Far-Ranging Mechanical Communication between Macrophages and Contractile Fibroblasts in Collagen Matrix

Pakshir, Pardis 1, 2 ;  Alizadehgiashi, Moien  3 ;  Miranda Coelho, Nuno  2 ;  Mohammadi, Hamid 2 ;  Guenther, Axel  3 ;  McCulloch, Christopher  2 ;  Hinz, Boris 1, 2

1. Laboratory of Tissue Repair and Regeneration; 2. Matrix Dynamics Group, Faculty of Dentistry; 3. Department of Mechanical and Industrial Engineering, University of Toronto

Background: Inflammation is crucial for the healing of lung injuries. Infiltration of macrophages (MΦ) to the wound site and their crosstalk with myofibroblast (MFs) that restore connective tissue architecture is part of the normal wound repair process. Dysregulated MΦ-to-MF communication however, stimulates MFs to excessively accumulate and contract collagen extracellular matrix (ECM) into scar tissue which can obliterate lung function. We found previously that spatial proximity of MΦ supports persistent activation of MFs. However, the initial guidance cues and mechanisms directing migratory MΦ towards MFs are unknown

Objective and Hypothesis: We hypothesize that local remodelling of collagen ECM by contracting MFs provides mechanical and physical cues in the ECM that are sufficient and essential for the attraction of MΦ. 

Materials and Methods: Primary mouse lung MFs were co-cultured with MΦ on collagen gels with implanted tracer beads. ECM deformation fields produced by MFs were analyzed over time by traction force videomicroscopy. Local mechanical loads were applied to collagen gels using an actuated microneedle to mimic dynamic pulling events of MFs for MΦ. Migration of individual MΦ, added at different times of establishment of deformation fields, was tracked and analyzed for speed, directionality and distance relative to the position of the force source (microneedle or MF). 

Results: MF-generated deformations in 2 mg/ml collagen ECM extended far beyond the cell diameter (1,600 µm within 6 h). When positioned within deformation fields, MΦ sensed and migrated towards mechanical forces exerted by contracting MFs and actuated microneedles. Microneedle experiments identified the presence of a dynamic force source as the critical signal initiating and directing M? migration in the ECM. In contrast, collagen fiber alignment resulting from MF remodelling activities was neither required nor sufficient to guide MΦ migration.

Conclusions: Dynamic MF contraction is transmitted through collagen ECM and critical to attract MΦ over distances that exceed the range of chemotactic gradients.

Significance/Impact: We propose a novel mechanism of far-ranged MΦ mechanosensing that integrates mechanical changes in the ECM over time rather than following consolidated changes (e.g., fibril alignment).