3C). Cell conjugates lasted for the full duration of the experiments, as demonstrated by DIC images taken at the end of the experiment (Fig. 3C). These results suggest that a physical interaction between BMMCs and Tregs is a key event involved in the inhibition of BMMC Ca2+ signaling. To gain insight DAPT nmr into MC morphological changes occurring while interacting with a Treg, we analyzed conjugates of these cells by transmission electron microscopy. Ten minutes after Ag stimulation, MCs and Tregs formed numerous cell conjugates. Examined at low magnification, BMMCs appeared as activated cells endowed with numerous surface filopodia and lamellopodia

which, in some

instances, seemed to embrace and envelope Tregs (Fig. 4A and B). Contact areas between BMMC and Treg plasma membranes were either contact points (Fig. 4A) or extended surface areas (Fig. 4B). Viewed at higher magnification, the latter exhibited the composite profile of true immunological synapses (Fig. 4C and D). They were arranged as alternating sites of tight membrane-to-membrane appositions and wider drug discovery intermembrane spaces that corresponded to the synaptic clefts. Here, the distance between the pre- and post-synaptic membranes was 100–150 nm (Fig. 4D). The close intermembrane appositions presented an intermembrane thickness ∼15 nm, which sealed the synaptic clefts apart. In a few instances, the synaptic cleft formed a kind of pocket where the Treg-coupled MC released the content of one or two secretory granules in a process of limited exocytosis (Fig. 4E and F). MCs challenged with Ag underwent classical compound exocytosis and extensive membrane ruffling

was observed: granules and plasma membranes fused, membrane pores were formed and membrane-free granule contents were released outside the cells (Fig. 5A). Interestingly, activated BMMCs interacting Mannose-binding protein-associated serine protease with Tregs exhibited cytoplasmic secretory granules with various degrees of content loss, i.e. granules with lucent areas in their cores, reduced electron density, disassembled matrices, residual cores and membrane empty containers (Fig. 5B). Empty or partially empty secretory containers could be recognized intermingled with granules, whose shape, size and density fell within normal range (Fig. 5B). The dilated granule containers maintained their limiting membranes, as no fusion events with the plasma membrane or with neighboring granule membranes occurred. In a small proportion of Treg-contacting MCs, 30–60 nm diameter lucent vesicles could be identified in the peripheral cytoplasm, next to granules or close to the plasma membrane (Fig. 5C and D).