In 1996, we demonstrated that the aging process induced a robust activation of NF-κB signaling system in the brain, heart, liver, and kidney in both rats and mice (Helenius et al., 1996a; Helenius et al., 1996b; Korhonen et al., 1997; Helenius et al., 2001). Given that the activation of NF-κB signaling is a hallmark of tissue inflammation, our observations indicated that chronic low-grade inflammation was associated with the aging process. Using EMSA assays, we demonstrated that with aging there was a constitutive increase in the nuclear binding activity of NF-κB complexes in several tissues. The identity of the NF-κB complex was verified by supershift and UV-crosslinking assays. The levels of p52 and p65 components were increased both in nuclear and cytoplasmic fractions. However, the aging process did not increase the mRNA expression of NF-κB components or inhibitory-κB proteins. These results clearly demonstrated that a chronic low-grade inflammation was associated with the aging process. This age-related increase in the inflammatory state was later termed inflammaging. We have reviewed in detail the activation mechanisms of innate immunity system during aging (Salminen et al., 2008; Salminen and Kaarniranta, 2009; Salminen and Kaarniranta, 2010).
Helenius M, Hänninen M, Lehtinen SK, Salminen A (1996a) Changes associated with aging and replicative senescence in the regulation of transcription factor nuclear factor-κB. Biochem. J. 318, 603-608. https://doi.org/10.1042/bj3180603.
Helenius M, Hänninen M, Lehtinen SK, Salminen A (1996b) Aging-induced up-regulation of nuclear binding activities of oxidative stress responsive NF-κB transcription factor in mouse cardiac muscle. J. Mol. Cell. Cardiol. 28, 487-498. https://doi.org/10.1006/jmcc.1996.0045.
Helenius M, Kyrylenko S, Vehviläinen P, Salminen A (2001) Characterization of aging-associated up-regulation of constitutive nuclear factor-κB binding activity. Antioxid. Redox Signal. 3, 147-156. https://doi.org/10.1089/152308601750100669.
Korhonen P, Helenius M, Salminen A (1997) Age-related changes in the regulation of transcription factor NF-κB in rat brain. Neurosci. Lett. 225, 61-64. https://doi.org/10.1016/s0304-3940(97)00190-0.
Salminen A, Huuskonen J, Ojala J, Kauppinen A, Kaarniranta K, Suuronen T (2008) Activation of innate immunity system during aging: NF-κB signaling is the molecular culprit of inflamm-aging. Ageing Res. Rev. 7, 83-105. https://doi.org/10.1016/j.arr.2007.09.002.
Salminen A, Kaarniranta K (2009) NF-κB signaling in the aging process. J. Clin. Immunol. 29, 397-405. https://doi.org/10.1007/s10875-009-9296-6.
Salminen A, Kaarniranta K (2010) Genetics vs. entropy: longevity factors suppress the NF-κB-driven entropic aging process. Ageing Res. Rev. 9, 298-314. https://doi.org/10.1016/j.arr.2009.11.001.
Inflammaging stimulates a compensatory immunosuppression which enhances immunosenescence and immune aging
There is convincing evidence that chronic inflammatory conditions generate a counteracting immunosuppressive state which is intended to suppress the harmful effects of excessive inflammation. The inflammaging state is associated with the expansion and activation of the immunosuppressive network involving myeloid-derived suppressor cells (MDSC) and the regulatory subtypes of T (Treg) and B (Breg) cells as well as regulatory phenotypes of macrophages (M2), dendritic (DCreg), natural killer (NKreg), and type II natural killer T (NKT) cells (Salminen et al., 2018a; Salminen, 2020). The presence of inflammation evokes the production of colony-stimulating factors, chemokines, and cytokines which stimulate the generation of MDSCs from the hematopoietic stem cells and subsequently MDSCs are released into the circulation and recruited into tissues (Salminen et al., 2018a). Immunosuppressive cells establish a co-operative network to enhance each other’s expansion and activity, e.g., by secreting anti-inflammatory cytokines, adenosine, and PGE2. Endoplasmic reticulum stress (Salminen et al., 2020), insulin/IGF-1 signaling (Salminen et al., 2021), and UV radiation (Salminen et al., 2022) promote the function of the immunosuppressive network, whereas AMPK activation (Salminen et al., 2019a) and many phytochemicals (Salminen et al., 2018b) inhibit excessive immunosuppressive activity. The age-related increase in the immunosuppressive activity is associated with many clinical pathologies, e.g., an increased risk for cancers and chronic infections, whereas the efficiency of vaccination and immunotherapy declines with aging (Salminen, 2022).
The aging process is associated with a gradual decline in the functional capacity of adaptive and innate immunity involving (i) a decline in the numbers of T cells and the T cell receptor repertoire, (ii) a decline in the cytotoxicity of CD8 T and NK cells reducing the immune surveillance capacity, (iii) a decline in B cell lymphopoiesis and antibody production, (iv) a reduced antigen uptake and presentation as well as a decrease in priming of T cells by dendritic cells, and (v) a reduced phagocytic activity and an increased production of cytokines and chemokines by macrophages (Salminen et al., 2019b; Salminen, 2021). These properties are the hallmarks of immunosenescence associated not only with aging but also with the chronic inflammatory conditions present in many inflammatory diseases. Immunosenescence and immune aging have been attributed to the suppression of the immune system induced by the cells of the immunosuppressive network in an attempt to prevent excessive inflammation and minimize tissue damage. Several experimental studies have revealed that for instance, the activation of MDSCs induces immunosuppression and immunosenescent properties in many immune effector cells (Salminen et al., 2019b). Immunosenescence is associated with many epigenetic changes which modify the functions of immune cells.
Salminen A (2020) Activation of immunosuppressive network in the aging process. Ageing Res. Rev. 57, 100998. https://doi.org/10.1016/j.arr.2019.100998.
Salminen, A (2021) Immunosuppressive network promotes immunosenescence associated with aging and chronic inflammatory conditions. J. Mol. Med. (Berl) 99, 1553-1569. https://doi.org/10.1007/s00109-021-02123-w.
Salminen A (2022) Clinical perspectives on the age-related increase of immunosuppressive activity. J. Mol. Med. (Berl) 100, 697-712. https://doi.org/10.1007/s00109-022-02193-4.
Salminen A, Kaarniranta K, Kauppinen A (2018a) The role of myeloid-derived suppressor cells (MDSC) in the inflammaging process. Ageing Res. Rev. 48, 1-10. https://doi.org/10.1016/j.arr.2018.09.001.
Salminen A, Kaarniranta K, Kauppinen A (2018b) Phytochemicals inhibit the immunosuppressive functions of myeloid-derived suppressor cells (MDSC): Impact on cancer and age-related chronic inflammatory disorders. Int. Immunopharmacol. 61, 231-240. https://doi.org/10.1016/j.intimp.2018.06.005.
Salminen A, Kauppinen A, Kaarniranta K (2019a) AMPK activation inhibits the functions of myeloid-derived suppressor cells (MDSC): impact on cancer and aging. J. Mol. Med. (Berl) 97, 1049-1064. https://doi.org/10.1007/s00109-019-01795-9.
Salminen A, Kaarniranta K, Kauppinen A (2019b) Immunosenescence: the potential role of myeloid-derived suppressor cells (MDSC) in age-related immune deficiency. Cell. Mol. Life Sci. 76, 1901-1918. https://doi.org/10.1007/s00018-019-03048-x.
Salminen A, Kaarniranta K, Kauppinen A (2020) ER stress activates immunosuppressive network: implications for aging and Alzheimer's disease. J. Mol. Med. (Berl) 98, 633-650. https://doi.org/10.1007/s00109-020-01904-z.
Salminen A, Kaarniranta K, Kauppinen A (2021) Insulin/IGF-1 signaling promotes immunosuppression via the STAT3 pathway: impact on the aging process and age-related diseases. Inflamm. Res. 70, 1043-1061. https://doi.org/10.1007/s00011-021-01498-3.
Salminen A, Kaarniranta K, Kauppinen A (2022) Photoaging: UV radiation-induced inflammation and immunosuppression accelerate the aging process in the skin. Inflamm. Res. 71, 817-831. https://doi.org/10.1007/s00011-022-01598-8.
Age-related immunosuppression promotes cellular senescence and tissue degeneration
Increased immunosuppression prevents excessive tissue damage due to inflammaging but simultaneously it can disturb the homeostasis within aged tissues (Salminen, 2021a). Immunosuppressive cells secrete anti-inflammatory cytokines (TGF-1β, IL-4, and IL-10), reactive oxygen and nitrogen species (ROS/NO), and express arginase 1 (ARG1) and indoleamine 2,3-dioxygenase (IDO) enzymes which not only deplete arginine and tryptophan amino acids required for protein synthesis, but also activate the kynurenine (KYN) pathway and trigger an integrated stress response (ISR). For instance, the age-related immunosuppression promotes immune aging, enhances myelopoiesis and osteoporosis, impairs autophagy and tissue proteostasis, damages the extracellular matrix, stimulates fibrosis, increases cellular senescence, and elicits age-related diseases, e.g., vascular diseases (Salminen, 2021a).
There is convincing evidence that age-related immunosuppression impairs the clearance of senescent cells from tissues (Salminen, 2021b). For example, immunosuppressive cells disturb the function of NK and CD8 T cells which have a crucial role in the immune surveillance of senescent cells in aged tissues. Immunosuppression especially impairs the NKG2D-mediated clearance of senescent cells and thus the subsequent inflammaging is associated with the accumulation of senescent cells within tissues. Senescent cells display a pro-inflammatory phenotype which is called the senescence-associated secretory phenotype (SASP). Senescent cells secrete colony-stimulating factors and many chemokines which stimulate the generation of MDSCs in the bone marrow and consequently leads to their recruitment into aging tissues (Salminen et al., 2018). There are studies indicating that both immunosuppressive cells and senescent cells secrete exosomal vesicles which are involved in the expansion of senescence into neighboring cells (Salminen et al., 2020). The age-related exosomes contain immune suppressive cargos which not only enhance immunosuppression in inflamed tissues but also expand the extent of cellular senescence and tissue degeneration. It seems that it is a feed-forward circuit formed by inflammaging, compensatory immunosuppression, and the expansion of senescent cells which is driving tissue degeneration (Salminen, 2021b).
Salminen A (2021a) Increased immunosuppression impairs tissue homeostasis with aging and age-related diseases. J. Mol. Med. (Berl) 99, 1-20. https://doi.org/10.1007/s00109-020-01988-7.
Salminen A (2021b) Feed-forward regulation between cellular senescence and immunosuppression promotes the aging process and age-related diseases. Ageing Res. Rev. 67, 101280. https://doi.org/10.1016/j.arr.2021.101280.
Salminen A, Kauppinen A, Kaarniranta K (2018) Myeloid-derived suppressor cells (MDSC): an important partner in cellular/tissue senescence. Biogerontology 19, 325-339. https://doi.org/10.1007/s10522-018-9762-8.
Salminen A, Kaarniranta K, Kauppinen A (2020) Exosomal vesicles enhance immunosuppression in chronic inflammation: Impact in cellular senescence and the aging process. Cell. Signal. 75, 109771. https://doi.org/10.1016/j.cellsig.2020.109771.
Aryl hydrocarbon receptor promotes immunosuppression, the aging process, and Alzheimer’s disease
Chronic inflammatory states, such as inflammaging, elevate the activity of indoleamine 2,3-dioxygenase (IDO) which stimulates the kynurenine (KYN) pathway. KYN and some of its metabolites, e.g., kynurenic and cinnabarinic acids, are potent activators of the aryl hydrocarbon receptor (AhR).
AhR signaling stimulates the expression of FoxP3, a master gene for immunosuppressive Treg cells (Salminen, 2022a). It is also known that AhR signaling can induce the differentiation of many immunosuppressive cells, such as MDSCs, Bregs, and M2 macrophages, as well as inducing several immune checkpoint proteins (Salminen, 2022a,b). There is clear evidence that the aging process activates the IDO1-KYN-AhR signaling and thus it increases the activity in the immunosuppressive network. Interestingly, the AhR transcription factor has revealed the properties of antagonistic pleiotropy in the regulation of the aging process (Salminen, 2022b). It means that the Ah receptor possesses many properties which are important during the organism’s developmental phase but later in life, they promote age-related degenerative processes. For instance, AhR signaling evokes a wide range of cellular stresses, e.g., it increases ROS and sphingolipid synthesis and also depletes NAD stores. AhR signaling promotes cellular senescence, aggravates extracellular matrix degeneration, and disturbs vascular homeostasis (Salminen, 2022b). Moreover, AhR signaling displays mutual antagonism with hypoxia-inducible factor-1α (HIF-1α), a well-known longevity factor (Salminen, 2022c). AhR signaling also impairs circadian rhythmicity, e.g., by interacting with the core BMAL1/CLOCK complex and disturbing the regulation of clock genes (Salminen, 2023a). The maintenance of circadian rhythms is impaired with aging thus disturbing metabolic regulation with aging.
The expression of AhR protein is significantly increased in the brains of Alzheimer’s patients. Its expression is enriched in the blood-brain barrier (BBB) and in fact, there are studies indicating that AhR signaling regulates the integrity of the BBB. For instance, (i) it controls blood flow in the brain via the renin-angiotensin system, (ii) it is able to induce pathological changes in microvessels, and (iii) it affects the circadian regulation of glymphatic flow. There is clear evidence that hypoperfusion of the brain can evoke immunosuppression of microglia, induce astrocyte senescence, and promote the accumulation of β-amyloid and tau proteins within the brains of Alzheimer’s patients (Salminen, 2021). Given that Ah receptors are the target of several indole molecules produced by gut host-microbiota, it seems that Ah receptors mediates the signaling between the gut and the brain (Salminen, 2023b). There are studies indicating that disturbances in the gut microbiota enhance the pathogenesis of Alzheimer’s disease. Moreover, AhR signaling can disturb glymphatic flow since (i) it affects the function of the BBB which is cooperatively connected with the glymphatic system and (ii) neuroinflammation in the brain and dysbiosis of gut microbiota produce potent activators of AhR signaling which are able to impair glymphatic flow (Salminen, 2023c).
Salminen A (2021) Hypoperfusion is a potential inducer of immunosuppressive network in Alzheimer's disease. Neurochem. Int. 142, 104919. https://doi.org/10.1016/j.neuint.2020.104919.
Salminen A (2022a) Role of indoleamine 2,3-dioxygenase 1 (IDO1) and kynurenine pathway in the regulation of the aging process. Ageing Res. Rev. 75, 101573. https://doi.org/10.1016/j.arr.2022.101573.
Salminen A (2022b) Aryl hydrocarbon receptor (AhR) reveals evidence of antagonistic pleiotropy in the regulation of the aging process. Cell. Mol. Life Sci. 79, 489. https://doi.org/10.1007/s00018-022-04520-x.
Salminen A (2022c) Mutual antagonism between aryl hydrocarbon receptor and hypoxia-inducible factor-1α (AhR/HIF-1α) signaling: Impact on the aging process. Cell. Signal. 99, 110445. https://doi.org/10.1016/j.cellsig.2022.110445.
Salminen A (2023a) Aryl hydrocarbon receptor (AhR) impairs circadian regulation: Impact on the aging process. Ageing Res. Rev. 87, 101928. https://doi.org/10.1016/j.arr.2023.101928.
Salminen A (2023b) Activation of aryl hydrocarbon receptor (AhR) in Alzheimer's disease: role of tryptophan metabolites generated by gut host-microbiota. J. Mol. Med. (Berl) 101, 201-222. https://doi.org/10.1007/s00109-023-02289-5.
Salminen A (2023c) Aryl hydrocarbon receptor (AhR) disturbs circadian regulation in Alzheimer’s disease: impact on glymphatic system dysfunction (Submitted
Tissue fibroblasts regulate the aging process in cooperation with the immunosuppressive network
Tissue fibroblasts are a heterogeneous population of mesenchymal cells which possess not only fibrogenic properties but can also act as potent immune regulators (Salminen, 2023 a,b). A high plasticity of fibroblasts enhances their adaptation to both acute injuries and chronic diseases in an attempt to maintain tissue homeostasis. Fibroblasts can display different phenotypes, such as quiescent fibroblasts, myofibroblasts, and senescent fibroblasts. We can also set apart fibrogenic and non-fibrogenic cells as well as pro-inflammatory and immunosuppressive phenotypes of fibroblats. Many age-related alterations, such as oxidative stress, inflammatory mediators, ECM stiffness, alarmins, and telomere shortening, induce differentiation of quiescent fibroblast into myofibroblasts. Moreover, many tissue insults and stresses can transdifferentiate monocytes, macrophages, adipocytes, pericytes, endothelial cells, and fibrocytes into fibrogenic and immunosuppressive myofibroblasts (Salminen, 2023 a).
In acute inflammatory state, fibroblasts secrete many chemokines which recruit immunosuppressive cells into inflamed tissues. Several anti-inflammatory cytokines, e.g., TGF-β, IL-6, and IL-10, secreted by immunosuppressive cells, such as MDSC, Treg, and M2 macrophages, induce both fibrogenic and immunosuppressive properties in myofibroblasts. These properties indicate that there is a close interaction and mutual cooperation between myofibroblasts and the cells of the immunosuppressive network (Salminen, 2023 b). There is convincing evidence that AMPK signaling inhibits the differentiation of myofibroblasts and thus attenuating the level of fibrosis in many pathological states, probably also in aging tissues (Salminen, 2024). Currently, the activation of AMPK signaling is recognized as a promising anti-aging therapy. It seems that tissue fibroblasts are crucial players in the aging process attributed to their intimate crosstalk with the immunosuppressive network.
Salminen A. (2023a) The plasticity of fibroblasts: A forgotten player in the aging process. Ageing Res. Rev. 89,101995. https://doi.org/10.1016/j.arr.2023.101995
Salminen A. (2023) The role of immunosuppressive myofibroblasts in the aging process and age-related diseases. J. Mol. Med. (Berl.) 101, 1169-1189. https://doi.org/10.1007/s00109-023-02360-1
Salminen A. (2024) AMPK signaling inhibits the differentiation of myofibroblasts: impact on age-related tissue fibrosis and degeneration. Biogerontology, https://doi.org/10.1007/s10522-023-10072-9