Transient efferocytosis-induced activation of IKKβ reprograms macrophages to promote tissue resolution
The clearance of apoptotic cells by macrophages, termed efferocytosis, reprograms macrophages to a resolution/repair phenotype, and pathologic defects in efferocytosis drive many chronic inflammatory diseases. Previous studies have elucidated numerous downstream pro-resolving pathways activated by efferocytosis, but whether there exists a common upstream trigger of these pathways remains unknown. Here, we report that efferocytosing macrophages surprisingly use a signaling module typically associated with inflammation to carry out this key initiating role in tissue resolution. The binding of apoptotic cells to the MerTK receptor triggers a rapid and transient activation of inhibitor of nuclear factor (NF) kappa-B kinase subunit beta (IKK{beta}), leading to NF{kappa}B and p38-signal transducer and activator of transcription 3 (STAT3) signaling and then activation of several key downstream pro-resolving pathways, including interleukin-10 (IL-10) production, continuing efferocytosis, and regulatory T (Treg) cell expansion. The upstream IKK{beta} pathway and the downstream resolution pathways are linked through several intermediary molecules, including the transcription factor Myc, the epigenetic modifier ten-eleven translocation-2 (TET2), and the immune checkpoint protein programmed cell death ligand 1 (PD-L1). Deletion of macrophage IKK{beta} in vivo blocks the above resolution pathways and compromises tissue repair in two efferocytosis-mediated repair settings: resolution of thymic injury after dexamethasone-induced thymocyte apoptosis; and, most importantly, atherosclerosis regression induced by low-density lipoprotein (LDL)-lowering, which is highly relevant to the prevention of cardiovascular disease in humans. These findings illustrate the existence of a unifying upstream signal for efferocytosis-induced resolution, which could suggest new therapeutic strategies to enhance multiple tissue resolution pathways and to optimize anti-inflammatory therapies by avoiding blocking IKK{beta}-NF{kappa}B/p38-mediated resolution.