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It is presently assumed that lethal hit delivery by cytotoxic T lymphocytes (CTLs) is mechanistically linked to centrosome polarization toward target cells, leading to dedicated release of lytic granules within a confined secretory domain. Here we provide three lines of evidence showing that this mechanism might not apply as a general paradigm for lethal hit delivery. First, in CTLs stimulated with immobilized peptide–MHC complexes, lytic granules and microtubule organizing center localization into synaptic areas are spatio-temporally dissociated, as detected by total internal reflection fluorescence microscopy. Second, in many CTL/target cell conjugates, lytic granule secretion precedes microtubule polarization and can be detected during the first minute after cell–cell contact. Third, inhibition of microtubule organizing center and centrosome polarization impairs neither lytic granule release at the CTL synapse nor killing efficiency. Our results broaden current views of CTL biology by revealing an extremely rapid step of lytic granule secretion and by showing that microtubule organizing center polarization is dispensable for efficient lethal hit delivery.

The molecular mechanisms used by regulatory T cells (Treg) to inhibit the effector phase of adaptive immune responses are still elusive. In the present work, we investigated the possibility that Treg may interfere with a basic biological function of T helper cells (T H ): polarization of secretory machinery for dedicated help delivery. To address this question, we visualized by confocal microscopy different parameters of activation in T H and Treg cells interacting simultaneously with individual antigen-presenting cells (APC). Our results show that, although productive TCR engagement in T H /APC conjugates was unaffected by the presence of adjacent Treg, the reorientation of T H secretory machinery toward APC was strongly inhibited. Blocking TGF-β completely reverted Treg induced inhibition of T H polarization. Our results identify a previously undescribed mechanism by which Treg inhibit effector T cells. TGF-β produced by adjacent Treg interferes with polarization of T H secretory machinery toward APC, thus affecting a crucial step of T H -mediated amplification of the immune response.