Neutrophil motility in extracellular matrix gels: Mesh size and adhesion affect speed of migration

Polymorphonuclear leukocyte (PMN) migration through tissue extracellular space is an essential step in the inflammatory response, but little is known about the factors influencing PMN migration through gels of extracellular matrix (ECM). In this study, PMN migration within reconstituted gels containing collagen type I or collagen type I supplemented with laminin, fibronectin, or heparin was measured by quantitative direct visualization, resulting in a random motility coefficient (mu, a quantititive index for rate of cell dispersion) for the migrating cell population. The random motility coefficient in unsupplemented collagen (0.4 mg/ml) gels was similar to 9 x 10(-9) cm(2)/s. Supplementing gels with heparin or fibronectin produced a significant decrease in mu, even at the lowest concentrations studied (1 mu g/ml fibronectin or 0.4 mu g/ml heparin), At least 100 mu g/ml of laminin, or 20% of the total gel protein, was required to produce a similar decrease in mu. Scanning electron microscopy revealed two different gel morphologies: laminin or fibronectin appeared to coat the 150-nm collagen fibers, whereas heparin appeared to induce fiber bundle formation and, therefore, larger interstitial spaces, The decrease in mu observed in heparin-supplemented gels correlated with the increased mesh size of the fiber network, but the differences observed in mu for fibronectin- and laminin-supplemented gels did not correlate with either mesh size or the mechanical properties of the gel, as determined by rheological measurements, However, PMNs adhered to fibronectin-coated surfaces in greater numbers than to collagen- or laminin-coated surfaces, suggesting that changes in cell adhesion to protein fibers can also produce significant changes in cell motility within an ECM gel.