HMGB1 mediates macrophage recruitment and regional intervertebral disc tissue functional and mechanical property changes following injury.

ObjectiveFrequently evaluated in musculoskeletal disease, damage associated molecular patterns (DAMPs) respond to tissue damage and cellular stress by facilitating an inflammatory response via macrophage activation and broad inflammatory pathway activation. In the context of disc degeneration (DD), high mobility group box 1 (HMGB1), a potent intracellular DAMP, is seen to be increased within severely degenerated human IVDs and to directly mediate inflammatory responses within disc cells in vitro. To further understand how HMGB1 mediated inflammation influences DD, this study evaluated the possible protective effect of an HMGB1 knockout on DD pathology following injury. MethodsUsing a needle puncture injury model in murine caudal IVDs we evaluated DD pathology within an IVD specific Hmgb1 knockout (KO) model. Structural and compositional changes in IVD cellularity, histopathology, disc height, and biomechanics were evaluated in addition to an assessment of disc inflammation via gene expression and macrophage presence throughout the course of degeneration. ResultsHMGB1 expression robustly increased shortly following needle puncture injury and elevated levels were sustained up to 28-days post injury both in injured IVDs and in the IVDs adjacent to the level of injury. IVD specific Hmgb1 KO mice had an increased disc height following injury both at the injured and adjacent to injury level compared to injured WT IVDs. Hmgb1 KO also protected against tissue mechanical property losses at both the injured (dynamic modulus) and adjacent to injury level (dynamic modulus, creep, and equilibrium modulus) compared to injured WT IVDs, however there was no significant effect on histopathologic scores post injury. Hmgb1 KO resulted in alterations in macrophage (F4/80+) recruitment to the IVD post injury in vivo. A lower macrophage migration was also observed in vitro in response to the secretome of an injured Hmgb1 KO IVD compared to injured WT IVDs. Hmgb1 KO had no effect on inflammatory gene expression changes following injury within adjacent to injury level or injury level IVDs. ConclusionOverall findings indicate that HMGB1 is upregulated regionally, at both the injured level and at the level adjacent to injury. Results suggest that HMGB1 plays a role in mediating structural, biomechanical, and inflammatory responses to IVD injury and serves as a potent chemoattractant, mediating macrophage recruitment to the IVD and overall migratory function.