Decreased calcitonin gene-related peptide expression in the dorsal root ganglia of TNF-deficient mice in a monoiodoacetate-induced knee osteoarthritis model

Background: The detailed mechanisms of knee osteoarthritis (OA) pain have not been clarified, but involvement of inflammatory cytokines such as tumor necrosis factor-alpha (TNF) has been suggested. The present study aimed to investigate the more detailed neurological involvement of TNF in joint pain using a TNF-knockout mouse OA model. Methods: The right knees of twelve-week-old C57BL/6J wild and TNF-deficient knockout (TNF-ko) mice (n=15, each group) were given a single intra-articular injection of 10 µg monoiodoacetate in 10 mL sterile saline. The left knees were only punctured as the control. Evaluations were performed immediately after the injection (baseline) and at 7, 14, and 28 days after the injection with a subsequent intra-articular injection of neurotracer into both knees. The animals were evaluated for immunofluorescence of the lumbar dorsal root ganglia (DRG) innervating the knee joints. The injected knees were observed macroscopically and mouse pain-related behaviors were scored. Results: Macroscopic observation showed similar knee OA development in both wild and TNF-ko mice. Calcitonin gene-related peptide (CGRP, a neuropeptide identified as a inflammatory pain-related biomarker) was significantly increased in DRG neurons innervating OA-induced knee joints with significantly less CGRP expression in TNF-ko animals. Pain-related behavior scoring showed a significant increase in pain in OA-induced joints, but there was no significant difference in pain observed between the wild and TNF-ko mice. Conclusions: The result of the present study indicates the possible association of TNF-alpha in OA pain but not OA development.     

Resolvin E1 inhibits dendritic cell migration in the skin and attenuates contact hypersensitivity responses

Resolvin E1 (RvE1) is a lipid mediator derived from ω3 polyunsaturated fatty acids that exerts potent antiinflammatory roles in several murine models. The antiinflammatory mechanism of RvE1 in acquired immune responses has been attributed to attenuation of cytokine production by dendritic cells (DCs). In this study, we newly investigated the effect of RvE1 on DC motility using two-photon microscopy in a contact hypersensitivity (CHS) model and found that RvE1 impaired DC motility in the skin. In addition, RvE1 attenuated T cell priming in the draining lymph nodes and effector T cell activation in the skin, which led to the reduced skin inflammation in CHS. In contrast, leukotriene B4 (LTB4) induced actin filament reorganization in DCs and increased DC motility by activating Cdc42 and Rac1 via BLT1, which was abrogated by RvE1. Collectively, our results suggest that RvE1 attenuates cutaneous acquired immune responses by inhibiting cutaneous DC motility, possibly through LTB4-BLT1 signaling blockade.Following the well-known epidemiological study conducted in Northwest Greenland in the 1970s (Dyerberg et al., 1978), several clinical assessments have indicated that a diet rich in ω3 polyunsaturated fatty acids (PUFAs) has beneficial effects in various inflammatory diseases, including asthma, psoriasis, inflammatory bowel diseases, and rheumatoid arthritis (Horrobin, 1987). Although it remains unclear how ω3 PUFAs exert such antiinflammatory effects, recent studies have identified several derivatives of ω3 PUFAs that possess strong antiinflammatory effects (Serhan et al., 2008; Tull et al., 2009). Resolvin E1 (RvE1) is one such antiinflammatory lipid mediator.RvE1 is known to exert its actions through two receptors, BLT1 and ChemR23 (Arita et al., 2007). RvE1 binds to BLT1, a G protein–coupled receptor for leukotriene B4 (LTB4), and inhibits BLT1 signals (Arita et al., 2007). In addition, RvE1 exhibits an agonistic activity toward ChemR23 (Arita et al., 2007), a G protein–coupled receptor for chemerin. The antiinflammatory effects of RvE1 have been demonstrated in acute innate immune inflammation, such as peritonitis (Arita et al., 2007) and colitis (Arita et al., 2005b). In these models, RvE1 exerted its antiinflammatory effects by inhibiting neutrophil infiltration into the inflammatory foci through a blockade of LTB4-BLT1 signaling in neutrophils (Haas-Stapleton et al., 2007). In contrast, few studies have been conducted on the effect of RvE1 on acquired immune responses, in which DCs and T cells play major roles in the development. In these studies, the attenuated cytokine production, such as IL-12 and IL-23, from DCs is considered as the major mechanism by which RvE1 exerts the antiinflammatory effects (Arita et al., 2005a; Haworth et al., 2008). However, the effect of RvE1 on DC motility has not been investigated in the context of acquired immunity.In the peripheral tissues such as the skin, DCs migrate in an amoeboid movement that requires actin polymerization via activation of the Rho family of small GTPases, such as Cdc42, Rac, and Rho A (Lämmermann and Germain, 2014). In acquired immunity such as contact hypersensitivity (CHS), upon uptake of foreign antigens, DCs migrate to the draining LNs (dLNs) via lymphatic vessels to establish sensitization by inducing the antigen-specific T cell differentiation (Honda et al., 2013). In elicitation, DC migration to form DC–T cell clustering is required for efficient antigen presentation in situ (Natsuaki et al., 2014). Thus, active DC motility is an essential factor for acquired immunity.In this study, we investigated the effects and underlying mechanisms of RvE1 on DC motility using a CHS model, which is a prototype of delayed-type hypersensitivity in the skin mediated by IFN-γ (Mori et al., 2008; Honda et al., 2013). RvE1 inhibited cutaneous DC migration into the dLNs and suppressed antigen-specific T cell induction in the sensitization phase. In addition, live imaging analysis revealed that RvE1 inhibited cutaneous DC motility and cluster formation in the skin, which subsequently attenuated activation of effector T cells in the skin in the elicitation phase of CHS. Intriguingly, LTB4 induced actin filament reorganization in DCs and increased DC motility by activating Cdc42 and Rac1 via BLT1, which was abrogated by RvE1. These results suggest that RvE1 exerts its antiinflammatory effects in cutaneous acquired immunity by inhibiting DC motility, possibly through an LTB4-BLT1 signaling blockade.