Amelie Niedermeier - Visualizing the immunological synapse at molecular resolution using cryo-electron tomography

Cytotoxic T lymphocytes eliminate virus-infected and cancer cells by forming immunological synapses with them. Cortical actin plays a critical role in stabilizing this interface, supporting receptor signaling and organizing the synapse architecture. However, the three-dimensional organization of the actin filaments underlying the plasma membrane at the immunological synapse, and its effect on plasma membrane geometry, remain poorly understood. Using cryo-electron tomography (cryo-ET), we analyzed immune synapses formed between human cytotoxic T cells and mouse cancer cells. We visualized the molecular architecture of the T cell interior, providing high-resolution snapshots of lytic granules transported by microtubules and curved actin filaments, as well as mitochondria and nuclei in close proximity to the synapse. Our data revealed uneven plasma membrane-plasma membrane spacing, with regions of very close apposition associated with actin nanofoci in the cytotoxic T cells. These nanofoci display a tent-like structure of oblique, branched filaments that are directed towards the plasma membrane, which is consistent with Arp2/3- mediated nucleation. The nanofoci deform the membrane of the cytotoxic T cells locally, suggesting that they play a role in receptor clustering and exclusion in the plasma membrane. Cryo-ET is uniquely capable of visualizing these contacts at a molecular level, capturing both these nanoscale events and the overall synaptic landscape. Our work provides structural insights that confirm assumptions and uncover unexpected details.