Follistatin-Like Protein 1 Contributes to Acute Traumatic Spinal Cord Injury by Enhancing Neuroinflammation Through the Modulation of Interferon Regulatory Factor 5-Mediated Macrophage Activation

Background and Purpose: Follistatin-like protein 1 (Fstl1) is a secreted glycoprotein with context-dependent inflammatory activity. We investigated whether Fstl1 is a reproducible feature of traumatic spinal cord injury (SCI) and whether it regulates macrophage activation through interferon regulatory factor 5 (IRF5). Methods: A target-blind analysis integrated four public SCI bulk transcriptomic cohorts with single-cell and spatial datasets. Findings were tested in wild-type and Fstl1+/− mice after SCI using spinal cord RNA sequencing, molecular and histological assays, behavioral testing, and bone marrow-derived macrophages with IRF5 gain- and loss-of-function. Adult-induced Postn-CreER-mediated Fstl1 deletion was evaluated in fibroblast–macrophage non-contact co-cultures. Results: The public-data screen reduced 440 cross-cohort candidates to four non-structural extracellular genes, including Fstl1. Fstl1 was consistently upregulated across SCI cohorts, correlated with myeloid signatures, and was predominantly associated with non-myeloid cell sources in single-cell data. Experimental SCI increased spinal Fstl1 and inflammatory cytokines, whereas partial Fstl1 depletion reduced tissue damage and improved locomotor recovery. Fibroblast Fstl1 deletion reduced macrophage iNOS, Il6, and Tnf and increased Arg-1 and Il10 in co-culture. Spinal cord RNA sequencing showed broad remodeling of immune and tissue-related programs after Fstl1 depletion. Computational perturbation and experimental validation identified IRF5 as a downstream mediator. Recombinant Fstl1 increased IRF5, iNOS, and CD80 while reducing Arg-1 and CD163; IRF5 silencing reversed these changes. Conclusions: Fstl1 is a reproducible SCI-associated extracellular candidate that supports IRF5-dependent inflammatory macrophage activation. Targeting the Fstl1-IRF5 axis may help limit secondary neuroinflammation after SCI.