Cytotoxic T lymphocytes (CTL) and Natural Killer cells (NK cells) patrol the human body in search of virus-infected and cancerous cells. Upon recognition of a target cell, the killer cell releases lytic granules at the immunological synapse and kills the target. For optimal immune function, the killer cell migration machinery has to be adjusted in order to maximize the likelihood of target cell encounter.

We used in vitro time lapse microscopy on primary human CTL to study the calcium dependence of migration. The analysis of trajectories in the absence of target cells revealed that CTL alternate between a mobile and a stationary state. The preference for each state depends on the external calcium concentration. On a short time scale (seconds/minutes), CTL show directional persistence, while migration on a long time scale (minutes/hours) is random. This behaviour cannot sufficiently be explained by pure Brownian motion but can be described by a model where migration is controlled by the combined forces of several motors with random direction and certain likelihoods for the ON and OFF state. We believe that this model can help us understand how cellular signalling can influence motor activity in order to optimize the immune response.