Paracrine factors of multipotent stromal cells ameliorate lung injury in an elastase-induced emphysema model

Abstract

Multipotent stromal cells (MSCs) ameliorate several types of lung injury. The differentiation of MSCs into specific cells at the injury site has been considered as the important process in the MSC effect. However, although MSCs reduce destruction in an elastase-induced lung emphysema model, MSC differentiation is relatively rare, suggesting that MSC differentiation into specific cells does not adequately explain the recuperation observed. Humoral factors secreted by MSCs may also play an important role in ameliorating emphysema. To confirm this hypothesis, emphysema was induced in the lungs of C57BL/6 mice by intratracheal elastase injection 14 days before intratracheal MSC or phosphate-buffered saline (PBS) administration. Thereafter, lungs were collected at several time points and evaluated. Our results showed that MSCs reduced the destruction in elastase-induced emphysema. Furthermore, double immunofluorescence staining revealed infrequent MSC engraftment and differentiation into epithelial cells. Real-time PCR showed increased levels of hepatocyte growth factor (HGF) and epidermal growth factor (EGF). Real-time PCR and western blotting showed enhanced production of secretory leukocyte protease inhibitor (SLPI) in the lung. In-vitro coculture studies confirmed the in vivo observations. Our findings suggest that paracrine factors derived from MSCs is the main mechanism for the protection of lung tissues from elastase injury.

Figure 1

Figure 1 MSCs reduced elastase-induced emphysema. (a) The timeline of experiments. Mice were intratracheally injected with elastase 14 days before intratracheal injection with 5 × 10⁵ MSCs, mouse fibroblasts (BLKCL4), or PBS. Mice were sacrificed at days 1, 2, 3, 4, 5, 6, 7, 14, and 21 as shown (n = ≥5/group). (b) Representative histological findings (hematoxylin and eosin staining). Treatment with MSCs reduced the elastase-induced emphysema, whereas BLKCL4 (fibroblast cell line) did not reduce these changes. Original magnification ×200. Bars = 50 µm. (c) The mean linear intercepts (Lm) for normal, elastase + PBS-, elastase + BLKCL4-, and elastase + MSC-treated groups. MSCs reduced the emphysematous changes; however, BLKCL4 did not. (d) MSCs restored the lung structure as assayed using the destructive index “DI” compared to normal, elastase + PBS-, and elastase + BLKCL4-treated groups. DI, destructive index; MSC, multipotent stromal cell; PBS, phosphate-buffered saline.

Figure 2

MSCs engrafted and differentiated into specific cells in the lung. (a) GFP⁺ MSCs engrafted in the lung. Arrows indicate GFP⁺ MSCs (shown at day 7; Original magnification ×200). (b) An MSC cell showed staining for two markers. Immunofluorescence staining at day 7 indicates a differentiated cell. Anti-GFP (green), anti-AQP5 (red), and merged (yellow). Original magnification ×200. (c) x–z confocal imaging of the cell shown in c. Original magnification ×400. (d) Immunofluorescence staining at day 7 shows that most MSCs had not differentiated because they continued to exhibit native green fluorescence only. White arrows show undifferentiated cells, blue dashed arrow show a differentiated cell. The proportions are shown in Table 1. Anti-GFP (green), anti-AQP5 (red), and merged (yellow) Original magnification ×200. The nuclei were stained with DAPI. AQP5, aquaporin 5; GFP, green fluorescent protein; MSC, multipotent stromal cell.

Figure 3

Inflammatory factor (IL-1β) was decreased in lungs treated by intratracheal administration of MSCs. (a) IL-1β concentration in BALF was measured by ELISA. IL-1β concentration was decreased to the normal levels of the WT groups compared to the elastase + PBS group. (b) Real-time PCR shows a significant decrease in mRNA levels of IL-1β in the MSC-treated groups. Representative data of the first week is shown. Levels of 1L-1β were not changed during the first week. AQP5, aquaporin 5; BALF, bronchoalveolar lavage fluid; ELISA, enzyme-linked immunosorbent assay; GFP, green fluorescent protein; IL-1β, interleukin-1β WT, wild-type.

Figure 4

Growth factors (HGF, EGF) and protection factor for epithelial cells (SLPI; secretory leukocyte protein inhibitor) were increased in lungs treated by intratracheal administration of MSCs. (a) mRNA levels of HGF were increased in the MSC-treated group. (b) mRNA levels of EGF were increased in the MSC-treated group. (c) Western blotting of whole lung extracts shows increased expression of SLPI in the treated mice compared with the nontreated mice. (d) The SLPI bands were further analyzed by densitometry in comparison with the bands of β-actin. (e) mRNA levels of SLPI were significantly increased in the MSCs-treated groups at days 1 and 3 and slightly higher at day 5. Representative data of the first week is shown as levels of SLPI were not changed dramatically during the first 5 days; however, it was downregulated thereafter. EGF, epidermal growth factor; HGF, hepatocyte growth factor; MSC, multipotent stromal cell; PBS, phosphate-buffered saline.

Figure 5

mRNA levels of HGF, EGF, and IL-1RA from MSCs were upregulated when cocultured with the mouse lung epithelial cell line MLE-12. (a) In the coculture experiments, MSCs were seeded at 1 × 10³/cm² in the upper chamber. MLE-12 cells were seeded at 1 × 10⁴/cm² in the lower chamber with or without 0.01 U/ml of elastase. Secreted proteins, but not cells, are able to pass through the membrane (pore size, 0.4 µm). (b) The HGF mRNA level of MSCs was increased in the coculture with MLE-12 cells. (c) The EGF mRNA level of MSCs was increased in the coculture with MLE-12 cells. (d) The SLPI mRNA level of MLE-12 cells was increased in the coculture with MSCs. (e) The IL-1RA mRNA level of MSC was increased in the coculture with MLE-12 cells. cDNA, complementary DNA; EGF, epidermal growth factor; HGF, hepatocyte growth factor; IL-1RA, IL-1 receptor antagonist; MSC, multipotent stromal cell; SLPI, secretory leukocyte protein inhibitor.

Figure 6

SLPI was decreased when EGF was knocked down in MSCs by siRNA. (a) siRNA knocked down EGF production in MSCs. (b) When MLE-12 cells were cocultured with MSCs transfected with siRNA against EGF or negative control, with or without elastase, the SLPI production of MLE-12 cells was decreased only in the case of MSCs transfected with siRNA against EGF and cultured with elastase. (c) Adding various concentrations of recombinant EGF (rEGF) to the culture medium containing elastase resulted in an increased production of SLPI in MLE-12 cells. Such increase was not noticed when rEGF or elastase alone were exist in the culture medium as assayed by real time PCR. EGF, epidermal growth factor; MSC, multipotent stromal cell; siRNA, small interfering RN; SLPI, secretory leukocyte protein inhibitor.