The human immune system uses several pathways for defence against various pathogens. After target cell recognition by the cytotoxic T-cell, a close contact is formed between them, called the immune synapse. The proteins involved in immune synapse formation are usually arranged in concentric rings, often referred to as a bull’s eye. The immune synapse exists only for a limited time during which the pore forming protein perforin along with granzymes are released by exocytosis of specialised lytic granules from the killer cell into the cleft between the cytotoxic and the target cell. Perforin pores are formed in the host cell, enabling the entry of granzymes, which rapidly kill the target cell by apoptosis.
Here we characterise the immune synapse in a cell culture model system by correlative fluorescence and electron tomography of sectioned, high pressure frozen and freeze-substituted cells, and by soft X-ray tomography of intact, plunge-frozen cells. The 3D reconstructions obtained by cryo X-ray tomography enabled us to examine the membrane structures in intact, frozen-hydrated cells forming immune synapses at different stages, while the electron tomograms of thin sections provide a more detailed view of the synapses. The lytic granules containing perforin and granzymes are clearly visible inside cytotoxic cells, adjacent to the synapse.
This work has provided an overview of the membrane structures in the complete immune synapse, and a more detailed 3D view of the cellular structures in thin sections.