Platelet-instructed SPP1+ macrophages drive myofibroblast activation in fibrosis in a CXCL4 dependent Manner

Fibrosis represents the common end-stage of chronic organ injury independent of the initial insult, destroying tissue architecture and driving organ failure. Here we discover a population of profibrotic macrophages marked by expression of Spp1, Fn1 and Arg1 (termed Spp1 macrophages), which expands after organ injury. Using an unbiased approach, we identified the chemokine (C-X-C motif) ligand 4 (CXCL4) to be among the top upregulated genes during profibrotic Spp1 macrophage differentiation. In vitro and in vivo studies demonstrated that loss of Cxcl4 abrogates profibrotic Spp1 macrophage differentiation and ameliorates fibrosis after both heart and kidney injury. Moreover, we uncovered that platelets, the most abundant source of CXCL4 in vivo, drive profibrotic Spp1 macrophage differentiation. Single nuclear RNA sequencing with ligand receptor interaction analysis revealed that macrophages orchestrate fibroblast activation via Spp1, Fn1 and Sema3 crosstalk. Finally, we confirmed that Spp1 macrophages expand in both human chronic kidney disease and heart failure.