Age and Genetic Differences in Immune Cell Levels of IL-1 and TNF Receptors

Aging is a complex biological process characterized by the progressive deterioration of physiological functions. A central pillar of this process is a chronic, low-grade, systemic inflammatory state called "inflammaging" [1]. This phenomenon is a key driver of pathology, significantly increasing the risk of a spectrum of age-related diseases, including cardiovascular disorders, neurodegenerative conditions like Alzheimer's, metabolic syndrome, and cancer [2,3,4]. Research shows that increases in inflammatory markers are non-specifically associated with aging [5]. Crucially, the biological impact of these cytokines is not solely dependent on their concentration but is profoundly shaped by the expression and function of their cellular receptors, which are, in turn, influenced by gene variants. A growing body of evidence now solidifies the link between polymorphisms in the genes encoding IL-1 and TNF receptors and the susceptibility to, and progression of, age-related decline [6,7].

The IL-1 family exerts some of the most potent inflammatory effects in the body, influencing everything from fever and leukocyte recruitment to tissue remodeling [8,9]. Its signaling is tightly regulated by a receptor duo—the signaling-competent IL-1 Receptor Type 1 (IL-1R1) and the non-signaling IL-1 Receptor Type 2 (IL-1R2)—the latter acting as a molecular "decoy" to sequester IL-1 and dampen its activity. The critical balance between these receptors determines the magnitude of IL-1 signaling. This balance is not identical in all individuals as it is modulated by genetics. Specific single-nucleotide polymorphisms (SNPs) in the IL1R1 and IL1R2 genes have been directly correlated with altered expression levels of their respective membrane-bound receptors [10]. This means that an individual's genetic code can predispose them to a hyperactive or hypoactive IL-1 response. The clinical relevance of this is clear: certain polymorphisms in these receptor genes have been associated with an increased risk of age-related hearing impairment, providing a direct molecular link between genetic variation in the IL-1 pathway and a common age-related condition [11].

TNF-α similarly plays an important role in inflammation and cellular homeostasis. Its effects are channeled through two distinct receptors: TNF Receptor 1 (TNFR1), which is ubiquitous and often triggers pro-inflammatory and apoptotic pathways, and TNF Receptor 2 (TNFR2), which is more restricted in expression and is frequently associated with cell survival, proliferation, and immunoregulation [12,13]. The aging immune system exhibits dysregulated expression dynamics for these receptors, and research shows nonlinear, age-dependent shifts in the percentage of cells expressing TNFR1 and TNFR2 on immune cells, disrupting the delicate balance between destructive and protective TNF signaling and thereby fueling the inflammaging process [14,15]. Adding another layer of complexity, genetic polymorphisms in the TNFR1 and TNFR2 genes directly affect the expression levels of these receptors on immune cells [16,17,18]. This genetic variation contributes to the considerable heterogeneity observed in how different individuals experience inflammatory aging.

The downstream consequences of this dysregulated cytokine–receptor axis are profound and multifaceted. Meta-analyses confirm that elevated circulating levels of IL-1β and TNF-α are consistently associated with a higher prevalence of numerous age-related diseases, including cardiometabolic and endocrine disorders, neurodegenerative and cognitive conditions, cancer, and autoimmune diseases [19]. At the cellular level, this chronic inflammatory environment drives immunosenescence—the aging of the immune system. For example, naïve and central memory CD8+ T-cells from older adults show heightened susceptibility to TNF-α-induced apoptosis, a mechanism that depletes the repertoire of adaptive immune cells and compromises the ability to fight new infections and malignancies [20]. Beyond immune exhaustion, the pro-inflammatory milieu creates a fertile ground for cancer development. Notably, IL-1-driven inflammation has been shown to promote aging-associated oncogenesis in the lung, directly linking the molecular mechanisms of inflammaging to tumorigenesis [21].

With age, chronic, low-grade systemic inflammatory activity (inflammaging) leads to dysregulation and a persistently elevated background of IL-1 and TNF. This chronic stimulation becomes pathological and contributes significantly to the development of age-related diseases. Excess IL-1 and TNF is associated with neuroinflammation and cognitive decline, insulin resistance, sarcopenia, osteoporosis, and atherosclerosis. Thus, IL-1 and TNF exemplify the dichotomic nature of aging: molecules essential for survival and protection in youth, upon prolonged and dysregulated activation, transform into primary drivers of the pathological processes that underpin aging and age-related diseases [22].

An individual's genetic profile for IL1R1, IL1R2, TNFR1, and TNFR2 sets a baseline for inflammatory responsiveness. With advancing age, this system becomes dysregulated, creating a self-perpetuating cycle of chronic inflammation that accelerates immunosenescence, increases cellular vulnerability, and fosters an environment conducive to age-related diseases and cancer. A detailed understanding of these genetic and molecular pathways is paramount for developing personalized, targeted anti-inflammatory interventions aimed at mitigating the burdens of inflammaging and promoting healthier longevity [6].

In conclusion, we hypothesize that the relationship between IL-1, TNF-α, and aging is deeply mechanistic, mediated through their specific receptor systems and modulated by genetic polymorphisms. Given the complex and poorly understood relationship between IL-1/TNF receptor expression, their polymorphisms, and age, the aim of this study is to determine the relative and absolute expression levels of these receptors on the main populations of immunocompetent cells in healthy donors of different age groups.

This study used an unsupervised clustering approach to stratify a cohort of 144 individuals into two distinct age groups: a young cluster (18–31 years, mean 23.4) and an older cluster (32–59 years, mean 45.1). Our central finding is that these age-defined clusters exhibit significant and cell-specific differences in the expression of receptors belonging to the TNF and IL-1 pro-inflammatory cytokine families. The most pronounced age-related differences were consistently observed in the monocyte population, which reveals a complex and compartmentalized reshaping of the immune landscape with age [24].

The most notable observation from our data is the predominant effect of age on monocyte receptor phenotypes. We found that young individuals displayed a significantly higher percentage of monocytes expressing both TNFR1 and TNFR2, as well as a higher overall level of TNFR1 expression. This suggests a heightened readiness of the early adult immune system to respond to TNF-mediated signaling. TNF is a crucial regulator of inflammatory and anti-apoptotic pathways, and its effects are critically dependent on the balance between its two receptors [25]. TNFR1 often mediates pro-inflammatory and apoptotic signals, while TNFR2 is frequently associated with tissue repair and immunoregulatory functions.

Our observation that TNFR1 is elevated in youth appears to contrast with some studies reporting increased inflammation, or "inflammaging," in older adults [26]; however, this discrepancy may be explained by a fundamental shift in receptor homeostasis. A compelling mechanism for the decreased TNFR surface expression in the older cohort is the age-associated dysregulation of receptor internalization and turnover. In robust, young immune systems, ligand-induced receptor internalization is a tightly regulated process that serves to attenuate signaling and allow for receptor recycling or degradation. With advancing age, however, chronic, low-grade inflammation—characterized by persistently elevated levels of circulating TNF-α—can lead to a state of perpetual receptor activation and subsequent internalization. Studies have shown that continuous exposure to TNF-α promotes the sustained internalization of TNFR1 into endosomal compartments, where it is targeted for lysosomal degradation rather than recycled to the plasma membrane [27,28,29]. This phenomenon, driven by the chronic inflammatory milieu of aging, results in a net reduction of surface-available TNFR1, as observed in our older cohort. Therefore, the observed decrease is not merely a passive loss but likely an active, maladaptive process of chronic stimulation, rendering monocytes less responsive to acute TNF challenges—a state of immune tolerance.

In this study, T-lymphocytes exhibited the inverse phenomenon, with a significantly greater percentage of TNFR2+ cells in the older cluster. This cell-type-specific regulation underscores the complexity of immunosenescence. On monocytes, the co-downregulation of TNFR1 and TNFR2 with age supports the hypothesis of generalized TNF receptor desensitization, potentially driven by the same mechanisms of chronic internalization. On the other hand, the upregulation of TNFR2 on T-cells could represent a distinct, compensatory mechanism or a marker of immunological aging. TNFR2 is a key costimulatory molecule for regulatory T-cells (Tregs) and is involved in the expansion and survival of this immunosuppressive population. Age-associated expansion of TNFR2+ T-cells has been linked to the accumulation of senescent T-cell phenotypes and a progressive decline in adaptive immune efficacy [14]. Thus, the increased quantity of TNFR2 on T-cells in the older cluster may not signify enhanced immunity but rather a shift towards a more immunoregulatory T-cell pool [30].

The observed differences were not confined to the TNF superfamily. A parallel reduction in both the percentage of IL-1R1+ monocytes and the level of its expression was found in the older cohort. The IL-1 family is another cornerstone of immunity, with IL-1β being a potent pyrogen and driver of inflammation. The coordinated downregulation of receptors for two major pro-inflammatory pathways (TNF and IL-1) strongly points toward a fundamental reprogramming of monocyte function [31]. The mechanism for IL-1R reduction may similarly involve ligand-induced internalization and degradation, as IL-1R signaling is also subject to tight regulation via endocytic processes [32]. This global dampening of primary inflammatory receptors could be a protective mechanism against chronic inflammation but comes at the cost of reduced vigilance against new infections.

Our results contribute a nuanced perspective to the concepts of "inflammaging" and immunosenescence. The classical view of inflammaging emphasizes a progressive increase in circulating pro-inflammatory cytokines [33]; however, our data highlight that the receptor landscape—the cellular "sensors" for these cytokines—undergoes significant alteration. We propose a model where systemic inflammaging (elevated circulating cytokines) directly drives the internalization and downregulation of receptors on immune cells like monocytes. This creates a paradoxical state of both chronic inflammation, as measured by plasma cytokines, and cellular immune deficiency, as measured by receptor availability and, likely, functional responsiveness.

The lack of significant differences in the other cell populations and soluble factors analyzed in our study further underscores these conclusions. It suggests that the monocyte compartment is particularly vulnerable to this age-related reprogramming, acting as a primary conductor of the immunological changes that characterize the transition from young adulthood to middle age.

Aging is associated with the accumulation of epigenetic modifications, such as DNA hypermethylation and altered histone acetylation, which can silence gene promoters. The promoters of TNFR1 and TNFR2 may become less accessible to transcription factors over time, leading to reduced mRNA production. This is supported by broader studies on immunosenescence showing that age-related epigenetic drift significantly impacts the expression of critical immune response genes [34,35,36]. Furthermore, chronic low-grade inflammation (inflammaging) can lead to a desensitization of key transcription factors like NF-κB, paradoxically resulting in the downregulation of certain target genes, including receptors, as part of a feedback mechanism to limit excessive signaling [37,38].

At the post-transcriptional level, regulatory mechanisms further modulate receptor levels. The stability and translation of TNFR mRNA can be influenced by age-associated microRNAs (miRNAs). For instance, miR-146a, a key inflammation-responsive miRNA that is often elevated with age, has been shown to target and repress the expression of various components of the immune signaling pathways, potentially including the mRNAs for TNF receptors, thereby contributing to their decreased synthesis [39]. This represents a layer of regulation where the inflammatory milieu of aging directly fine-tunes receptor availability.

The stability of the receptors themselves is also a critical factor. The process of proteolytic shedding, where membrane-bound receptors are cleaved by metalloproteinases like ADAM17 to release soluble forms (sTNFR), is a key regulatory node [40]. With advancing age, increased proteolytic activity or expression of these sheddases could lead to accelerated cleavage of TNFR1 and TNFR2 from the monocyte surface, effectively reducing their membrane-bound levels while increasing their soluble counterparts in the circulation [41,42].

Finally, cell-intrinsic changes and shifts in monocyte subsets contribute to the observed phenotype. Aging drives remodeling of the hematopoietic system, leading to alterations in the output and phenotype of immune cells. There is a documented shift in monocyte subsets with age, often towards a more inflammatory classical phenotype [43]; however, these cells may exist in a state of chronic pre-activation or "exhaustion," which can be associated with downregulation of certain surface receptors as a form of desensitization. Therefore, the overall reduction in TNFR expression across the monocyte pool may reflect both a change in the transcriptional program of individual cells and an alteration in the proportional representation of different subsets.

Our previous research has shown that receptor gene polymorphisms are associated with both the percentage of cells expressing these receptors and the number of receptors per cell [44]. Furthermore, studies have indicated that receptor expression on cells correlates with age and differs across TNF receptor gene polymorphisms [14]. In our current study, we demonstrate that the two age clusters, when further stratified by TNF and IL-1 receptor gene polymorphisms, show differences in receptor expression levels on various immune cells. Table 1 presents data on the frequency of specific polymorphisms and their associated changes in receptor expression.

For the young cluster, significant differences were observed in both the quantity and percentage of B-lymphocytes expressing TNF and IL-1 receptors; in contrast, the older cluster exhibited more pronounced differences in monocytes. These disparities may be attributed, first, to differences in subpopulation composition. It has been shown that the absolute count of B-cells is higher in young individuals compared to older age groups, while the percentage of naïve B-cells is significantly reduced in older groups, with an increased percentage of atypical memory B-cells [45]. Research has also demonstrated that the expression of both types of TNF receptors and IL-1 receptors varies across B-lymphocyte subpopulations [46,47]. The differences we obtained show the influence of polymorphisms in the genes for both cytokine receptors on receptor expression in B-lymphocytes, suggesting an important regulatory role for these cells during youth. This is manifested, for example, in a better response to vaccination [48].

Our data indicate that cytokine receptor gene polymorphisms are associated with differences in receptor expression on monocytes in the older cohort. Monocytes also undergo age-related changes in their subpopulation composition; studies show that as adults age, the frequency and absolute count of intermediate and non-classical monocyte subsets increase [31]. Furthermore, it has been shown that different monocyte subpopulations express different levels of TNF receptors, which influences their differentiation and functions [49,50]. Monocytes isolated from older individuals have been found to produce larger amounts of pro-inflammatory cytokines [51].

While our study provides valuable insights, several limitations should be acknowledged. First, the cohort size of 144 individuals, while sufficient for the initial clustering, may limit the statistical power for more detailed subgroup analyses, especially when stratifying by both age cluster and multiple genetic polymorphisms. Second, our analysis focused on surface receptor expression and genetic variation but did not include direct functional assays to measure downstream signaling activity (e.g., NF-κB activation) or cellular responses to ligand stimulation. Consequently, the functional consequences of the observed receptor changes are inferred from the literature rather than directly demonstrated. Third, the age range of our "older" cluster (32–59 years) captures early to middle adulthood and may not fully represent the more pronounced immunological alterations characteristic of advanced age (e.g., 65+ years). Finally, while we discuss potential mechanisms such as epigenetic regulation and receptor shedding, these were not directly measured in our study, and their specific contributions remain to be experimentally validated.

This study demonstrates the complex and multifaceted influence of age and genetic variation on the expression of receptors for key pro-inflammatory cytokines, IL-1 and TNF, in the human immune system. The central finding is the identification of significant, cell-specific disparities in the expression of TNF (TNFR1 and TNFR2) and IL-1 (IL-1R1) receptors between two distinct age clusters ("young," 18–31 years, and "older," 32–59 years). The most pronounced age-related differences were consistently observed in the monocyte compartment. The young cohort exhibited a significantly higher percentage of monocytes expressing TNFR1 and TNFR2, as well as a greater number of IL-1R1 and TNFR1 receptors on these cells. This suggests a heightened baseline readiness for cytokine signaling in early adulthood. In contrast, the observed downregulation of these receptors in monocytes from the older group likely reflects an active, maladaptive process driven by chronic, low-grade inflammation ("inflammaging"), potentially involving ligand-induced internalization, degradation, and epigenetic silencing, leading to a state of cellular desensitization. Conversely, T-lymphocytes exhibited an inverse pattern for TNFR2, with a higher percentage of positive cells in the older cluster. This cell-type-specific regulation underscores the complexity of immunosenescence, possibly indicating a shift towards a more immunoregulatory T-cell pool. Furthermore, we established that genetic polymorphisms in the TNFR1, TNFR2, IL1R1, and IL1R2 genes significantly modulate receptor expression levels, and crucially, these genetic effects were often age-dependent. In the young cohort, polymorphisms primarily influenced receptor expression on B-lymphocytes, whereas in the older cohort, the most significant associations were observed in monocytes. This highlights that genetic control of the cytokine receptor landscape is not static but evolves with the aging immune system.

In summary, our results support a model in which systemic inflammation, characterized by elevated circulating cytokine levels, triggers compensatory downregulation of receptors on immune cells such as monocytes. This creates a paradoxical state of chronic inflammation coupled with cellular immunodeficiency. Individual genetic predisposition to these receptors sets a baseline inflammatory response that deteriorates with age, contributing to the heterogeneity of immunosenescence. A detailed understanding of these age- and genotype-dependent pathways is crucial for the development of personalized, targeted anti-inflammatory interventions aimed at reducing the burden of inflammation and promoting healthier longevity.