Polarization of Macrophages in Epidural Inflammation Induced by Canine Intervertebral Disc Herniation

Introduction: Canine interverterbral disc (IVD), although physiologically acellular, displays an inflammatory cell population consisting almost exclusively of macrophages (M phi) when acutely herniated. M phi encompass a heterogenous cell population, roughly divided into classically (M1) or alternatively activated (M2)M phi. Polarization into M1 M phi leads to strong antimicrobial activity and pro-inflammatory response. In contrast, M2M phi exibit anti-inflammatory function and regulate wound healing. The purpose of this study was to characterize the phenotype of the M phi population present in naturally occurring IVD herniation. Materials and Methods: IVD material of dogs with IVD disease was collected during standard decompressive surgery. A negative control consisting of IVD material of dogs without IVD degeneration and a positive control consisting of canine liver and lymph node samples were also included. All samples were embedded in OCT and shock frozen. Eight micrometer cryostat sections were prepared, air dried and immunostained without prefixation or permeabilization. CD14 was used as marker M phi, MHCII for M1M phi and CD206 for M2M phi. Results: Fifteen samples of dogs with IVD herniation, 10 negative, and 5 positive control samples were obtained. No positive cell was found in the negative control group. The positive control group displayed several MHCII and CD206 positive cells, all of them being simultaneously positive to CD14. All herniated samples displayed a mixed population of M1M phi and M2M phi, and some sparse M phi displaying markers for both M1 and M2M phi simultaneously. Conclusion: The mixed phenotype encountered shows the plasticity and dynamism of M phi and evidences the chronic component of IVD disease despite its acute clinical presentation.