Abstract
Low back pain is a highly prevalent clinical problem and intervertebral disc (IVD) degeneration is now accepted as the major pathophysiological mechanism responsible for this condition. Accumulating evidence suggests that inflammation plays a crucial role in the progression of human IVD degeneration, with macrophages being pointed as the key immune cell players in this process since their infiltration in degenerated IVD samples has been extensively demonstrated. Since they are highly plastic, macrophages can play different roles depending on the microenvironmental cues. The study of inflammation associated with IVD degeneration has been somehow neglected and one of the reasons is related with lack of adequate models. To overcome this, we established and characterized a new model of IVD organ culture under pro-inflammatory conditions to further dissect the role of macrophages in IVD associated immune response. For that, human monocyte-derived macrophages were co-cultured either with bovine caudal IVD punches in the presence of the pro-inflammatory cytokine IL-1β, or IVD-conditioned medium (CM), to investigate how IVD-produced factors influence macrophage phenotype. After 72 h, metabolic activity, gene expression and cytokine profile of macrophages and IVD cells were measured. Our results show that macrophages and IVDs remain metabolically active in the presence of IL-1β, significantly upregulate CCR7 gene expression and increase production of IL-6 on macrophages. When treating macrophages with IL-1β-IVD-CM, CCR7 upregulation follows the same trend, while for IL-6 an opposite effect was observed. On the other hand, macrophages interfere with IVD ECM remodeling, decreasing MMP3 expression and downregulating aggrecan and collagen II gene expression in the presence of IL-1β. Overall, the co-culture model established in this study can be considered a suitable approach to address the cellular and molecular pathways that regulate macrophage-IVD crosstalk, suggesting that degenerated IVD tissue tends to polarize human macrophages toward a more pro-inflammatory profile, which seems to aggravate IVD degeneration. This model could be used to improve the knowledge of the mechanisms that link IVD degeneration and the immune response.
Highlights
Low back pain (LBP) is a common clinical problem affecting about 70–85% of the world population [1]
We investigated whether macrophage viability would be affected by the proinflammatory/degenerative intervertebral disc (IVD) environment and whether IVDs viability would be compromised by the presence of macrophages
The results showed that mitochondrial metabolic activity of macrophages was not affected when IL-1β was added to the media, but slightly decreased when macrophages were co-cultured with IVD in the absence of pro-inflammatory stimuli
Summary
Low back pain (LBP) is a common clinical problem affecting about 70–85% of the world population [1]. Macrophages were identified in human herniated IVD samples in several studies, associated with increased disc degeneration [5, 7,8,9], but they are suggested to have an important role in the spontaneous hernia regression, a rare event occurring in some LBP patients [10]. These immune cells are implicated in non-herniated IVD degeneration, this topic remains poorly understood [5, 11,12,13]. These apparent controversial results can be explained by the high plasticity of macrophages, that can express different functional profiles in response to distinct environmental cues, from the classic pro-inflammatory M1 to a more pro-regenerative M2 phenotype
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