This project has been successfully completed, including the PhD defence of Lanette Kempers (thesis: A vascular affair, UvA, 28 September 2022). In particular, the development of the blood vessel on a chip proved to be a fruitful track. With that technology, we found a hitherto undiscovered migration signalling pathway for neutrophils deficient for the actin regulatory ArpC1B. This illustrates that new pathways can be identified with such new technology, potentially leading to new therapeutic interventions.

This work led to a re-evaluation of our understanding of endothelial cell connections. We found that endothelial cells partially overlap in junctional regions, i.e. the plasma membrane extends beyond the VE-cadherin boundaries. This phenotype was confirmed in vivo using so-called confetti knock-in endothelium-specific mouse models, giving individual endothelial cells a different staining. This allowed us to effectively identify endothelial membrane overlaps. We detected these overlaps in the liver, lungs and skin. Further analysis showed that these were characterised by strong PECAM-1/CD31 staining rather than VE-cadherin. They were additionally found to be transmigration hotspots for neutrophils: the larger the overlap, the more neutrophils preferred such sites to pass through the endothelium. Using advanced real-time microscopy called lattice light-sheet microscopy in close collaboration with the Janelia Research Institute in Ashburn, USA, we found that the endothelial transmigration membrane generated tunnels for leukocytes to pass through.

This work shows that the current dogma of how leukocytes pass through the endothelium by temporarily uncoupling two adjacent endothelial cells may not be correct or at least incomplete and needs to be modified. This will lead to new concepts and insights into how leukocytes manage to cross the endothelium while maximising the integrity of the endothelial barrier.
The research results have been well received internationally, leading to publications in leading journals (including J. Cell Sci. and J. Mol. Sci.) and to Lanette Kempers' PhD. The research is now being continued by another PhD student in our group.

THESIS 2022: A vascular affair - Lanette Kempers