Cortical actin recovery at the immunological synapse leads to termination of lytic granule secretion in cytotoxic T lymphocytes

AT Ritter, SM Kapnick, S Murugesan… - Proceedings of the …, 2017 - National Acad Sciences
AT Ritter, SM Kapnick, S Murugesan, PL Schwartzberg, GM Griffiths, J Lippincott-Schwartz
Proceedings of the National Academy of Sciences, 2017National Acad Sciences
CD8+ cytotoxic T lymphocytes (CTLs) eliminate virally infected cells through directed
secretion of specialized lytic granules. Because a single CTL can kill multiple targets,
degranulation must be tightly regulated. However, how CTLs regulate the termination of
granule secretion remains unclear. Previous work demonstrated that centralized actin
reduction at the immune synapse precedes degranulation. Using a combination of live
confocal, total internal reflection fluorescence, and superresolution microscopy, we now …
CD8+ cytotoxic T lymphocytes (CTLs) eliminate virally infected cells through directed secretion of specialized lytic granules. Because a single CTL can kill multiple targets, degranulation must be tightly regulated. However, how CTLs regulate the termination of granule secretion remains unclear. Previous work demonstrated that centralized actin reduction at the immune synapse precedes degranulation. Using a combination of live confocal, total internal reflection fluorescence, and superresolution microscopy, we now show that, after granule fusion, actin recovers at the synapse and no further secretion is observed. Depolymerization of actin led to resumed granule secretion, suggesting that recovered actin acts as a barrier preventing sustained degranulation. Furthermore, RAB27a-deficient CTLs, which do not secrete cytotoxic granules, failed to recover actin at the synapse, suggesting that RAB27a-mediated granule secretion is required for actin recovery. Finally, we show that both actin clearance and recovery correlated with synaptic phosphatidylinositol 4,5-bisphosphate (PIP2) and that alterations in PIP2 at the immunological synapse regulate cortical actin in CTLs, providing a potential mechanism through which CTLs control cortical actin density. Our work provides insight into actin-related mechanisms regulating CTL secretion that may facilitate serial killing during immune responses.
National Acad Sciences