  Cytotoxic T lymphocytes (T cells) and natural killer cells form a tight
contact, the immunological synapse (IS), with target cells, where they release
their lytic granules containing perforin/granzyme and cytokine containing
vesicles. During this process the cell repolarizes and moves the microtubule
organizing center (MTOC) towards the IS. In the first part of our work we
developed a a computational model for the molecular-motor-driven motion of the
MT cytoskeleton during T cell polarization and analyzed effects of cortical
sliding and capture-shrinkage mechanisms. Here we use this model to analyze the
dynamics of the MTOC repositioning in situations in which a) the IS is in an
arbitrary position with respect to the initial position of the MTOC and b) the
T cell has two IS at two arbitrary positions. In the case of one IS, we found
that the initial position determines which mechanism is dominant and that the
time of repositioning does not rise monotonously with the MTOC-IS distance. In
the case of two IS, we observe several scenarios that have also been reported
experimentally: the MTOC alternates stochastically (but with a well defined
average transition time) between the two IS; it wiggles in between the two IS
without transiting to one of the two; or it is at some point pulled to one of
the two IS and stays there. Our model allows to predict which scenario emerges
in dependency of the mechanisms in action and the number of dyneins present. We
report that the presence of capture-shrinkage mechanism at least in one IS is
necessary to assure the transitions in every cell configuration. Moreover, the
frequency of transitions does not decrease with the distance between the two IS
and is the highest when both mechanisms are present in both IS.
