P- and E-selectins are expressed on the surface of endothelial cells and may contribute to neutrophil recruitment following injurious lengthening contractions of skeletal muscle. Blunting neutrophil, but not macrophage, accumulation after lengthening contractions may provide a therapeutic benefit as neutrophils exacerbate damage to muscle fibers, while macrophages promote repair. In this study, we tested the hypothesis that P- and E-selectins contribute to neutrophil, but not macrophage, accumulation in muscles after contraction-induced injury, and that reducing neutrophil accumulation by blocking the selectins would be sufficient to reduce damage to muscle fibers. To test our hypothesis, we treated mice with antibodies to block P- and E-selectin function and assessed leukocyte accumulation and damage in muscles 2 days after lengthening contractions. Treatment with P/E-selectin blocking antibodies reduced neutrophil content by about half in muscles subjected to lengthening contractions. In spite of the reduction in neutrophil accumulation, we did not detect a decrease in damage 2 days after lengthening contractions. We conclude that P- and/or E-selectin contribute to the neutrophil accumulation associated with contraction-induced muscle damage and that only a portion of the neutrophils that typically accumulate following injurious lengthening contractions is sufficient to induce muscle fiber damage and force deficits. Thus, therapeutic interventions based on blocking the selectins or other adhesion proteins will have to reduce neutrophil numbers by more than 50% in order to provide a benefit.
© 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.

One of the greatest challenges in cell therapy is to minimally invasively deliver a large quantity of viable cells to a tissue of interest with high engraftment efficiency. Low and inefficient homing of systemically delivered mesenchymal stem cells (MSCs), for example, is thought to be a major limitation of existing MSC-based therapeutic approaches, caused predominantly by inadequate expression of cell surface adhesion receptors. Using a platform approach that preserves the MSC phenotype and does not require genetic manipulation, we modified the surface of MSCs with a nanometer-scale polymer construct containing sialyl Lewis(x) (sLe(x)) that is found on the surface of leukocytes and mediates cell rolling within inflamed tissue. The sLe(x) engineered MSCs exhibited a robust rolling response on inflamed endothelium in vivo and homed to inflamed tissue with higher efficiency compared with native MSCs. The modular approach described herein offers a simple method to potentially target any cell type to specific tissues via the circulation.

This investigation sought to determine if P-selectin-mediated mechanisms contributed to macrophage localization in damaged muscle, an essential process for muscle regeneration. Mice were injected intravenously (i.v.) with soluble P-selectin glycoprotein ligand-1 (sPSGL-1) at 5, 50, or 200 microg/mouse or with 100 microl vehicle alone, and then, lengthening contractions were induced in hindlimb plantar-flexor muscles. The contractions caused fiber damage in soleus muscles, with maximal invasion by CD11b+ mononuclear cells at 24 h post-injury and substantial accumulation of CD11b+ mononuclear cells in the extracellular matrix up to 7 days post-injury. sPSGL-1 treatment caused a dose-dependent decrease in the number of regenerating fibers (P=0.021), as determined by developmental myosin heavy chain (dMHC) expression. This expression was reduced 93% at 7 days post-injury by the highest dose of sPSGL-1, which had no significant influence on intrafiber or extracellular accumulation of cells expressing CD11b, a general marker for phagocytic cells. Additional mice were injected i.v. with 20 microg anti-P-selectin or isotype-control immunoglobulin G and were then subjected to lengthening contractions as before. At 7 days post-injury, soleus muscles from anti-P-selectin-treated mice contained 48% fewer mononuclear cells that bound ER-BMDM1 (P=0.019), a marker for mature macrophages and dendritic cells, and 84% fewer fibers expressing dMHC (P = 0.006), compared with muscles from isotype-injected, control mice. The number of CD11b+ cells was not significantly different between groups. The results are consistent with the concept that P-selectin is involved in the recruitment, maturation, and/or activation of cells that are critical for muscle fiber regeneration.