A discrete 'early-responder' stromal-cell subtype orchestrates immunocyte recruitment to injured tissue, thereby promoting regeneration [scRNA-seq]. A discrete 'early-responder' stromal-cell subtype orchestrates immunocyte recruitment to injured tissue, thereby promoting regeneration [scRNA-seq]

Following acute injury, stromal cells promote tissue regeneration by diverse mechanisms. Time-resolved single-cell RNA sequencing of muscle mesenchymal stromal cells (MmSCs) responding to cardiotoxin(CTX)-induced injury identified an “early-responder” subtype that spiked on day 1 and expressed a striking array of transcripts encoding immunomodulators. IL-1b, TNFa and oncostatin M, primarily produced by myeloid cells, each strongly and rapidly induced MmSCs transcribing most of these immunomodulators. Transfer of the inflammatory MmSC subtype, tagged with a unique surface marker, into healthy hindlimb muscle induced inflammation primarily driven by neutrophils and macrophages. Among the abundant inflammatory transcripts produced by this subtype, Cxcl5 was stroma-specific and highly upregulated with injury. Depletion of this chemokine early after injury revealed a non-redundant impact on recruitment of neutrophils, a prolongation of inflammation at later times, and ineffectual tissue regeneration. MmSC subtypes expressing a comparable inflammatory program were found in a mouse model of muscular dystrophy and in several other tissues and pathologies in both mice and humans. These “early-responder” MmSCs, already in place, permit rapid and coordinated mobilization and amplification of critical cell collaborators during tissue regeneration. Overall design: We investigated the transcriptional heterogeneity of flow-cytometrically sorted MmSCs from hindlimb muscles (tibialis anterior, gastrocnemius and quadriceps) of 6- to 8-wk-old male Foxp3GFP mice at homeostasis and at day 0.5, 1, 3, 7 and 14 following CTX-induced injury; 2.5-, 5- and 12.5-wk-old male Dmdmdx mice; or 2.5- and 12.5-wk-old male DBA/2J mice. Cells were encapsulated using the Chromium Single Cell 5′ v3 platform (10x Genomics). For sequencing of MmSCs from Dmdmdx and DBA/2J mice, individual samples were tagged (hashed) with DNA-coded anti-biotin antibodies (Biolegend).