Amniotic Mesenchymal Stromal Cell Administration Prevents and Stops Lung Fibrosis and Is Associated with Distinct Macrophage Signatures

Background/Objectives: Mesenchymal stromal cells from the amniotic membrane (hAMSCs) counteract fibrosis progression, primarily via anti-inflammatory effects like promoting macrophage polarization toward an anti-inflammatory, pro-regenerative phenotype. Methods: We investigated hAMSCs’ ability to prevent and halt lung fibrosis in a bleomycin-induced fibrosis murine model. We focused on their impact on recruitment and polarization of different macrophage populations, including SPARC- and CD169-expressing macrophages, implicated in resolving pulmonary inflammation and fibrosis. Results: hAMSCs, administered early (concomitant with bleomycin, during acute inflammation), or late (at day 7 post-bleomycin, during established fibrosis), showed anti-fibrotic activity, preserving alveolar area, reducing the extent of lung fibrosis, and decreasing α-SMA levels. These preventive and late anti-fibrotic effects of hAMSCs are associated with a context-dependent presence of distinct macrophage signatures. Early treatment reduced macrophage recruitment and increased levels of Arg1+/iNOS− macrophages, curbing injury-induced inflammation. Late treatment uniquely increased the lung levels of CD169+ macrophages, suggesting their contribution to hAMSCs’ anti-fibrotic effect. We hypothesized a potential involvement of lung CD169+ macrophages in promoting recruitment of regulatory T cells (Tregs) to the lungs. Although these macrophages can establish a CCL22-CCR4 axis with Tregs, and treatment effectively boosted Treg lung levels, the Treg increase is not directly attributable to higher CD169+ macrophage numbers, implying other potentially IL-10-driven mechanisms. Conclusions: hAMSC treatment effectively prevents and blocks lung fibrosis. These effects are associated with distinct lung macrophage marker profiles, suggesting a potential involvement of different macrophage populations in a time-dependent manner; thus highlighting administration timing’s role in optimizing therapeutic synergy.