Title: Evaluation of Inflammatory Biological Drivers and the Role of the Innate Immune Response During Intervertebral Disc Degeneration
Description: Lower back pain is the leading cause of disability and is thought to be driven primarily by intervertebral disc degeneration (DD) [1]. Studies suggest DD is associated with increases in inflammatory and catabolic signaling and is characterized by a loss of structural integrity [2, 3]. These degenerative changes ultimately compromise disc mechanics and produce a loss of pressurization. Prior studies have identified increases in pro-inflammatory signaling molecules (IL1β, TNFα, HMGB1) during DD [4-6]. Furthermore, the presence of infiltrating immune cells, such as monocyte/macrophages have also been observed in injured and degenerated discs [7, 8]. Though an increased inflammatory signaling microenvironment is thought to be a hallmark of DD, studies have yet to identify if inflammatory signaling alone is capable of driving degeneration. Further complicating this, the complex mechanical environment and the immune privileged nature of the IVD has left unanswered questions regarding the role that innate immune response plays in propagating disease pathology.

In following studies, we evaluated the inflammatory signaling milieu produced by needle puncture injury. Within this study we utilized a connective tissue specific genetic knockout model of a primary inflammatory candidate following injury, the potent damage associated molecular pattern, HMGB1. We identified regional activation of HMGB1 to have roles in tissue structure homeostatic changes and recruiting innate immune cells to the disc following tissue damage. To next answer whether inflammatory biological factors alone are capable of initiating DD, we utilized a connective tissue specific genetic mouse model. In this broad approach of producing an inflammatory microenvironment we identified how prolonged activation of NF-κB, a master transcription factor regulator of inflammatory responses and immune cell recruitment, affects disc integrity. In vivo analyses of the inflammatory disc microenvironment revealed that NF-κB over-activation within IVD cells produced severe degeneration, possibly initiated by an increase in chemotactic proteins and recruitment of inflammatory macrophages.

Lastly, directed by findings of significant monocyte/macrophage infiltration following NF-κB over-activation and tissue damage, we examined the response of macrophages to the mechanical hydrostatic pressure (HP) loading present in the IVD microenvironment [9]. Using a novel bioreactor system, we observed macrophages to be mechanoresponsive to physiologically relevant HP loading magnitudes via activation of an inflammatory resolving functional state. We characterized this HP activated macrophage by distinct transcriptome profile changes, increased anti-inflammatory cytokine release, and phagocytic activity. These findings reveal IVD homeostatic and inflammatory functions primarily mediated by HMGB1, both basally and following injury. Further, findings provide evidence that NF-κB signaling is capable of producing severe DD in the absence of a physical injurious initiating event. Within inflammatory over-activation and puncture injury models, we have identified multiple avenues, dictated by inflammatory signaling or tissue damage, in which innate immune cells are recruited to the IVD.

Lastly, using a novel HP bioreactor system we have characterized an inflammatory resolving functional macrophage activated via healthy HP loading magnitudes. These findings suggest that a loss of pressurization within the disc may contribute to a lack of inflammatory resolution and frustrated healing.