The interactions between haematopoietic and stromal cells are profoundly altered by leukaemias, contributing to the phenomena of resistance to myeloablative treatments. In this study, we followed the dynamics of JAM adhesion molecules at the membrane between leukaemic and stromal cells by videonanoscopy in order to study the establishment and evolution of these cellular junctions. The trajectories of JAMs were analysed with near-nanometer precision using a dedicated MTT (Multi-Target Tracing, Sergé et al. Nature Methods 2008) algorithm extended to 2 colours, which allows to reveal the signature of interaction and stabilization events at cell contacts. We have thus characterised the involvement of JAMs in the interaction mechanisms of tumour cells as well as the maintenance of stem cells in bone marrow niches through enhanced interaction. From a therapeutic perspective, we destabilised leukaemic stem cells using blocking antibodies opening opportunities for disrupting LSC resistance mechanisms.

https://doi.org/10.1242/jcs.258736

(B) Maximum projection of a 500-frame videonanoscopy acquisition, showing JAM-B and JAM-C positions over time (left). Maps of JAM-B and JAM-C trajectories represented by gradients of green and magenta, respectively, according to time, and superimposed on the transmission image of the cells (right). Inserts show magnifications of the framed areas. Spatiotemporal colocalizations are denoted by white circles with a size proportional to duration. Several concentric circles correspond to successive colocalizations at a nearby locations but with different durations. (C) Images from the same videonanoscopy acquisition corresponding to the area framed in B, with colocalization events or not (white circle or green/magenta arrowheads, respectively).

This article presents the evidence that immune cells are regulating very rapidely, even before classical signalling times as recorded by calcium fluxes, their mechanical properties when encountering an activating substrate, being either artificial (bead) or physiological (APC). For this, we developed a micropipette-based rheometer to track cell viscous and elastic properties. We have shown that leukocytes become up to 10 times stiffer and more viscous during their activation. Elastic and viscous properties evolve in parallel, preserving a ratio characteristic of the leukocyte subtype. These mechanical measurements set up a complete picture of the mechanics of leukocyte activation and provide a signature of cell function

https://doi.org/10.1016/

Activation of three types of leukocytes studied with the micropipette rheometer. Left (a): T cell, middle (b): B cell, right (c): PLB cells. All bars represent 5 μm.

SIGNIFICANCE Leukocytes have a ubiquitous capacity to migrate on or in solid matrices and with or without adhesion, which is instrumental to fight infections. The precise mechanisms sustaining migration remain, however, arguable. It is for instance widely accepted that leukocytes cannot crawl on two-dimensional substrates without adhesion. In contrast, we showed that human lymphocytes swim on nonadherent two-dimensional substrates and in suspension. Furthermore, our experiments and modeling suggest that propulsion hardly rely on cell body deformations and predominantly on molecular paddling by transmembrane proteins protruding outside the cell. For physics, this study reveals a new type of microswimmer, and for biology, it suggests that leukocyte’s ubiquitous crawling may have evolved from an early machinery of swimming shared by various eukaryotic cells.

Biophysical Journal 119, 1157–1177, September 15, 2020 1157

This paper was commented on in Science (link), CellPress (link), Eurekalert (link) and Science&Vie (link) and CNRS (link).