Involvement and mechanism of fecal microbiota transplantation on endotoxic acute lung injury in rats

Objective To observe the effects of fecal microbiota transplantation (FMT) on acute lung injury (ALI) induced by lipopolysaccharide (LPS) in rats and to investigate the possible mechanism in order to provide new thoughts for the treatment of acute lung injury. Methods Forty-five adult healthy male SPF level SD rats were randomly divided into three groups of normal saline (NS), LPS and FMT with 15 in each group. ALI was induced by intraperitoneal injection of rats with 5 mg/kg of LPS. The FMT group was given 10 ml/kg of fecal microbiota solution intervention (twice a day for two consecutive days) after ALI induction. Five rats in each group were randomly selected and sacrificed at 24 h, 48 h and 72 h after intervention. Pathological changes in lung tissues were examined with hematoxylin and eosin (HE) staining and pathological scores were assessed. Right lung samples were weighed to measure wet/dry (W/D) ratios. Abdominal aorta blood samples were collected for PaO2 analysis. The levels of tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin 6 (IL-6) in bronchoalveolar lavage fluid (BALF) were detected by enzyme linked immunosorbent assay (ELISA). Expression of transforming growth factor-β (TGF-β) and intracellular signal transduction protein Smads (Smad3 and Smad7) at mRNA level was analyzed by reverse transcription-polymerase chain reaction (RT-PCR). Expression of ERK1/2 and p-ERK1/2 at protein level was detected by Western blot (WB). Rat fecal samples were collected to extract DNA and the PCR products of V3 and V4 regions were sequenced by Illumina MiSeq. Bioinformatics analysis on microbiota was conducted with Illumina MiSeq based on operational taxonomic unit (OTU) clustering. Results Compared with the NS group, the LPS group showed significantly widened alveolar septa, massive inflammatory cell infiltration and some alveolar collapse in lung tissues. Compared with the LPS group, the FMT group showed significantly alleviated inflammatory cell infiltration, reduced swelling in pulmonary interstitial tissues, and less damage to pulmonary structure. Compared with the NS group, the W/D lung ratio, PaO2 concentration and TNF-α, IL-1 and IL-6 levels in BALF in the LPS group significantly increased at 24 h after intervention and then gradually decreased, but still higher than those in the NS group (P<0.05). The W/D lung ratio, PaO2 concentration and TNF-α, IL-1 and IL-6 levels in BALF of the FMT group were significantly lower than those of the LPS group (P<0.05). Compared with the NS group, the expression of TGF-β1 and Smad3 at mRNA level in lung tissues increased at all time points in the LPS group, while the expression of Smad7 at mRNA level decreased (P<0.05). The expression of TGF-β1 and Smad3 at mRNA level in the FMT group were significantly lower than that in the LPS group (P<0.05), while the expression of Smad7 at mRNA level was higher (P<0.05). Compared with the NS group, p-ERK expression significantly increased in the LPS group with the highest expression at 24 h and gradually decreased at 48 h and 72 h (P<0.05). The expression of p-ERK in the FMT group was significantly lower than that in the LPS group (P<0.05). Results of the intestinal microbiota sequencing revealed 1 362 OTU in the NS group, 1 443 OTU in the LPS group and 1 510 OTU in the FMT group. There were 336 OTU shared by them accounting for 26.4%. The constitution of intestinal microbiota in the LPS group was significantly different with that of normal rats and characterized by high diversity with increased Firmicutes/Bacteroidetes ratio and decreased gene abundance of intestinal microbiota. After the intervention of fecal bacteria transplantation, the constitution of intestinal microbiota in the FMT group was similar to that of the NS group, showing decreased Firmicutes/Bacteroidetes ratio and increased gene abundance of the bacterial community (P<0.05). Conclusions FMT might inhibit immune inflammation, reduce the production and secretion of inflammatory markers in the body, alleviate alveolar epithelial damage and abnormal repair through regulating intestinal microbiota and TGF-β1/Smads/ERK pathway to improve the LPS-induced endotoxic ALI in rats. Key words: Acute lung injury; Fecal microbiota transplantation; High-throughput Sequencing; Lipopolysaccharide