Macrophages Do Not Represent the Main Cell Type in Sequestrated and Extruded Intervertebral Discs: Evidence for Their Involvement in Disc Resorption, Rather than Initiation of an Immune Response

Abstract

Introduction Herniated disc tissue have been reported to be infiltrated predominately by macrophages.1,2 Number of macrophages in herniated disc tissue was found to be dependent on type of disc herniation. In more detail, sequestrated discs have been observed to be infiltrated stronger by macrophages as compared with subligamentous extrusions. Infiltration of herniated disc tissue with macrophages is thought to be due to breakage of nucleus pulposus through the outer layers of the annulus fibrosus and/or posterior longitudinal ligament and hence treatment of this part of the intervertebral disc as foreign. Initiation of an immune response has been assumed to be the main function of macrophages in association with disc herniation.3,4 Since macrophages are not able to activate naïve CD4+T cells, this type of antigen-presenting cell (APC) is probably not involved in initiation of an immune response directed toward herniated nucleus pulposus. Plasmacytoid dendritic cells (PDCs) have been characterized as an additional APC, but unique in activating naïve CD4+T cells.5 Hence, PDCs are predisposed to initiate an immune response directed toward herniated nucleus pulposus. The aim of the present study was to assess the importance of macrophages and plasmacytoid dendritic cells for initiation of an immune response by examining numbers of these cell types and memory T cells in disc infiltrates of sequestrated and extruded discs. Materials and Methods Fourteen patients with sequestrated discs and four patients with extruded discs underwent surgery after their neurological examinations. Disc material was harvested, weighted and digested. Separated single cells were counted, stained for plasmacytoid dendritic cells (CD123+CD4+), macrophages (CD14+CD11c+) and memory T cells (CD4+CD45RO+) and analyzed by flow cytometry. Patients having sequestrated discs were compared with patients displaying subligamentous extrusions in cells proportions, age, preoperative duration of radicular pain and severity of muscle weakness. Results Patients with sequestrated discs were significantly older than the ones displaying subligamentous extrusions ( p< 0.02). There was a trend in patients with sequestrated discs of having more frequently severe muscle weakness than patients with extruded discs ( p< 0.14). Both subgroups did exhibit a similar preoperative duration of radicular pain ( p < 0.48). While patients with sequestrated discs did not differ from patients having subligamentous extrusions in their cell proportions, proportions of plasmacytoid CD123+CD4+dendritic cells did predominate over proportions of CD14+CD11c+macrophages ( p< 0.007) and proportions of memory CD4+CD45RO+T cells ( p< 0.026) in disc infiltrates of both sequestrated and extruded discs. Conclusion The current study did characterize disc infiltrates of both sequestrated and extruded discs as comprising predominately plasmacytoid dendritic cells along with few macrophages and memory T cells. This finding provides evidence for manifestation of an immune response after penetration of nucleus pulposus and/or other disc tissue though the outer layers of the annulus fibrosus and/or posterior longitudinal ligament. Coexistence of plasmacytoid dendritic cells with memory T cells in disc infiltrates of both sequestrated and extruded discs characterizes this immune response as a stage between an innate and an adaptive immune response. Detection of plasmacytoid dendritic cells and macrophages in high and low proportions, respectively in disc infiltrates is indicative of their different functions within manifestation of an immune response. Predominance of plasmacytoid dendritic cells in disc infiltrates points to involvement of this cell type in initiation of an immune response, while macrophages may mediate disc resorption at a later stage of the manifested immune response. Disc herniation has been considered to be preceded by disc degeneration. Disc degeneration is caused, in part, by vascularisation of the disc.6 Accordingly, it is conceivable that plasmacytoid dendritic cells are also involved in initiation of immune response in association with disc degeneration.1,7 Disclosure of Interest None declared References Shamji MF, Setton LA, Jarvis W, et al. Proinflammatory cytokine expression profile in degenerated and herniated human intervertebral disc tissues. Arthritis Rheum 2010;62(7):1974–1982 Virri J, Grönblad M, Seitsalo S, Habtemariam A, Kääpä E, Karaharju E. Comparison of the prevalence of inflammatory cells in subtypes of disc herniations and associations with straight leg raising. 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