Sphingosine-1-Phosphate Prevents Egress of Hematopoietic Stem Cells From Liver to Reduce Fibrosis.

BACKGROUND & AIMS: There is growing interest in the use of bone marrow cells to treat liver fibrosis, however, little is known about their antifibrotic efficacy or the identity of their effector cell(s). Sphingosine-1-phosphate (S1P) mediates egress of immune cells from the lymphoid organs into the lymphatic vessels; we investigated its role in the response of hematopoietic stem cells (HSCs) to liver fibrosis in mice. METHODS: Purified (c-kit+/sca1+/lin-) HSCs were infused repeatedly into mice undergoing fibrotic liver injury. Chronic liver injury was induced in BoyJ mice by injection of carbon tetrachloride (CCl4) or placement on a methionine-choline-deficient diet. Some mice were irradiated and given transplants of bone marrow cells from C57BL6 mice, with or without the S1P antagonist FTY720; we then studied HSC mobilization and localization. Migration of HSC lines was quantified in Transwell assays. Levels of S1P in liver, bone marrow, and lymph fluid were measured using an enzyme-linked immunosorbent assay. Liver tissues were collected and analyzed by immunohistochemical quantitative polymerase chain reaction and sphingosine kinase activity assays. We performed quantitative polymerase chain reaction analyses of the expression of sphingosine kinase 1 and 2, sphingosine-1-phosphate lyase 1, and sphingosine-1-phosphate phosphatase 1 in normal human liver and cirrhotic liver from patients with alcohol-related liver disease (n = 6). RESULTS: Infusions of HSCs into mice with liver injury reduced liver scarring based on picrosirius red staining (49.7% reduction in mice given HSCs vs control mice; P < .001), and hepatic hydroxyproline content (328 mg/g in mice given HSCs vs 428 mg/g in control mice; P < .01). HSC infusion also reduced hepatic expression of α-smooth muscle actin (0.19 ± 0.007-fold compared with controls; P < .0001) and collagen type I α 1 chain (0.29 ± 0.17-fold compared with controls; P < .0001). These antifibrotic effects were maintained with infusion of lymphoid progenitors that lack myeloid potential and were associated with increased numbers of recipient neutrophils and macrophages in liver. In studies of HSC cell lines, we found HSCs to recruit monocytes, and this process to require C-C motif chemokine receptor 2. In fibrotic liver tissue from mice and patients, hepatic S1P levels increased owing to increased hepatic sphingosine kinase-1 expression, which contributed to a reduced liver:lymph S1P gradient and limited HSC egress from the liver. Mice given the S1P antagonist (FTY720) with HSCs had increased hepatic retention of HSCs (1697 ± 247 cells in mice given FTY720 vs 982 ± 110 cells in controls; P < .05), and further reductions in fibrosis. CONCLUSIONS: In studies of mice with chronic liver injury, we showed the antifibrotic effects of repeated infusions of purified HSCs. We found that HSCs promote recruitment of endogenous macrophages and neutrophils. Strategies to reduce SIP signaling and increase retention of HSCs in the liver could increase their antifibrotic activities and be developed for treatment of patients with liver fibrosis.

The biology of equine mesenchymal stem cells: phenotypic characterization, cell surface markers and multilineage differentiation.

Mesenchymal stem cells (MSCs) are multipotent stem cells that can give rise to a range of connective tissue cells including osteoblasts, chondrocytes and adipocytes. MSCs have been isolated from humans and a variety of animal species including rodents, dogs, horses and rabbits. There is currently no consensus on how these cells are identified and characterized. This is partly due to the lack of standardized specific cell surface markers for MSCs. The aim of this review is to examine the literature on equine MSCs and establish whether there is a well-defined phenotype for these cells. Equine MSCs have been obtained from four main sources, bone marrow, adipose tissue, umbilical cord (blood and matrix) and peripheral blood. MSCs from these tissue sources have been shown to undergo chondrogenic, adipogenic and osteogenic differentiation. However the markers used to identify these cells vary significantly in the literature. Despite this, CD90 and CD34 seem to be reliable positive and negative markers respectively. Our understanding of the biology of equine MSCs will benefit from better reagents for their phenotypic characterization. The antibodies and molecular probes needed for the reliable identification of equine MSCs are not standardized and this is a high priority for future research.