurs at a preliminary level (central training) with the involvement of the hematopoietic stem (HSCs) and progenitor cells (HSPCs) (33). These cells, residing in the bone marrow have a longer life span and respond rapidly to protect the host against chronic infections (34, 35). In addition to the bone marrow-derived HSCs and HSPCs, monocytes and granulocytes with respectively, Ly6C⁺ and Ly6G⁺/Gr-1⁺ phenotypes, are associated with the trained immunity-mediated effector functions, including degranulation and release of proinflammatory molecules upon infection by various pathogens (36–38). These immune responses can retain immunological memory between the self and non-self which leads to the establishment of a long-term trained immunity (7, 39).

Epidemiological observation studies in humans vaccinated with BCG, intended to protect against TB, indicated a non-specific cross-protection against sepsis and respiratory tract infections in vaccinated individuals (40). Similar observations of cross-protection were noted for the measles vaccine and smallpox vaccine against general mortality in children and leprosy or whooping cough, respectively (41, 42). Further, the existence of trained immunity has been reported in various in vivo studies, including treating mice with different antigens and stimulants, which would protect against infection. For instance, administration of β-glucan, a fungal cell-surface ligand protected normal and leukemic mice against systemic sepsis caused by Staphylococcus aureus infection (43, 44). Similarly, intraperitoneal administration of CpG, an oligo deoxy nucleotide, protects against meningitis caused by E. coli infection in neutropenic mice models (45). A study on SCID mice that lack functional T cells and B cells showed that BCG vaccination protected the mice against Candidiasis (46, 47). Further, BCG vaccination in Rag^-/- knockout and athymic mice lacking T cells were protected against re-infection with C. albicans (48, 49). Similarly, β-glucan induces trained immunity in splenectomized mice, and removal of the spleen did not modulate the expression of pro-inflammatory cytokines or circulating monocytes or NK cells (50). In another study, activation of trained immunity through intraperitoneal injection of LPS and BCG was shown to be associated with the induction of inflammation and fibrosis in Balb/c mice, marked by increased expression of cytokines (IL-6, IL-1β, IL-6 and IL-10), chemokines (CCR2, CCR4, TLR-2, and TLR-4), inflammatory (Ly6C and CD43) and co-stimulatory receptors (CD80 and iCOS) by the stimulated splenocytes (51). It should be noted that in standard mice models, the contributions of innate and adaptive immunity are intertwined, making it challenging to dissect the specific roles of each component. However, the SCID and Rag-/- mice lack functional T and B lymphocytes, rendering them incapable of mounting adaptive immune responses (52–54). Therefore, SCID and Rag-/- mice allow researchers to study the role of innate immunity, including trained immunity, in host defense innate immune response, without the confounding effects of adaptive immunity. Furthermore, due to their reliance on innate immune responses, SCID and Rag-/- mice may exhibit heightened sensitivity to stimuli that induce trained immunity (52–54). This increased sensitivity can facilitate the detection of subtle changes in innate immune function and provide insights into the mechanisms underlying trained immunity. Thus, data from studies obtained in transgenic mice models that are defective in adaptive immunity, such as lack of secondary lymphoid organ (i.e. spleen) or T and B cells, highlights the functional contribution of trained immunity in mounting an effective immune response and/or protecting the infected host. However, findings from studies in SCID and Rag-/- mice should be interpreted in the context of their immunodeficient status and may not fully recapitulate immune responses in normal physiological conditions (52–54).

The immunostimulants of trained immunity, including β-glucan, oxidized low-density lipoprotein (oxLDL), and BCG induce epigenetic reprogramming through histone modifications and metabolic shifts in innate immune cells, such as macrophages. These stimuli trigger changes in proinflammatory gene expression profiles and immunometabolic pathways, enhancing immune activation and host defense mechanisms (18). For example, β-glucan activates Dectin-1 signaling, promoting glycolysis and pentose phosphate pathway while suppressing oxidative phosphorylation (OXPHOS). Similarly, oxLDL engages scavenger receptors, inducing epigenetic alterations and pro-inflammatory responses, while BCG induces epigenetic reprogramming and shifts macrophage metabolism toward glycolysis, contributing to trained immunity and improved host defense (18). An in vitro study on human monocytes using β-glucan, oxLDL, and BCG as stimulants, induced trained immunity with increased reactive oxygen species (ROS) production and metabolic shift towards glycolysis, which activated a proinflammatory response of these APCs (55). It should be noted that activation of trained immunity by different stimuli may lead to varied trained responses that have potential implications for disease resistance and homeostasis. For example, in patients with systemic lupus erythematosus (SLE), excessive inflammation triggered by necrotic debris of neutrophils and macrophages, including nucleic acids and proteins, impairs the innate immune functions, leading to poor phagocytosis, formation of immune activation complex (IAC) and auto-antigens (18). In addition, histone modifications were also impaired in SLE patients, which alters the epigenetic and immunometabolic reprogramming of APCs. Since these processes and pathways are directly associated with key immunological responses of trained immunity, activation of these biological functions through trained immune activation of APCs would further worsen the disease pathology, as seen in SLE patients (18). Overall, various antigenic stimuli, such as PAMPs, DAMPs, and vaccines can modulate the trained immunity of innate immune cells to different extents through the interplay between epigenetic remodeling, cellular metabolism, and immune function.

Apart from infection, physical exercise can also impact the ability of innate immune cells to develop trained immunity. Regular exercise has been shown to enhance trained immunity through various mechanisms, including improved immune cell function, anti-inflammatory effects (switching from proinflammatory M1 to anti-inflammatory M2 phenotype), metabolic adaptations (elevating mitochondrial quality and function), stress reduction, and epigenetic modifications (56, 57). These effects have significant implications for human health and disease prevention strategies, as exercise may reduce the risk of infections, autoimmune diseases, and chronic inflammatory conditions. To demonstrate the impact of trained immunity on APCs’ function in the context of exercise, C57BL/6 mice were subjected to daily exercise at 1 hour per day on a treadmill for 8 weeks, and BMDMs were isolated from these mice and stimulated with LPS to elicit trained immunity. Interestingly, BMDM from exercised mice induced a higher NF-κB activation and associated proinflammatory gene expression, compared to the control mice. This suggests that moderate exercise reprograms the metabolic pathways related to trained immunity in BMDMs (58). Thus, activation of trained immunity during regular physical activity could promote immune health and protect the host against the burden of infectious and inflammatory diseases.

Trained immunity may contribute to the pathogenesis of chronic inflammatory conditions, such as atherosclerosis, rheumatoid arthritis, and inflammatory bowel disease (7, 18). In these conditions, dysregulated trained immunity can lead to excessive inflammation and tissue damage, exacerbating disease pathology (7, 18). Conversely, modulating trained immunity through targeted interventions may offer therapeutic opportunities for managing chronic inflammation. For example, dampening trained immune responses could help mitigate inflammation in conditions like atherosclerosis, while enhancing trained immunity may promote immune surveillance and tissue repair (7, 18). In a study by Bhattarai et al, a dystrophin-deficient mice BMDM, which lacks a chemokine receptor, CCR2, was used to study the effect of trained immunity on Duchenne muscular dystrophy (DMD). In this study, stimulation of BMDMs isolated from DMD mice with β-glucan resulted in TLR-4-dependent functional and epigenetic changes, inducing a memory response with the release of pro-inflammatory markers of trained immunity (59, 60).

Similarly, trained immunity has been implicated in the pathogenesis of autoimmune diseases, in which the immune system mistakenly targets self-antigens and mounts an inflammatory response. Dysregulated trained immune responses may contribute to the breakdown of immune tolerance and the perpetuation of autoimmunity as described above for SLE (7, 18). Modulating trained immunity could be explored as a potential strategy for managing autoimmune diseases. For instance, dampening trained immune responses might help mitigate autoinflammatory processes, while enhancing regulatory mechanisms could promote immune tolerance and reduce autoimmunity (7, 18, 60).

Trained immunity also plays a critical role in host defense against microbial infections by providing a faster and more robust immune response. Vaccines, such as BCG, measles, and monkeypox can induce trained immunity and enhance protection against unrelated infections (7, 18). Leveraging trained immunity in vaccine development and immunization strategies may improve vaccine efficacy, particularly in populations with impaired adaptive immune responses, such as the elderly or immunocompromised individuals. Understanding the mechanisms underlying trained immunity in specific infectious diseases could inform the development of novel therapeutic approaches and adjuvants for enhancing immune responses and combating infections.

Trained immunity is durable and can exhibit long-lasting effects, persisting for weeks to months after the initial stimulus (7, 18, 61). This durability allows for sustained protection against infections and may contribute to vaccine efficacy. However, the duration of trained immunity responses can vary depending on factors such as the nature of the stimulus and the strength of the elicited immune response (7, 18, 62). In addition, there is considerable variability exists in the magnitude and duration of trained immunity responses among individuals. Some of the factors contributing to this variability include genetic background, age, sex, environmental exposures, and pre-existing health conditions. While trained immunity is beneficial for enhancing host defense, overactivation of innate immune responses can lead to chronic inflammation and tissue damage. Dysregulated trained immunity has been implicated in the pathogenesis of chronic inflammatory conditions, autoimmune diseases, and metabolic disorders. Therefore, understanding the factors that influence individual responses and balancing the activation of trained immunity with regulatory mechanisms is important for optimizing therapeutic interventions and vaccine strategies to prevent excessive inflammation and maintain immune homeostasis.

Since the inception of the trained immunity concept, researchers have provided experimental evidence for various mechanistic aspects of the ability of innate immune cells to develop memory responses ( Figure 1B ). Modulations in the immunological, epigenetic, and metabolic aspects of innate immune cells upon stimulation with antigens, have been implicated as the primary mechanisms in establishing trained immunity. These three mechanisms are interdependent and regulated by multiple crisscrossing cellular signaling pathways and networks that ultimately culminate in the effector response changes observed in trained innate immune cells ( Figure 2 ).

Neutrophils (polymorphonuclear cells) are produced in the bone marrow and are the first responders to any injury or infection of the host. Although neutrophils have a shorter lifetime, they are the most abundant innate immune cells that provide significant and non-specific broad protection against related or unrelated pathogens (20, 131, 132). Although neutrophils are traditionally thought to lack long-term memory, recent studies suggest that they can retain epigenetic memory. Neutrophils are among the key effector cells of trained immunity with antigen-presenting functions, secreting cytokines that induce inflammatory factors, degranulate, and eliminate microbes through phagocytosis (133). A study reported that BCG vaccination in healthy individuals reprogrammed the neutrophils with increased expression of surface markers CD11b and CD66b, with concomitant dampening of CD62L and PDL1 expression, which persisted for at least 3 months, through epigenetic modifications (76). In this study, in vitro stimulation of blood-derived neutrophils, obtained after 3 months of BCG vaccination, with LPS or Staphylococcus aureus was shown to induce the expression of neutrophil activation markers CD11b and IL-8 (76). These trained neutrophils also showed increased degranulation and phagocytosis in vitro when stimulated with Candida albicans, Mtb, or LPS (76). Intranasal BCG vaccination of mice showed increased accumulation of neutrophils in the lungs, which helped to control subsequent Mtb infection by promoting antimicrobial responses (134).

In a zebrafish larvae model of Shigella infection, stimulation with BCG or β-glucan before infection elicited trained immunity in neutrophils, which showed epigenetic alterations, elevated ROS production, and antimicrobial responses (135). Importantly, induction of trained immunity in neutrophils has been shown to occur in a paracrine manner (136). In this study, treatment of neutrophils with soluble factors, secreted by mesenchymal stromal cells upon stimulation with CpG-ODN, a TLR-9 ligand, induced trained immunity marked by characteristic histone modifications and elevated granulopoiesis (136). Similarly, neutrophils isolated from mice treated with zymosan, which contains β-glucan, had elevated IL-6 levels upon ex vivo re-stimulation with LPS (35, 137). These trained neutrophils also showed elevated myeloperoxidase levels and improved killing of intracellular L. monocytogenes (35). Thus, these studies suggest that induction of trained immunity in neutrophils by various stimuli can promote neutrophil activation, which is useful for effective bacterial clearance (137). In contrast to these findings, subcutaneous vaccination of C57BL/6 mice with BCG was reported to control Mtb at 7 days post-infection through a non-trained immunity mechanism, involving neutrophils and macrophages (138).

Neutrophils, despite their short lifespan, inherit epigenetic modifications from the myeloid progenitor cells during hematopoiesis (101, 139). Neutrophils can undergo activation-induced epigenetic changes in response to antigenic stimulation or microbial challenge. These activation-induced epigenetic modifications can persist even after the stimulus is removed, allowing neutrophils to retain a primed/activated state and mount rapid and robust response upon re-stimulation (101, 139). For example, elevated H3K4me3 at the promoters of JAK/STAT signaling (e.g., STAT4), PI3K/AKT pathway as well as proinflammatory (IL-8, IL-1B) and metabolic (mTOR, HK1 and PFKB) network genes noted in the peripheral neutrophils of BCG-vaccinated individuals, compared to non-vaccinated controls (101). Induction of these pathways is the hallmark response of trained immunity (i.e., proinflammatory and antimicrobial responses), suggesting that histone methylation underpins the long-term trained response of neutrophils (101). Other epigenetic reprogramming markers, such as DNA methylation and histone modifications can be established during neutrophil development (granulopoiesis) and maintained throughout their lifespan in both circulation and at specific tissues (101, 139). Emerging evidence suggests that epigenetic modifications acquired by neutrophils in response to environmental cues or inflammatory signals may be transmitted to their progeny (139). For example, neutrophils can release extracellular vesicles containing epigenetic regulators such as microRNA and histones which may influence the landscape of neighboring cells or circulating progenitors (139). A recent study revealed an association between the epigenetic reprogramming of granulopoiesis as well as neutrophils and trained immunity induced by β-glucan in a murine model of cancer (140). In this study, mice treated with β-glucan displayed an anti-tumor phenotype mediated by trained neutrophils through a type I IFN signaling, and independent of the host adaptive immune response. Importantly, the trained immunity-mediated anti-tumor effect can be established in the naïve mouse upon adoptive transfer of neutrophils or transplantation of bone marrow from the β-glucan treated mice (140). These transgenerational epigenetic inheritance mechanisms could contribute to the persistence of epigenetic changes in neutrophils and their progeny, potentially contributing to immune memory across generations of immune cells.

Since trained immunity involves reprogramming of innate myeloid cells to induce robust and long-term efficient immune response upon pathogen encounter, several potential avenues of this pathway can be used as targets to develop intervention strategies ( Figure 4 ). For example, trained immunity is regulated by epigenetic and metabolic changes in the innate immune cells relying on specific pathways and networks (18). Among the epigenetic modifications, histone methylation, acetylation, and DNA methylation are the key targets to tweak trained immunity. Drugs that target histone modification enzymes, such as HATs, HDACs, HMTs and HDMs can modulate the epigenetic landscape of trained cells and their effector functions (18). For example, HATs and H3K4Me3 activating drugs can promote histone acetylation and promoter access to enhance the production of proinflammatory (e.g., TNF-α, IL-6) and effector molecule (e.g., cathelicidins) production and the response of trained cells. Similarly, HDAC, HMT and DNMT inhibitors can be used, respectively, to influence histone methylation dynamics and enhance the stability and maintenance of DNA methylation marks to facilitate long-term memory-like responses associated with trained immunity. Metabolic pathways such as glucose metabolism and lipid metabolism, the shift in pentose phosphate pathway, aerobic glycolysis, and alterations in FAO might also be ideal targets to activate trained immunity (18). Increased glycolytic flux, fueled by glucose uptake and utilization, provides energy and biosynthetic precursors for enhanced effort functions and cytokine/chemokine production in trained cells. Furthermore, metabolic intermediates, such as α-KG, succinate and acetyl-CoA derived from metabolic pathways serve as substrates for epigenetic modifications and regulate the gene expression profile of trained cells. Thus, inhibitors and modulators of key enzymes of glycolysis or OXPHOS, as well as amino acid and fatty acid metabolism can be used to elevate the energy metabolism of trained cells for rapid and robust effector functions against pathogenic challenge. Furthermore, PRRs such as TLRs and nucleotide binding receptors are critical in recognizing the antigens and pathogens and interacting with trained immune cells. Transcription factors such as signal transducer and activator proteins (STAT) influence immune cell functions and proinflammatory signaling pathways such as IL-1β and IFN also might act as potential targets to enhance trained immunity.

Similarly, since trained immunity induces a broader but effective immune response, particularly against a range of pathogenic microbes, understanding the pathways and molecules involved in the activation of trained immunity can be useful in developing potential vaccine candidates against infectious diseases. Targeting pathways of trained immunity, such as histone modifications, metabolic reprogramming, and innate immune signaling, with specific agonists or antagonists cold enhance innate effector responses. This can be achieved by incorporating molecules, such as epigenetic or metabolic modulators that can induce trained immunity, into vaccine formulation. For example, incorporating small molecule agonists or adjuvants that promote trained immunity, such as β-glucan, BCG, or specific cytokines (e.g. IL-1β, IL-6) could be used to boost immune memory and improve vaccine efficacy. Similarly, adjuvants and live attenuated vaccines that can interact with PPRs, such as TLRs, NLRs and CLRs can augment vaccine-induced lasting immunity. These molecules act as strong adjuvants or immune stimulants during vaccine formulation and help in priming the immune system at a stronger level to encounter primary or secondary infection by divergent pathogens or in combating different types of cancer.

Since chemotherapy for cancer treatment is often highly cytotoxic, alternates such as immunotherapy have been sought as better treatment options. Importantly, the efficacy of immunotherapy can be improved by combining with agonists such as β-glucan, which can train and activate myeloid cells for better anti-tumor effector functions. This concept has recently gained attention in the treatment of neuroblastoma (NB) and metastatic pancreatic ductal adenocarcinoma (PDAC) treatment. For example, in a recent Phase II randomized clinical trial of patients with NB, adjunctive oral administration of β-glucan during bivalent, GD2 lactone/GD3 lactone-keyhole limpet hemocyanin conjugate vaccination was shown to increase the anti-GD2 IgG1 antibody titer without elevating toxicity, which was associated with better survival of vaccinated patients (141, 142). Moreover, an ongoing phase II clinical trial (Clinical trial NCT04936529) is evaluating the protective efficacy of this bivalent vaccine (OBT-821) combined with β-glucan as a dietary supplement and granulocyte-macrophage colony-stimulating factor (GM-CSF), against NB. The idea of including GM-CSF along with β-glucan is to increase the number of granulocytes, such as neutrophils by GM-CSF, while empowering those cells through β-glucan-mediated trained immunity to effectively control NB cells. Similarly, in a Phase II study (Clinical trial NCT00874848), BTH1677, a β-glucan immune modulator was shown to improve the efficacy of cetuximab, carboplatin, and paclitaxel as first-line treatment for non-small cell lung cancer (143). In addition, the tolerability and efficacy of β-glucan combined with a CD40 agonistic monoclonal antibody (CDX-1140) is being tested in a Phase 1b study on patients with PDAC (Clinical trial NCT04834778). The logic of this approach is that both CDX-1140 and β-glucan can promote the activation and maturation of APCs through non-redundant myeloid signaling pathways that shift the immune milieu of the tumor microenvironment (TME) and facilitate better clearance and control of cancer cells. Although the results of the PDAC trial are pending, these clinical studies indicate the potential application of trained immunity-based concepts to devise novel and improved treatment modalities for various diseases. It is worth noting that the cytokine/chemokine-induced trained immunity (e.g., IL-1b, GM-CSF, M-CSF) can potentially be useful as combination therapy in preventing/alleviating treatment-associated (e.g., chemo-/radio-/immune-therapy) or disease-associated complications. In a clinical study, treatment of GM-CSF in combination with rituximab and cyclophosphamide/doxorubicin/prednisone/vincristine improved the survival of patients with de novo diffuse large B-cell lymphoma (144). Similarly, G-CSF (e.g., pegfilgrastim) is already in clinical use for treating neutropenia occurring during the myelosuppressive chemotherapeutic regimen for cancer treatment (145). Thus, there is a higher potential for chemokines such as M-CSF to be used as an immune trainer (e.g. post-chemotherapy or after stem cell transplantation), which can induce epigenetic rewiring in HSCs in vivo (28), and thus may be helpful for disease management.

Trained immunity might be useful in designing vaccines against pathogens that can mutate or develop resistance over time; an activated innate memory response can effectively recognize and respond to the evolving strains. In addition, stimulators of trained immunity can be combined with traditional vaccines or immunotherapies to enhance persistent, longer host-protective immune responses. For example, epigenetic modulators, such as HDAC or HMT inhibitors, as well as metabolic modulators targeting glycolysis, OXPHOS can be combined with vaccines to improve the durability of trained immune response. Nutritional interventions, including dietary supplements such as vitamins, amino acids, or fatty acids, as well as mitochondrial-targeting antioxidants, can enhance the effectiveness of vaccines and boost immune memory. Thus, understanding individual variations in trained immunity responses, influenced by genetics, age, and environmental factors could be useful for developing personalized vaccines tailored to elicit a specific immune response. Moreover, biomarkers of trained immunity, such as epigenetic and metabolic signatures, and cytokine profiles, can be used to assess vaccine responsiveness and guide to improvise personalized vaccination strategies, particularly in vulnerable populations. Large-scale clinical studies are needed to identify genetic determinants, biomarkers, and other factors of variations in trained immunity. This approach would aid in developing personalized precision medicine based on trained immunity for the effective treatment of infectious and chronic diseases.

Although the concept of trained immunity emerged recently, it has gained significant momentum in explaining the host response to microbial infections and chronic diseases, with the potential for clinical applications. However, trained immunity generates a non-specific, immune response towards unrelated pathogens, compared to the antigen-specific, targeted response of adaptive immunity. This lack of specificity might result in an overstimulated immune system causing unnecessary inflammation and tissue damage. Unlike adaptive immunity, trained cells do not discriminate between pathogen-specific antigens, this lack of antigen specificity could lead to non-specific or inappropriate immune responses. In addition, the duration and persistence of trained immunity are yet to be fully unraveled as it is unclear how long the enhanced protective immunity would last and whether it would have any long-term side effects. The non-specific nature of trained immune cells also raises the possible risk of the immune system targeting self-antigens, mistakenly leading to immunopathology, autoimmunity, or chronic inflammatory conditions. In this scenario, the identification of targetable negative regulators or checkpoints that modulate trained immunity pathways and maintain immune homeostasis would reduce the risk of autoimmune reactions. Furthermore, designing newer vaccines based on trained immunity might be challenging, due to non-specificity, as traditional vaccines are designed to induce antigen-specific adaptive immune responses, and attempting to replicate this specificity with a trained immunity concept might be complex and complicated. Therefore, understanding the interconnectedness of pathways/networks involved in the molecular and cellular processes of trained immunity, including epigenetic modifications, metabolic reprogramming, and immune response, is vital for identifying specific, context-dependent intervention targets. Moreover, the effector and regulatory functions of trained immune responses can vary significantly between individuals and populations based on factors such as age, sex, prior exposure to microbes, and genetic variations, which makes it difficult to predict and control the trained immune responses as it leads to hyperinflammation. Therefore, accounting for this heterogeneity and developing personalized approaches, tailored to individual immune profiles may be necessary for optimizing clinical outcomes. The duration and persistence of trained immune responses may vary, and the longevity of memory-like responses is currently not well understood. Therefore, studies on the molecular pathways and epigenetic mechanisms governing the durability of trained memory and enhancing memory persistence are urgently needed to devise improved interventions for long-lasting immune protection.

Ethical considerations pose an additional challenge in implementing interventions based on trained immunity, as inducing a non-specific immune response, without understanding the long-term effects is unlikely to be accepted by the community. Due to the potential off-target effects of interventions targeting trained immunity, ensuring the safety of such interventions and minimizing adverse effects is critical for clinical translation, particularly in vulnerable populations. Thus, designing clinical trials to evaluate the safety and efficacy of interventions targeting trained immunity poses unique challenges. Identifying appropriate biomarkers of trained immunity, defining clinically relevant endpoints, and conducting long-term follow-up studies are essential for patient stratification, treatment selection, assessing therapeutic efficacy, and establishing clinical utility. In this regard, collaboration between basic scientists, clinicians, immunologists, pharmacologists, and other stakeholders is essential for advancing translational research efforts. Integrating expertise from this multidisciplinary group can accelerate the translational research to clinical applications. In addition, compliance with regulatory approval, including patient consent, privacy, data sharing, and protection of personal data, per ethical guidelines is essential for advancing clinical translation efforts. Patient-centered approaches that prioritize patient needs, preferences, and values through the engagement of patients, caregivers, and advocacy groups that balance scientific rigor and patient welfare, can enhance awareness and support for trained immunity-based therapies.