Pediatric tumors often develop in highly proliferative tissues with structural and vascular differences from adults. The ECM in pediatric tumors is enriched with laminin, fibronectin, and collagen III and serves as a dynamic supporting matrix that fosters tumor growth and immune modulation (1, 6). Unlike the rigid ECM in adults, pediatric ECM is plastic and more susceptible to remodeling. Similarly, basement membrane scaffolds remain immature and loosely organized during tissue morphogenesis, facilitating tumor cell invasion (3, 11).

Non-cellular mediators include both soluble factors (VEGF, TGF-β, IGF-1) and insoluble components like ECM proteins and glycoproteins that establish biochemical gradients influencing tumor behavior (3, 8). The chemokine and cytokine networks also differ between pediatric and adult tumors, which often rely on CXCL12/CXCR4 signaling to drive migration and angiogenesis, whereas adult TMEs exhibit proinflammatory cytokine profiles (IL-6, TNF-α) that support immune evasion (9, 18).

The immune contexture differs as well, with pediatric TMEs displaying fewer antigen-presenting cells and a predominance of immunosuppressive populations like Tregs and M2 macrophages compared to adult tumors (6, 18). These immune cell populations play critical but opposing roles in tumor immunity as illustrated in Figure 2,

The pediatric tumor microenvironment (TME) undergoes developmental changes that significantly influence tumor progression, immune modulation, and therapeutic responsiveness. As children mature, the evolving immune and stromal landscape within the TME reflects both intrinsic developmental processes and tumor-induced reprogramming. These changes underscore the need for age-adjusted therapeutic approaches in pediatric oncology.

Age-specific immune responses and the gradual maturation of immune cells shape the developmental pathways underlying immune evasion in pediatric tumors. Cancers in children arise within a distinct immunological environment, where the maturation of the immune system interacts with tumor cells in ways that differ markedly from adults. This interaction is defined by evolving immune cell profiles and functional responses that change throughout early childhood. For example, infants possess an immature, hypo-inflammatory immune system, which may limit their ability to mount effective anti-tumor responses (27). Koutsogiannaki (2023) demonstrated that innate immune responses dominate during early development and that leukocyte functions and transcriptomic profiles undergo significant changes from infancy to school age. Consistent with this, many pediatric tumors exhibit rapid growth and minimal leukocyte infiltration; the immune infiltrate is often enriched in M2-like macrophages and naïve-like T cells, both of which are less capable of initiating robust anti-tumor activity (28). This distinctive immunological landscape suggests that therapeutic strategies aimed at immune modulation particularly those promoting a pro-inflammatory environment could enhance treatment efficacy in children (29). Furthermore, the dynamic nature of the paediatric immune system may offer unique opportunities to develop innovative immunotherapeutic strategies that leverage its developmental characteristics. Recent studies show that immune cell activity in pediatric solid tumors changes as children age. Longitudinal transcriptomic profiling demonstrates a progressive decline in antitumor immune gene expression and an increase in tolerogenic and immunosuppressive signatures as children age (30). This shift reflects a transition from an immunologically active (“hot”) to an immune-depleted or tolerogenic (“cold”) TME, which may contribute to reduced responsiveness to immunotherapies and increased tumor resilience over time.

Beyond immune components, fibroblasts within the TME also undergo age-related changes that influence tumor behavior. Studies in adult pancreatic cancer models have shown that aged fibroblasts promote enhanced tumor cell proliferation, invasiveness, and ECM remodeling, suggesting that stromal aging can exacerbate malignancy (3, 6). Although direct data in pediatric tumors are limited, similar fibroblast-driven modifications to the TME are likely to occur, potentially altering tumor growth dynamics and immune accessibility as children grow.

Organ-specific age-specific biological environments within the TME play a critical role in shaping tumorigenesis, cellular plasticity, and therapeutic response, particularly in pediatric cancers. Far from being a passive structural backdrop, the TME actively regulates tumor phenotype through complex interactions between resident cells and environmental cues. In pediatric gliomas, for example, distinct regional microenvironments give rise to intra- and inter-tumoral heterogeneity, highlighting how local context can drive divergent tumor behaviors (4). In pancreatic cancer, regulatory T cells (Tregs) can either support or suppress tumor growth depending on how they interact with local stromal and immune cells, showing how flexible immune responses can be in different tissue environments (13). These differences are also influenced by normal developmental processes, such as branching morphogenesis, which shapes organ architecture through signaling cues and ECM remodeling. While these processes are crucial for healthy tissue formation, they can reappear in tumors, influencing shape, growth, and progression (31). Studies comparing organ development have uncovered shared regulatory patterns that drive organ-specific adaptations, highlighting how local context can determine tumor behavior (32). Even though tissue-specific traits are important, finding pathways that are shared across organs could reveal therapeutic targets with wider relevance (33). Understanding these organ-specific TMEs is especially critical in children, where ongoing development and immature tissues add extra complexity to tumor biology.

In pediatric kidney tumors, such as Wilms tumor, the TME is strongly shaped by kidney-specific developmental processes. Kidney development relies on carefully coordinated interactions between epithelial and mesenchymal cells, guiding nephron formation and vascular patterning processes that tumors can take advantage of (34). The pathways such as WT1, WNT, and IGF2 are often dysregulated in tumor and the surrounding stroma (31). Early childhood is a critical period because kidney growth and function closely follow age, making the pediatric renal TME highly sensitive to developmental cues (4). Environmental factors, like nutrition or toxin exposure, can further affect normal development and tumor susceptibility. To interpret tumor-stroma interactions and for designing therapies that are tailored to young patients in pediatric nephro-oncology it is important to understand the age-dependent environment.

Hormonal signaling plays a multifaceted and developmentally sensitive role in shaping the pediatric TME, particularly in relation to growth and neurodevelopment. Central to this process is the growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis, which is essential for postnatal skeletal growth and tissue development. Disruption of this axis, as demonstrated in Tmem263 knockout mice, results in severe dwarfism and skeletal dysplasia due to its regulation of GH receptor expression and IGF-1 levels (35). These alterations have systemic implications, including on stromal remodeling and vascular development key TME processes.

In pediatric patients with growth hormone deficiency (GHD), neuroimaging has revealed reduced gray matter volume and disrupted white matter microstructure, correlating with cognitive and emotional impairments (36). Hormonal imbalances during puberty, especially fluctuations in steroid hormones like estrogen and testosterone, may further modulate neuroimmune circuits and influence the immune composition of the TME (37). Additionally, the growing concern over endocrine-disrupting chemicals emphasizes that environmental factors may exacerbate or mimic hormonal effects, further complicating the developmental trajectory of pediatric TMEs (9). These hormone-driven mechanisms underscore the need for precision in age- and sex-specific treatment approaches in pediatric oncology.

The paediatric TME is profoundly shaped by interactions with the microbiome, which modulates immune function, infection susceptibility, and therapeutic efficacy. In early childhood, the developing microbiota plays a vital role in immune education and inflammation regulation—factors that are particularly relevant for children undergoing chemotherapy or immunotherapy. Dysbiosis, or microbial imbalance, has been associated with increased infection risk, compromised mucosal immunity, and greater treatment-related toxicity in paediatric cancer patients (38, 39).

Microbiome composition can significantly influence treatment response across multiple body sites (40). Specific microbial signatures have been linked to differential outcomes in chemotherapy, immunotherapy, and antibiotic exposure, impacting drug metabolism, immune surveillance, and local TME dynamics (38, 39). In hematopoietic stem cell transplant (HSCT), balancing the microbiome can improve tolerance and reduce risk of graft-versus-host disease (GVHD) (38). For instance, Akkermansia muciniphila promotes anti-tumor immunity by activating T lymphocytes and macrophages, demonstrating direct microbiome-immune crosstalk within the TME (41).

Beyond the gut, the oral and skin microbiomes contribute to both local mucosal and systemic immune responses. In atopic dermatitis (AD), patients exhibit lower microbial diversity across oral, skin, and gut compartments compared to healthy children, with increased colonization by Staphylococcus aureus that aggravates inflammation and weakens barrier function (42, 43). Early probiotic use has been associated with reduced AD occurrence, highlighting the microbiome’s role in immune tolerance during early life (43). The oral microbiome influences not only dental health but also systemic inflammatory states (44, 45), while the skin microbiome evolves rapidly during infancy, shaped by delivery mode, antibiotic use, and environmental exposures (46). Bacterial genera such as Cutibacterium and Staphylococcus play dual roles—either maintaining homeostasis or promoting inflammatory conditions that may alter the local TME in rare paediatric cutaneous malignancies.

Emerging therapeutic strategies such as fecal microbiota transplantation (FMT) and probiotic supplementation show promise in restoring microbial balance and improving overall treatment tolerance and efficacy (47, 48). Manipulating the microbiome to reduce systemic inflammation or enhance immunotherapy outcomes represents a novel frontier in paediatric oncology. However, the dynamic nature of the paediatric microbiome, particularly in immunocompromised children undergoing treatment, poses risks including infection and treatment-related complications due to microbial imbalances. Despite potential benefits, it is crucial to consider downsides such as increased inflammation or development of antibiotic resistance. Further research is needed to establish safe and effective microbiome-based approaches tailored to paediatric oncology contexts.

Differences in nutrient availability and metabolic programming between paediatric and adult TMEs also reflect developmental distinctions. Children’s TMEs are influenced by rapid growth demands and higher nutrient needs relative to body weight, affecting both composition and metabolism of the microenvironment (49). Maternal nutrition during gestation can epigenetically program offspring tissues, influencing nutrient availability and TME properties in childhood cancers. Paediatric tumors often rely on altered metabolic pathways, such as aerobic glycolysis (Warburg effect), supporting rapid proliferation despite lower energy yield (50). These early-life exposures may induce lasting epigenetic changes affecting cancer risk and therapeutic response, whereas adult TMEs are shaped by cumulative environmental exposures, chronic inflammation, and lifestyle factors necessitating age-specific therapeutic approaches.

Sex-specific differences critically influence the paediatric tumor microenvironment (TME), shaping immune composition, stromal behavior, and therapy response particularly during puberty when hormonal and metabolic shifts occur. Estrogen and androgen signaling modulate T cell activation, cytokine profiles, and stromal remodeling, while X-linked immune genes further contribute to sexual dimorphism in immune regulation (51, 52). Estrogen enhances antibody and Th2 responses (52), whereas sex-dependent variations in CD8+, Treg, and myeloid cell populations alter tumor immunity (51, 52).

Despite well-documented sex-based disparities in immunotherapy outcomes among adults, paediatric oncology trials rarely include sex as a biological variable. The growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis shows sexual dimorphism with implications for postnatal growth and tissue development (35), while sex-specific drug metabolism via cytochrome P450 enzymes affects efficacy and toxicity. Incorporating sex-stratified analyses in future paediatric TME studies is essential to enable sex-informed precision therapies that improve treatment outcomes while reducing developmental toxicity (51, 52).

Key Points: Pediatric TME Overview.

The extracellular matrix (ECM) and associated physical features create substantial obstacles to drug delivery and immune cell infiltration. The dense ECM, enriched with fibronectin and collagen, physically prevents drug penetration into tumors (22, 56). Beyond acting as a barrier, ECM components interact with cancer cells to trigger survival signals that increase drug resistance (22). Dynamic ECM remodeling, particularly pronounced in paediatric tumors where fibroblasts actively shape stromal architecture (5–7), continuously adapts to therapeutic pressure.

Hypoxia represents a critical physical characteristic driving multiple resistance mechanisms. Low oxygen conditions typical of solid tumors activate hypoxia-inducible factor (HIF) pathways, which enhance cancer cell adaptability, promote epithelial-mesenchymal transition (EMT), and facilitate immune evasion (22). Hypoxia-driven metabolic reprogramming allows tumors to survive in nutrient-depleted environments while simultaneously excluding immune effector cells.

Drug efflux mechanisms further limit therapeutic efficacy. Many cancer cells produce large amounts of ATP-binding cassette (ABC) transporters, which actively expel drugs from cells, lowering intracellular drug concentrations and reducing treatment effectiveness (56). Cancer stem cells possess particularly robust drug efflux capabilities, helping tumors survive therapy and develop resistance (56). The TME’s acidic conditions further contribute to resistance by altering pH partitioning at cell membranes, leading to extracellular accumulation of chemotherapeutic agents that would otherwise passively diffuse into cancer cells (57).

Immune evasion mechanisms within the TME significantly impairs therapeutic responses. While some tumors respond well to immune checkpoint inhibitors, others do not, highlighting intrinsic differences in resistance development patterns. In paediatric solid tumors, high levels of TGF-β, IL-10, and VEGF contribute to immune suppression and T cell exclusion. Immune checkpoint pathways including PD-1/PD-L1 and CTLA-4, while well-validated targets in adult cancers, show limited efficacy in paediatric contexts, with response rates consistently below 5% in sarcomas (58).

New immunotherapies are attempting to reshape the tumor environment to improve treatment efficacy in paediatric cancers. Targeting stromal TME components with FAK inhibitors or TGF-β blockers, often in combination with immune checkpoint inhibitors (ICIs), has shown promise in improving T cell infiltration. This mechanistic interplay between the TME and immune modulation is critical for advancing effective immunotherapies in paediatric oncology. The predominance of immunosuppressive populations regulatory T cells (Tregs), M2 macrophages, and MDSCs creates a tolerogenic microenvironment that actively suppresses anti-tumor immunity through secretion of IL-10, TGF-β, and other inhibitory signals (6, 18, 59).

Metabolic reprogramming driven by the TME represents a key contributor to therapy resistance (60). Cancer cells adapt to metabolic stress induced by treatments through several interconnected mechanisms. The shift to aerobic glycolysis (Warburg effect) involves increased glucose uptake and lactate production to support rapid proliferation and survival under therapeutic stress (61). Enhanced lipid metabolism in both primary and metastatic tumors provide essential energy and structural components, reinforcing resistance mechanisms. Alterations in amino acid metabolism sustain protein synthesis and energy production under therapeutic pressure (61).

The TME induces stress response secretory programs (SRSPs), where non-cancerous stromal cells secrete cytokines and growth factors that protect tumor cells during treatment. Increased inflammatory signaling within the TME strengthens cancer cell survival and fosters acquired resistance (17). Cancer-associated fibroblasts (CAFs) release signaling molecules that alter tumor cell treatment responses, creating a protective niche that enhances therapeutic resistance (8, 15).

Cancer cells exhibit lineage plasticity—the ability to alter their phenotype under therapeutic stress allowing them to evade treatment by adopting less differentiated or alternative cell states (62, 63). This plasticity enables tumors to bypass targeted therapies by shifting to phenotypes not recognized by the therapeutic agent. Lineage plasticity is closely linked to metabolic reprogramming and is often accompanied by epigenetic changes, where metabolites directly influence chromatin modifications, thereby promoting tumor progression and therapeutic resistance (63). Stromal cells, particularly CAFs, further promote adaptive cellular programs such as epithelial-mesenchymal transition (EMT), enhancing tumor invasiveness and therapy resistance (56).

Epigenetic reprogramming plays a crucial role in treatment resistance, particularly in aggressive paediatric and adult malignancies such as gliomas. These modifications do not alter DNA sequence but modify gene expression patterns, allowing cancer cells to adjust quickly to treatment and immune pressures. Histone acetylation and DNA methylation changes can silence tumor-suppressor genes and activate cancer-promoting pathways, helping tumor cells grow, survive, and evade the immune system (41, 64). These modifications also shape the immune environment around tumors, creating suppressive settings enriched with regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs) (41, 65). Consequently, fewer cytotoxic T cells can enter and function properly, lowering immunotherapy success rates.

Drugs targeting epigenetic changes show promise. Entinostat, an HDAC inhibitor, helps the immune system identify tumor cells more easily and improves immunotherapy efficacy (65). Preclinical research demonstrates that combining epigenetic drugs with immune checkpoint inhibitors or cell-based therapies can boost antitumor effects, reduce tumor size, and extend survival. However, tumors remain adaptable and can develop new resistance mechanisms even during epigenetic therapy. Further research is needed to determine how long these treatments remain effective and how the tumor environment changes over time.

While targeting metabolic pathways represents a promising therapeutic approach, it may inadvertently promote further plasticity or new resistance mechanisms, highlighting the complexity of this strategy. The interconnected nature of these resistance mechanisms—physical barriers, immune suppression, metabolic adaptation, and epigenetic flexibility—creates robust, multi-layered defense systems that challenge single-agent therapies. Despite formidable TME-driven resistance mechanisms, emerging strategies such as combination therapies and precision medicine approaches are being developed to counteract these barriers and improve therapeutic outcomes (66).

Resistance in pediatric cancers evolves dynamically as the TME remodels under therapeutic pressure. Early chemotherapy induces tumor cell death and damage-associated molecular pattern (DAMP) release, triggering inflammatory immune infiltration that creates a transient window for immune activation. However, surviving tumor cells simultaneously upregulate stress response programs, while cancer-associated fibroblasts (CAFs) secrete protective factors that shield residual disease from subsequent treatment cycles. Between treatment cycles, vascular normalization occurs approximately 1–3 days after anti-VEGF therapy, potentially enhancing drug delivery if chemotherapy is optimally timed. However, prolonged anti-angiogenic pressure triggers compensatory angiogenesis through alternative pathways (FGF, PDGF), explaining why initial responses cannot be sustained. The pediatric TME, with inherently higher vascular plasticity due to ongoing development, may undergo more rapid remodeling than adult tumors. Longitudinal immune profiling reveals progressive shifts from activated to exhausted phenotypes (67). In pediatric gliomas, this transition from immunologically “hot” to “cold” TME occurs over 6–12 months (30), suggesting immunotherapy may be most effective when introduced early rather than reserved for relapsed disease. Epigenetic reprogramming accelerates during treatment, with each cycle selecting for increasingly adaptable clones. Matched diagnosis-relapse pairs in sarcomas confirm accumulated modifications affecting DNA repair, drug efflux, and immune evasion pathways. The kinetics of resistance evolution are influenced by treatment schedules. Studies in pathogenic bacteria demonstrate that temporal variation in drug exposure—fluctuating between high and low concentrations—can slow resistance evolution compared to constant intermediate exposure (68). Fluctuating selection pressures prevent consistent directional selection, instead favoring phenotypic plasticity over fixed resistance mutations (68). Translating these principles to cancer therapy, intermittent dosing schedules may limit selection for stable resistance mechanisms. Given rapid developmental remodeling in pediatric tissues, real-time monitoring via circulating biomarkers, liquid biopsies, or imaging is vital to capture TME evolution and optimize treatment timing. Future clinical trials should incorporate planned biopsies at multiple timepoints or employ circulating markers to identify optimal temporal windows for introducing TME-targeted interventions. Understanding not just what mechanisms drive resistance, but when they emerge during treatment, will enable rational sequencing of therapies to preempt rather than merely respond to resistance evolution.

Key Points: TME-Driven Therapy Resistance (Table 2).

The pediatric TME exhibits profound heterogeneity across tumor types, directly influencing therapeutic response and necessitating tumor-specific treatment approaches. Understanding these differences is essential for rational treatment selection and predicting clinical outcomes. Current FDA-approved and clinical-stage therapies targeting the pediatric TME are listed in Table 3.

Developmental toxicity represents the foremost concern. Anti-angiogenic agents target VEGF signaling essential for growth plate ossification, dental development, and organ maturation (32, 58). Preclinical studies demonstrate impaired bone growth and delayed sexual maturation in juvenile animals, necessitating dose reductions that compromise efficacy. Checkpoint inhibitors cause higher rates of severe immune-related adverse events in children than adults: thyroiditis (18% vs 8%), type 1 diabetes (5% vs 1%), hypophysitis (12% vs 3%) (70, 75). These differences occur because developing immune systems have narrower therapeutic windows between anti-tumor activity and autoimmunity. These endocrine toxicities may require lifelong hormone replacement, an unacceptable burden for cured paediatric patients. Long-term cardiovascular, fertility, and neurocognitive impacts remain inadequately studied.

Pharmacokinetic differences across paediatric age groups complicate dosing. Infants have immature hepatic enzymes causing prolonged drug half-lives, children aged 2–12 require higher mg/kg dosing due to faster metabolism, and adolescents experience unpredictable drug distribution during puberty’s rapid body composition changes (76). Over 80% of TME-targeted therapies lack paediatric pharmacokinetic data, and body surface area extrapolation from adults fails to account for developmental TME differences.

Small patient populations create insurmountable statistical barriers. Most pediatric solid tumors affect fewer than 200 new US patients annually (19). Ethical constraints prohibit placebo controls when effective chemotherapy exists, necessitating demonstration of added benefit without increased toxicity. Age stratification further fragments samples, while standard endpoints fail to capture 30-50-year developmental impacts.

Economic and regulatory barriers limit investment. Small market size provides insufficient incentive for pediatric-specific formulations despite high development costs. FDA pediatric exclusivity provisions have proven inadequate. Many agents are used off-label based solely on adult data, but insurance often denies coverage for off-label pediatric use. Adult TME biomarkers (PD-L1 expression, tumor mutational burden) poorly predict pediatric response, yet developing pediatric-specific companion diagnostics requires separate regulatory approval (75).

Biological complexity of the rapidly evolving pediatric TME poses unique challenges. The microenvironment changes over months as children grow, creating temporal dynamics that complicate treatment timing (77). Preclinical models inadequately recapitulate pediatric TME: adult mice lack developmental context, juvenile models exist for few tumor types, patient-derived xenografts lose pediatric characteristics in adult mice, and organoids lack immune/stromal components (78, 79). Limited data on normal pediatric tissue TME across age spectrum creates uncertainty about developmental impact of TME-targeted interventions.

What this really means is that we can’t just give children smaller doses of adult cancer drugs. Paediatric TME-targeted therapy needs its own approach one that accounts for how children’s bodies develop and grow, while also thinking about quality of life for kids who survive into adulthood.

Effective TME-targeted therapy in paediatric oncology depends on biomarker-guided precision strategies rather than empirical selection. Immune infiltration metrics (e.g., CD8+/FOXP3+ ratios, spatial exclusion) and composite immunoscores predict immunotherapy response, though pediatric validation remains incomplete (80). Stromal and angiogenic signatures such as CAF markers, collagen profiles, and the Xerna™ TME Panel differentiate subtypes with distinct responses to FAK or TGF-β blockade (25, 80).

Integrative multi-omics models and AI-based platforms (e.g., Lunit SCOPE IO) improve predictive power (16), while adaptive and Bayesian trial designs address small paediatric cohorts and imperfect biomarker accuracy (81). Paediatric tumors’ low mutational burden, fusion-driven biology, and age-dependent immunity demand tailored biomarker interpretation (80). Standardized, minimally invasive monitoring using ctDNA, CTCs, and extracellular vesicles (78, 79) can support real-time, biomarker-informed decisions, transforming paediatric oncology into a precision-guided therapeutic framework (80, 81).

Key Points: TME-Targeted Therapies: Clinical Evidence.

Successes:

Challenges:

Ongoing Research: