Since any immune response to the presence of senescent cells may possibly be similar to that of cancer cells, the following is a brief outline describing the key points in the removal process (Ullrich et al, 2007, Vulink et al, 2008, Wesa and Storkus, 2008, Chan and Housseau, 2008).
Dendritic cells are antigen-presenting cells found in all tissues of the body and are crucial for stimulating a naïve T-lymphocyte response in the removal of tumour cells.
In the presence of tumour cells, dendritic cells capture (by engulfing portions of the tumour cell) and process tumour-specific molecules (antigens) so that they become presented on their cell surface. Dendritic cells start to mature as they migrate to the lymph nodes, a process which enables dendritic cells to present the tumour information. The maturation process is needed as additional co-stimulatory molecules are required so that they can be recognised by other immune cells. When mature dendritic cells reach the lymph nodes, they interact with cytotoxic T-lymphocytes (CTLs), and pass on the tumour information, causing CTLs to become activated and consequently proliferate. The large numbers of CTLs then circulate the body, recognising the tumour-specific antigens, binding to them and destroying tumour cells by the release of enzymes.
If a cancer cell (or a senescent cell) is not removed by the immune system, then something isn’t working as it should. From the brief overview above, there are a number of points between cell recognition and removal that could have failed. These are:
(1) Dendritic cells did not recognise the cancer/senescent cell.
(2) The dendritic cells did not display cancer/senescent specific markers on it’s surface.
(3) Lymphocytes were not activated in response to dendritic cells.
The changes which occur as we age which may have an impact on the removal of tumour cells/senescent cells will be discussed next.
.Review Paper: Physiological and pathological consequences of cellular senescence