Proteoglycan-4 is an Essential Regulator of Synovial Macrophage Polarization and Inflammatory Macrophage Joint Infiltration

Authors

Marwa Qadri, Chapman UniversityFollow
Gregory D. Jay, Rhode Island Hospital
Ling X. Zhang, Rhode Island Hospital
Tannin A. Schmidt, University of Connecticut
Jennifer Totonchy, Chapman UniversityFollow
Khaled A. Elsaid, Chapman UniversityFollow

Document Type

Article

Publication Date

9-14-2021

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 (Prg4^GT/GT) murine knee joints.

Methods

Macrophage phenotyping in Prg4^GT/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 Prg4^_GT/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 Prg4^GT/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 Prg4^GT/GT animals.

Comments

This article was originally published in Arthritis Research & Therapy, volume 23, in 2021. https://doi.org/10.1186/s13075-021-02621-9

Recommended Citation

Qadri, M., Jay, G.D., Zhang, L.X. et al. Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint infiltration. Arthritis Res Ther 23, 241 (2021). https://doi.org/10.1186/s13075-021-02621-9

Copyright

The authors

Creative Commons License

This work is licensed under a Creative Commons Attribution 4.0 License.