Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration

Abstract

Background Synovial macrophages perform a multitude of functions that include clearance of cell debris and foreign bodies, tissue immune surveillance, and resolution of inflammation. The functional diversity of macrophages is enabled by distinct subpopulations that express unique surface markers. Proteoglycan-4 (PRG4) is an important regulator of synovial hyperplasia and fibrotic remodeling, and the involvement of macrophages in PRG4’s synovial role is yet to be defined. Our objectives were to study the PRG4’s importance to macrophage homeostatic regulation in the synovium and infiltration of pro-inflammatory macrophages in acute synovitis and investigate whether macrophages mediated synovial fibrosis in Prg4 gene-trap (Prg4GT/GT) murine knee joints. Methods Macrophage phenotyping in Prg4GT/GT and Prg4+/+ joints was performed by flow cytometry using pan-macrophage markers, e.g., CD11b, F4/80, and surface markers of M1 macrophages (CD86) and M2 macrophages (CD206). Characterizations of the various macrophage subpopulations were performed in 2- and 6-month-old animals. The expression of inflammatory markers, IL-6, and iNOS in macrophages that are CD86+ and/or CD206+ was studied. The impact of Prg4 recombination on synovial macrophage populations of 2- and 6-month-old animals and infiltration of pro-inflammatory macrophages in response to a TLR2 agonist challenge was determined. Macrophages were depleted using liposomal clodronate and synovial membrane thickness, and the expression of fibrotic markers α-SMA, PLOD2, and collagen type I (COL-I) was assessed using immunohistochemistry. Results Total macrophages in Prg4GT/GT joints were higher than Prg4+/+ joints (p<0.0001) at 2 and 6 months, and the percentages of CD86+/CD206− and CD86+/CD206+ macrophages increased in Prg4GT/GT joints at 6 months (p<0.0001), whereas the percentage of CD86−/CD206+ macrophages decreased (p<0.001). CD86+/CD206− and CD86+/CD206+ macrophages expressed iNOS and IL-6 compared to CD86−/CD206+ macrophages (p<0.0001). Prg4 re-expression limited the accumulation of CD86+ macrophages (p<0.05) and increased CD86−/CD206+ macrophages (p<0.001) at 6 months. Prg4 recombination attenuated synovial recruitment of pro-inflammatory macrophages in 2-month-old animals (p<0.001). Clodronate-mediated macrophage depletion reduced synovial hyperplasia, α-SMA, PLOD2, and COL-I expressions in the synovium (p<0.0001). Conclusions PRG4 regulates the accumulation and homeostatic balance of macrophages in the synovium. In its absence, the synovium becomes populated with M1 macrophages. Furthermore, macrophages exert an effector role in synovial fibrosis in Prg4GT/GT animals.

Multi-omics provide evidence for an anti-inflammatory immune signature and metabolic alterations in patients with Long COVID Syndrome – an exploratory study

“…Angiogenin (ANG) is an angiogenic protein also described as anti-bacterial protein secreted by macrophages ( 26 ). Proteoglycan-4 (PRG4) is mainly expressed by fibroblast-like cells but serves as an important regulator of inflammation ( 27 ) and has been described to act as an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration ( 28 ). Biotinidase has been described to be essential for basic macrophage functions.…”

Noncovalent hyaluronan crosslinking by TSG-6: Modulation by heparin, heparan sulfate, and PRG4

“…PRG4 has been shown to bind CD44 ( Al-Sharif et al, 2015 ) (and inhibit HA stimulated signaling ( Sarkar et al, 2019 )), and more recently MMP9 ( Menon et al, 2021 ) (which associates with CD44 as well ( Yu and Stamenkovic, 1999 )). Given PRG4’s anti-inflammatory ( Qadri et al, 2018 ; Das et al, 2019 ; Menon et al, 2021 ; Krawetz et al, 2022 ), immunomodulatory ( Qadri et al, 2021 ; Krawetz et al, 2022 ), and anti-fibrotic properties ( Qadri et al, 2020 ; Krawetz et al, 2022 ), it is intriguing to consider what type of additive, synergistic, or inhibitory biological properties TSG-6 bound PRG4 might have in various tissues and diseases.…”

Recombinant Human Proteoglycan 4 Regulates Phagocytic Activation of Monocytes and Reduces IL-1β Secretion by Urate Crystal Stimulated Gout PBMCs

“…Our in vivo observations should be rationalized in the context of our recent discoveries regarding PRG4’s role in tissue macrophage homeostasis and function. We have recently shown that in Prg4 GT/GT mice, the synovial microenvironment skews the balance of resident macrophages towards a pro-inflammatory M1 phenotype with age-dependent progressive decline in the numbers of anti-inflammatory wound-healing macrophages ( 51 ). Furthermore, macrophages from Prg4 GT/GT animals had reduced anti-inflammatory interleukin-10 (IL-10) expression at baseline and in response to TLR2 ligand stimulation ( 51 ).…”

“…We have recently shown that in Prg4 GT/GT mice, the synovial microenvironment skews the balance of resident macrophages towards a pro-inflammatory M1 phenotype with age-dependent progressive decline in the numbers of anti-inflammatory wound-healing macrophages ( 51 ). Furthermore, macrophages from Prg4 GT/GT animals had reduced anti-inflammatory interleukin-10 (IL-10) expression at baseline and in response to TLR2 ligand stimulation ( 51 ). Extending these findings to our peritoneal model, it is plausible to expect that the impaired expression of IL-10 in Prg4 GT/GT mice contributed to the lack of adequate resolution of inflammation ( 27 ).…”

“…Extending these findings to our peritoneal model, it is plausible to expect that the impaired expression of IL-10 in Prg4 GT/GT mice contributed to the lack of adequate resolution of inflammation ( 27 ). Furthermore, the re-introduction of PRG4 may have blunted the overall pro-inflammatory tissue microenvironment in Prg4 GT/GT animals ( 51 ), therefore allowing for more NCM influx and clearance of neutrophils.…”