Sex-specific divergences in the types and timing of infiltrating immune cells during the intervertebral disc acute injury response and their associations with degeneration

Objective: Inadequate repair of the intervertebral disc (IVD) contributes to low back pain. Infiltrating immune cells into damaged tissues are critical mediators of repair, yet little is known about the identities, roles, and temporal regulation following IVD injury. By analyzing transcripts of immune cell markers, histopathologic analysis, immunofluorescence, and flow cytometry, we aimed to define the temporal cascade of infiltrating immune cells and their associations with IVD degeneration.

Methods: Caudal IVDs from 12-week-old C57BL6/J mice were injured and monitored for 42 days post-injury. Transcriptional markers identifying myeloid, B, and T immune cells, and angiogenic factors were measured from the IVDs every 2-3 days. Histopathologic degeneration of the IVD was measured throughout. Flow cytometry and immunofluorescence were used to identify and localize cells including B, T, natural killer T (NKT) cells, monocytes, neutrophils, macrophages, eosinophils, and dendritic cells.

Results: The injured IVD revealed distinct phases of inflammation and proliferation. Robust temporal oscillation in the myeloid and T cell transcripts was observed in females. Cd3+ T cells were more abundant in females than in males. The Cd3+Cd4-Cd8- T cells that dominate the female cascade contain rare γδ T cells. Injury-mediated degeneration was prevalent in both sexes but more severe in males.

Conclusions: This study defines the coordinated infiltration of immune cells in the IVD following injury. We report the discovery of γδ T cells in the female IVD, and this was associated with less severe degeneration. γδ T cells have potent anti-inflammatory roles and may suppress degeneration following IVD injury.