M2a macrophages identified as a potential therapeutic candidate for intervertebral disc herniation regression

Lumbar disc herniation (LDH) is one of the most common spine pathologies and its spontaneous regression has been attributed to the activation of immune cells by inflammatory processes and extracellular matrix remodelling. This study aims to identify a macrophage candidate with potential for physiological treatment of LDH.

In vitro co-culture model of phagocytosis using human surgical hernia explants and monocyte-derived macrophages. Lumbar disc hernia tissues were collected from patients who underwent discectomy for LDH and co-cultured in transwell assays with M0, M1, M2a and M2d macrophages derived from blood monocytes. The macrophage phenotypes and phagocytic activity were analyzed by flow cytometry using pHrodo™ BioParticles™ conjugates and immunostaining for classical M1 and M2 markers. Explants were analyzed histologically with Alcian blue/Sirius red.

A total of 21 samples were included, patient's average age was 48.8 ​± ​12.9 years and the most common levels were L4-5 (42.9 ​%) and L5-S1 (33.3 ​%). After three days co-culture with monocyte-derived macrophages, explants maintained the structure and high proteoglycans/collagen ratio. Flow cytometry analysis revealed that the explants modified the immune profile of macrophages, increasing the proportion of pro-inflammatory CD86⁺ cells, while reducing the percentage of anti-inflammatory CD163⁺ cells. Additionally, the M2a phenotype presented larger cells and had the highest phagocytic profile towards the explants.

M2a macrophages showed the highest potential towards the design of a macrophage-based therapy for LDH. This study holds great promise for translational medicine aimed at accelerating spontaneous LDH using a physiological treatment that could prevent need for surgical intervention.