Immediate icing disrupts the very early MCP-1/CCR2 recruitment window and delays muscle regeneration in rats.

Icing is a widely used initial intervention for skeletal muscle injury in sports settings. However, accumulating evidence suggests that icing impairs muscle regeneration, potentially via delayed monocyte/macrophage accumulation. To elucidate the mechanisms underlying this icing-induced delay, we investigated whether icing modulates the monocyte chemoattractant protein 1 (MCP-1)/CC chemokine receptor 2 (CCR2) axis governing monocyte/macrophage recruitment. We comprehensively characterized inflammatory cell dynamics in a rodent crush-injury model using immunostaining and flow cytometry. During the very early phase (up to 5 h postinjury), neutrophils were the dominant inflammatory cell population within the lesion. Subsequently, MCP-1 concentrations and CCR2+ monocytes/macrophages increased and became prominent within 24 h, suggesting that MCP-1/CCR2 signaling prominently contributes to monocyte/macrophage recruitment during this early phase. Consistently, CD68midCD163lo monocytes/macrophages, which exhibited the highest Ccr2 expression, predominated during the first 24 h. Immediate icing transiently suppressed MCP-1 production by neutrophils and monocytes/macrophages during this very early phase and was associated with attenuated subsequent recruitment of circulating monocytes. Consequently, monocyte/macrophage accumulation peaked at 48 h in the nonicing group but was delayed until 72 h in the immediate icing group. Notably, the late-icing model demonstrated that withholding icing during the early phase preserved normal monocyte/macrophage dynamics. Collectively, these results show that immediate icing disrupts the early MCP-1/CCR2-associated recruitment phase, likely by reducing the number of inflammatory cells and their production of MCP-1, thereby contributing to delayed monocyte/macrophage accumulation. These findings provide a crucial mechanistic basis for reconsidering and optimizing the timing of cryotherapy in clinical and sports settings.NEW & NOTEWORTHY Icing after severe muscle injury provides analgesic benefits but delays monocyte/macrophage-mediated regeneration. We demonstrate that immediate icing disrupts early MCP-1 production by neutrophils and infiltrating monocytes/macrophages during the very early postinjury phase, which likely contributes to the subsequent attenuation of circulating monocyte recruitment. These findings provide clinical insight into the importance of optimizing the timing of icing and avoiding its use during the very early phase.