Background: Gout is an inflammatory arthritis caused by monosodium urate monohydrate (MSU) crystals' joint deposition. MSU phagocytosis by resident macrophages is a key step in gout pathogenesis. MSU phagocytosis triggers nuclear factor kappa B (NFκB) activation and production of cytokines and chemokines. Proteoglycan-4 (PRG4) is a glycoprotein produced by synovial fibroblasts and exerts an anti-inflammatory effect in the joint mediated by its interaction with cell surface receptor CD44. PRG4 also binds and antagonizes TLR2 and TLR4. The objective of this study is to evaluate the efficacy of recombinant human PRG4 (rhPRG4) in suppressing MSU-induced inflammation and mechanical allodynia in vitro and in vivo.
Methods: THP-1 macrophages were incubated with MSU crystals ± rhPRG4 or bovine submaxillary mucin (BSM), and crystal phagocytosis, cytokines and chemokines expression and production were determined. NFκB p65 subunit nuclear translocation, NLRP3 induction, caspase-1 activation and conversion of proIL-1β to mature IL-1β were studied. MSU phagocytosis by Prg4+/+ and Prg4-/- peritoneal macrophages was determined in the absence or presence of rhPRG4, BSM, anti-CD44, anti-TLR2, anti-TLR4 and isotype control antibodies. Rhodamine-labeled rhPRG4 was incubated with murine macrophages and receptor colocalization studies were performed. Lewis rats underwent intra-articular injection of MSU crystals followed by intra-articular treatment with PBS or rhPRG4. Weight bearing and SF myeloperoxidase activities were determined.
Results: rhPRG4 reduced MSU crystal phagocytosis at 4 h (p < 0.01) and IL-1β, TNF-α, IL-8 and MCP-1 expression and production at 6 h (p < 0.05). BSM did not alter MSU phagocytosis or IL-1β production in human and murine macrophages. rhPRG4 treatment reduced NFκB nuclear translocation, NLRP3 expression, caspase-1 activation and generation of mature IL-1β (p < 0.05). MSU-stimulated IL-1β production was higher in Prg4-/- macrophages compared to Prg4+/+ macrophages (p < 0.001). rhPRG4, anti-CD44, anti-TLR2 and anti-TLR4 antibody treatments reduced MSU phagocytosis and IL-1β production in murine macrophages (p < 0.05). rhPRG4 preferentially colocalized with CD44 on Prg4-/- peritoneal macrophages compared to TLR2 or TLR4 (p < 0.01). rhPRG4 normalized weight bearing and reduced SF myeloperoxidase activity compared to PBS in vivo.
Conclusion: rhPRG4 inhibits MSU crystal phagocytosis and exhibits an anti-inflammatory and anti-nociceptive activity in vitro and in vivo. rhPRG4's anti-inflammatory mechanism may be due to targeting CD44 on macrophages.
Keywords: CD44; Gout; Lubricin; Macrophages; Proteoglycan-4; TLR2; TLR4; Urate.