Actin Depletion Initiates Events Leading to Granule Secretion at the Immunological Synapse

Summary

Cytotoxic T lymphocytes (CTLs) use polarized secretion to rapidly destroy virally infected and tumor cells. To understand the temporal relationships between key events leading to secretion, we used high-resolution 4D imaging. CTLs approached targets with actin-rich projections at the leading edge, creating an initially actin-enriched contact with rearward-flowing actin. Within 1 min, cortical actin reduced across the synapse, T cell receptors (TCRs) clustered centrally to form the central supramolecular activation cluster (cSMAC), and centrosome polarization began. Granules clustered around the moving centrosome within 2.5 min and reached the synapse after 6 min. TCR-bearing intracellular vesicles were delivered to the cSMAC as the centrosome docked. We found that the centrosome and granules were delivered to an area of membrane with reduced cortical actin density and phospholipid PIP2. These data resolve the temporal order of events during synapse maturation in 4D and reveal a critical role for actin depletion in regulating secretion.

Graphical Abstract

Highlights

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  4D imaging elucidates the order of events leading to secretion

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  Actin depletion initiates events leading to centrosome polarization and secretion

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  Lattice light-sheet imaging reveals a rearward flow of actin away from the synapse

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  Both centrosome and granules are delivered to an area of membrane depleted of actin

Abstract

The key sequence of events by which cytotoxic T lymphocytes establish an immunological synapse to kill target cells remains unclear. Griffiths and colleagues have resolved this by using high-resolution 4D imaging, revealing a critical role for actin reorganization in initiating centrosome polarization and granule secretion.