According to the World Health Organization, infectious diseases (ie, pneumonia, diarrhea, measles and malaria), followed by preterm birth complications, accounted for approximately 5.0 million deaths of children (<5 years) worldwide in 2020. Particularly, in low- and middle-income countries, pediatric infectious diseases remain a significant public health concern. During early development, infants rely mainly on the activity of the innate immune responses in the defense against invading pathogens. Until recently, the dogma has been that the innate immune system lacks memory regardless of successive exposures. This paradigm has been recently challenged by increasing data indicating that innate immune cells may develop nonspecific (off-target) memory-like responses, a process termed trained immunity or trained innate memory (TRIM).1,2 This responsive feature enables innate immune cells to exhibit rapid action with increased efficacy against subsequent infections. Here we will discuss the role of TRIM and its implications for infectious diseases in childhood.

The Concept of Trained Innate Memory

Although lacking an adaptive immune system, plants and invertebrates are well-recognized to exhibit nonspecific resistance responses against reinfections.1 Correspondingly, it would be plausible that such a critical evolutionary trait of the innate immune system responding with memory features, would not be restricted and may occur also in highly advanced species like vertebrates. Several studies have demonstrated that immunization with the live attenuated vaccine against tuberculosis, Bacille Calmette-Guerin (BCG)—an activator of multiple pathogen recognition receptors including toll-like receptors, protects not only from tuberculosis mycobacteria, but has a myriad of off-target immunologic effect against infectious diseases due to other pathogens, boosting the immune system and contributing to decreased overall mortality in mammals.3 More recently, Netea et al clarified these observations by introducing the concept of TRIM as a nonspecific responsive feature of innate immune cells as well as tissue-resident stem cells in vertebrates.1,2 Taken together, reports highlight a new characteristic trait of innate immune (monocytes, macrophages, NK cells, microglia and mast cells) and nonimmune cells (ie, epithelial cells), undergoing functional reprogramming characterized by epigenetic and metabolic rewiring to exhibit memory-like signals marked by heightened resistance responses when faced with secondary challenges.1,3

To date, a wide array of signals, including both pathogen-derived stimuli (eg, β-glucan, lipopolysaccharide and BCG) and diverse nonpathogen-associated endogenous stimuli (eg, heme, urate), have been identified as potential triggers for reprogramming the innate immune system, resulting in enhanced protective effects1,2 (Fig. 1). Moreover, microbiota-derived nano-sized biovesicles carrying bioactive components such as nucleic acids, proteins and metabolites promote the induction of TRIM in murine granulocytes in vitro.2,4

FIGURE 1.: Schematic illustration of different factors (immunization, probiotics and microbiome) that may support TRIM during childhood and the putative role of increased/decreased TRIM affecting crucial infectious events during early development (pros/cons).1,3

At the cellular level, TRIM is defined by enhanced immune reactivity, characterized by increased production of pro-inflammatory mediators (ie, Tumor necrosis factor-α, IL-6, IL-1β and reactive oxygen species) and increased phagocytic as well as antimicrobial activities, mediated particularly by phosphoinositide 3-kinase/mechanistic target of rapamycin pathway.1,4 A growing body of evidence suggests that TRIM results from epigenetic and metabolic reprogramming of innate immune and nonimmune cells, including mono- or trimethylation (ie, H3K4me1, H3K4me3) and acetylation (ie, H3K27ac) of histones, with resulting changes in metabolism such as increased glycolysis, glutaminolysis or accumulation of fumarate.1,4 Circulating innate immune cells have a defined lifespan, and the induction of TRIM occurs in hematopoietic progenitor cells, contributing to prolonged defense capacity.1

The Impact of Innate Memory During Childhood Infectious Diseases

Depending on various factors (eg, pathogen type, disease), TRIM can be either beneficial or harmful, and its modulation is of clinical interest to enhance protective reactions or mitigate extensive inflammatory events. To date, the concept of TRIM has gained increasing interest as a potential mechanism to improve innate immune responses against various infectious agents, especially in preterm and term neonates with enhanced vulnerability to infectious diseases due to impaired immune function.

Vaccination represents one of the most effective and promising strategies for reducing the burden of pediatric infectious diseases. As mentioned previously, the BCG vaccine was one of the first and most studied vaccines to support broader protection of host against various infectious diseases.1 Induction of TRIM by BCG in a murine model of neonatal sepsis was shown to protect mice against microbial sepsis in vivo, indicated by elevated inflammatory cytokines, increased neutrophil recruitment, and increased antimicrobial activity driven particularly by macrophages.5 Initial observations showed that these antibacterial and antiviral effects induced by BCG are mainly triggered by the reprogramming of innate immune cells, especially monocytes/macrophages through an IL-1β-dependent pathway.1 Notably, these effects were promoted by increased epigenetic modification on the promoters of H3 histones (like H3K4me3 and H3K27Ac) and increased glycolysis.5

Clinical evidence indicating possible TRIM effects during early life in humans comes from the observation that chorioamnionitis—an intrauterine inflammatory condition—was associated with a reduced risk of developing Staphylococcus epidermidis bacteremia as well as late-onset bacterial sepsis in preterm neonates.3 An epidemiologic study showed that BCG vaccination was associated with decreased hospitalization rates of newborn infants with respiratory infections and sepsis, indicating possible indirect evidence of TRIM.6 There are increasing number of studies on the potential off-target effects of BCG to reduce the burden of several viral infections, including yellow fever, malaria, and lately was also reported for COVID-19 infections.1,2,7 These studies demonstrated that BCG-vaccinated healthy adults had lower levels of the experimental yellow fever virus loads as well as Plasmodium falciparum parasitemia, driven by the memory traits of myeloid cells in a cytokine-dependent [ie, IL-1β, interferon (IFN)-γ] manner. Interestingly, an in vivo study revealed that adults immunized by a single dose of adenoviral vector COVID-19 vaccine (AZD1222) exhibited also TRIM features to subsequent unrelated stimuli in monocytes for up to 3 months after vaccination.7,8 In a randomized clinical trial, healthy adults receiving the measles, mumps and rubella vaccine exhibited profound long-term functional changes with increased levels of tumor necrosis factor-α and IFN-γ—TRIM in γδ T cells, affording broader protection against unrelated infections.2 Moreover, children infected by SARS-CoV-2 viruses manifested less severe COVID-19 symptoms and had faster recovery compared to adults, which may be driven by routine exposures of children to vaccination schedules, including here BCG vaccine, promoting better resistance mechanisms (Fig. 1).1 As shown in an ex vivo model of TRIM, these off-target protective effects of human neonatal monocytes exposed to BCG were driven by elevated IFN signatures accompanied mainly by long-term DNA methylation marks at the promoters of inflammatory genes.7 However, clinical data remain conflicting. A recent randomized trial where BCG vaccination did not result in improved outcomes in COVID-19 disease in healthcare workers.6

It is well-known that the immune system of newborns is marked by impaired responses of the adaptive immune system, especially impaired T helper (Th) cell functions. In line with this, Hong et al9 showed that in utero hepatitis B viral exposure provokes TRIM effects in cord blood immune cells, characterized by elevated levels of IL-12p40 and IFN-α2 triggering the maturation of the innate immune system and development of Th1 driving responses against unrelated microbial pathogens in vitro. Despite its role as a pathogen for local and disseminated infections, Candida albicans cell wall constituent β-glucan is a potent immune modulator that may support resistance reactions against secondary infections by various pathogenic agents during early life.1

The role of microbiota in regulating the development and function of the innate and adaptive immune system is increasingly recognized. Furthermore, microbiome-produced metabolites such as short-chain fatty acids directly influence myeloid cells, bolstering their protective ability by regulating epigenetic signatures (eg, inhibition of histone deacetylase) and metabolic rewiring (eg, lipid metabolism).1,2 While the gut microbiome is known as a crucial modulator of vaccine responses, its role in BCG-induced protection remains elusive. An ex vivo study found that Roseburia species (a short-chain fatty acid-producing bacteria) may alter TRIM reactions induced by BCG.10 In line with this, a study in human dendritic cells in vitro revealed a greater impact of gut microbiome, particularly probiotic bacteria Limosilactobacillus reuteri, in inducing a memory-like phenotype resulting in Th cell differentiation.11

Inappropriate induction of TRIM may trigger maladaptive effects resulting in the development of several inflammatory pathologies (ie, sepsis, autoimmune diseases, atherosclerosis).12 Sepsis represents a life-threatening and extremely challenging condition where patients experience simultaneously both hyper-inflammatory and immunosuppressive features. To date, there are few data about the role of TRIM in sepsis in general and specifically for young patients, however, we would speculate that TRIM can be beneficial for septic patients during the recovery state by boosting protection against secondary infections, but may also worsen the early phase of sepsis by resulting in increased inflammatory cytokines leading to organ dysfunction.1,3 Altogether these studies reveal how the early-life innate immune system adapts to microbes, developing TRIM against severe infections.

CONCLUSIONS

Pediatric infectious diseases continue to be a global health concern. Effective public strategies, including vaccination programs, are essential for mitigating the impact of these diseases and reducing their burden on children. TRIM as a novelty of innate immune cells may serve as a compelling therapeutic approach for the prevention of several infectious diseases during childhood; however, the current knowledge about TRIM in pediatric infectious diseases, underlying mechanisms and applications are limited. Critical future research should focus on elucidating the molecular mechanisms, long-term effects, and disease-specific applications of TRIM in pediatric infectious diseases, prioritizing tailored vaccinations, microbiome fluctuations, age-specific studies and effective interventions.

ACKNOWLEDGMENTS

The authors acknowledge Prof. Dr. Sotirios G. Zarogiannis for his collegial review of the manuscript.

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