Time course of chemokine expression and leukocyte infiltration after acute skeletal muscle injury in mice

Innate pro-inflammatory processes, such as chemokine signaling and leukocyte infiltration, predominate during the first 48h after an acute skeletal muscle injury. However, the time course of chemokine expression and its relationship to leukocyte infiltration after acute muscle injury within this early post-injury time period has not been investigated. In this study, 46 anesthetized female C57BL/6NHsd mice underwent a closed crush injury of the gastrocnemius muscle and were euthanized 4, 8, 24 and 48h post-injury. Microarray analysis found 14 chemokine genes to be up-regulated during this period, 12 of which are involved in macrophage or neutrophil chemotaxis, with up-regulation peaking at either 8 or 48h. RT-PCR analysis on select chemokines confirmed the microarray activation pattern. Neutrophil infiltration patterns mirrored the time course of neutrophil-related chemokines with Gr-1-, 1A8- and 7/4-positive neutrophils infiltrating the muscle 4h after injury, decreasing at 48h. Conversely, gene expression and relative quantification levels of macrophage-related chemokines Ccl2 and Ccl7 peaked at 8h, preceding the infiltration of CD68- and F4/80-positive macrophages, and protein expression of Ccl2 in the muscle. The up-regulation of other macrophage-related chemokines and their receptors peaked at 48h post-injury.