Prevention of LPS-induced acute lung injury in mice by mesenchymal stem cells overexpressing angiopoietin 1

Abstract

Background: The acute respiratory distress syndrome (ARDS), a clinical complication of severe acute lung injury (ALI) in humans, is a leading cause of morbidity and mortality in critically ill patients. ALI is characterized by disruption of the lung alveolar-capillary membrane barrier and resultant pulmonary edema associated with a proteinaceous alveolar exudate. Current specific treatment strategies for ALI/ARDS are lacking. We hypothesized that mesenchymal stem cells (MSCs), with or without transfection with the vasculoprotective gene angiopoietin 1 (ANGPT1) would have beneficial effects in experimental ALI in mice.

Methods and findings: Syngeneic MSCs with or without transfection with plasmid containing the human ANGPT1 gene (pANGPT1) were delivered through the right jugular vein of mice 30 min after intratracheal instillation of lipopolysaccharide (LPS) to induce lung injury. Administration of MSCs significantly reduced LPS-induced pulmonary inflammation, as reflected by reductions in total cell and neutrophil counts in bronchoalveolar lavage (BAL) fluid (53%, 95% confidence interval [CI] 7%-101%; and 60%, CI 4%-116%, respectively) as well as reducing levels of proinflammatory cytokines in both BAL fluid and lung parenchymal homogenates. Furthermore, administration of MSCs transfected with pANGPT1 resulted in nearly complete reversal of LPS-induced increases in lung permeability as assessed by reductions in IgM and albumin levels in BAL (96%, CI 6%-185%; and 74%, CI 23%-126%, respectively). Fluorescently tagged MSCs were detected in the lung tissues by confocal microscopy and flow cytometry in both naïve and LPS-injured animals up to 3 d.

Conclusions: Treatment with MSCs alone significantly reduced LPS-induced acute pulmonary inflammation in mice, while administration of pANGPT1-transfected MSCs resulted in a further improvement in both alveolar inflammation and permeability. These results suggest a potential role for cell-based ANGPT1 gene therapy to treat clinical ALI/ARDS.

Figure 1. Characterization of MSCs Isolated from C57Bl/6J Mice, and Experimental Design for In Vivo Study

(A) Undifferentiated MSCs (P8) were seeded in a T75 flask at a density of 100 cells/cm² and found to expand readily in a fashion that started from a single cell to colony. (B) Staining with oil red-O was used to detect MSCs that differentiated into adipocytes, identified by perinuclear red staining of fat globules. (C) Staining with alizarian red was used to detect MSCs that differentiated into osteocytes. (D) MSCs that differentiated into chondrocytes were stained with toluidine blue. Photomicrographs were obtained with a 10× (A and C) or a 20× (B and D) objective using a Nikon Eclipse TS100 inverted microscope. (E) Flow cytometry was performed for surface marker expression on cultured MSCs. All data are presented as mean ± SEM. (F) C57Bl/6J mice initially received LPS by intratracheal instillation, followed by intravenous injection 30 min later with saline, cultured MSCs, MSCs-pFLAG, or MSCs-pANGPT1. Mice were then sacrificed 15 min and 3 d after to assess cell retention, or 3 d after to evaluate the therapeutic efficacy. A photomicrograph of MSCs-pANGPT1 was obtained using a Nikon Eclipse TS100 inverted microscope with a 10× objective 1 d after transfection and prior to injection into animals.

Figure 2. Therapeutic Potential of MSCs, Alone or Transfected with pANGPT1, on LPS-Induced Lung Inflammation in Mice

(A and B) Total cell (A) and neutrophil (B) counts were performed on BAL fluid to evaluate lung airspace inflammation. There was a 19-fold increase in total inflammatory cells in BAL fluid collected 3 d after LPS, which was reduced by 53% in MSCs-treated mice (non-/null-transfected), and by 96% with MSCs-pANGPT1. Group comparisons were analyzed by one-way ANOVA with Dunnett post hoc test. *p < 0.05 and **p < 0.01, compared between LPS/saline versus each treated group (MSCs, MSCs-pFLAG, or MSCs-pANGPT1). n = 5 per group. (C) Histological evaluation of therapeutic potential of MSCs and MSCs-pANGPT1 on LPS-induced lung injury in mice. Representative images of hematoxylin and eosin stained lung sections from six experimental groups. Lungs were fixed with 4% paraformaldehyde, embedded in paraffin, and then cut into 5-μm thick sections before being stained. Photomicrographs were obtained with a Nikon Eclipse E800 microscope with a 40× objective. Scale bar = 20 μm.

Figure 3. Levels of Proinflammatory Cytokines and Chemokines in BAL Fluid

Levels of the proinflammatory cytokines IFN-γ, TNF-α, IL6, and IL1β in BAL fluid were measured using ELISA. In addition, chemokine levels (Cxcl2, JE [murine homolog of human CCL2], and KC [murine IL8 homolog]) in BAL fluid were measured by multiplex immunoassay. Group comparisons were analyzed by one-way ANOVA with Dunnett post hoc test. *p < 0.05 and **p < 0.01, LPS/saline versus each treated group (MSCs, MSCs-pFLAG, or MSCs-pANGPT1). n = 5 per group.

Figure 4. Levels of Proinflammatory Cytokines and Chemokines in Lung Homogenate

Cytokine (TNF-α and IL6) and chemokine (Cxcl2, JE [murine homolog of human CCL2], and KC [murine IL8 homolog]) levels in lung homogenates were measured by multiplex immunoassay. Group comparisons were analyzed by one-way ANOVA with Dunnett post hoc test. *p < 0.05 and **p < 0.01, LPS/saline versus each treated group (MSCs, MSCs-pFLAG, or MSCs-pANGPT1). n = 5 per group.

Figure 5. Effect of MSCs and MSCs-pANGPT1 on LPS-Induced ALI

Therapeutic efficacy was assessed by measurement of total protein, albumin, and IgM (biomarkers of pulmonary vascular leakage resulting from disruption of the alveolar–capillary membrane barrier) in BAL fluid. (A) Total protein concentration was measured by Bradford assay (A); albumin was measured using a mouse-specific albumin ELISA (B); and IgM was measured using a mouse IgM ELISA kit (C). Group comparisons were analyzed by one-way ANOVA with Dunnett post hoc test. *p < 0.05 and **p < 0.01, LPS/saline versus each treated group (MSCs, MSCs-pFLAG, or MSCs-pANGPT1). n = 5 per group.

Figure 6. Retention of Injected MSCs in Mice With or Without LPS-Induced ALI

MSCs were labeled with the cell tracing dye CFDA SE (green) prior to injection. Nuclei were stained with TO-PRO-3 (blue). Scale bars in photomicrographs = 20 μm. White arrows indicate labeled MSCs. (A) Labeled MSCs were observed in 5 μm, PFA-fixed lung sections from LPS-injured mice sacrificed at 15 min (initial retention). Image obtained with a Leica laser scanning confocal microscope with a 20× objective. (B) A three-dimensional lung section from an animal that received labeled MSCs. Lung was inflated, stored in OCT, and cut into 50 μm thick sections. Z-series images (30 sections, total thickness of the tissue scanned = 17.71 μm) were collected with a 63× oil objective and projected in different axes, as shown. Red autofluorescence shows outline of the alveolus. (C) pANGPT1-transfected MSCs labeled with the green fluorescent cell tracker CFDA SE were observed in lung section from LPS-injured mice killed at 3 d. Photomicrographs were taken in z-axis with a 20× objective, then images were stacked using Leica confocal software. (D) Lung lobes (left upper and all right lobes) from each animal were enzyme-digested into single cells before MSCs were counted by flow cytometry. n = 5 per group.