Centrosome polarization delivers secretory granules to the immunological synapse

Abstract

Cytotoxic T lymphocytes (CTLs) destroy virally infected and tumorigenic cells by releasing the contents of specialized secretory lysosomes--termed 'lytic granules'--at the immunological synapse formed between the CTL and the target. On contact with the target cell, the microtubule organizing centre of the CTL polarizes towards the target and granules move along microtubules in a minus-end direction towards the polarized microtubule organizing centre. However, the final steps of secretion have remained unclear. Here we show that CTLs do not require actin or plus-end microtubule motors for secretion, but instead the centrosome moves to and contacts the plasma membrane at the central supramolecular activation cluster of the immunological synapse. Actin and IQGAP1 are cleared away from the synapse, and granules are delivered directly to the plasma membrane. These data show that CTLs use a previously unreported mechanism for delivering secretory granules to the immunological synapse, with granule secretion controlled by centrosome delivery to the plasma membrane.

A dual role for IQGAP1 in regulating exocytosis

“…7A). Although in the immune system delivery of secretory lysosomes requires the clearing of both actin and IQGAP1 from the target site of the plasma membrane (Stinchcombe et al, 2006), depletion of IQGAP1 in mast cells only mildly enhances agonist-stimulated histamine secretion (Psatha et al, 2007). Together, these data support the concept that the essential role of IQGAP1 in secretion is regulatory.…”

CD4+T-cell synapses involve multiple distinct stages

“…What this might be is suggested by the fact that these invasive pseudopodia would significantly increase the surface area of T cell-APC contact, at least fivefold over a "flat" interaction, thus enabling the T cell to sample much more of the possible antigens on the APC surface. T cells are very sensitive to their peptide-MHC ligands, able to detect even one molecule (32,33), and although this is sufficient for the cell to stop, more peptide-MHC agonist ligands are usually needed (3)(4)(5)(6)(7)(8)(9)(10) to make a full response (2). Interestingly, this remarkable probing activity has not been seen in fluorescence studies of T cell-APC interactions, perhaps because of a lack of resolution and/or its rapidity.…”

“…microtubule organizing center | centrosome A prominent feature of many T-lymphocyte interactions with other cells on which it recognizes a particular antigen is the formation of an immunological synapse (IS). The formation of this structure correlates with robust signaling, lineage commitment, and fate determination of T cells (1)(2)(3)(4)(5)(6) and the directed secretion of cytokines and/or cytotoxic molecules. There is a wholesale reorganization of the T cell's cytoskeleton, surface molecules, and organelles, and the loss of polarity regulators or guidance cues that negatively affect T-cell activation (7,8).…”

“…Here electron microscopic studies are particularly valuable for their very high resolution, especially with the power of 3D tomography and highpressure freezing, which preserves cell structure more faithfully. Although a number of high-resolution electron microscopy studies of T cell-APC interactions have been reported (3,(12)(13)(14)(15), these involve studies of cytotoxic T or natural killer cells and their targets for only the first few minutes of CD4 + T-cell recognition (16). In earlier work using fluorescence microscopy, Kupfer et al…”

The small GTPase Rab29 is a common regulator of immune synapse assembly and ciliogenesis

“…It is noteworthy that Rab29 also interacts with Kif3A. Although the centrosome polarizes very close to the IS membrane, 30 recent evidence has highlighted the presence of short microtubules emanating from the polarized centrosome and directed toward the IS membrane in cytotoxic T cells, on which transport is ensured by kinesin 1/Kif5b. 24 Our finding that Rab29 interacts with Kif3A suggests the possibility that it may further participate in the pathway by coupling the TCR + endosomes that have reached the centrosome to these short microtubules for their kinesin-dependent transport to the IS.…”