Impact of Ageing on Adipose, Muscle and Systemic Inflammation

Adipose tissue is a highly dynamic organ that produces a wide array of adipokines which can affect the function of other tissues throughout the body. The physiology of adipose tissue is a relatively new and exciting area of research and researchers are learning more about its complexity, in particular the way in which adipose tissue plays a dynamic and active role in various normal and pathological processes. Comparatively little is known about the changes that occur within adipose tissue over the natural course of ageing - and adipose dysfunction could play a role in ageing-related chronic systemic inflammatory diseases such as type 2 diabetes and cardiovascular disease.

In this study, the investigators would like to investigate inflammatory and metabolic changes that occur within adipose tissue with ageing. the investigators would also like to examine whether age-related changes in adipose tissue are specific to this particular tissue type by comparing adipose-resident immune cell populations and measures of inflammation and metabolism to those in muscle tissue and blood.

By exploring the immune dysfunction that occurs with ageing in adipose tissue and relating them to inflammatory and metabolic differences in muscle and blood, this work may potentially reveal causal mechanisms in the development of ageing-related chronic inflammatory diseases and ultimately lead to the development of better treatment/management strategies.

BACKGROUND Adipose tissue is sizeable endocrine organ and is highly dynamic, producing a wide array of adipokines which can affect a range of physiological processes including regulation of appetite, energy expenditure, insulin sensitivity, inflammation, endocrine and reproductive systems and bone metabolism. Ageing is a process that is associated with adipose tissue accumulation, changes in adipose tissue distribution and its dysfunction which in turn are linked to the development of chronic inflammatory disorders such as type 2 diabetes and cardiovascular disease.

Adipose tissue inflammation may be key Adipose tissue consists not only of adipocytes, but also many other cell types including endothelial cells, preadipocytes, immune cells such as macrophages and lymphocytes such that adipocytes themselves may only represent 60-70 % cell numbers in adipose tissue. Research from over the last decade or so suggests that the presence of immune cells within the adipose tissue itself are important in regulating both local and systemic inflammation/production of adipokines. For example, adipose tissue macrophages contribute the majority of the pro-inflammatory cytokine TNFα and ~50% IL6 secreted by adipose tissue, which show increased secretion with adipose tissue dysfunction and are implicated in the development of chronic inflammatory disorders. Changes in adipose resident immune cells have been relatively well studied in obesity but there are comparatively few studies in humans examining changes in immune cell populations and their potential impact on adipose tissue inflammation in the context of ageing. Given the time-course of adipose tissue accumulation in obesity compared to ageing, there is the potential for differences in the underlying mechanisms of adipose tissue dysfunction to occur.

In humans, there is overwhelming evidence (including work by the investigator's group) that macrophages accumulate in subcutaneous adipose tissue with obesity and are important mediators of adipose tissue inflammation. Additionally, work performed by this group has also shown that T cells in adipose tissue are more activated with obesity and this may also be related to levels of adipose tissue inflammation. Only one study has attempted to investigate the impact of ageing on immune cells in adipose tissue, however, this was in a very specific population (Pima Indians) and only up-to the age of 45 years so it is not known how immune cells change with further increases in age where chronic inflammatory disorders become more prevalent. Studies using mouse models indeed suggest that there are discrepancies in immune cell populations within adipose tissue following diet-induced obesity compared to ageing. In mice, it is not clear if there is a change in the number or just a change from an anti- to pro-inflammatory phenotype which would be in contrast to obesity, where there is evidence of both an increase in number and skewing towards a pro-inflammatory phenotype. Mice also show an increase in T-cells in adipose tissue with ageing - especially in T-regulatory cells, however, in diet-induced obesity there may be an increase in the number of pro-inflammatory effector cells (e.g. CD8+).

The potential role of adipose tissue inflammation in the development of age-related chronic inflammatory disorders and how this compares to the investigators' previous work in the context of obesity is something the investigators are keen to address in the present study.

Investigating tissue specific changes with ageing With ageing there is an increased deposition of adipose tissue within muscle and this can have profound effects on muscle tissue including inducing insulin resistance and lipotoxicity. Considering that muscle is one of the major sites for glucose uptake, impairment in this process can have a profound effect on systemic glucose concentrations and whole body insulin resistance. Like adipose tissue, muscle itself is host to resident immune cells, but it is not known whether these differ in terms of proportion or function to those found in adipose tissue. Furthermore, it is not known how these immune cells in muscle tissue may affect local and systemic inflammation in the development of ageing-related chronic inflammatory diseases. It is important to consider the role of different tissues in disorders that affect the body as a whole as there may be specific responses to ageing that need to be considered when trying to understand the underlying pathophysiology of ageing-related chronic diseases. Thus, the impact of ageing on muscle immune cells and inflammation is another factor that the investigators are keen to address in the present study. the investigators are especially interested in examining whether ageing affects adipose and muscle in a similar or different way.

Purpose The purpose of this study is to investigate inflammatory and metabolic changes in adipose tissue that occur with ageing and to compare these changes to those in muscle and blood.

By comparing metabolic and inflammatory parameters within adipose tissue, muscle and the circulation in younger and older individuals, the investigators hope to gain vital clues regarding the potential pathogenic mechanisms involved in the development of ageing-related chronic inflammatory diseases. With greater understanding of these mechanisms the investigators hope to prepare the ground for new and more effective means of prevention/treatments for ageing-related chronic inflammatory diseases.