Periodic Reporting for period 1 - ICSIS (The interaction and clearance of senescent vascular cells by the innate immune system)
Reporting period: 2015-06-01 to 2017-05-31
The overall objective of this study was to evaluate the impact of an early (acute) versus a late (chronic) senescent cell secretome on monocyte/macrophage function. It was suspected that if senescent cells were not killed by immune cells and persisted, their secretory phenotype would become altered (chronic) which could promote disease. Specific objectives included the assessment of (1) immunogenic factors on senescent cells, (2) the senescent secretome on markers of monocyte/macrophage activation/differentiation, (3) the impact of the senescent secretome on physiological responses by monocytes/macrophages including migration, adhesion and phagocytosis, and (4) the impact of a chronic senescent secretome on pathological responses by monocytes/macrophages.
This research demonstrated that an acute senescent secretome promotes the differentiation of monocytes into anti-inflammatory M2-like macrophages associated with increased macrophage migration, adhesion and phagocytosis. However, a chronic senescent secretome promoted pathological features in monocytes/macrophages that has the potential to promote disease development.
Following on from these findings which focused specifically on an acute senescent secretome, we evaluated the response of monocytes/macrophages to a chronic senescent secretome. Reduced cell migration was observed and associated with a reduction in cell surface receptor expression (CXCR1 and CXCR2) in monocytes/macrophages. These receptors are important for sensing certain secreted compounds. No significant difference in monocyte/macrophage adhesion or phagocytosis in response to a chronic senescent secretome was observed when compared to an acute senescent secretome.
We also observed a further elevation in the expression of CD36, a lipid transporter involved in pathological foam cell formation. Foam cells are macrophages loaded with fats such as oxidised low-density lipoprotein (ox-LDL). Such cells are known to play a role in the development of atherosclerosis-thickening of the arteries. Our data suggests that senescent cells may promote elevated extracellular ox-LDL which leads to increased CD36 expression and ox-LDL uptake in monocytes/macrophages.
Monocytes treated with a chronic senescent secretome also reduced T-cell proliferation. L-arginine is required for T-cell proliferation. However, a chronic senescent secretome promotes the upregulation of arginase 1 expression, an enzyme that breaks down L-arginine. This suggests that a reduction in T-cell proliferation may be due to lack of L-arginine. This immunosuppressive response is also linked to an elevation in TGFβ expression and upregulation of PPARγ in monocytes/macrophages.
Overall, these findings demonstrate that the senescent secretome regulates monocyte/macrophage function and that a chronic senescent secretome can promote a pathological, immunosuppressive environment. These findings clearly demonstrate that the senescent secretome can change with time, becoming detrimental. By further understanding the specific changes and the mechanisms involved, future therapeutic interventions could be targeted to reverse this process. In doing so, it may be possible to improve immune clearance of senescent cells with age. This data will be used to apply for further funding to investigate just that. In addition, a manuscript is currently in progress for submission to a scientific journal. Following publication, these findings will be made available in an easy to read format suitable for the public in the form of a blog (cellsen.wixsite.com/senescentcell). It is anticipated that this data will also be presented at conferences such as the annual International Cell Senescence Association (www.cellsenescence.info) meetings.
Fellowship Impact: Previous to this research fellowship, the Fellows primary scientific experience was concerned with molecular cell biology, with few skills involving immune biology. Through the undertaking of this project, the Fellow received training in the field of immunology that places him in a unique position to undertake further study in an emerging field of research focused on immunosurviellance of senescent cells. Specific scientific training, acquired data, student mentoring, lab management, conference attendance, networking, scientific writing, grant reviewing and grant writing undertaken during this fellowship places the Fellow in a strong position for the successful application of further funding and appointment as a principle investigator. A scientific manuscript is currently in preparation.