Abstract: BACKGROUND: Preliminary studies of the research team have shown that bone marrow mesenchymal stem cells can reduce pulmonary inflammation and pulmonary fibrosis in rats caused by silica dust, and no obvious rejection or side effects were found. However so far, almost all reports on the treatment of silicosis animals with bone marrow mesenchymal stem cells are single infusion, and there is no report comparing the effect of multiple repeated infusions and single infusions.  
OBJECTIVE: To investigate the effect of frequency of bone marrow mesenchymal stem cell infusion on pulmonary fibrosis in rats exposed to silica dust. 
METHODS: SD rats were randomly divided into normal control group, model group, single infusion group, and multiple infusion group, with 6 rats in each group. Rats in the latter three groups were treated with 1 mL SiO2 suspension at a concentration of 30 mg/mL by tracheal injection, and rats in normal control group were treated with equal volume of normal saline. The single infusion group was injected with 5×109 L-1 0.5 mL bone marrow mesenchymal stem cell suspension via caudal vein on day 28 after dust exposure; the multiple infusion group was injected with bone marrow mesenchymal stem cell suspension on day 28, 35 and 42 after dust exposure; the model group and the normal control group were injected with equal volume of normal saline. After 84 days of experiment, lung CT scan was performed to weigh and calculate lung coefficient in each group. Hematoxylin-eosin staining and Masson staining were performed to detect the levels of hydroxyproline in lung tissue and transforming growth factor β1 in serum.  
RESULTS AND CONCLUSION: (1) Lung CT images showed that no abnormality was detected in rat lungs of the normal control group. In model group, small granular high-density shadows with diffused distribution and different sizes were observed in both lungs, as well as reticular fiber shadow or cable shadow. The density of the granular shadow distribution and the fiberoptic area were all decreased in the rat lung field of the single-infusion and multiple-infusion groups. (2) Hematoxylin-eosin staining showed that diffuse silicon nodules were seen in the model group, which were fused into films in a large area and infiltrated by lymphocytes. In the single infusion group, there were silicon nodules, local fusion, fibrous tissue hyperplasia, and lymphocyte infiltration in alveolar septum. In the multiple infusion group, a small number of silicon nodules, interstitial fibrous hyperplasia and lymphocyte infiltration were observed. (3) Masson staining showed that in the model group, massive collagen proliferation was seen; small collagen fiber was found in the widened alveolar interval and bronchus and vessels the in multiple infusion group; collagen fibers in single infusion group were between model group and multiple infusion group. (4) Hydroxyproline content in lung tissue, lung collagen volume fraction and serum transforming growth factor β1 level from low to high were normal control, multiple infusion, single infusion, and model groups. Lung hydroxyproline content and collagen volume fraction varied significantly between the groups (P < 0.05). There was significant difference in transforming growth factor β1 level between all the other groups (P < 0.05) except for the model group and the single infusion group as well as single and multiple infusion groups (P > 0.05). (5) Results indicate that multiple infusion of bone marrow mesenchymal stem cells is more effective than single infusion on pulmonary fibrosis induced by silica dust.

Key words: mesenchymal stem cell, silicosis, rat, pulmonary fibrosis, infusion frequency, cell transplantation