Long‐ and short‐term intravital imaging reveals differential spatiotemporal recruitment and function of myelomonocytic cells after spinal cord injury

<jats:title>Key points</jats:title><jats:p><jats:list list-type="explicit-label"> <jats:list-item><jats:p>Inflammatory cells such as myelomonocytic cells are key players in the progression and recovery from spinal cord injury (SCI).</jats:p></jats:list-item> <jats:list-item><jats:p>However, the precise spatiotemporal distributions and roles of different subpopulations of myelomonocytic cells remain unclear in part because their dynamics have not been examined <jats:italic>in vivo</jats:italic>.</jats:p></jats:list-item> <jats:list-item><jats:p>Using chronic <jats:italic>in vivo</jats:italic> two‐photon microscopy techniques in adult transgenic mice with SCI, we show that infiltrating and resident myelomonocytic cells have differential spatiotemporal distribution patterns to injury sites.</jats:p></jats:list-item> <jats:list-item><jats:p>We also show that infiltrating myelomonocytic cells are associated with the collapse of certain cut axon terminals thus potentially impeding recovery, whereas resident myelomonocytic cells clear axon debris, which may be important for axon regrowth and recovery.</jats:p></jats:list-item> <jats:list-item><jats:p>These results set a framework to understand the roles of different subpopulations of myelomonocytic cells in SCI, and may be important for the development of therapies that target specific immune cell populations at precise times post‐injury.</jats:p></jats:list-item> </jats:list></jats:p><jats:p><jats:bold>Abstract </jats:bold> After spinal cord injury (SCI), resident and peripheral myelomonocytic cells are recruited to the injury site and play a role in injury progression. These cells are important for clearing cellular debris, and can modulate the retraction and growth of axons <jats:italic>in vitro</jats:italic>. However, their precise spatiotemporal recruitment dynamics is unknown, and their respective roles after SCI remain heavily debated. Using chronic, quantitative intravital two‐photon microscopy of adult mice with SCI, here we show that infiltrating lysozyme M (LysM(+)) and resident CD11c(+) myelomonocytic cells have distinct spatiotemporal recruitment profiles, and exhibit changes in morphology, motility, phagocytic activity and axon interaction patterns over time. This study provides the first <jats:italic>in vivo</jats:italic> description of the influx of inflammatory and resident myelomonocytic cells into the injured spinal cord and their interactions with cut axons, and underscores the importance of precise timing and targeting of specific cell populations in developing therapies for SCI.</jats:p>