University of Geneva reveals details of immune system's killer cells

A collaboration between the University of Geneva and Lausanne University Hospital has shown in new detail how the body's defenses fight back against malignant cells.

Those defenses involve T lymphocytes, or T cells, which are dedicated to eliminating infected or cancerous cells, through the release of active molecules intended to destroy the target cells and leave neighboring ones unharmed. Until now the fine organization of these events remained difficult to observe, commented the project.

The research, published in Cell Reports, used a version of expansion microscopy to visualize these mechanisms in three dimensions in a near-native state, revealing how the molecular organization of cytotoxic T cells underpins their function.

Expansion microscopy (ExM) is a route to bioimaging in which the distance between fluorescently labelled species of interest is physically increased so as to reveal cellular processes more clearly without disturbing the cellular structure. Cross-linked swellable hydrogels infused into the tissues and then expanded are one such ExM technique.

"No single technique has so far combined near-native structural preservation, quantitative nanoscale resolution, multicolor molecular specificity, and whole-cell three-dimensional coverage," noted the project in its paper.

The Geneva solution involved a cryo-expansion microscopy (cryo-ExM) operation, which combines instantaneous cryo-immobilization with an ExM operation.

Although cryo-ExM is not applicable to live imaging, it is uniquely suited for quantitative, volumetric mapping of mechanically fragile structures across entire cells and large sample cohorts, complementing live-cell super-resolution approaches rather than replacing them, wrote the team.

Improved therapeutic strategies 

"This technique involves instantaneously freezing cells at very high speed, placing them in a so-called vitreous state, where water solidifies without forming crystals and thus faithfully preserves biological structures," explained Virginie Hamel from the University of Geneva. "The samples are then physically expanded using an absorbent hydrogel, making it possible to observe their internal organization with great precision while maintaining their near-native architecture."

In trials using human T cells, the cryo-ExM method successfully preserved actin, microtubules, membranes and fine membrane protrusions with high fidelity. The team then used the same approach to capture the sequence of events at the immunological synapse, a dynamic interface where T cell receptors actively recognize antigens and trigger remedial action.

One finding was an unexpected adhesion-dependent dome-like space beneath activated T cells on stimulatory surfaces, highlighting how mechanical and adhesive cues shape synapse topology in the anti-cancer mechanisms.

"Our work reveals that at the point of contact between the immune cell and its target, the membrane forms a kind of dome, whose structure appears to be linked to adhesion interactions and the internal organization of the cell," noted Florent Lemaître from the University of Geneva.

By providing a 3D and near-native view of these processes, this work establishes a reference framework for analyzing how immune cells function, said the project. It could help improve therapeutic strategies, particularly in immuno-oncology, by enabling a better understanding of the mechanisms that determine the effectiveness or limitations of the immune response.