Immunological memory is important to protect humans against recurring diseases.
Memory CD8+ T cells are required for quick expansion into effector cells but also
provide immediate cytotoxicity against their targets. Whereas many functions of the two
main cytotoxic subtypes, effector memory CD8+ T cells (TEM) and central memory CD8+ T
cells (TCM), are well defined, single TEM and TCM cell cytotoxicity has not been quantified.
To quantify cytotoxic efficiency of TEM and TCM, we developed a FRET-based single cell
fluorescent assay with NALM6 target cells which allows analysis of target cell apoptosis,
secondary necrosis following apoptosis, and primary necrosis after TEM- or TCM-target cell
contact. Both, single cell and population cytotoxicity assays reveal a higher cytotoxic
efficiency of TEM compared to TCM, as quantified by target cell apoptosis and secondary
necrosis. Perforin, granzyme B, FasL, but not TRAIL expression are higher in TEM
compared to TCM. Higher perforin levels (likely in combination with higher granzyme
levels) mediate higher cytotoxic efficiency of TEM compared to TCM. Both, TEM and TCM
need the same time to find their targets, however contact time between CTL and target,
time to induce apoptosis, and time to induce secondary necrosis are all shorter for TEM. In
addition, immune synapse formation in TEM appears to be slightly more efficient than in
TCM. Defining and quantifying single TEM and TCM cytotoxicity and the respective
mechanisms is important to optimize future subset-based immune therapies
