New Insights into Aging and Immune System Decline: Causes and Possible Treatments

The cGAS-STING signaling pathway can recognize abnormal DNA within cells, as well as DNA fragments from invading pathogens, producing type-I interferons and other inflammatory cytokines to exert immune functions [43]. In recent years, activation of the cGAS-STING pathway has been reported to be involved in regulating the cellular aging process [44,45]. However, whether the cGAS-STING pathway can directly cause cellular senescence in tissues and age-related inflammation in the body remains unclear.

In August 2023, Andrea Ablasser et al. from the Swiss Federal Institute of Technology Lausanne published a study in Nature [29]. Using a natural aging mouse model, they discovered that administering the STING inhibitor H-151 or knocking out STING could significantly improve age-related inflammation. Furthermore, they investigated the impact of the cGAS-STING pathway on brain aging. They found abnormalities in mitochondrial morphology in the microglia of aged mice, with mitochondrial DNA released into the cytoplasm to activate the cGAS-STING pathway.

This study, which started with naturally aging mice, revealed that activation of the cGAS-STING pathway is a major promoter of age-related inflammation. Targeting the cGAS-STING pathway may become a viable strategy for improving or even reversing age-related damage.

Extensive research has been conducted on the regulation of aging by protein-coding genes, but the regulatory role of noncoding regions of the genome in aging has also gradually attracted increasing attention. Reports have revealed that silent long-interspersed element-1 (LINE1) retrotransposons are activated during the aging process and trigger an innate immune response associated with aging phenotypes [46,47].

Endogenous retroviruses (ERVs) are another type of transposable element that is dormant within our genome [48]. In January 2023, Guang-Hui Liu et al. from the Institute of Zoology at the Chinese Academy of Sciences published a research paper in the journal Cell revealing that ERVs are reactivated during the aging process and promote aging [23]. The researchers used various aging systems to discover that epigenetic derepression in SCs leads to the transcriptional activation and translation of viral proteins from a young ERV subfamily, HERVK (HML-2), resulting in the formation of retrovirus-like particles (RVLPs). On the one hand, the reverse transcription products of HERVK activate the cGAS-STING pathway, inducing cellular senescence and inflammation. On the other hand, the RVLPs released by aging cells are transmitted between cells and amplify aging signals, ultimately inducing senescence in young cells.

This study first described the reactivation of a young ERV subfamily during the cellular aging process, proposed the theory that ancient virus revival mediates the programmed and infectious aspects of aging, and innovatively developed multidimensional intervention strategies to delay aging by blocking the revival and spread of ERVs.

As aging progresses, the integrity of the nuclear envelope is compromised, allowing a large amount of cytoplasmic chromatin fragments (CCFs) to leak from the nucleus into the cytoplasm. These CCFs promote the SASP in aging cells by activating the cGAS-STING pathway [49,50].

Thioredoxin reductases (TXNRDs) can inhibit oxidative stress and are among the most important antioxidant proteins in cells [51]. The cytoplasmic isoform of TXNRDs, TXNRD1, is considered to play a role in tissue aging related to its enzymatic activity, as oxidative damage can lead to tissue aging and inflammation [52]. However, research published in Nature Aging by Rugang Zhang’s team at the MD Anderson Cancer Center revealed that TXNRD1 colocalizes with cGAS in SCs, promoting the binding of cGAS to DNA, which in turn drives the SASP and inflammation [53].

Researchers found that knocking down TXNRD1 in SCs or using its specific inhibitor Tri-1 significantly reduced SASP levels. Further studies using two different TXNRD1 inhibitors showed that only inhibiting the interaction between TXNRD1 and cGAS could reduce SASP levels, thereby indicating that the regulation of the SASP by TXNRD1 does not depend on its enzymatic activity but rather on its interaction with cGAS. In the future, strategies targeting the interaction between TXNRD1 and cGAS hold significant potential for the development of clinical treatments for senescence and age-related diseases.

Rapamycin modulates the aging process by acting on downstream autophagy and S6 protein kinase (S6K) through target of rapamycin complex 1 (TORC1) [54]. Although early studies have shown that the absence of S6K can extend the lifespan of mice [55], the specific mechanisms of action involved remain unclear.

In February 2024, Linda Partridge et al. published research in the journal Nature Aging that revealed the mechanisms by which S6K regulates aging [56]. Using fruit flies as a model, the research team identified the adipose tissue of adult fruit flies as the key tissue where the TORC1-S6K pathway regulates aging. By inhibiting S6K activity in adipose tissue, they were able to reduce age-related inflammation mediated by the IMD pathway in the adipose tissue of elderly fruit flies and improve immunosenescence caused by aging. Furthermore, they discovered that syntaxin 13 (Syx13), which is associated with cell membrane fusion, mediates the effects of the TORC1-S6K signaling pathway on the morphology of endolysosomes and inflammation, thereby showing that the TORC1-S6K-Syx13 signaling pathway regulates aging and age-related inflammation through the endolysosomal system.

This study established the endolysosomal system as a novel cellular mechanism that mediates the regulatory effects of rapamycin and S6K on immunosenescence and lifespan, thus providing a new direction for future research and treatment related to aging.

The two main hallmarks of T cell senescence are immunodeficiency and inflammaging [57]. Inflammaging is a systemic chronic inflammatory state that occurs without obvious infection and is capable of causing widespread tissue dysfunction, frailty, and premature death [14,58,59]. However, the mechanisms underlying inflammaging are currently not well understood.

Previous studies have indicated that the majority of the effector molecules involved in T-cell-mediated inflammation are cytokines. Recently, a study published in Nature Aging by Jorg J. Goronzy et al. revealed that activated T cells in elderly individuals can mediate strong inflammatory responses by releasing large amounts of mitochondrial DNA [60]. They found that increased expression of cytokine-inducible SH2-containing protein (CISH) promotes the proteasomal degradation of ATP6V1A, an essential component of the proton pump V-ATPase, leading to a diminished lysosomal acidification capacity and thus inhibiting lysosomal degradation functions. A decrease in lysosomal function prevents the clearance of upstream autophagosomes and late endosomes, causing them to accumulate and fuse within the cell to form amphisomes, which subsequently merge with the cell membrane and release their contents, including mitochondrial DNA, to the exterior of the cell. By further utilizing murine infection and immunization models, the researchers have shown that silencing CISH in mouse T cells reduces the serum mitochondrial DNA and inflammatory cytokine levels while enhancing virus clearance and antibody production.

This study suggested that downregulating CISH expression in T cells is a promising strategy for enhancing lysosomal function, alleviating inflammaging in elderly individuals, and ultimately boosting immunity.

The thymus plays a crucial role in the development and function of the immune system, and thymic involution is one of the main hallmarks of T cell senescence [57,61]. Severe infectious diseases often cause acute thymic involution, [42,62] which triggers immunosuppression [63]. However, the exact molecular basis of acute thymic involution during severe infections and the related functional impairments in T cell senescence are not yet fully understood.

In November 2022, Xiaojun Chen et al. published an article in Nature Communications revealing that IL-33 induces immunosuppression by causing thymic involution associated with naive T cell functional impairment and aberrant expression of aging-related genes [64]. Using mouse disease models of schistosomiasis and sepsis, the research team discovered that IL-33 is a critical factor that causes host thymic involution and T cell senescence. Further studies revealed that IL-33 induces the excessive production of medullary thymic epithelial cell (mTEC) IV (thymic tuft cells) in a Pou2f3-dependent manner, disrupting the mTEC/cortical TEC (cTEC) compartment and thereby leading to thymic involution. Knockout of IL-33 or its receptor ST2, as well as treatment with an IL-33 neutralizing antibody, could eliminate host thymic involution, thereby restoring T cell responses and ultimately enhancing host infection resistance.

In summary, reversing thymic involution is a potential therapeutic strategy that can restore T cell immune responses in order to better control severe infections.

Many studies have confirmed that aging in organisms is accompanied by changes in the gut microbiota composition, leading to age-related functional decline and the development of diseases [65,66,67]. However, the mechanisms and reasons for these age-related changes in the gut microbiota composition are still not well understood.

In May 2023, Eiji Hara et al. revealed that the presence of commensal bacteria can induce aging in B cells located in the germinal center (GC) of the gut [68]. The researchers initially used bioluminescence imaging (BLI) and single-cell RNA sequencing (scRNA-seq) to analyze aging markers in specific pathogen-free (SPF) and germ-free (GF) mice. They found that only the GC B cells in the ileal lymphoid follicles of aging SPF mice exhibited significant signs of aging, indicating that the presence of gut bacteria could induce B cell aging by upregulating the expression of p16INK4a and p19ARF. Further studies revealed that lipopolysaccharide (LPS) from gram-negative bacteria could penetrate into ileal tissues. LPS-treated cultured B cells induced B cell hyperproliferation and upregulated p16INK4a expression, along with causing DNA damage in B cells.

Overall, this study revealed that with increasing age, the likelihood of gram-negative bacteria invading the ileum increases, which in turn causes B cell aging. The aging of GC B cells reduces the production and abundance of IgA, thereby affecting the composition of the gut microbiota.

Previous studies have focused on clearing SCs using pharmacological or genetic approaches, which have significant side effects [17,69]. Considering that SCs produce SASP components, they could serve as targets for the immune system. However, researchers have not yet clearly determined how the immune system combats SCs in the human body.

Recently, a research team led by Shadmehr Demehri at Massachusetts General Hospital and Harvard Medical School published an article in Cell titled “Cytotoxic CD4⁺ T cells eliminate senescent cells by targeting cytomegalovirus antigen” [25]. The researchers studied human skin samples of various ages and found that the chemokine CXCL9 expressed by keratinocytes recruits CD4⁺ CTLs to the skin. Senescent fibroblasts express HLA-II and human cytomegalovirus glycoprotein B (HCMV-gB). HLA-II then presents HCMV-gB as an antigen on the fibroblast surface, which can be directly targeted and cleared by CD4⁺ CTLs.

This study revealed the interaction between the human immune system and viruses, providing a theoretical basis for designing immunotherapies targeting HCMV antigens to clear SCs.

SCs are key drivers of organismal aging, and clearing these cells can delay or alleviate many age-related diseases [6]. Therefore, identifying which types of SCs contribute most significantly to aging and targeting them as a priority for treatment remain critical issues.

Laura J. Niedernhofer et al. published a study in Nature revealing that senescent immune cells are the most perilous type of SCs, accelerating the aging of other organs and thus promoting systemic aging [33]. To determine the impact of immunosenescence on organismal aging, the researchers selectively deleted the Ercc1 gene, which encodes a key DNA repair protein, in the hematopoietic cells of mice [70]. This deletion increased the burden of endogenous DNA damage exclusively within the immune system, leading to premature aging of the immune system alone. However, these mice also exhibited increased aging and damage in non-lymphoid organs. Furthermore, transplanting spleen cells from Ercc1 knockout mice or aged wild-type mice into young mice accelerated aging in the recipients, while transplanting young immune cells mitigated aging. This result further indicates that senescent immune cells can promote systemic aging.

In summary, this study suggested that the senescence of immune cells may be the most detrimental to the organism. Therefore, senescent immune cells have become a key therapeutic target for extending a healthy lifespan.

As early as 2020, a research team led by Professor Li Qiang at Columbia University first confirmed that adipose tissue is the tissue initiating aging through metabolomic analysis of multiple tissues in mice [71]. Adipose tissue plays a central role in longevity, and interventions targeting adipose tissue may influence the lifespan [72].

Unlike IgA or IgM, IgG has an especially long half-life due to its unique recycling mechanism [73]. Beyond traditional immune functions, the role of IgG in aging and metabolism remains unclear. In February 2024, Professor Li Qiang’s research team published an article in Cell Metabolism revealing that IgG is an aging factor that causes fibrosis in adipose tissue and metabolic decline [74]. The researchers discovered a significant enrichment of IgG in the visceral fat of aging mice through quantitative proteomics and found that administering exogenous IgG to mice on caloric restriction (CR) could reverse the improvements in adipose tissue function induced by CR. Further mechanistic studies showed that IgG activates macrophages via the Ras signaling pathway and induces fibrosis in white adipose tissue (WAT) through the TGF-β/SMAD pathway, thereby impairing the metabolic function of adipose tissue. The neonatal Fc receptor (FcRn) is the recycling receptor for IgG in macrophages [73], and conditional knockout of this receptor can prevent the accumulation of IgG during aging, thereby extending a healthy lifespan.

In summary, this study revealed that IgG begins to accumulate systemically early in aging, particularly in adipose tissue, leading to fibrosis and metabolic damage in the tissue. Therefore, intervening in the accumulation of IgG represents a viable therapeutic strategy for delaying aging.

As the number of cell divisions increases, telomeres become progressively shorter, eventually leading to cellular senescence [75]. Although T cells can utilize telomerase to mitigate telomere shortening caused by rapid clonal expansion, the activation of telomerase is not sufficient to prevent T cell exhaustion, ultimately still resulting in the production of senescent T cells [76,77,78].

Recently, researchers at University College London (UCL) discovered that some T cells can extend their own telomeres by acquiring telomeres from extracellular vesicles (EVs) secreted by antigen-presenting cells (APCs) [78]. Further mechanistic studies revealed that when some T cells interact with APCs, the APCs degrade shelterin to provide telomeres. These telomeres are then cleaved by the telomere trimming factor TZAP and transferred in EVs at the immunological synapse. The telomere vesicles retain the Rad51 recombinase, which enables the telomeres to fuse with the ends of the T cell chromosomes, extending them by an average of approximately 3000 base pairs.

Overall, this study revealed that the transfer of telomeres from APCs to T cells can protect T cells from senescence. When T cells acquire telomeres from APCs during antigen presentation, they transition to a state similar to central long-lived memory T cells, providing long-term immune protection for the organism.

Quercetin has a wide range of biological activities, such as antioxidant, anticancer, and anti-inflammatory effects [80]. In 2015, quercetin was first discovered to be a senolytic that can effectively kill senescent human endothelial cells and mouse bone marrow-derived mesenchymal stem cells (BM-MSCs) [18]. In 2019, researchers at the Mayo Clinic observed that the combination of dasatinib and quercetin (D&Q) successfully cleared SCs in patients with diabetic nephropathy, significantly reducing the burden of SCs in adipose and skin tissues [81]. Furthermore, a recent article published in Science reported that the neural innervation of the heart weakens with age, but this effect can be reversed with treatment using the D&Q senolytics combination [82]. However, quercetin must be used in conjunction with dasatinib to exhibit effective senolytic activity [18,81].

Fisetin is another flavonoid that has shown strong antitumor activity by inhibiting cancer cell proliferation and inducing apoptosis in cancer cells [83]. Research has shown that the anti-proliferative and pro-apoptotic effects of fisetin are limited to cancer cells and have a much weaker impact on normal cells [84]. In 2017, James L. Kirkland et al. first discovered that fisetin selectively induces apoptosis in SCs. It induced apoptosis in senescent but nonproliferating human umbilical vein endothelial cells (HUVECs) without affecting proliferating HUVECs, making it a cell-specific senolytic [85]. Later, Matthew J. Yousefzadeh et al. conducted in vivo studies on the effects of fisetin on aging in aged mice, confirming the senolytic activity of fisetin, which can reduce senescence markers in multiple tissues [86].

Additionally, ongoing basic research and clinical studies are further exploring senolytics such as quercetin, dasatinib, fisetin, piperlongumine, and EF24. Several clinical trials involving senolytics are currently underway or planned (Table 2), and these senolytics have provided evidence of clearing SCs in various pathologies.

Reengineering patients’ own T cells to selectively target and eliminate tumor cells has cured patients with otherwise untreatable hematological cancers [89]. However, evidence from both basic and clinical research has highlighted the potential of CAR T therapy to go beyond oncology, addressing autoimmune diseases, chronic infections, cardiac fibrosis, age-related diseases, and other conditions [90,91].

In 2020, a team led by Scott W. Lowe at the Memorial Sloan Kettering Cancer Center published an article in Nature proposing a method to treat age-related pathologies using CAR T cells [19]. Researchers identified urokinase-type plasminogen activator receptor (uPAR) as a specific SCs surface marker and developed uPAR CAR T cells. They found that these cells could safely and effectively clear SCs in several induced young mouse models and reverse liver fibrosis in a liver disease model. This study confirmed the potential of senolytic CAR T cells to treat age-related diseases and provided a new direction for the future treatment of age-related conditions.

Recently, Scott W. Lowe’s team published their latest research findings in Nature Aging [92]. They demonstrated that uPAR-positive SCs accumulate during the aging process in mice and can be safely targeted and eliminated by CAR T cells. Treatment with CAR T cells targeting uPAR improved the mobility of aged mice and ameliorated metabolic dysfunction in both aged mice and mice on a high-fat diet (HFD) without causing any tissue damage or toxic effects. More importantly, due to the memory capacity and longevity of T cells, this senolytic CAR T cells therapy requires only a single administration to achieve long-term therapeutic and preventive effects.

NKG2D ligands (NKG2DLs), which include MICA, MICB, and ULBP1-6, are highly expressed in tumor cells, and numerous cancer therapies targeting these ligands have entered clinical trials without the discovery of serious side effects [93,94,95]. In humans, NKG2DLs also act as triggers for the NK cell-mediated killing of SCs [96]. Therefore, in anti-aging research prioritizing safety, NKG2DLs are undoubtedly ideal targets for the elimination of SCs.

In August 2023, Xudong Zhao’s team published a cover article in Science Translational Medicine detailing NKG2D-CAR T cells targeting NKG2DLs as effective and safe senolytic agents [28]. Researchers first reported the significant upregulation of NKG2DLs in various stress-induced SCs and then engineered NKG2D-CAR T cells that recognize NKG2DLs. In vitro experiments showed that NKG2D-CAR T cells significantly killed SCs induced by DNA damage, replicative exhaustion, oncogene activation, or tumor suppressor gene inactivation in a dose-dependent manner. In vivo, the researchers developed mNKG2D-CAR T cells for mice and hNKG2D-CAR T cells for macaques, which significantly reduced the number of SCs in aging animals and did not cause severe side effects.

In summary, this study, based on the emerging CAR T immunotherapy for clearing SCs, demonstrated the feasibility of using NKG2DLs as anti-aging targets. It lays the foundation for developing more effective, safe, and precise anti-aging treatment methods.

Traditionally, vaccines have primarily been used for the prevention of infectious diseases and the treatment of cancer [97,98]. In 2020, a research paper published in Nature Communications reported for the first time the use of a vaccine as a therapeutic tool to eliminate SCs [32].

Senescent T cells, which increase with age, are defined as CD4⁺ CD44^high CD62L^low PD-1⁺ CD153⁺ cells and accumulate in the visceral adipose tissue (VAT) of obese individuals, directly correlating with the development of various diseases [99]. Researchers developed a CD153-CpG vaccine and confirmed that vaccination increased and sustained anti-CD153 antibody levels for several months. They found that the number of aging T cells in the VAT of HFD-induced obese mice vaccinated with the CD153-CpG vaccine was significantly reduced, and these mice exhibited improved glucose tolerance and lower insulin resistance. Complement-dependent cytotoxicity (CDC) assays further revealed that mouse IgG2 antibodies produced in mice vaccinated with the CD153-CpG vaccine successfully reduced the number of senescent T cells [32]. This study innovatively developed a senolytic vaccination capable of removing senescent T cells from the body, demonstrating the new potential applications of vaccines.

Subsequently, Tohru Minamino et al. identified a target for senolytic therapy—glycoprotein nonmetastatic melanoma protein B (GPNMB) [20]. GPNMB is a transmembrane protein that is enriched on the surface of some SCs. The research team developed a peptide vaccine for GPNMB. Upon injection, the vaccine-generated antibodies bind only to the GPNMB protein on the surface of SCs, thereby marking these cells for destruction. Then, these antibodies induce antibody-dependent cell-mediated cytotoxicity (ADCC) to kill SCs, an effect that was validated in models of atherosclerosis and aging mice.

In summary, these studies suggest that vaccines may be a viable tool for the treatment of aging and age-related diseases. Senolytic vaccination could become a new and suitable therapeutic approach, although its clinical application requires further assessment and the management of safety.

Many studies have shown that the immune system can clear SCs induced by various causes [25,30,39]. However, despite this capability, SCs still accumulate in various tissues and organs with aging [100,101]. Currently, little is known about the molecular mechanism underlying the accumulation of SCs.

In the study of cancer, scientists have discovered that the immune surveillance of tumor cells is negatively regulated by immune checkpoints [102,103]. In November 2022, a team led by Makoto Nakanishi from the University of Tokyo published an article in Nature [21] where they discovered that SCs heterogeneously express the immune checkpoint programmed death-ligand 1 (PD-L1). They found that PD-L1⁺ SCs accumulate in the body with age. Through in vitro T-cell killing assay, the researchers showed that PD-L1⁺ SCs are resistant to T cell immune surveillance, suggesting that the expression of PD-L1 in SCs is essential for evading T cell immunity. Based on these findings, the researchers tested the anti-aging effects of PD-1 antibodies. Treating naturally aged mice or mice with nonalcoholic steatohepatitis (NASH) with PD-1 antibodies significantly reduced the population of PD-L1⁺ cells in the body and improved various age-related phenotypes in a manner dependent on activated CD8⁺ T cells.

Therefore, treating the accumulation of aging PD-L1⁺ cells through ICB therapy represents a more promising treatment strategy compared to traditional anti-aging therapies.

In summary, immunotherapy for cancer has already achieved significant initial success, with targeted therapies eliminating tumor cells and curing patients with otherwise untreatable hematological cancers [89]. Compared to the use of immunotherapy for cancer treatment, the use of immunotherapy to alleviate aging and age-related diseases may have greater advantages. By exploring the molecular mechanisms of cellular senescence and immunosenescence, researchers may identify more senoantigens, thereby developing new types of senolytics with high specificity, low toxicity, and high activity, such as senolytic CAR T cells, senolytic vaccines, and ICB (Figure 2). These immunotherapies, which clear accumulated SCs, may represent a more promising treatment strategy than traditional anti-aging therapies.