Low-molecular-weight chondroitin sulfate attenuates hyperalgesia via immunomodulation of macrophage polarization.

Osteoarthritis is the most common form of arthritis, for which effective and safer treatments are needed. While chondroitin sulfate (CS) has been reported to exert analgesic effects, its cellular targets and immunomodulatory mechanisms remain unclear. This study aimed to clarify the effects of low-molecular-weight CS-rich extract (LMW-CSE) derived from shark cartilage on the hyperalgesia observed in adjuvant-induced arthritis (AIA) mice and to investigate the role of peripheral macrophage (MΦ) subsets in its analgesic action. LMW-CSE was orally administered to mice at doses up to 300 mg/kg twice daily, starting 14 days before AIA with an injection of complete Freund's adjuvant (CFA), and continuing until the completion of behavioral testing. LMW-CSE significantly attenuated hyperalgesia in AIA mice. In addition, the increase in the number of activated excitatory neurons in the spinal superficial dorsal horn of AIA mice was attenuated by LMW-CSE treatment. Flow cytometric analysis revealed that LMW-CSE suppressed the accumulation of proinflammatory M1 MΦs and increased the proportion of immunoregulatory M2 MΦs at inflammatory sites, accompanied by reduced expression of proinflammatory cytokines. These findings suggest that LMW-CSE exerts anti-hyperalgesic effects, at least in part through modulation of peripheral macrophage subsets, supporting its potential as a therapeutic strategy for inflammatory pain.