An exploratory simulation of the optimization of leukocyte transmigration, indicating that barrier routing is governed by context-dependent biochemical cues rather than hardwired programs

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Abstract
Introduction: Leukocyte infiltration across the blood-brain barrier contributes to many neuroinflammatory disorders, yet the physical mechanisms that determine how immune cells cross the barrier remain incompletely understood. In particular, it is unclear how leukocytes choose between paracellular migration through endothelial junctions and transcellular migration through the endothelial cell body. This question is especially relevant during central nervous system inflammation, where different T-lymphocyte subsets, including Th1 and Th17 cells, appear to preferentially use distinct migration routes. Previous studies have examined individual mechanical forces or upstream biochemical signaling pathways, but these approaches have not provided a unified framework that considers the energetic contributions of both the leukocyte and the deforming endothelium. Methods: To address this, we developed a two-route energy minimization model that incorporates an anisotropic, fiber-reinforced hyperelastic model, junctional unzipping, Helfrich membrane bending, caveolar vesicle fusion, and cytokine-dependent signaling. Model parameters were optimized, and route switching was evaluated using Sobol global sensitivity analysis to determine which specific mechanical and signaling parameters independently drive route switching rather than merely scaling overall energy magnitude. Results: The model reproduced the experimental validation conditions used in this study and identified distinct regimes of mechanical control. Under IFN-γ-dominant conditions, route selection was influenced primarily by ICAM-1 adhesion kinetics. Under IL-17-dominant conditions, control shifted toward PECAM-1-related physical properties and their nonlinear interactions, which accounted for 76% of the variance in route switching. Discussion: These exploratory findings suggest that leukocyte transmigration is shaped by local biochemical conditions together with the mechanical properties of the endothelial barrier, rather than by a single fixed migration program. Mapping the associated energetic landscape may help clarify the physical factors governing immune-cell entry into the central nervous system and provide a framework for studying mechanisms that regulate leukocyte trafficking during neuroinflammation.

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