Macrophage Infiltration Is a Causative Factor for Ligamentum Flavum Hypertrophy through the Activation of Collagen Production in Fibroblasts

Takeyuki Saito, Masamitsu Hara, Hiromi Kumamaru, Kazu Kobayakawa, Kazuya Yokota, Ken Kijima, Shingo Yoshizaki, Katsumi Harimaya, Yoshihiro Matsumoto, Kenichi Kawaguchi, Mitsumasa Hayashida, Yutaka Inagaki, Keiichiro Shiba, Yasuharu Nakashima, Seiji Okada

Ligamentum flavum (LF) hypertrophy causes lumbar spinal canal stenosis, leading to leg pain and disability in activities of daily living in elderly individuals. Although there have been previous studies on LF hypertrophy, its pathomechanisms have not been fully elucidated. In this study, we demonstrated that infiltrating macrophages were a causative factor for LF hypertrophy. Induction of macrophages into the mouse LF by applying a micro-injury resulted in LF hypertrophy along with collagen accumulation and fibroblasts proliferation at the injured site, which were very similar to the characteristics observed in the severely hypertrophied LF of human. However, we found that macrophage depletion by injecting clodronate-containing liposomes counteracted LF hypertrophy even with micro-injury. For identification of fibroblasts in the LF, we utilized collagen type 1 alpha 2 linked to green fluorescent protein (COL1a2-GFP) transgenic mice, and selectively isolated GFP-positive fibroblasts from the micro-injured LF using laser microdissection (LMD). A quantitative RT-PCR on LMD samples showed that the gene expression of collagen markedly increased in the fibroblasts at the injured site with infiltrating macrophages compared to the uninjured location. These results suggested that macrophage infiltration was crucial for LF hypertrophy by stimulating collagen production in fibroblasts, providing better understanding the pathophysiology of LF hypertrophy.