Experimental dissection of the pro- and anti-ageing roles of different macrophage subpopulations

Our immune system can both prevent and drive pathology as seen in many age-related diseases (e.g., cancer, COPD, neurodegeneration).

Immunosenescence contributes to increased susceptibility to infection and ageing more generally through decreased immune surveillance.

During ageing, the immune system can also become overactive or dysregulated, causing inflammation, an important contributor to accelerated ageing.

These aspects have been highlighted by the COVID-19 pandemic, with the elderly and immunocompromised at high risk, but interestingly patients developing overly strong immune reactions to COVID are at high risk of mortality.

Indeed, dexamethasone (and other corticosteroids) are now proven treatments for COVID, most likely through their anti-inflammatory properties.

To improve health and combat ageing, it will thus be crucial to understand which immune components require potentiation and which require suppression during ageing.

Currently, our understanding of this is limited by the absence of tractable experimental manipulations of subpopulations of immune cells in vivo during ageing. Only when we understand the fundamental biology of the ageing immune system can we develop rejuvenation therapies. To achieve this we will use the fruit fly as a model, building on data obtained using tools we have recently developed.

The fruit fly, Drosophila melanogaster has been used in biomedical research for over 100-years and has played a key role in our understanding of innate immune signalling and activation, for example the role of Toll-like receptors in this process.

In mammals, macrophages play a key role in development, host defence and homeostasis and their dysfunction contributes to a wide-range of human diseases that increase in their prevalence as we age.

These white blood cells are highly heterogeneous - there are many different types of tissue resident macrophages and macrophages can also become activated to a range of states.

We have recently identified a number of functionally-distinct macrophage subpopulations in the fly, that exhibit plasticity in their numbers and localisation across the lifecourse of the fly, including during ageing.

As part of this work, we have developed genetic tools that enable imaging and manipulation of these subpopulations in the context of a genetically-tractable in vivo model that is highly amenable to ageing studies due to its short lifespan.

Moreover, the molecular mechanisms identified in both macrophage specification and subpopulation identity, as well as ageing, share strong similarity between the fly and ourselves.

As further proof of the utility of flies as a model in this space, we recently identified pro-inflammatory macrophage subpopulations that become more prominent during the ageing process.

Therefore, our fly model now uniquely allows a large-scale experimental and empirical in vivo test of the importance of macrophage subpopulations during ageing.

Using the functional genetic tools we have developed, we will directly manipulate the macrophage subpopulations in the fly, increasing or decreasing their numbers during the ageing process to test their function during ageing in vivo.

We will also compare the transcriptomes of macrophage subpopulations in young and aged flies to help identify the molecules that determine the actions of these cells.

Assays on functional performance will help identify which aspects of physiology are impacted by these subpopulations of macrophages.

This study will therefore set the foundations for uncovering the roles of different types of white blood cell during ageing and the molecules involved in driving the changes we see at a whole organismal level.

As such it will provide the first proof-of-principle as to whether enhancing or suppressing the function of certain immune cells can be beneficial to the ageing process and whether enhancing immunity in old age is a viable approach or can lead to unintended negative consequences.

In the longer term, this work has the potential to identify mechanisms and molecules involved in how macrophages contribute to/protect against the age-related decline organisms experience. Such insights will help generate new precise anti-ageing therapies and life-style advice.

Crucially, the immune system is highly amenable to pharmacological manipulation and interventions can be envisioned that are highly targeted, but with large-scale systemic anti-ageing effects.

A downside with most of the currently promoted anti-ageing drugs is that they are aimed at many cells in the body and not targeted subsets, inviting negative side-effects.