Inflammatory cells are traditionally associated with pain, heat, redness and swelling. However, accumulating studies have shown that some of these cells can also contribute to tissue repair. Indeed, neutrophils and macrophages can contribute to the resolution of inflammation and to skeletal muscle regeneration via the release of cytokines and growth factors. We recently showed that tryptase, the most abundant mediator in mast cell granules, could potentially support muscle regeneration by increasing skeletal muscle cell proliferation.
PURPOSE: To evaluate if mast cells can stimulate skeletal muscle cell proliferation.
METHODS: In vitro: mast cells were isolated from peritoneal cavity of female Wistar rats. L6 muscle cells were cultured with either mast cells activated with compound 48/80 or mast cellderived conditioned media. L6 cell number was determined with CellTiter assay 24h post-seeding. In vivo: muscle injury was induced through a bupivacain injection into the right EDL muscle. Rats received a daily intra-peritoneal injection of 5 bromo-2’deoxyuridine (BrdU) and were treated or not with the mast cell stabilizer cromolyn from 24h before injury. Rats were sacrificed 48 h post injury and immunohistochemistry analyzes were performed.
RESULTS: In vitro proliferation of L6 cells cultured with either activated mast cells or mast cell-conditioned media was significantly increased above control (1.30±0.08 fold and 1.24±0.04 fold), respectively. The proliferative effect of conditioned media was lost when APC-366, a tryptase inhibitor, was added. In vivo results shown that, compared to control, mast cell stabilization increased the density of proliferating cells (109,033±8,186 vs 79,678±10,833 cells/mm3), neutrophils (34,116±6,167 vs 15,636±4,201 cells/mm3), macrophages ED1 (35,426±7,517 vs 13,075±4,108 cells/mm3) and macrophages ED2 (21,671±1,676 vs 16,922±715 cells/mm3), respectively. P<0.05.
CONCLUSION: Activated mast cells can stimulate skeletal muscle cell proliferation via tryptase release in vitro. However, in vivo this effect was masked by the influence of mast cells on the recruitment of other mitogenic cells such as neutrophils and macrophages. Supported by grants from NSERC
