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Senescent Macrophages: A Unique Target

These cells cannot be targeted like other senescent cells.

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In Aging, researchers have published a new study on a tool meant for analyzing macrophage senescence along with differences between inflammaging and regular inflammation.

Macrophages are driven to senescence

Previous work has found that excessive senescent cells are uniquely harmful to macrophages, recruiting them into senescence and causing them to perpetuate the chronic age-related inflammation known as inflammaging [1]. For example, macrophages that come into contact with senescent cells in the peritoneum, the membrane that lines the abdomen, begin expressing SA-β-gal and p16INK4A, two well-known markers of senescence [2].

Compounding the problem, inflammaging reduces, rather than increases, the macrophages’ ability to handle harmful bacteria, including in the gut [3]. Macrophages taken from older animals are less able to clear pathogens and debris (phagocytosis) than macrophages taken from young ones [4].

To get a better handle on this problem, the researchers intensively studied macrophage aging on the cellular level, looking for the details in how these cells age and what can be done about it.

Senescence through division

Fortunately for researchers but unfortunately for older people, it is not difficult to get macrophages to become senescent. After 14 days of culture, mouse macrophages derived from the peritoneum had substantially greater gene expression of p16INK4A and p21CIP1, another well-known senescence marker. p53 levels also went up despite not having an increased expression in mRNA. SA-β-gal, interestingly, was relatively high at day 2 of culture but remained static throughout the experiment.

Having high senescence markers meant that the macrophages had little ability to proliferate, but very few of them died by the cellular self-destruction known as apoptosis. The researchers noted that this appears to be due to the expression of p21, which inhibits apoptosis [5].

Interestingly, while SASP biomarkers do increase in living two-year-old mice, many SASP biomarkers increase far more in this cellular culture than in these older animals, even despite the cultured cells being derived from young mice. This appears to be connected to macrophage polarization: the cultured macrophages became rapidly polarized towards inflammation rather than healing over the two weeks, with metabolic markers supporting this transition. Despite this polarization and in accordance with previous research [4], however, these cultured macrophages were less able to conduct phagocytosis.

Senolytics ineffective

Just like in previous research [6], the senolytic combination of dasatinib and quercetin was completely ineffective against senescent macrophages. However, previous work has found that Trx-1, which is naturally formed in the human body, is effective in reducing macrophage inflammation [7]. This is upstream of CB3, a peptide that this team has found to be directly effective in reducing key inflammatory factors [8].

However, that study was conducted in the context of high-fat diets, not aging. In this study, the effects were visible, but mixed: some SASP components were decreased, but another, IL-1β, was increased. p21 was decreased, and proliferation was restored, but phagocytosis was not, and polarization appeared to still be inflammatory.

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While these are largely negative results, such results are crucial for understanding why some anti-inflammatory approaches that work well against short-term inflammation in younger people, such as that caused by sprains or other injuries, may be completely ineffective or even counterproductive in attempting to treat inflammaging. Furthermore, as this research confirms, directly dealing with macrophages cannot be done in the same way as with other senescent cells. Therefore, a pointed and specific effort must be made in order to defeat inflammaging at the macrophage level or otherwise neutralize the SASP’s effects on it.

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Literature

[1] Hall, B. M., Balan, V., Gleiberman, A. S., Strom, E., Krasnov, P., Virtuoso, L. P., … & Gudkov, A. V. (2016). Aging of mice is associated with p16 (Ink4a)-and β-galactosidase-positive macrophage accumulation that can be induced in young mice by senescent cells. Aging (Albany NY), 8(7), 1294.

[2] Hall, B. M., Balan, V., Gleiberman, A. S., Strom, E., Krasnov, P., Virtuoso, L. P., … & Gudkov, A. V. (2017). p16 (Ink4a) and senescence-associated β-galactosidase can be induced in macrophages as part of a reversible response to physiological stimuli. Aging (Albany NY), 9(8), 1867.

[3] Thevaranjan, N., Puchta, A., Schulz, C., Naidoo, A., Szamosi, J. C., Verschoor, C. P., … & Bowdish, D. M. (2017). Age-associated microbial dysbiosis promotes intestinal permeability, systemic inflammation, and macrophage dysfunction. Cell host & microbe, 21(4), 455-466.

[4] Aprahamian, T., Takemura, Y., Goukassian, D., & Walsh, K. (2008). Ageing is associated with diminished apoptotic cell clearance in vivo. Clinical & Experimental Immunology, 152(3), 448-455.

[5] Gartel, A. L., & Tyner, A. L. (2002). The role of the cyclin-dependent kinase inhibitor p21 in apoptosis. Molecular cancer therapeutics, 1(8), 639-649.

[6] Xu, M., Pirtskhalava, T., Farr, J. N., Weigand, B. M., Palmer, A. K., Weivoda, M. M., … & Kirkland, J. L. (2018). Senolytics improve physical function and increase lifespan in old age. Nature medicine, 24(8), 1246-1256.

[7] Billiet, L., Furman, C., Larigauderie, G., Copin, C., Brand, K., Fruchart, J. C., & Rouis, M. (2005). Extracellular human thioredoxin-1 inhibits lipopolysaccharide-induced interleukin-1β expression in human monocyte-derived macrophages. Journal of Biological Chemistry, 280(48), 40310-40318.

[8] Canesi, F., Mateo, V., Couchie, D., Karabina, S., Nègre-Salvayre, A., Rouis, M., & El Hadri, K. (2019). A thioredoxin-mimetic peptide exerts potent anti-inflammatory, antioxidant, and atheroprotective effects in ApoE2. Ki mice fed high fat diet. Cardiovascular Research, 115(2), 292-301.

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About the author
Josh Conway
Josh Conway
Josh is a professional editor and is responsible for editing our articles before they become available to the public as well as moderating our Discord server. He is also a programmer, long-time supporter of anti-aging medicine, and avid player of the strange game called “real life.” Living in the center of the northern prairie, Josh enjoys long bike rides before the blizzards hit.