Probing the biomechanical contribution of the endothelium to lymphocyte migration: diapedesis by the path of least resistance

Abstract

Immune cell trafficking requires the frequent breaching of the endothelial barrier either directly through individual cells (a ‘transcellular’ route) or through the inter-endothelial junctions (a ‘paracellular’ route). What determines the loci/route of breaching events is an open question with important implications for overall barrier regulation. We hypothesized that basic biomechanical properties of the endothelium might serve as critical determinants of this process. By altering junctional integrity, cytoskeletal morphology and, as a consequence, local endothelial cell stiffness of different vascular beds, we could modify diapedesis route preference. In particular, high barrier function was coupled with predominant transcellular migration, whilst negative modulation of junctional integrity resulted in switch to paracellular diapedesis. Furthermore, we showed that lymphocytes dynamically probe the underlying endothelium by extending invadosome-like protrusions (ILP) into its surface that deform the nuclear lamina, distort actin filaments and ultimately breach the barrier. Fluorescence imaging and pharmacologic depletion of F-actin demonstrated that lymphocyte barrier breaching efficiency was inversely correlated to local endothelial F-actin density/stiffness. Together, these data support the hypothesis that lymphocytes are guided by the mechanical ‘path-of-least-resistance’ as they transverse the endothelium, a process we term ‘tenertaxis’ (Latin, tener, soft).