Targeted Deletion of CD11b+ Macrophage Subtypes alter the Acute Inflammatory Cytokine Response In Bone Healing

Abstract

Abstract Any immunological or inflammatory dysregulation can impair the bone healing process. Macrophages (MØ) are immune cells specialized for phagocytosis and assist in all stages of bone repair. MØs are often divided into two main subsets, MØ1, pro-inflammatory M1 macrophages and MØ2, anti-inflammatory or resolving M2 macrophages. Our preliminary data demonstrates that MØ1 to MØ2 polarization occurs following femur fracture during the first 7 days post-fracture and is associated with a specific cytokine expression profile over time. Other studies have demonstrated that dysregulated inflammation can impair fracture healing outcomes in normal mice. We hypothesize that targeted depletion of CD11b+ MØ cells will reduce the acute inflammatory response, alter the normal cytokine expression profile, and interfere with the bone healing response. Therefore, the bone marrow of mice depleted of CD11b+ MØ cells were compared to normal mice following femur fracture at days 0, 1, 2, and 4 using established methods. MØ subtypes were identified using flow cytometry and cytokine expression was quantified using Luminex. The data demonstrated that after fracture, the CD11b+ MØ depleted group had significantly less MØ1s, F480+/CD86+/MHC classII+/CD11b+ cells when compared to controls at all timepoints. MØ cytokine expression profiles were found to be altered in the control fractures when compared to the CD11b+ MØ depleted fractures. Our results demonstrate that MØ subsets contribute substantially to the signaling mechanisms that guide the acute inflammatory phase of bone healing. Proper modulation of macrophage polarization may be useful to develop novel treatment strategies to improve healing when the immune system or inflammation is dysregulated.