Myostatin and CXCL11 promote nervous tissue macrophages to maintain osteoarthritis pain.

Pain is the most debilitating symptom of knee osteoarthritis (OA) that can even persist after total knee replacement. The severity and duration of pain do not correlate well with joint tissue alterations, suggesting other mechanisms may drive pain persistence in OA. Previous work identified that macrophages accumulate in the dorsal root ganglia (DRG) containing the somas of sensory neurons innervating the injured knee joint in a mouse OA model and acquire a M1-like phenotype to maintain pain. Here we aimed to unravel the mechanisms that govern DRG macrophage accumulation and programming. The accumulation of F4/80+iNOS+ (M1-like) DRG macrophages was detectable at day 3 after mono-iodoacetate (MIA)-induced OA in the mouse. Depletion of macrophages prior to induction of OA resolved pain-like behaviors by day 7 without affecting the initial development of pain-like behaviors. Analysis of DRG transcript identified CXCL11 and myostatin. CXCL11 and myostatin were increased at 3 weeks post OA induction, with CXCL11 expression partially localized in satellite glial cells and myostatin in sensory neurons. Blocking CXCL11 or myostatin prevented the persistence of OA pain, without affecting the initiation of pain. CXCL11 neutralization reduced the number of total and F4/80+iNOS+ DRG macrophages, whilst myostatin inhibition diminished the programming of F4/80+iNOS+ DRG macrophages. Intrathecal injection of recombinant CXCL11 did not induce pain-associated behaviors. In contrast, intrathecal myostatin increased the number of F4/80+iNOS+ DRG macrophages concurrent with the development of mechanical hypersensitivity that was prevented by macrophages depletion or CXCL11 blockade. Finally, myostatin inhibition during established OA, resolved pain and F4/80+iNOS+ macrophage accumulation in the DRG. In conclusion, DRG macrophages maintain OA pain, but are not required for the induction of OA pain. Myostatin is a key ligand in neuro-immune communication that drives the persistence of pain in OA through nervous tissue macrophages and represent a novel therapeutic target for the treatment of OA pain.

NLRP3 inflammasome activation in sensory neurons promotes chronic inflammatory and osteoarthritis pain.

Pain is one of the most debilitating symptoms in rheumatic diseases. Pain often persists after total knee replacement in osteoarthritis, or when inflammation is minimal/absent in rheumatoid arthritis. This suggests that pain transitions to a chronic state independent of the original damage/inflammation. Mitochondrial dysfunction in the nervous system promotes chronic pain and is linked to NLRP3 inflammasome activation. Therefore, we investigated the role of mitochondrial dysfunction and NLRP3 inflammasome activation in the transition from acute to persistent inflammation-induced nociplastic pain and in persistent monoiodoacetate-induced osteoarthritis pain. Intraplantar injection of carrageenan in mice induced transient inflammatory pain that resolved within 7 days. A subsequent intraplantar PGE2 injection induced persistent mechanical hypersensitivity, while in naive mice it resolved within one day. Thus, this initial transient inflammation induced maladaptive nociceptor neuroplasticity, so-called hyperalgesic priming. At Day 7, when mice were primed, expression of NLRP3 inflammasome pathway components was increased, and dorsal root ganglia (DRG) neurons displayed signs of activated NLRP3 inflammasome. Inhibition of NLRP3 inflammasome with MCC950 prevented the transition from acute to chronic pain in this hyperalgesic priming model. In mice with persistent monoiodoacetate-induced osteoarthritis pain, DRG neurons displayed signs of mitochondrial oxidative stress and NLRP3 inflammasome activation. Blocking NLRP3 inflammasome activity attenuated established osteoarthritis pain. In males, NLPR3 inhibition had longer-lasting effects than in females. Overall, these data suggest that NLRP3 inflammasome activation in sensory neurons, potentially caused by neuronal oxidative stress, promotes development of persistent inflammatory and osteoarthritis pain. Therefore, targeting NLRP3 inflammasome pathway may be a promising approach to treat chronic pain.

Macrophages transfer mitochondria to sensory neurons to resolve inflammatory pain.

The current paradigm is that inflammatory pain passively resolves following the cessation of inflammation. Yet, in a substantial proportion of patients with inflammatory diseases, resolution of inflammation is not sufficient to resolve pain, resulting in chronic pain. Mechanistic insight into how inflammatory pain is resolved is lacking. Here, we show that macrophages actively control resolution of inflammatory pain remotely from the site of inflammation by transferring mitochondria to sensory neurons. During resolution of inflammatory pain in mice, M2-like macrophages infiltrate the dorsal root ganglia that contain the somata of sensory neurons, concurrent with the recovery of oxidative phosphorylation in sensory neurons. The resolution of pain and the transfer of mitochondria requires expression of CD200 receptor (CD200R) on macrophages and the non-canonical CD200R-ligand iSec1 on sensory neurons. Our data reveal a novel mechanism for active resolution of inflammatory pain.

Dorsal Root Ganglia Macrophages Maintain Osteoarthritis Pain.

Pain is the major debilitating symptom of osteoarthritis (OA), which is difficult to treat. In OA patients joint tissue damage only poorly associates with pain, indicating other mechanisms contribute to OA pain. Immune cells regulate the sensory system, but little is known about the involvement of immune cells in OA pain. Here, we report that macrophages accumulate in the dorsal root ganglia (DRG) distant from the site of injury in two rodent models of OA. DRG macrophages acquired an M1-like phenotype, and depletion of DRG macrophages resolved OA pain in male and female mice. Sensory neurons innervating the damaged knee joint shape DRG macrophages into an M1-like phenotype. Persisting OA pain, accumulation of DRG macrophages, and programming of DRG macrophages into an M1-like phenotype were independent of Nav1.8 nociceptors. Inhibition of M1-like macrophages in the DRG by intrathecal injection of an IL4-IL10 fusion protein or M2-like macrophages resolved persistent OA pain. In conclusion, these findings reveal a crucial role for macrophages in maintaining OA pain independent of the joint damage and suggest a new direction to treat OA pain.SIGNIFICANCE STATEMENT In OA patients pain poorly correlates with joint tissue changes indicating mechanisms other than only tissue damage that cause pain in OA. We identified that DRG containing the somata of sensory neurons innervating the damaged knee are infiltrated with macrophages that are shaped into an M1-like phenotype by sensory neurons. We show that these DRG macrophages actively maintain OA pain remotely and independent of joint damage. The phenotype of these macrophages is crucial for a pain-promoting role. Targeting the phenotype of DRG macrophages with either M2-like macrophages or a cytokine fusion protein that skews macrophages into an M2-like phenotype resolves OA pain. Our work reveals a mechanism that contributes to the maintenance of OA pain distant from the affected knee joint and suggests that dorsal root ganglia macrophages are a target to treat osteoarthritis chronic pain.