Macrophage polarization and macrophage-related factor expression in hypertrophy of the ligamentum flavum.

Purpose: Owing to the unknow types of infiltrating macrophages and the corresponded factors, we aimed to investigate the specific types of infiltrating macrophages involved in HLF and the expression of macrophage-related factors.

Methods: The ligamentum flavum was obtained from patients with lumbar spinal stenosis (HLF group; n = 15) and lumbar disc herniation (non-hypertrophic ligamentum flavum [NLF] group; n = 15). Ligamentum flavum specimens were paraffin embedded, followed by histological and immunohistochemical staining to identify the macrophage type and expression of macrophage-related factors.

Results: The HLF group demonstrated CD206 marker expression, while the NLF group did not (P < 0.0001; n = 11). CD68 marker was expressed in both groups (P > 0.05; n = 11). CCR7 was not expressed in either group. The expression levels of the extracellular matrix proteins aggrecan (Agg), type I collagen (Coll1), and type II collagen (Coll2) were higher in the HLF group than in the NLF group (P < 0.0001; n = 11). The aging markers p21, p16, and p53 were expressed in the HLF group, but not in the NLF group (P < 0.0001; n = 11). The expression levels of the inflammatory factors TNF-α and IL-1β were higher in the HLF group than in the NLF group (P < 0.0001; n = 11). Similarly, the expression level of the fibrosis factor TGF-β1 was higher in the HLF group than in the NLF group (P < 0.0001; n = 11).

Conclusions: The infiltration of M2 macrophages may be involved in HLF, while involvement of M1 macrophages may only occur early in inflammation. The expression of extracellular matrix proteins and macrophage-related factors was increased. Aging may also be associated with HLF.