Immunosenescence: signaling pathways, diseases and therapeutic targets

NF-κB is a transcription factor that can be activated during cellular damage and stress. The activity of NF-κB increases with aging and aging-related diseases due to the accumulation of endogenous DNA damage and oxidative stress. In aged mice, NF-κB activation has been observed in a variety of cell types.¹⁰ Genetic depletion or pharmacological inhibition of NF-κB decreases oxidative DNA damage and stress in aged mice, leading to delayed cellular senescence and age-related symptoms and pathologies.¹⁰ Persistent NF-κB activation drives inflammaging, which impairs immune surveillance, reduces T cell diversity, and promotes tissue degeneration.¹¹ Reactive oxygen species (ROS) accumulation during aging can activate NF-κB via IκBα phosphorylation or IKK modulation at critical cysteine residues, impairing its DNA-binding capacity and disrupting redox homeostasis.^10,12,13 Furthermore, NF-κB suppresses autophagy by transcriptionally activating the mechanistic target of mTOR, a key inhibitor of autophagic flux.¹⁴ Impaired autophagy in aged immune cells leads to the accumulation of damaged mitochondria and protein aggregates, exacerbating oxidative stress and inflammasome activation.¹⁵ Additionally, NF-κB enhances the survival of dysfunctional immune cells by upregulating antiapoptotic proteins, preventing the clearance of SnCs.¹⁶ This apoptotic resistance contributes to reduced immune diversity and increased cancer risk during aging. These findings underscore NF-κB as a central driver of immune aging, linking oxidative stress, inflammaging, autophagy suppression, and immune cell survival dysregulation.

The mTOR signaling pathway serves as a central regulator of cell survival and growth and cell cycle progression.¹⁷ mTOR functions through two distinct complexes, mTOR complex 1 (mTORC1) and mTOR complex 2 (mTORC2), each of which play unique roles in cellular metabolism and immune function.¹⁸ mTORC1 integrates signals from nutrients and growth factors to regulate various anabolic processes, including protein synthesis, nucleotide production, lipid biosynthesis, and glycolysis, while inhibiting catabolic processes such as autophagy by phosphorylating ULK1/2 and sequestering lysosomal enzymes.^19–23 The activation of mTORC1 has long been associated with cellular senescence²⁴. However, in senescent CD8⁺ T cells, autophagy is not restored despite reduced mTORC1 activity, partly due to alternative inhibitory mechanisms such as chronic p38 MAPK activation, which impairs autophagy independently of mTORC1 and contributes to immune decline in senescent CD8⁺ T cells.^23,25 Additionally, cytoplasmic p53 inhibits autophagy via mTOR activation under basal conditions, whereas nuclear p53 promotes autophagy through mTOR-independent mechanisms under stress.^26,27 In addition to its role in autophagy regulation, mTORC1 also plays an important role in T cell function. In aged mice, in vitro inhibition of mTORC1 with metformin or everolimus increased IL-2 production and T cell proliferation and reduced oxidative stress in CD4⁺ T cells²⁸. Moreover, in elderly humans, low-dose mTORC1 inhibition with RAD001 and BEZ235 reduced infection rates over 12 months, indicating an enhancement of immune function.²⁹

mTORC2 has been increasingly recognized for its role in immune aging. In aged murine CD4⁺ T cells, increased mTORC2 signaling is associated with impaired TCR responsiveness and reduced proliferative capacity. These functional defects are related to mTORC2-mediated dysregulation of cytoskeletal organization (including actin polymerization) and cell survival pathways.³⁰ In vivo studies in lymphocytic choriomeningitis virus-infected mice have shown that mTORC2 prevents ferroptosis in virus-specific memory CD4⁺ T cells by limiting lipid peroxidation and mitochondrial ROS accumulation, primarily through activation of AKT and inhibition of GSK3β, thereby supporting their long-term survival.³¹ Moreover, mTORC2 regulates CD8⁺ T cell differentiation via the SGK1-Foxo1 axis. The inhibition or genetic deletion of SGK1, a downstream effector of mTORC2, promotes the formation of memory precursors of CD8⁺ T cells and enhances their long-term survival.³²

The JAK-STAT signaling pathway is fundamental to immune regulation and plays key roles in infection defense, immune tolerance, and tumor surveillance. During aging, the JAK-STAT signaling pathway is dysregulated, contributing to immunosenescence by driving persistent inflammation and altering immune cell function. JAK-STAT pathway alterations impair immune homeostasis by affecting immune cell development and function. Hyperactivation of STAT3 enhances the production of proinflammatory cytokines, including interleukin (IL)-6 and IL-23, promoting the senescence-associated secretory phenotype (SASP) and sustaining inflammatory signaling.³³ JAK1/2 activation further amplifies the SASP, accelerating immune aging. Additionally, JAK3 and STAT5B mutations impair Foxp3 expression, disrupting regulatory T cell (Treg)-mediated immune tolerance.³⁴ JAK3 mutations result in defective T and natural killer (NK) cell maturation, weakening immune responses against infections and tumors. Overactivation of STAT3 skews the immune balance by promoting T helper 17 (Th17) cell expansion while suppressing Treg function. Moreover, excessive JAK-STAT activation disrupts hematopoietic stem cell (HSC) differentiation, favoring myeloid rather than lymphoid lineage commitment, a hallmark of immune system aging.³⁵

The cGAS-STING pathway senses cytosolic DNA under certain conditions, such as DNA damage, mitochondrial dysfunction, or nuclear envelope disruption. The accumulation of damaged DNA in the cytoplasm, which serves as a damage-associated molecular pattern (DAMP) to be recognized by DNA sensors, including the cGAS-STING pathway, is observed in aging cells.³⁶ Through the cGAMP–STING–TBK/IKK axis, it induces NF-κB-dependent expression of inflammatory cytokines such as IL-6 and CXCL10, promoting the SASP.³⁷ Notably, aging immune cells exhibit impaired secretion of type I interferons (IFN-I), which are downstream effector molecules of cGAS-STING signaling. For example, aging plasmacytoid dendritic cells (pDCs) exhibit limited production of IFN-I due to impaired IRF7 phosphorylation, resulting in decreased presentation of antigens to T lymphocytes.³⁸

AMPK is a pivotal serine/threonine protein kinase that plays an extensive role in the regulation of cellular energy metabolism³⁹. It is critically involved in maintaining cellular homeostasis, mitigating oxidative stress, promoting cell survival and growth, and modulating cell death and autophagy⁴⁰. As a central energy sensor, AMPK extends lifespan by promoting autophagy via mTOR inhibition and ULK1 activation, enhancing mitochondrial function through PGC-1α/SIRT1 signaling and improving the NAD⁺/NADH balance, suppressing inflammatory responses via NF-κB inhibition, and modulating stress resistance via the FOXO3/p53 pathways, thereby linking metabolic regulation to aging suppression⁴¹. The activation of AMPK broadly suppresses proinflammatory signaling pathways, including the JAK-STAT, NF-κB, C/EBPβ, CHOP, and HIF-1α pathways, which in turn inhibits the expansion and immunosuppressive function of myeloid-derived suppressor cells (MDSCs)⁴². Moreover, AMPKα1 is essential for CD8⁺ T cell memory formation because it senses glucose deprivation and suppresses mTORC1 activity, as AMPKα1-deficient CD8⁺ T cells fail to survive metabolic stress during immune contraction and exhibit impaired secondary responses⁴³. In highly differentiated human CD4⁺ T cells (the CD27⁻CD28⁻ subset), AMPK activation under metabolic stress or DNA damage recruits TAB1 to induce p38 autophosphorylation, leading to telomerase suppression and proliferative arrest⁴⁴. Given its regulatory effects on immune signaling and inflammation, AMPK may play a role in modulating immunosenescence, although further studies are needed to elucidate this potential connection.

Melatonin, a hormone produced by the pineal gland that plays a pivotal role in regulating circadian rhythms, significantly decreases in level as age progresses, manifesting as a deterioration of circadian rhythmicity.⁴⁵ Melatonin counteracts immunosenescence via a multitarget regulatory network spanning cytokine balance, oxidative stress defense, immune cell functional restoration, signaling pathway crosstalk, disease-specific interventions, and circadian rhythm integration. Specifically, melatonin suppresses proinflammatory cytokines (e.g., IL-1β, IL-6, tumor necrosis factor-α (TNF-α), and interferon-γ (IFN-γ)) and enhances anti-inflammatory cytokines (e.g., IL-4 and IL-10) by inhibiting the NF-κB pathway, although its low-dose transient proinflammatory effects highlight dose- and pathology-dependent dynamics.^46,47 It directly scavenges ROS and upregulates the expression of antioxidant enzymes such as superoxide dismutase (SOD) and glutathione peroxidase (GPX), reducing oxidative damage and SASP accumulation in immune cells.^46–48 Through an “antioxidant cascade”, melatonin metabolites such as N1-acetyl-N2-formyl-5-methoxykynuramine (AFMK) and N1-acetyl-5-methoxykynuramine (AMK) further neutralize free radicals, protecting mitochondrial integrity, proteins, and DNA from oxidative destruction. At the immune cell level, melatonin enhances CD4⁺/CD8⁺ T cell proliferation and antigen responsiveness, modulates the Treg/Th1/Th2 balance, promotes macrophage polarization toward the anti-inflammatory M2 phenotype, and augments NK cell cytotoxicity.^46,47 These effects are mediated by SIRT1 pathway activation, which optimizes mitochondrial function and autophagy, and by miRNA-dependent regulation (e.g., miR-146a targeting Nrf2/NF-κB pathways), although the mechanisms vary across cell types and microenvironments.^46,47 Epigenetically, melatonin may inhibit histone deacetylases (HDACs) and modulate miRNA expression to reverse age-associated proinflammatory gene silencing in senescent immune cells, restoring functional competence.⁴⁹ As a core circadian regulator, melatonin stabilizes clock genes (e.g., CLOCK protein), reduces cortisol-mediated immunosuppression, and coordinates rhythmic immune cell activities (e.g., diurnal fluctuations in macrophage phagocytosis), systemically delaying immunosenescence.^46,49

Sirtuin family proteins play crucial roles in immune aging by regulating mitochondrial function, oxidative stress, and NF-κB signaling, thereby maintaining immune homeostasis across various immune cell types. In HSCs, sirtuin 3 (SIRT3) preserves genomic stability and mitochondrial integrity, delaying cellular aging by increasing superoxide dismutase 2 (SOD2) activity and reducing oxidative stress.⁵⁰ Restoring NAD⁺ levels can further reactivate SIRT3, improving stem cell reprogramming and lifespan extension.⁵¹ Within the innate immune system, SIRT1 and SIRT6 mitigate inflammatory responses in macrophages by inhibiting NF-κB signaling and suppressing excessive TNF-α and IL-1β expression, promoting endotoxin tolerance.⁵² In dendritic cells (DCs), SIRT1 modulates autophagy and cytokine secretion, enhancing antiviral responses while preventing excessive Th2/Th17-mediated inflammatory reactions.⁵³ In NK cells, SIRT2 and SIRT6 contribute to exhaustion in colorectal cancer, suppressing NK cytotoxicity by downregulating glycolysis and mitochondrial respiration. Silencing these proteins restores the antitumor function of NK cells.^54,55 Within adaptive immunity, SIRT1 is essential for T cell activation and peripheral tolerance, preventing autoimmune diseases by suppressing AP-1 transcription and IL-2 production in the absence of CD28 costimulation.⁵⁶ Furthermore, both SIRT1 and SIRT7 regulate B cell class-switch recombination (CSR), influencing immunoglobulin (Ig) maturation.⁵⁷ Overall, the sirtuin family acts as a critical regulator of immune aging by fine-tuning metabolic and inflammatory pathways in multiple immune cell types. Targeting these proteins may provide novel therapeutic strategies to mitigate immune decline and promote healthier aging.

Immunosenescence is closely linked to various metabolic disorders, particularly type 2 diabetes (T2D).²⁴⁶ The immune aging process results in functional alterations in immune cells, increasing the susceptibility of elderly individuals to metabolic dysfunction. Metabolic diseases such as T2D, obesity, and metabolic syndrome are associated with low-grade chronic inflammatory states (Fig. 5). This inflammation is called metabolic inflammation and is similar to the chronic inflammatory process that occurs during aging.²⁴⁷ Increased fat mass, metabolic dysfunction, and systemic inflammatory responses are key features of these metabolic diseases and can further exacerbate immune dysfunction associated with aging.¹⁶⁴ In T2D, a hallmark of immune aging is a reduction in the proportion of naïve CD4⁺ T cells alongside increased memory CD4⁺ T cells and effector CD4⁺ and CD8⁺ T cells. These effector cells are the main producers of proinflammatory cytokines, including IFN-γ and TNF-α, leading to increased systemic inflammation.^164,248 Additionally, an increase in the number of senescent T cells, including CD8⁺CD57⁺ and CD8⁺CD28⁻ T cells, is considered a predictor of hyperglycemia development in humans.²⁴⁸ Apart from adaptive immunity, aging induces defects in the function and activation of the innate immune system in T2D. In T2D patients, the phagocytic capacity and TLR responsiveness of peripheral blood mononuclear cells (PBMCs), which are closely related to endoplasmic reticulum stress and poor blood sugar control, are significantly impaired.^248,249 Obesity is another metabolic disorder associated with metabolic inflammation and has a similar phenotypic spectrum as aging, such as dysfunctional mitochondria, weakened immunity, and elevated systemic inflammation.²⁵⁰ Obesity significantly impacts B cell function. Studies have demonstrated that B cells from obese individuals exhibit a reduced capacity to generate pathogen-specific antibodies.²⁵¹ Furthermore, both obesity and aging drive the expansion of double-negative (DN) B cells (CD27⁻IgD⁻), which display a proinflammatory phenotype marked by autoimmune antibody secretion and upregulated expression of activation markers (e.g., CD11c and T-bet).²⁵² Exposure to plasma from obese individuals promoted the apoptosis, DNA damage, and mitochondrial dysfunction of PBMCs, resulting in increased production of IL-1β and IL-8.²⁵³ Moreover, CD8⁺ T cells present an immunosenescent phenotype with reduced expression of CD28, indicating that chronic systemic inflammation in individuals with obesity promotes immune system dysfunction and aging.²⁵³

Immunosenescence in the elderly population is closely related to increased susceptibility to infectious diseases such as severe acute respiratory syndrome (SARS-CoV) and cytomegalovirus (CMV) infection (Fig. 5).¹¹³ As people age, the immune system undergoes functional decline. During CMV infection, the oligoclonal expansion of CMV-specific CD8⁺ T cells is inhibited in elderly individuals, which limits the ability to combat viral infection.²⁵⁴ The coronavirus disease 2019 (COVID-19) pandemic has starkly illustrated the clinical consequences of immunosenescence. Elderly patients infected with SARS-CoV-2 frequently experience rapid disease progression due to high expression of PD-1 and Tim-3 on CD8⁺ T cells and dysregulated innate immunity, culminating in cytokine storms dominated by IL-6 and granulocyte‒macrophage colony‒stimulating factor (GM-CSF).^255,256 Moreover, immunosenescence compromises vaccine efficacy. Although mRNA vaccines (e.g., BNT162b2) partially mitigate age-related immune deficits by enhancing GC reactions and memory B cell generation, their protective efficacy against COVID-19 in older adults remains suboptimal.²⁵⁷ Strategic booster doses, however, significantly improved cross-protection against variants such as Omicron, highlighting the necessity of age-tailored vaccination protocols.²⁵⁸ Similarly, elderly individuals exhibit weaker antibody responses upon immunization with influenza and pneumococcal vaccines, underscoring the need for enhanced immunization strategies.^257,259 Moreover, JAK3 mutations have been shown to lead to severe immunodeficiencies, whereas STAT3 mutations impair IL-17 production, which increases susceptibility to bacterial and fungal infections. These findings suggest that specific genetic mutations may contribute to the diminished ability of the immune system to effectively fight infections in elderly individuals, potentially compounding the challenges posed by immunosenescence.³⁴

In chronic obstructive pulmonary disease (COPD), aging-associated immune dysfunction exacerbates lung damage through multiple pathways: epithelial barrier integrity deterioration, impaired mucus clearance, and alveolar macrophages with low phagocytic capacity.^260,261 These changes amplify inflammation triggered by environmental insults such as cigarette smoke, while accelerated cellular aging (e.g., telomere shortening in alveolar cells) further drives oxidative stress and apoptosis.²⁶² A parallel mechanism is observed in idiopathic pulmonary fibrosis (IPF), where telomere attrition in lung fibroblasts, combined with chronic inflammation and TGF-β pathway activation, promotes excessive collagen deposition and irreversible structural damage.^263,264

Rheumatoid arthritis (RA) is a chronic inflammatory disorder characterized by symmetrical and destructive inflammation of joints and other organs/tissues. As individuals age, the immune system shifts toward a more proinflammatory state, marked by the accumulation of CD28^– T cells. These senescent T cells disrupt immune tolerance, promote autoreactivity against self-antigens, and exacerbate disease severity, particularly in RA patients with extra-articular manifestations (Fig. 5).²⁶⁵ In addition, genetic factors, such as STAT3/STAT4 polymorphisms, contribute to the development of autoimmune diseases such as RA by impairing immune tolerance and increasing susceptibility to autoreactivity.³³ Increased telomere attrition in these T cells exacerbates the inflammatory state and is associated with the development of cardiovascular disease in RA patients, indicating the systemic effects of immunosenescence.^262,266 The complex interplay between immunosenescence and autoimmunity highlights the importance of further research and the development of novel therapeutic approaches to treat autoimmune diseases in the elderly population.

Aging is the most significant risk factor for CVD, which remains the leading cause of death worldwide.²⁶⁷ CVD encompasses a range of heart and vascular diseases, which are associated with the biological process of aging, the loss of homeostasis, and increased morbidity and mortality rates (Fig. 5).²⁶⁸ Inflammaging is a key risk factor for CVD and involves elevated levels of proinflammatory cytokines, leading to endothelial damage, impaired vascular remodeling,²⁶⁹ and atherosclerosis.²⁷⁰ These inflammatory molecules are secreted primarily by senescent T cells and proinflammatory macrophages.^271,272 This reflects the body’s inability to properly regulate immune responses during aging, driving tissue dysfunction and pathological alterations. During cardiac stress, ischemic injury, and metabolic syndrome in the cardiovascular system, necrotic cells release DAMPs, which are recognized by pattern recognition receptors on innate immune cells, triggering strong inflammatory responses.²⁷³ This leads to the secretion of proatherosclerotic cytokines, ROS, and reactive nitrogen species (RNS), amplifying oxidative stress. These cytokines also stimulate the proliferation of vascular smooth muscle cells (VSMCs) and the accumulation of oxidized low-density lipoprotein (LDL) particles, which are then captured by foam cells in vessel walls.²⁷⁰ Senescent T cells, particularly cytotoxic CD8⁺ T cells, also contribute to the pathophysiology of CVD. The expansion of CD8⁺CD28⁻ T cells is a risk factor for vascular dysfunction.²⁷⁴ Studies have shown that the expansion of peripheral late-differentiated CD4⁺CD28⁻ T cells that produce IFN-γ after persistent antigenic stimulation is observed in unstable angina.²⁷⁵ In older men, CMV infection-related atherosclerosis may be mediated by an increased proportion of memory CD4⁺ T cells.²⁷⁶

There is substantial evidence demonstrating that the mTOR signaling pathway is a critical target for antiaging interventions. Inhibition of mTORC1 promotes autophagy, which facilitates the clearance of unwanted cytoplasmic proteins and reduces the accumulation of toxic metabolites, thereby mitigating cellular stress and extending lifespan.^290,291 Another proposed mechanism is that mTOR regulates the crosstalk between mitochondria and the nucleus, stabilizing the key protein Clk-1, which is essential for mitochondrial signaling communication.^292–294 The first-generation mTOR inhibitor rapamycin has been shown to extend lifespan across various organisms, making it the only known pharmacological agent that directly modulates aging (Table 1).^295–297 However, Cloughesy et al. reported that while rapamycin reduces the proliferation of glioblastoma, its effects are not sustained.²⁹⁸ Consequently, the primary goal of second-generation mTOR inhibitors is to simultaneously target both the mTORC1 and mTORC2 signaling pathways, as well as their feedback loops, which were not addressed by first-generation inhibitors.²⁹⁹ By providing more comprehensive inhibition of the mTOR signaling pathway, second-generation mTOR inhibitors, including PP242, KU0063794, AZD3147, and eCF309, have shown promising results in preclinical and clinical trials.^300,301 The most recent advancement, RapaLink-1, structurally resembles rapamycin and binds to mTOR, effectively inhibiting mTORC1.³⁰² Compared with rapamycin, RapaLink-1 has been reported to exert a stronger inhibitory effect on T cell proliferation.³⁰² Phase IIa and IIb clinical trials investigating the combination of the mTORC1 inhibitors RTB101 and RAD001 revealed that this regimen could reduce immunosenescence and enhance the response to influenza vaccination.^29,303 However, the phase III trial of RTB101 alone failed to achieve the desired outcomes.³⁰³

During the aging process, sustained activation of the JAK-STAT signaling pathway induces the expression of the SASP, promoting cellular senescence and apoptosis, as well as impairing the function of T cells, B cells, and macrophages. Targeting the JAK-STAT signaling pathway can alleviate chronic inflammation, improve immune cell function, and delay the onset of aging-related phenotypes.^34,304 Notably, the JAK pathway is more active in the adipose tissue of aged animals than in that of their younger counterparts. Compared with that in younger mice, the efficacy of the JAK inhibitor ruxolitinib in aged mice is superior, manifesting in the clearance of SnCs, enhancement of physical performance, and maintenance of adipose tissue homeostasis (Table 1). This evidence suggests that JAK inhibitors confer beneficial effects by modulating SnCs.^304,305 The percentage of SnCs was significantly reduced in mice treated with the JAK inhibitor NVP-BSK805 in combination with docetaxel, thereby enhancing the antitumor response to docetaxel.³⁰⁶ Additionally, JAK inhibitors may promote hair growth by stimulating the activation and proliferation of stem cells.³⁰⁷ The underlying mechanism could be leveraged to directly target tissue stem cells and their respective niches.³⁰⁷ The administration of JAK inhibitors leads to a significant increase in satellite cell populations, facilitates robust muscle regeneration, and elevates functional capabilities, thereby presenting a viable and innovative therapeutic strategy for addressing muscle wasting conditions.³⁰⁸

Metformin is a well-known activator of AMPK that functions by reducing the ADP/ATP and AMP/ATP ratios (Table 1).³⁰⁹ This reduction is achieved through partial inhibition of Complex I of the mitochondrial ETC. Consequently, this inhibition leads to direct or indirect activation of AMPK.³¹⁰ In age-related diseases, metformin beneficially enhances mitochondrial function, increases Nrf2 activity, induces autophagy, or ameliorates accelerated aging defects in Hutchinson–Gilford progeria syndrome (HGPS) cells by altering gene splicing through its activation mechanisms.³¹¹ Metformin can also mitigate age-related hearing loss and neurodegeneration symptoms in D-galactose-induced aging rats by modulating the AMPK/extracellular signal-regulated kinase 1/2 (ERK1/2) signaling pathway.³¹² In addition, we cannot overlook the role that resveratrol plays in the AMPK pathway. It has been reported in the literature that resveratrol can prevent oxidative stress-induced aging and proliferation damage by activating the AMPK/FOXO3 signaling pathway.³¹³ Oleanolic acid induces autophagy and apoptosis in colon cancer cells by modulating the AMPK-mTOR signaling pathway, thereby exerting therapeutic effects against colorectal cancer.³¹⁴

The mechanisms by which melatonin-related signaling pathways exert their antiaging effects can be summarized as follows (Table 1): (1) Antioxidant function. Melatonin can directly scavenge ROS through its electron-rich indole ring, and it can also induce antioxidant enzymes such as GPX, SOD, and catalase to achieve antioxidative effects. Furthermore, melatonin enhances the efficacy of antioxidant vitamins. (2) Stabilization of mitochondrial function and enhancement of autophagy. As previously mentioned, mitophagy is a crucial step in protecting aging cells from damage caused by endogenous waste stress.³¹⁵ Melatonin enhances mitophagy, which can alleviate the accumulation of dysfunctional mitochondria and restore mitochondrial quality.^316,317 (3) Anti-inflammatory effects. Melatonin exerts its neuroprotective effects by inhibiting the release of cytochrome and the activation of caspases, as well as negatively regulating the production of proinflammatory cytokines.³¹⁸ Early melatonin intervention disrupts the vicious cycle of Aβ-ROS-induced neuroinflammation in AD and ameliorates metabolic inflammation in T2D.^47,319 The outcomes of clinical trials in these areas have been highly encouraging, underscoring its efficacy. However, while preclinical investigations of cardiovascular diseases have yielded promising results, melatonin has not met anticipated efficacy benchmarks in translational studies involving patients with established cardiovascular conditions.^320,321 Nonetheless, it has shown notable protective effects against cardiovascular risk factors.³²¹ Consequently, a deeper exploration of its mechanistic pathways and therapeutic potential is warranted, aiming to elucidate novel strategies and substantiate evidence for the prevention and management of these diseases.

Sirtuins are a family of NAD⁺-dependent deacetylases that serve as crucial regulators in delaying cellular senescence and extending organismal lifespan.³²² With advancing age, there is a notable decline in sirtuin activity, particularly in SIRT1 and SIRT3.^323,324 The overexpression of SIRT1 has been demonstrated to inhibit the aging of nucleus pulposus cells, promote cell proliferation, and suppress apoptosis.³²⁵ Moreover, the activation of SIRT1 also inhibits the senescence of dermal fibroblasts induced by ultraviolet irradiation.³²⁶ On the basis of the aforementioned research, increasing interest has been directed toward various NAD⁺ precursors or small-molecule activators, which aim to increase the activity of sirtuins.³²³ Intriguingly, the first SIRT1 activator to capture widespread attention was resveratrol, which exerts its effects through several mechanisms (Table 1).³²⁷ For example, it can modulate the deacetylation state of the core autophagy protein ATG9A by activating SIRT1, thereby delaying age-related hearing loss.³²⁸ Additionally, resveratrol improves motor function in senescence-accelerated mice by attenuating the negative regulation of insulin and apoptotic signaling through the SIRT1/FOXO1 pathway.³²⁹ These findings underscore the capacity of resveratrol to exert its antiaging effects across diverse biological pathways.

Notably, SIRT1, which serves as a pivotal modulator in the promotion of healthspan, captures scientific intrigue largely because of its ability to orchestrate a multitude of signaling cascades. SIRT1 interacts with the RelA/p65 subunit of NF-κB, leading to the deacetylation of lysine residues on this subunit. This interaction ultimately diminishes the transcriptional activity of the NF-κB complex, and as a result, SIRT1-mediated deacetylation serves to inhibit NF-κB signaling.³³⁰ Concerning another significant pathway that promotes aging—the mTOR pathway—SIRT1 can obstruct mTOR signaling, thereby rectifying autophagy impairments induced by oxidative stress and consequently enhancing the survival rate of embryonic stem cells.³³¹ In alignment with this, SIRT1 and AMPK can reciprocally enhance each other’s activity. SIRT1 deacetylates LKB1, thereby activating AMPK and inhibiting senescence³³². Conversely, AMPK can increase intracellular NAD⁺ levels, which in turn boosts SIRT1 activity, ultimately delaying the aging process of chondrocytes in osteoarthritis.^333,334

Dasatinib, a common drug for targeting SnCs, is a second-generation tyrosine kinase inhibitor that can downregulate the expression of senescence-associated biomarkers, including β-galactosidase, p16, and p21.³³⁵ Quercetin, which targets the Bcl-2 family member hypoxia-inducible factor-1α (HIF-1α) and the antiapoptotic PI3K/AKT and p21 signaling pathways, has demonstrated synergistic effects (Table 1).³³⁶ This combination has been shown to mitigate adipose tissue inflammation and cellular senescence in aged mice, concurrently enhancing systemic metabolic performance.^337,338 It has been shown to prevent the progression of age-dependent intervertebral disc degeneration,³³⁹ reduce intestinal SnCs and excessive inflammation in aged mice,³⁴⁰ and effectively induce apoptosis, thereby eliminating SnCs and the SASP burden in adipose tissue.³³⁷ Additionally, navitoclax (also known as ABT-263) is a drug classified as a “Bcl-2 family inhibitor.” It specifically inhibits the antiapoptotic proteins Bcl-2, Bcl-xL, and Bcl-w within the Bcl family and induces apoptosis through the activation of caspase signaling pathways.³⁴¹ For example, the oral administration of Navitoclax in mice subjected to sublethal irradiation or natural aging effectively depletes senescent hematopoietic stem cells and senescent muscle stem cells, thereby rejuvenating stem cell function.³⁴² However, its side effects, such as transient thrombocytopenia and neutropenia, remain obstacles to clinical translation.^342–344 It has been reported that p53 is a senescence marker and that the p53 signaling pathway regulates cellular senescence.¹⁸¹ In another study, FOXO4 was found to be elevated in SnCs and contributed to the senescence process.^345,346 FOXO4-DRI, a cell-permeable peptide, disrupted the FOXO4-p53 interaction, thereby reducing senescence-induced FOXO4 activity and selectively targeting SnCs for p53-dependent apoptosis (Table 1).³⁴⁶ FOXO4-DRI inhibits renal tubular senescence and restores renal function in vivo³⁴⁷. Additionally, it effectively eliminates senescent chondrocytes in vitro and ameliorates pathological changes and collagen deposition in the lungs of mice with pulmonary fibrosis.^348,349

Dysfunction of HSCs may serve as a fundamental basis for the aging of the myeloid and lymphoid lineage systems.³⁵⁰ For example, aged HSCs exhibit a diminished capacity to adhere to stromal cells, which may perturb the interactions between stem cells and the hematopoietic niche, ultimately leading to alterations in the functionality of aging HSCs.^351,352 Therefore, the underlying causes of stem cell aging are likely to represent potential therapeutic targets. One contributing factor to the changes observed in senescent stem cells may be the age-dependent acquisition of defects in telomeres, genomic DNA, or mitochondrial DNA.^353–356 During the process of replicative aging of HSCs in both humans and mice, telomeres progressively shorten.³⁵⁷ The function of telomerase in HSCs is to counteract the rate of telomere shortening during cell division, thereby preventing premature telomere attrition and extending the replicative capacity of HSCs.³⁵⁷ The in vitro culture of telomere-binding protein protection of telomeres 1 (Pot1) in HSCs has been demonstrated to maintain self-renewal capacity and regulate the activity of HSCs in elderly individuals, which has significant implications for the ex vivo cultivation of human HSCs.³⁵⁸ Similarly, gene therapy employing adeno-associated virus (AAV) 9 vectors to overexpress telomerase reverse transcriptase (TERT) has been demonstrated to alleviate age-related telomere damage and delay senescence in mouse models of cancer resistance.³⁵⁹

The diminished efficacy of HSCs can be attributed not only to telomere shortening but also to an increased risk of DNA damage³⁶⁰ and elevated levels of ROS production.³⁶¹ In the context of ataxia-telangiectasia mutated (ATM) deficiency, treatment with the permeable thiol antioxidant N-acetyl-l-cysteine (NAC) was shown to restore the reconstitution capacity of HSCs, thereby preventing bone marrow failure.³⁶¹ Furthermore, inhibition of the upregulated tumor suppressors p16^INK4a and p19^ARF in the context of ATM deficiency has been demonstrated to restore the replicative function of HSCs.³⁶¹ However, p16^INK4a does not universally induce HSC senescence in all contexts, and its role in maintaining HSC homeostasis has not been reported in other studies.³⁶² A new study revealed that depleting aged murine HSCs with myeloid-biased outputs (my-HSCs) via antibodies such as anti-CD150 can restore features of a younger immune system in mice. This intervention increased naïve T cells and mature B cells in aged mice while reducing the levels of inflammatory markers.³⁶³ Interestingly, in aging tissues, the inhibition of key molecules with high activity in oxidative stress and strong ROS inducers, such as p38, can restore the function of HSCs.^350,364 SIRT3, a prominent member of the sirtuin family, enhances antioxidant activity and exerts an inhibitory effect on ROS. Upregulation of SIRT3 rescues functional impairments in aged HSCs.³⁶⁵ Notably, mTOR and cell division control protein 42 (CDC42) are closely associated with the aging process and have been identified as promising targets for rejuvenating HSCs.^{295,366–368} The small-molecule CDC42-targeting drug CASIN has significant potential in mobilizing murine HSCs, thereby contributing to overall lifespan extension.^369,370 Furthermore, targeted therapies against these proteins may not only lead to the rejuvenation of HSCs but also potentially induce systemic rejuvenation of the entire organism.^371,372 Inflammation associated with aging induces the expression of interleukin-27 receptor α (IL27Ra) on the surface of HSCs through the TNF-α-ERK-ETS1 signaling pathway, leading to HSC senescence.³⁷³ The pharmacological targeting of key molecules within this signaling pathway may provide new therapeutic avenues for combating immune aging and related diseases caused by HSC senescence.³⁷³ In summary, the concept of revitalizing tissue-specific stem cells to restore tissue function and, ultimately, rejuvenating the whole organism represents a promising avenue worthy of further exploration. Hematopoietic stem cell transplantation is a treatment method with promising therapeutic outcomes. An intervention involving the shared circulation of blood between two individuals of different ages has been reported to rejuvenate the state and functionality of stem cells in the brain, liver, and muscle tissues of older individuals.^374–376 However, subsequent studies demonstrated that this rejuvenation effect is attributed to noncellular factors present in the blood rather than the cells themselves.^375,377,378

Direct targeting of pathogenic T cells constitutes a widely investigated therapeutic strategy (Fig. 7). For example, the proliferation and functional restoration of T cells can be stimulated through the use of cytokines. IL-7 modulates the size of the peripheral T cell pool and plays a pivotal role in regulating T cell homeostasis. In the context of hematopoietic cell transplantation, the administration of exogenous IL-7 can facilitate T cell reconstitution, thereby promoting the homeostatic proliferation of T cells and conferring antiapoptotic effects on peripheral T cells.^385,386 In addition to IL-7, several other cytokines that play pivotal regulatory roles in T cell regeneration have been identified in preclinical studies. IL-12 acts as an enhancer cytokine, potentiating the IL-2 and IL-7 signaling pathways, thereby sustaining thymic T cell function and supporting their development during aging.³⁸⁷ IL-15 is critical for enhancing the development and functional efficacy of CD8⁺ T cells.^388,389 IL-21 has demonstrated efficacy in promoting the reconstitution of the peripheral naïve T cell compartment across diverse models of immune injury.³⁸⁹ Additionally, IL-22 facilitates the development of thymic-derived peripheral T cells.³⁹⁰ Together, these cytokines collectively contribute to the maintenance and restoration of T cell homeostasis, underscoring their therapeutic potential in immune regeneration and repair processes.

On the basis of the previously discussed evidence, keratinocyte growth factor (KGF) has been demonstrated to significantly enhance thymic regeneration. The therapeutic administration of recombinant KGF expedites the recovery of the peripheral T cell compartment and replenishes T cell populations subsequent to immune insults such as irradiation, cyclophosphamide administration, and dexamethasone treatment. In clinical practice, the utilization of KGF holds promise for the efficient reconstitution of the T cell compartment, thereby reinstating the ability to elicit a potent adaptive immune response.^391,392 In an experimental study evaluating the effects of KGF therapy on thymic architecture and T cell immune reconstitution following myeloablative total body irradiation and autologous peripheral blood progenitor cell (PBPC) transplantation in healthy rhesus macaques, KGF treatment significantly increased de novo T cell production and the recovery of naïve T cells. These findings underscore the potential of KGF to promote robust thymic regeneration and immune restoration in the context of intensive conditioning regimens.³⁹³ Furthermore, the combined administration of KGF and androgens resulted in the accelerated reconstitution of naïve CD4⁺ and CD8⁺ T cells with a diverse Vβ repertoire, significantly contributing to the restoration of peripheral T cell populations.³⁹⁴

The capacity of sex hormones to influence lymphocyte development has been recognized for decades. In particular, the ability of estrogen to suppress postnatal thymocyte development and T cell production has been well-documented.³⁹⁵ Sex steroid ablation has been widely demonstrated to be beneficial for T cell reconstitution. A study showed that castration in 9-month-old mice rapidly reversed thymic involution, restoring the numbers of CD4⁺ and CD8⁺ cells to those of 2-month-old mice.³⁹⁶ Therapeutically, strategies that target sex steroid-related pathways, such as luteinizing hormone-releasing hormone (LHRH), or directly block sex steroid receptors can also achieve similar effects. A nonrandomized trial indicated that administration of the LHRH agonist goserelin (Zoladex) increased the numbers of various T cell subsets, particularly naïve CD4⁺ T cells, and improved the restoration of TCR repertoire diversity.³⁹⁷ LHRH agonists represent an effective and rational strategy for enhancing thymic function, not only in immunocompromised patients but also during normal aging.³⁹⁸ AR inhibitors and LHRH antagonists, which have the advantage of bypassing the sex steroid surge observed with LHRH agonists, may offer a greater opportunity for T cell reconstitution.³⁹⁹ However, it is worth considering that the regenerative effects of sex steroid inhibition on T cell development may be sustained only if sex steroid levels remain suppressed and that sex may also influence the efficacy of these interventions.⁴⁰⁰

The CD153 vaccine has been demonstrated to attenuate the presence of CD153⁺ senescent T cells within the adipose tissue of diet-induced obese mice through the induction of mouse IgG2 antibodies following administration.⁴⁰¹ Additionally, the adoptive transfer of in vitro expanded or genetically engineered CD4⁺ T cells has emerged as a promising therapeutic approach to rejuvenating immune function in the context of immunosenescence. Notably, specific pharmacological interventions have shown potential in mitigating immunosenescence via T cell modulation. For example, dasatinib, a senolytic agent that inhibits tyrosine kinase (TK) activity and regulates the SASP, has been shown to promote the differentiation of CD4⁺ T cells toward a more juvenile phenotype.⁴⁰² Furthermore, the mitochondrion-targeted antioxidant plastoquinonyl-decyltriphenylphosphonium has been shown to increase the intrathymic CD4⁺ T-to-CD8⁺ T cell ratio and prolong lifespan in diverse species.⁴⁰³ In elderly populations, metformin administration may ameliorate age-related impairments in autophagy and its downstream effects on CD4⁺ T cells.⁴⁰⁴ Recent studies have reported that micro/nanomaterial-based artificial antigen-presenting cell platforms can induce T memory stem cells in vitro and sustain long-term T cell immunity, thereby providing durable immune competence in elderly individuals.⁴⁰⁵

Experimental evidence indicates that mice lacking MyD88, a critical adapter molecule for TLR signaling, lack ABCs and that pharmacological inhibition of this signaling axis effectively impedes the commitment of B cells to the ABC lineage.^{116,406–408} The downstream mediators of TLR7 and TLR9 signaling include IFN-γ and IL-21. In the context of IFN-γ deficiency, IL-4 exerts a suppressive effect on ABC differentiation, and targeted inhibition of IFN-γ or IL-21 represents a potential therapeutic avenue.⁴⁰⁶ Several studies have shown that follicular B (FO B) cells can serve as progenitors for ABCs, and their findings further revealed that neither MHC-II-deficient nor CD40-deficient FO B cells are capable of generating ABCs.^408–410 Moreover, CD154-deficient mice exhibit an age-dependent failure to develop natural ABCs. Another recently characterized age-associated B cell population is aged adipose B cells (AABs), which localize to fat-associated lymphoid clusters and display a phenotypic profile distinct from that of ABCs.⁴¹¹ The expansion of AABs is mediated by Nlrp3 inflammasome activation.⁴¹¹ Therapeutic strategies involving the blockade of IL-1 signaling to inhibit Nlrp3-dependent B cell accumulation or the targeted depletion of B cells within adipose tissue via anti-CD20 antibodies have been shown to mitigate metabolic dysfunction in aged AT.⁴¹¹ CASIN, which is utilized for the treatment of aged HSCs, has also been shown to restore B cell populations and extend the posttransplant lifespan.³⁶⁶ Exogenous supplementation with spermidine can effectively promote the translation of TFEB, a transcription factor involved in autophagosome and lysosome biogenesis, through hypusinated EIF5A. This process reverses the age-dependent decline in the EIF5A-TFEB-autophagy axis and restores B cell function.⁴¹²

Therapeutic blockade of NKG2A, an inhibitory receptor that modulates NK cell cytotoxic activity, enhances NK cell-mediated immunosurveillance and significantly diminishes senescent cell accumulation in aged murine models.^413,414 The development and functionality of NK cells are regulated by a variety of miRNAs. Among these, the expression of miR-181a-5p notably decreases with age.¹²⁸ This reduction is partially mediated through the upregulation of NLK and BCL2, which may contribute to the functional impairment observed in NK cells during aging.^128,415 Consequently, the targeted delivery of miR-181a-5p to NK cells represents a clinically significant avenue of research aimed at enhancing the maturation and functional efficacy of NK cells in elderly individuals. Furthermore, it is well established that immunosenescence and inflammation are intricately interconnected. Immune cells that mediate the initiation and progression of inflammatory responses consequently represent promising therapeutic targets for intervention in age-related immune dysfunction. In monocyte/macrophage lineages, pharmacological modulation via the metabolic agent metformin achieves dual therapeutic effects by impairing monocyte-to-macrophage differentiation, consequently attenuating age-related macrophage senescence and systemic immune dysfunction.⁴¹⁶ Pharmacological intervention through p38-MAPK signaling pathway inhibition or COX-2 suppression has demonstrated efficacy in restoring monocyte functional competence.⁴¹⁷ Notably, cytokine-based intervention with IL-4 administration confers cytoprotective effects on macrophages while enhancing their survival capacity, ultimately ameliorating age-associated physiological decline and increasing health span parameters in geriatric mouse cohorts.⁴¹⁸ Interestingly, recent studies have revealed that immunoglobulin G (IgG) induces both senescence and a proinflammatory state in macrophages from mice and humans. An antisense oligonucleotide targeting Fcgrt (encoded by FcRn depletion, which reduces tissue IgG levels) decreases IgG levels and reduces senescence-associated marker levels.⁴¹⁹ Moreover, transplantation of eosinophils from young donors into aged recipient mice transiently reversed age-associated alterations in the HSC pool, leading to measurable improvements in physiological and immune health parameters that were partially mediated by eosinophil-derived IL-4.⁴²⁰ Brahmakshatriya and colleagues demonstrated that adoptive transfer of ex vivo activated bone marrow-derived DCs into aged mice enhances postimmunization GC formation and Tfh cell responses.⁴²¹ Importantly, a separate study revealed that the reduction in the number of type 2 conventional dendritic cells (cDC2s) within the DC compartment may account for the attenuated vaccine efficacy observed in aged populations.⁴²² Topical administration of the TLR7 agonist imiquimod was shown to increase antigen-bearing cDC2 populations, resulting in partial restoration of vaccination efficacy through targeted phenotypic restoration.⁴²²

Thymic involution is indeed one of the key characteristics of immunosenescence. As people age, the thymus gradually atrophies, leading to a decrease in T cell production, which in turn affects the function of the immune system. Therefore, thymic regeneration is considered a potential strategy to combat immunosenescence. Thymic epithelial cells (TECs) primarily maintain thymic development to ensure their integrity and support T cell development (Figs. 6, 7). Therefore, it is feasible to develop a developmental niche within the thymus by amplifying TECs and to concentrate on the interplay between TECs and thymocytes to increase the flux of thymopoiesis.^423,424 Importantly, Forkhead box N1 (FOXN1) regulates the formation and expansion of TECs, making it a key regulatory factor in thymic development and even regeneration.⁴²⁵ Therefore, for the rejuvenation of the aging thymus, our strategies focused on the FOXN1-TEC axis, such as (1) the enhancement of thymic rejuvenation by the exogenous expression of FoxN1 in TECs via FoxN1 cDNA plasmids and FoxN1 transgenic models;⁴²⁶ (2) the implantation of FOXN1-reprogrammed embryonic fibroblasts (FREF) into the senescent native thymus to restore thymic function and potentially counteract age-related inflammation;^427,428 (3) extracellular vesicles and exosomes derived from young, healthy serum might contribute to the recovery of thymic aging by enhancing FoxN1 expression, characterized by partial reversal of thymic involution and enhanced negative selection signals;⁴²⁹ and (4) furthermore, key cytokines and growth factors essential for maintaining TEC function, including mesenchymal-derived KGF, macrophages and T lymphocyte-derived IGF-1, and thymic stromal cell-derived bone morphogenetic protein-4 (BMP4), as well as critical TEC functions such as the production of cytokines (e.g., IL-7 and Kit ligands) and chemokines (e.g., SDF-1 and TECK), can improve thymic function in mice.^{424,430–434} Deficiency of the epithelium-specific microRNA-205 (miR-205) can lead to significant thymic involution.^435,436

Bone marrow (BM) can provide niches for plasma cells and memory T cells and is influenced by aging. Immunosenescence is widely acknowledged to play a role in the pathogenesis of AD.⁴³⁷ A previous study revealed that young bone marrow transplantation (BMT) can rejuvenate peripheral cells, alleviate aging-associated signaling pathways, restore altered intercellular communication in senescent PBMCs, and reduce the levels of SASP factors in the blood (Fig. 6).⁴³⁸ Pangrazzi et al. revealed that the expression of molecules responsible for sustaining immune memory in the human BM undergoes age-related alterations. On the basis of this insight, the application of antioxidants could mitigate inflammatory levels within the BM and promote the generation of adequate “appropriate” survival factors for memory cells. Consequently, such interventions may enhance the preservation of immune memory during aging.⁴³⁹ Most importantly, clarifying the concept of the hematopoietic niche, which refers to the spatial localization and unique microenvironment of HSCs and immune cells within the bone marrow, is essential. The critical role of HSCs in immunosenescence is clear. As the bone marrow serves as the site of the hematopoietic niche, targeting senescent niches within the bone marrow holds broad application prospects. For example, targeting HIF transcription factors (involved in ROS production pathways) has been shown to improve outcomes in patients with hematopoietic disorders by reconstructing the niche composition. Additionally, supplementation with β3-adrenergic receptor agonists can enhance sympathetic innervation in the bone marrow and promote hematopoietic function.

Although thymic regeneration represents a promising therapeutic strategy for addressing immunosenescence, emerging evidence suggests that age-associated fibrosis in lymph nodes may counteract the benefits conferred by thymic rejuvenation (Fig. 6).⁴⁴⁰ Consequently, lymph node fibrosis has emerged as a potential therapeutic target in the context of rejuvenating the aged immune system. Alternatively, the restoration of lymphatic and immune cell functions through the utilization of functional synthetic lymphoid organoids composed of stromal progenitor cells and decellularized extracellular matrix scaffolds represents a novel and innovative approach.⁴⁴¹ In addition, as the largest secondary immune organ in the body, the spleen is closely associated with immune aging.⁴⁴² Reports indicate that transplanting splenic cells from young mice can reduce aging and tissue damage in aged mice.⁴⁴³ Furthermore, systemic AAV vector-mediated LAV-BPIFB4 gene transfer has been shown to redirect circulating SnCs to the spleen and increase the activity of immune cells within the spleen, thereby effectively eliminating senescent immune cells.⁴⁴⁴ However, immune organs do not function in isolation, and targeting a single organ still has numerous limitations. Therefore, combining this approach with other strategies is essential to achieve long-lasting restoration of immune function.