Peripheral nerve injury reduces macrophage efferocytosis to facilitate neuropathic pain

For reasons not fully understood, pro-resolving immune processes sometimes fail to engage after peripheral nerve injury (PNI), leading to enhanced neuropathic pain and inflammation. Here, we implicate reduced efferocytosis due to proteolytic cleavage of surface MER tyrosine kinase (MERTK) from macrophages at the site of PNI. After PNI, the proportion of macrophages expressing MERTK progressively decreased, while soluble (cleaved) MER increased. Using male and female knock-in mice encoding cleavage-resistant Mertk, we demonstrated that cleavage of MERTK from macrophages at the PNI site led to exaggerated pain-related behaviors. PNI-induced hyperactivity of TRPV1+ sensory neurons and damage to myelin and myelinated axons was exacerbated by MERTK cleavage. Cleavage of MERTK led macrophages to adopt a proinflammatory phenotype. It also reduced their efferocytotic capacity, increasing accumulation of TUNEL+ (apoptotic) and RIPK3+ (necroptotic) cells at the injury site. The pronociceptive damage-associated molecular patterns (DAMPs) interleukin-33 and heat shock protein 90 were increased, consistent with passive release from uncleared cell corpses. These corpses can also release de novo antigens along with DAMPs to trigger autoimmunity, recently implicated in neuropathic pain through a mechanism involving secretion of immunoglobulin G (IgG). Indeed, MERTK cleavage led to accumulation of IgG at the injury site and dorsal root ganglia. All outcomes were further worsened when Mertk was conditionally deleted from macrophages. Our findings identify cleavage of MERTK from macrophages at the injury site as a pivotal regulator of pro-nociceptive and tissue-damaging neuroimmune signaling after PNI.