MerTK Expression and ERK Activation Are Essential for the Functional Maturation of Osteopontin‐Producing Reparative Macrophages After Myocardial Infarction

BACKGROUND: We previously reported that osteopontin plays an essential role in accelerating both reparative fibrosis and clearance of dead cells (efferocytosis) during tissue repair after myocardial infarction (MI) and galectin‐3(hi)CD206(+) macrophages is the main source of osteopontin in post‐MI heart. Interleukin‐10– STAT3 (signal transducer and activator of transcription 3)–galectin‐3 axis is essential for Spp1 (encoding osteopontin) transcriptional activation in cardiac macrophages after MI. Here, we investigated the more detailed mechanism responsible for functional maturation of osteopontin‐producing macrophages. METHODS AND RESULTS: In post‐MI hearts, Spp1 transcriptional activation occurred almost exclusively in MerTK (Mer tyrosine kinase)(+) galectin‐3(hi) macrophages. The induction of MerTK on galectin‐3(hi) macrophages is essential for their functional maturation including efferocytosis and Spp1 transcriptional activity. MerTK(+)galectin‐3(hi) macrophages showed a strong activation of both STAT3 and ERK (extracellular signal‐regulated kinase). STAT3 inhibition suppressed the differentiation of osteopontin‐producing MerTK(+)galectin‐3(hi) macrophages, however, STAT3 activation was insufficient at inducing Spp1 transcriptional activity. ERK inhibition suppressed Spp1 transcriptional activation without affecting MerTK or galectin‐3 expression. Concomitant activation of ERK is required for transcriptional activation of Spp1. In Il‐10 knockout enhanced green fluorescent protein–Spp1 knock‐in mice subjected to MI, osteopontin‐producing macrophages decreased but did not disappear entirely. Interleukin‐10 and macrophage colony‐stimulating factor synergistically activated STAT3 and ERK and promoted the differentiation of osteopontin‐producing MerTK(+)galectin‐3(hi) macrophages in bone marrow–derived macrophages. Coadministration of anti‐interleukin‐10 plus anti–macrophage colony‐stimulating factor antibodies substantially reduced the number of osteopontin‐producing macrophages by more than anti–interleukin‐10 antibody alone in post‐MI hearts. CONCLUSIONS: Interleukin‐10 and macrophage colony‐stimulating factor act synergistically to activate STAT3 and ERK in cardiac macrophages, which in turn upregulate the expression of galectin‐3 and MerTK, leading to the functional maturation of osteopontin‐producing macrophages.