Chondrocyte innate immune myeloid differentiation factor 88–dependent signaling drives procatabolic effects of the endogenous toll‐like receptor 2/toll‐like receptor 4 ligands low molecular weight hyaluronan and high mobility group box chromosomal protein 1 in mice

Abstract

Objective Toll‐like receptor 2 (TLR‐2)/TLR‐4–mediated innate immunity serves as a frontline antimicrobial host defense, but also modulates tissue remodeling and repair responses to endogenous ligands released during low‐grade inflammation. We undertook the present study to assess whether the endogenous TLR‐2/TLR‐4 ligands low molecular weight hyaluronan (LMW‐HA) and high mobility group box chromosomal protein 1 (HMGB‐1), which are increased in osteoarthritic (OA) joints, drive procatabolic chondrocyte responses dependent on TLR‐2 and TLR‐4 signaling through the cytosolic adaptor myeloid differentiation factor 88 (MyD88). Methods We studied mature femoral head cap cartilage explants and immature primary knee articular chondrocytes from TLR‐2/TLR‐4–double‐knockout, MyD88‐knockout, and congenic wild‐type mice. Generation of nitric oxide (NO), degradation of hyaluronan, release of HMGB‐1, matrix metalloproteinase 3 (MMP‐3), and MMP‐13, and protein expression of type X collagen were assessed by Griess reaction and Western blotting analyses. Expression of messenger RNA for type II and type X collagen, MMP‐13, and RUNX‐2 was examined by real‐time quantitative reverse transcription–polymerase chain reaction. Results Interleukin‐1β and TLR‐2 and TLR‐4 ligands induced both HMGB‐1 release from chondrocytes and extracellular LMW‐HA generation in normal chondrocytes. TLR‐2/TLR‐4^−/− and MyD88^−/− mouse cartilage explants and chondrocytes lost the capacity to mount procatabolic responses to both LMW‐HA and HMGB‐1, demonstrated by >95% suppression of NO production (P < 0.01), and attenuated induction of MMP‐3 and MMP‐13. Combined deficiency of TLR‐2/TLR‐4, or of MyD88 alone, also attenuated release of NO and blunted induction of MMP‐3 and MMP‐13 release. MyD88 was necessary for HMGB‐1 and hyaluronidase 2 (which generates LMW‐HA) to induce chondrocyte hypertrophy, which is implicated in OA progression. Conclusion MyD88‐dependent TLR‐2/TLR‐4 signaling is essential for procatabolic responses to LMW‐HA and HMGB‐1, and MyD88 drives chondrocyte hypertrophy. Therefore, LMW‐HA and HMGB‐1 act as innate immune cytokine‐like signals with the potential to modulate chondrocyte differentiation and function in OA progression.