Intra-articular 2.5% polyacrylamide hydrogel alters synovial immune pathways in equine experimental osteoarthritis.

Overview

• This study investigated how injecting a 2.5% polyacrylamide hydrogel (2.5% iPAAG) into horse joints with experimentally induced osteoarthritis (OA) affects gene expression in the joint’s synovial tissue and fluid cells.
• The research aimed to understand molecular changes linked to immune responses and other cellular pathways after treatment with this synthetic hydrogel designed to manage OA.

Background and Purpose

• Osteoarthritis is a degenerative joint disease characterized by cartilage damage and inflammation within the joint, including synovial tissue changes.
• Polyacrylamide hydrogel is a biocompatible, nondegradable synthetic polymer that has potential as an intra-articular (IA) treatment to improve joint function and reduce OA symptoms.
• The study’s goal was to determine how treatment with 2.5% iPAAG affects gene expression in synovial tissue and synovial fluid cells, focusing on immune system pathways and other functional signaling networks involved in joint health.

Study Design and Methods

• Sixteen horses aged 2 to 5 years had osteoarthritis surgically induced in one middle carpal joint by creating an osteochondral fragment; the opposite joint underwent a sham operation to serve as control.
• Post-surgery, horses exercised 5 days a week on a high-speed treadmill to simulate mechanical stress relevant to OA progression.
• Fourteen days after OA induction, the affected joint was injected either with 2 mL of 2.5% iPAAG or saline (8 horses per group).
• Synovial tissue biopsies were collected at baseline (day 0) and day 70, while synovial fluid samples were taken periodically (days 0, 14, 28, 42, 56, and 70).
• RNA was extracted from synovium and synovial fluid cells and analyzed via transcriptomic sequencing to determine differences in gene expression and enriched biological pathways.
• The study was conducted from July to October 2024.

Key Findings

• At day 70, transcriptomic analysis of synovial tissue from joints treated with 2.5% iPAAG showed:
• Upregulation (increased activity) of innate immune activation pathways, indicating enhanced early immune responses.
• Downregulation (reduced activity) of metabolic pathways, cell cycle processes, and growth factor signaling, suggesting altered cellular proliferation and metabolic states.
• Synovial fluid cells showed the most significant transcriptomic changes at day 28 (two weeks post-injection), characterized by:
• Downregulation of tumor necrosis factor-alpha (TNF-α) inflammatory pathways, which are commonly involved in OA-associated inflammation.
• Reduced signaling related to T-cells and lymphocyte activation, indicating modulation of adaptive immune responses.
• These expression changes suggest that 2.5% iPAAG influences the joint environment by activating innate immune responses in the synovium yet suppressing key inflammatory pathways in synovial fluid immune cells over time.

Implications and Limitations

• The findings reveal a potential mechanism of action for 2.5% iPAAG where it alters immune and signaling pathways to influence disease progression or symptomatology in OA.
• By integrating into the synovium, 2.5% iPAAG seems to regulate immune activity and cell cycle pathways balancing inflammation and tissue response.
• The study’s model used an acute, surgically induced form of osteoarthritis in horses, which may not fully replicate chronic or naturally occurring disease conditions, limiting direct clinical extrapolation.
• Further studies are needed to assess long-term outcomes, functional effects on joint health, and applicability to other species including humans.

Summary

• Intra-articular administration of 2.5% polyacrylamide hydrogel affects the molecular environment of osteoarthritic joints in horses.
• The treatment modulates immune pathways differently in synovial tissue versus synovial fluid cells, enhancing innate immune responses in tissue while suppressing some inflammatory signals in fluid cells.
• This dual effect could underpin its therapeutic potential for managing osteoarthritis by modulating the joint’s local immune and cellular activity.