Introduction: Hepatic fibrosis may evolve to liver cirrhosis which may ultimately develop into hepatocellular carcinoma. Mesenchymal stem cells (MSCs) have potential to trans-differentiate into hepatic cells and thus may be a curative option in liver cirrhosis. Natural killer (NK) cells are a component of the innate immune system that destroy compromised host cells and have critical antifibrotic role. Aim: To show the effects of bone marrow MSCs transferred via the tail vein to the rats having hepatic fibrosis produced by common bile duct ligation (CBDL) with a special focus on the changes of the NK population distribution. Methods: Rats were divided into three groups; 1- CBDL rats that were injected with phosphate-buffered saline (PBS), 2- CBDL rats injected with MSCs, 3- healthy rats that were sham operated and given MSCs. All rats were sacrificed at the end of the experiment. We analyzed the in vivo functional outcomes by measuring the serum glucose, bilirubin and ALT levels. Splenocytes isolated from the collected spleens were cultured and the supernatants were analyzed for IL (interleukin)-1α, TNF (tumor necrosis factor)-α, IFN (interferon)-γ and IL-10 using flow cytometry. Results: Liver fibrosis developed in CBDL rats while the healthy rats did not show any alteration in liver architecture. Although healthy group had significantly lower serum bilirubin, ALT and glucose levels than CBDL rats, the difference regarding those parameters did not differ between groups 1 and 2. T-lymphocyte proliferation was significantly suppressed in group 2 compared to group 1. IL-1α, TNF-α and IFN-γ levels in supernatants of cultured lymphocytes in group 2 were significantly higher than group 3 while they tended to be lower than group 1. NK and Treg cells in peripheral blood significantly increased in group 2 compared to group 1. Histologically, hepatic fibrosis scores were significantly alleviated in group 2 compared to group 1 (p=0.039). NK cells in the general hepatic parenchyma and in the portal areas were both increased significantly in group 2 compared to group 1. Conclusion: Our findings suggest bone marrow derived MSCs may be beneficial in alleviating the hepatic fibrosis by suppressing the T cell proliferation and the proinflammatory cytokines and inducing NK cell recruitment to liver parenchyma in rats. Thus, MSC treatment may appear promising in liver injury and future clinical therapies are warranted. Immunophenotyping of leukocytes. CD3+CD4+ T lymphocyte, CD3+CD8+ Cytotoxic T cell, CD4+CD25+ T regulatory cell and CD161a NK cell in all groups by flow cytometry. (B) CD3+CD4+ % T lymphocyte, CD3+CD8+ % Cytotoxic T cell, CD4+CD25+ %T regulatory cell and CD161a NK cells were displayed statistically in all groups..*P < 0.05, **P < 0.01.Figure
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