The evolution of cell killing: when a target cell became “invited” to choose how to die.

During evolution, in the world of cytotoxic reactions a new mechanism has emerged that was based upon the direct "responsibility" of targets in their death. In other words, effector cells thought targets to use their own genetic material to trigger their suicide. To investigate this hypothesis, we have used an experimental model represented by Eisenia foetida coelomocytes that kill human cells, and asked whether coelomocytes can trigger apoptosis, or are able to provoke their death via necrosis, or even both, Using a strategy based upon a technique we have developed, that evaluates the expression of different forms of Fas (CD95/APO-1, i.e. the proapoptotic membrane form and the anti-apoptotic soluble form) mRNA in target cells, we tested the capacity of either coelomocytes from E. foetida or supernatant from E. foetida coelomocyte cultures to kill human cell lines of different origin. When target cells were incubated with coelomocytes, an upregulation of the membrane form of Fas occurred, along with an increase of the total form. On the contrary, cells treated with supernatant (containing cytotoxic molecules) had a significant reduction of the production of both forms of Fas mRNA, suggesting that mechanisms devoted to triggering of apoptosis were downregulated. Our data suggest that the production of soluble mediators, that could be considered the ancestors of humoral immunity, causes death of the foreign cell because of an aspecific activity of lytic molecules, that likely bind target membrane for physicochemical reasons, and determine necrosis. The development of cellular mechanisms to kill targets, i.e. likely representing the onset of cellular immunity, could act by modulating the expression of genes involved in apoptosis, determining an increase of total Fas expression.