Proteoglycan-4 is an essential regulator of synovial macrophage polarization and inflammatory macrophage joint
Marwa Qadri1, Gregory D Jay2, Ling X Zhang2
1Department of Pharmacology, College of Pharmacy, Jazan University, Jazan, 82826, Kingdom of Saudi Arabia.
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.
Insights
Proteoglycan-4 (PRG4) absence leads to increased M1 macrophages and synovial fibrosis. Macrophage depletion reduces synovial hyperplasia and fibrosis markers, indicating macrophages mediate fibrosis in PRG4-deficient joints.
Area of Science:
- Immunology
- Cell Biology
- Biochemistry
Background:
- Synovial macrophages are crucial for joint homeostasis, immune surveillance, and inflammation resolution.
- Proteoglycan-4 (PRG4) influences synovial hyperplasia and fibrosis, but its role in macrophage regulation is unclear.
- This study investigates PRG4's impact on synovial macrophage populations and their role in fibrosis.
Purpose of the Study:
- To determine PRG4's role in synovial macrophage homeostasis.
- To investigate PRG4's effect on pro-inflammatory macrophage infiltration in synovitis.
- To assess if macrophages mediate synovial fibrosis in Prg4-deficient mice.
Main Methods:
- Macrophage phenotyping using flow cytometry (CD11b, F4/80, CD86, CD206) in Prg4 gene-trap (Prg4GT/GT) and wild-type (Prg4+/+) mice.
- Analysis of inflammatory (IL-6, iNOS) and fibrotic markers (α-SMA, PLOD2, COL-I) via immunohistochemistry.
- Macrophage depletion using liposomal clodronate.
Main Results:
- Prg4GT/GT joints showed increased total macrophages and a higher proportion of M1-like (CD86+) macrophages.
- PRG4 re-expression normalized macrophage populations, reducing M1-like cells.
- Macrophage depletion significantly reduced synovial hyperplasia and expression of fibrotic markers.
Conclusions:
- PRG4 is essential for maintaining synovial macrophage balance, preventing M1 macrophage accumulation.
- Macrophages play a critical role in mediating synovial fibrosis in the absence of PRG4.
- Targeting macrophages may be a therapeutic strategy for PRG4-related synovial fibrosis.
More Related Videos
09:18Author Spotlight: Isolation and Culture of Primary Synovial Macrophages and Fibroblasts from Murine Arthritis Tissue
Published on: February 24, 2023
07:15In vivo Macrophage Imaging Using MR Targeted Contrast Agent for Longitudinal Evaluation of Septic Arthritis
Published on: October 20, 2013
Related Concept Videos
01:05Proteoglycans
01:24T Cell Types and Functions
Th1 cells stimulate dendritic cells to express necessary co-stimulatory molecules on their surfaces for...
01:28Inflammatory Response
Inflammation can be triggered by various stimuli, such as impact, abrasion, chemical irritation, infections, and extreme hot or cold temperatures. These can damage cells and connective tissue fibers,...
01:23Glycosaminoglycans
GAGS are found in the extracellular matrix of vertebrates, invertebrates, and bacteria. Due to their polar nature they attract water, and serve as excellent lubricants or shock absorbers in an animal body.
Hyaluronic...
01:24Inflammatory Response II: Inflammatory Exudate and Tissue Repair
The typical wound exudate is odorless, transparent, straw-colored, thin, and watery. Exudate, however, can differ depending on the state of wound healing. Likewise, the...
01:21Matrix Proteoglycans and Glycoproteins