Publishing in Aging, a team of researchers from SENS Research Foundation has described a new method of enriching natural killer (NK) cells to fight senescent cells.
Culturing NK cells to fight senescence
Previous experiments have shown that natural killer (NK) cells are partially responsible for the clearance of senescent cells from the human body . While some senescent cells have ways of avoiding detection and clearance , NK cells are attracted to certain parts of the SASP, which trigger them to kill the cells expressing it. Techniques are being developed to use this senolytic ability of NK cells as a potential therapy.
The researchers lament two problems with the experiments used to develop some of these techniques: in many cases, the co-culturing lasted only 2 to 6 hours, and there was an extremely high ratio of NK (“effector”) cells to senescent (“target”) cells. Some of these experiments had 20 or more NK cells for every senescent cell . The researchers hold that such experiments are not physiological: they do not match the conditions of the human body.
Enriching and using NK cells
After taking NK cells out of whole blood, the researchers sought to change the distribution of these cells. NK cells express different amounts of CD56 and CD16. NK cells that express high CD56 but low CD16 are immature and secrete interferon-γ; NK cells with low CD56 and high CD16 are responsible for cytotoxicity: the actual killing of other cells. The enrichment process, which involved activating the cells through the cytokine IL-2, substantially increased the percentages of both of these cell types.
These enriched cells were found to be very good at selectively eliminating senescent cells after 16 hours. In an experiment where there was only one NK cell for every senescent cell, 15% of normal fibroblasts and 43% of senescent fibroblasts died. These numbers remained largely the same regardless of how senescence was induced, and endothelial cells yielded similar results to fibroblasts.
Doubling or tripling the number of NK cells did kill more senescent cells; however, it also increased the number of normal cells being killed in the process. Therefore, instead of using more NK cells, the researchers increased the time in co-culture. This proved to be extremely helpful; while the number of normal fibroblasts dying remained low, only 10% of senescent cells survived after four days’ exposure to fresh, enriched NK cells. If the NK cells had been previously frozen (a common storage technique), 30% of senescent cells survived.
While the power of these cells, as measured by expression of cytotoxic factors, varied slightly by donor, all donor cells were significantly more cytotoxic towards senescent cells than normal cells.
The researchers describe the four-day experiment as follows:
a 4-day co-culture in which virtually all senescent cells were killed whereas viability of non-senescent cells with NK cell effectors was visually indistinguishable from negative control cells untouched by effector cells.
Even with this success, it’s easy to see potential ways in which this technique could be improved; it may be possible to adjust the protocol to increase specificity or attack different types of senescent cells. However, even in its current form, this technique could be explored in animal models to ascertain its potential therapeutic value.
Additionally, while this research is specific to this particular problem, the researchers are correct that lab conditions should match the conditions of the human body as closely as possible. Biology is an extremely complicated system, and if what cells experience in cell cultures (or animal models) do not match the experiences of cells in human beings, the results could be useless or misleading.
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 Sagiv, A., Biran, A., Yon, M., Simon, J., Lowe, S. W., & Krizhanovsky, V. (2013). Granule exocytosis mediates immune surveillance of senescent cells. Oncogene, 32(15), 1971-1977.
 Pereira, B. I., Devine, O. P., Vukmanovic-Stejic, M., Chambers, E. S., Subramanian, P., Patel, N., … & Akbar, A. N. (2019). Senescent cells evade immune clearance via HLA-E-mediated NK and CD8+ T cell inhibition. Nature communications, 10(1), 1-13.