Cytotoxic T Lymphocyte Activation Signals Modulate Cytoskeletal Dynamics and Mechanical Force Generation*

Cytotoxic T lymphocytes (CTLs) play a key role in the adaptive immune response by killing infected cells. CTL activation requires antigen presenting cells (APCs) to present pathogenic peptides and provide co-stimulatory signals to receptors on the CTL surface. Activated CTLs secrete lytic granules (LGs), containing enzymes that trigger target cell death, at the CTL-target junction or the immune synapse (IS). LGs are transported along microtubules to the IS, where secretion occurs in zones of actin depletion. Actomyosin-mediated force exertion at the IS further promotes target death. Cytokines such as interleukin-12 (IL-12), produced by APCs, act as a third signal for activation and enhance CTL function. We hypothesized that IL-12 modulates cytoskeletal dynamics at the IS, thus potentiating CTL function. We used TIRF microscopy to study cytoskeletal dynamics in murine CTLs activated in the presence of peptide and co-stimulation (two signals), or additionally with IL-12 (three signals). We found that three signal-activated CTLs display altered actomyosin flows, microtubule growth and LG speeds at the IS. We further showed that three signal-activated CTLs exert higher forces. Our results indicate a potential physical mechanism by which cytokines can augment the immune response.