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Rigid and viscous, the truth about white blood cells

Thanks to their expertise in cell micromanipulation, Julien Husson and his team were able to demonstrate that the mechanical properties of white blood cells change when they come into contact with pathogens. These changes in viscosity and rigidity are thought to play a role in their mobility and interaction with their targets.

Credits : Julien Husson, LadHyX, https://cellmechanics.jimdofree.com/

In the continuation of their research on the physical properties of cells, Julien Husson, professor at the École Polytechnique and researcher at the Hydrodynamics Laboratory (LadHyX*), his team, and several other teams collaborating on this study, have shown that the rigidity and viscosity of leukocytes (white blood cells) increase drastically when they are brought into contact with their targets. This work is published in the Biophysical Journal on Tuesday 16 March, and are part of the studies carried out by Julien Husson when he was nominated as a 2019 laureate of the Biomedical Engineering Seed Grant Program.

From the study of a single cell...

To reach this conclusion, the researchers used a rheometer based on the use of micropipettes. This instrument makes it possible to manipulate a single cell and measure the changes in its mechanical properties. They were thus able to select leukocytes one by one in order to study their variations precisely.

Key cells of the immune system, leukocytes have various subtypes whose roles are to fight pathogens and infected cells, to remove cellular debris in the body, or to exchange information with other cells. It is as a result of this exchange of information that they can identify antigens that indicate the presence of a threat in the body, and then coordinate the appropriate immune response. These antigens are elements usually present on the surface of viruses and bacteria, and which lead to the activation of leukocytes and the immune system. In this study, the researchers used beads covered with antigens to artificially activate them.

...to the generalisation of a mechanism

Within seconds of the leukocytes coming into contact with the beads, Julien Husson and his team were able to observe an increase in the rigidity and viscosity of the cells. These changes, which peaked within a few minutes, were sufficiently stereotyped to distinguish leukocyte subtypes. Although the ratio of viscosity to stiffness remained constant in each subtype, the researchers show that the evolution of mechanical properties between leukocytes is variable. For some subtypes, stiffness and viscosity were increased tenfold!

As the viscoelasticity of cells is a key parameter in their mobility and interaction with other cells, the authors conclude that these changes are important keys to understanding the activation of immune cells and their response mechanisms to pathogens.

* LadHyX: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris