Polarization of infiltrating macrophages in the outer annulus fibrosus layer associated with the process of intervertebral disc degeneration and neural ingrowth in the human cervical spine.

Background Context: As no infiltrating macrophages exist in healthy discs, understanding the role of infiltrating macrophages including their polarity (M1 and M2 phenotypes) in intervertebral discs (IVDs) is important in the assessment of the pathomechanisms of disc degeneration.

Purpose: To determine the relationship between infiltrating macrophage polarization and the progression of human cervical IVD degeneration.

Study Design: Histopathological study using harvested human cervical IVDs.

Methods: IVDs collected during anterior cervical decompression from 60 patients were subjected to immunostaining and immunoblotting. The samples were classified as type 0-3 according to the percentage of CD16- and CD206-positive cells to CD68-positive cells in the outer annulus fibrosus layer. The number of vessels and nerve fibers and the severity of chronic inflammation with a focus on inflammatory cell infiltration, fibrosis, and capillary proliferation were also assessed.

Results: The number of CD16-positive cells was the highest in type 2 IVDs, and was suppressed following the infiltration of CD206-positive cells. The degree of chronic inflammation was significantly higher in type 2 and type 3 IVDs, and the number of nerve fibers was significantly higher in type 3 IVDs. The endothelial cells of small vessels were positive for nerve growth factor, brain-derived neurotrophic factor, and neurotrophin-3 expression. Staining for tropomyosin receptor kinase (Trk)-A, Trk-B, and Trk-C was positive in aberrant fibers. In immunoblot analysis, the expression levels of these neurotrophic factors and receptors were significantly higher in type 2 and 3 IVDs.

Conclusions: The polarity of macrophages around newly developed microvasculature might be altered with cervical IVD degeneration. A higher number of infiltrating M1 macrophages around the vessels was associated with chronic inflammation; however, their number got suppressed following the infiltration of M2 macrophages. The expression of neurotrophins in the capillaries of small vessels might contribute to neural ingrowth into degenerated IVDs.

Clinical Significance: Clarifying macrophages polarity change around new microvasculature associated with progression of IVD degeneration could enhance our understanding of the underlying mechanisms of neural ingrowth into degenerated IVDs and lead to development of a novel therapeutic target for prevention of IVD.