Phase-dependent efficacy of intravenous amniotic mesenchymal stem cells in a rat spinal cord injury model

Abstract

Background: Spinal cord injury results in profound neurological disability driven initially by primary mechanical damage and subsequently by secondary injury processes characterized by progressive neuroinflammation. Intravenous administration of human amniotic mesenchymal stem cells (MSC) has emerged as a promising therapeutic approach; however, the optimal timing of administration and its relationship to dynamic immune responses remain unclear. Methods: A rat contusion model of spinal cord injury was used to evaluate the effects of intravenous MSC administration at three post-injury time points: days 1, 3, and 7. Functional and histological assessments were performed for each group. Systemic inflammatory responses were evaluated through blood analysis of neutrophil and macrophage counts, systemic inflammation index (SII), and plasma proteomics. Local immune responses were assessed by quantifying infiltrating immune cells within the injured spinal cord. Results: The most substantial improvement in locomotor function was observed in the day-1 treatment group, followed by the day-7 group, whereas the day-3 group showed minimal benefit. The day-3 group also demonstrated a trend toward greater lesion length and increased macrophage infiltration 28 days after injury. MSC administration reduced SII in the day-1 and day-7 groups but not in the day-3 group, which instead showed an increased systemic inflammatory response. Analysis of spinal cord tissue demonstrated that MSC treatment on day-1 effectively reduced neutrophil infiltration, which peaks at this time point, while day-7 administration reduced macrophage infiltration during its peak phase. In contrast, MSC administration on day-3 failed to attenuate either neutrophil or macrophage accumulation. Plasma proteomic profiling revealed enhanced complement and coagulation pathway activation specifically on day-3. Conclusions: The therapeutic efficacy of intravenously administered MSC is highly dependent on the timing of intervention. Optimal benefit is achieved when treatment coincides with peak activation of a dominant target immune cell population and avoids the peak of complement and coagulation signaling. These findings support a phase-matched therapeutic strategy to maximize MSC effectiveness following spinal cord injury.

1. Introduction

Spinal cord injury (SCI) is a highly disabling disorder of the nervous system. Patients frequently experience enduring deficits in motor, sensory, and autonomic function and bear a substantial socioeconomic burden [1, 2]. Beyond the initial mechanical insult, a subsequent “secondary injury” cascade—including ischemia, excitotoxicity, oxidative stress, and a dynamically evolving neuroinflammatory response—drives progressive tissue loss and functional deterioration. After SCI, the inflammatory milieu evolves rapidly: microglial activation leads to neutrophils infiltration, followed by infiltration of monocytes/macrophages, while astroglial and fibrotic scarring develops over days to weeks. Accordingly, strategies that modulate these secondary processes, particularly those that remodel the immune microenvironment, constitute a central focus of contemporary research in neuroprotection and neural repair [3, 4].

Mesenchymal stem/stromal cells (MSC) have emerged as promising candidates for SCI therapy owing to their low immunogenicity, secretion of trophic and anti-inflammatory factors, release of extracellular vesicles, and capacity to influence glial and immune cell phenotypes to promote neuroprotection and plasticity [5–7]. Among MSC sources, amniotic MSCs (AMSCs) have attracted special interest because of their perinatal origin, robust paracrine activity, and favorable safety profile [8, 9]. Although a substantial fraction of intravenously administered cells undergoes pulmonary first-pass entrapment,10 systemic immunomodulation, altered leukocyte homing/migration, and indirect actions on the injured cord can nonetheless yield functional benefits across multiple animal models [11–13].