The ageing effect of immune system and adipose tissue chatting

Author: José R. Pineda got his Ph.D. from University of Barcelona in 2006. Since 2007 he has worked for Institut Curie and The French Alternative Energies and Atomic Energy Commission. Currently he is a researcher of the UPV/EHU. He investigates the role of stem cells in physiologic and pathologic conditions.

There is strong evidence that inflammation is an important hallmark for health span and lifespan. Ageing is associated with a malfunctioning immune system, characterized by immunosenescence (decline in immune function) and thymic involution (shrinking of the thymus), along with the activation of innate immune genes. Thus, ageing is characterized by a chronic inflammatory state also known as “inflammaging” ¹, usually with increased weight and adiposity ². It has been correlated that inflammatory mediators called adipokines and cytokines can be released by adipose tissue and led to the inflammatory state during ageing ³ increasing the risk of metabolic syndrome. Indeed, beneficial effects of calorie restriction (CR) in modulating metabolic pathways and preventing inflammation ⁴. On the other hand, removing damaged cells from our body or induce autophagy also has been emerged as an indirect approach to reduce inflammatory signals ⁵. Now, Anissa A. Widjaja and collaborators have advanced in a next step published in Nature the discovery of the pro-inflammatory cytokine IL-11 (member of the IL-6 family) that has a negative effect on age-associated disease and lifespan of mice. Moreover, they propose anti-IL-11 therapy to determine the effects of IL-11 inhibition on ageing pathologies in older people ⁶. How did they come to this conclusion, let us take a closer look…

It is known that IL-11 upregulates across cell types and tissues as mice age. IL-11 can regulate several signaling pathways such as ERK (involved in cell cycle and proliferation), AMPK (plays a role in cellular energy homeostasis, leading the use of glucose and fatty acid uptake and oxidation) and mTORC1 (functions as a nutrient/energy/redox sensor and controls protein synthesis). Altogether, these pathways are critical for inflammation signaling, but they are also crucial to regulate lifespan across species ⁷⁸. Indeed, the ERK-AMPK-mTORC1 axis is known to modulate cellular, tissue- and organism-level ageing pathologies. The AMPK–mTORC1 axis is crucial for maintaining metabolic health, especially in adipose tissue, and inhibiting mTOR has been shown to extend lifespan in mice. Interestingly, mice develop ageing-related pathologies similar to those in humans becoming more complex than simpler organisms, like yeast, worms, and fruit flies. However, extending lifespan does not necessarily mean extending the period of life spent in good health (health span). There is a growing need for studies that examine the effects of interventions on both health span and lifespan. Mice are particularly suitable for such studies.

They hypothesized that IL-11 could promote ageing-related pathologies and shorten lifespan. This hypothesis was based on studies showing that IL-11 could activate ERK–mTORC1 and/or JAK–STAT3 pathways. This upregulation in older individuals has been linked to cellular senescence (a key hallmark of ageing). Using genetic and pharmacological methods, researchers tested the impact of IL-11 signaling on health span and lifespan in mice. They found that IL-11 expression increases with age in the liver (hepatocytes), adipose tissue (white adipose tissue), and skeletal muscle (gastrocnemius muscle cells). They used Il11-EGFP reporter mice to detect the expression by immunohistochemistry in tissues from aged finding its presence in stromal, epithelial and endothelial cells across tissues and in nerves in skeletal muscle of very old mice. This upregulation in the very old animals was associated with the activation of the ERK–mTORC1 pathway, a signaling modulation involving increased levels of phosphorylated proteins like ERK, p90RSK, mTOR, and p70S6K, and reduced activity of the LKB1–AMPK pathway. This diverse expression suggested that IL-11 had a widespread effect across different tissues as animals’ age.

To explore the relationship between IL-11 and ageing, Widjaja studied mice lacking the IL-11 receptor (Il11ra1^−/−). These mice showed reduced activation of the ERK–mTORC1 pathway and lower levels of senescence markers compared to normal (wild-type) mice. Additionally, Il11ra1^−/− mice had lower body weights, reduced fat mass, higher lean mass, and improved metabolic profiles with lower serum cholesterol and triglycerides. They also showed preserved telomere lengths and mitochondrial DNA copy numbers, indicators of slower biological ageing.

Next step was to use human cells to try to recapitulate the differences observed in animals. To achieve that, they used human fibroblasts or human hepatocytes that were stimulated with IL-11 treatment. They found that ERK–mTOR pathways were activated, and they found increased levels of p16 and p21, and reduced expression of PCNA and cyclin D1 (cell cycle regulation), which was prevented by pharmacological drugs U0126 (an ERK inhibitor) or rapamycin (an mTOR inhibitor). Interestingly, supernatants of IL-11 treated cells were rich in secretory factors such IL-6, IL-8 (strong proinflammatory cytokines), LIF, VEGFA, HGF, and the chemokines CCL2, CCL2033, CXCL1, CXCL5, CXCL6 (involved in white blood cell recruitment). The majority of these factors are regulated by ERK–mTORC1 pathways.

Having observed their implication in human cells and having determined the effects of the lack of IL-11 receptor (Il11ra1^−/−)-deleted mice, next step was to characterize the effects of the total absence of IL-11. To do that, they used genetically-engineered animals with no expression of IL-11 (Il11 full deletion: Il11^−/−). They compared young (3-month-old) and aged (2-year-old) Il11^−/− mice respect their controls (Il11^+/+ or wild type mice). Old Il11−/− mice had lower body weights and fat mass and preserved lean mass and mildly increased body temperature. At behavioral level, Il11^−/− mice had improvements in tremor (unchanged brown adipose tissue mass respect young animals), reduced loss of fur color, improvement on gait disorders and vestibular disturbance and increased muscle strength compared with the age-matched controls. Furthermore, old Il11−/− mice exhibited better levels of serum AST, ALT, cholesterol and triglycerides respect wild-type mice aged.

To examine the potential role of de novo lipogenesis in aged tissue, they compared the expression of fatty acid synthesis genes in visceral white adipose tissue (vWAT) and found their expression to be increased with age in old wild-type mice but not in old Il11^−/− ones. Interestingly, pro-inflammatory gene expression was increased in vWAT of old wild-type mice but not in that of old Il11^−/− animals. In conclusion, all these data suggested that even having the same age, biologically these mice were…: younger.

Telomere length and mitochondrial DNA are the hallmarks and key identifiers of real biological age. Telomere length shortens with age leading to senescence, apoptosis, or oncogenic transformation and mitochondrial mRNA expression reduces during ageing provoking a decline in mitochondrial activity. Thus, the authors decided to check the status of old Il11^−/− mice respect the age-matched control. They found that in liver, skeletal muscle and vWAT telomere lengths and mtDNA content reduction was more pronounced in old wild-type mice respect old Il11^−/− mice.

Having observed the benefits in the overall effects of the absence of IL-11, the researchers sought a potential therapeutic approach by administering neutralizing antibodies to IL-11 known as “X203” (neutralizing antibodies binds and prevents the function of their antigen). Compared with controls, mice receiving X203 from 75 to 100 weeks of age progressively lost body weight that was defined by a reduction in indexed fat mass, even for vWAT. Treated animals also had an improved glucose metabolism, and increased muscle strength. It also reduced markers of inflammation and senescence across various tissues, including telomere length and mtDNA copies. These benefits were observed even when treatment was started late in life, indicating the potential of anti-IL-11 therapy to improve health span and lifespan. At molecular level, mice receiving X203 had the most significant gene set enrichment scores for hallmarks of oxidative phosphorylation and metabolism, whereas scores for markers of inflammation. A more detailed study of the vWAT transcriptome revealed that the gene that was most upregulated by anti-IL-11 genome-wide was Ucp1, which is important for the development of thermogenic ‘beige’ adipocytes in white adipose tissue (WAT). They also found an increase of UCP1 and PGC1α proteins. This data was complemented with histological examination observing a reduction of lipid droplet area in vWAT.

However, as we said at the beginning, not only fat is responsible for signaling with pro-inflammatory molecules, the immune system is also a major culprit. To this end, the researchers decided to analyze the surface gene markers CD68, CD4 and CD19 responsible of the infiltration of the immune system finding a downregulation and fewer resident CD68⁺ macrophages after X203 treatment.

Finally, Kaplan-Meier studies showed that mice lacking IL-11 (Il11^−/−) had significantly longer lifespans compared to normal mice. Similar benefits were observed when aged mice received anti-IL-11 therapy. This intervention not only extended lifespan but also improved various aspects of health span, such as reduced frailty and better metabolic health. In addition, the most interesting of this study: the gross autopsy study revealed fewer macroscopic tumours (15%) in mice treated with X203 respect the 61% observed in control mice (cancers are a common cause of death in old mice, as well as in humans).

In conclusion, this study demonstrates 1) the danger and vicious loop of pro-inflammatory IL-11 raising levels, cumulative visceral adipose tissue and inflammatory profile of our immune system; 2) IL-11 plays a significant role in ageing and 3) targeting IL-11 can improve both health span and lifespan in mice. This finding opens up possibilities for developing therapies aimed at inhibiting IL-11 to combat ageing, fat mass, and, why not? Maybe to delay the development of cancer and extend healthy life.