The Pivotal Roles of Macrophages and Microglia in Mesenchymal Stromal/Stem Cell-Derived Small Extracellular...

Spinal cord injury (SCI) initiates a complex secondary cascade characterized by disruption of the blood-spinal cord barrier (BSCB), infiltration of peripheral immune cells, and chronic neuroinflammation. Within this response, macrophages and microglia act as key effectors that critically influence both the progression of injury and the subsequent repair processes. Mesenchymal stromal/stem cell-derived small extracellular vesicles (MSC-sEVs) have recently gained recognition as a promising cell-free therapeutic approach that acts through multiple paracrine mechanisms, including but not limited to immunomodulation. This review summarizes current evidence elucidating how macrophages and microglia contribute to the multifaceted therapeutic actions of MSC-sEVs in the context of SCI. Following intravenous administration, MSC-sEVs preferentially localize to the lesion site, where they are internalized by CD206⁺ macrophages. This interaction initiates a multifaceted therapeutic program. First, MSC-sEVs not only reprogram myeloid cells toward an anti-inflammatory, M2-like phenotype but also sustain this reparative state, thereby stabilizing a pro-resolving immune environment, attenuating the production of proinflammatory cytokines such as TNF-α and IL-6, and enhancing the expression of anti-inflammatory mediators, including Interleukin-10 (IL-10) and Transforming growth factor-beta (TGF-β). This polarization is partly driven by transferred microRNAs that suppress central inflammatory signaling hubs, notably the Toll-like receptor 4 (TLR4)/Nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and NOD-like receptor protein 3 (NLRP3) inflammasome pathways. Second, MSC-sEVs augment macrophage phagocytic capacity, facilitating the removal of myelin debris and apoptotic cells and thereby creating a permissive microenvironment for regeneration. Third, soluble factors released from reprogrammed myeloid cells confer neuroprotective and trophic support to neurons and oligodendrocytes, mitigating secondary degeneration. Finally, these cells contribute to the re-establishment of BSCB integrity by promoting tight junction protein re-expression, reconstituting pericyte-endothelial interactions, and enhancing microvascular remodeling. Collectively, these coordinated mechanisms suppress neuroinflammation, preserve neural tissue, and support functional recovery. The therapeutic benefits of MSC-sEVs in SCI, therefore, depend substantially on the reprogramming of macrophages and microglia. Elucidating this bidirectional communication between MSC-sEVs and myeloid cells provides critical insight into SCI pathophysiology and identifies macrophage and microglial modulation as a strategic target for next-generation sEV-based neuroregenerative interventions.