Published online Feb 27, 2018. doi: 10.4254/wjh.v10.i2.287
Peer-review started: September 20, 2017
First decision: October 31, 2017
Revised: October 31, 2017
Accepted: February 5, 2018
Article in press: February 5, 2018
Published online: February 27, 2018
The resulting post-hepatectomy liver failure is a major manifestation of the small-for-size liver remnant syndrome, when the critically reduced liver mass is insufficient to maintain normal liver function. The problem can be solved by some specific controllable stimulation of compensatory growth combined with intensive liver care and management for acute hepatic failure. Multipotent stromal cells may therefore represent a reasonable choice not only in cases of extensive hepatectomy but also for other types of severe liver damage (e.g. cirrhosis). Multipotent stromal cells are found to be a helpful supplement in various repair processes in mammals, particularly in solid organs. These cells are considered as a promising tool of regenerative medicine. Their ability to stimulate reparative regeneration of damaged liver has been confirmed, but the mechanisms remain uncertain.
The experimentally proven enhancement of liver regeneration by multipotent stromal cells (MSCs) must be essentially paracrine, because their transplantation to residual livers causes an increase in concentrations of HGF and several other growth factors within the regenerating tissues. Therapeutic activity of MSCs is apparently related to their anti-inflammatory properties, which also represent a sort of paracrine regulation, as manifested by a local increase in IL-4, IL-13, and TSG-6 production paralleled by relative shortage of TNFα and IL-6. Modulation of inflammatory reactions may be implemented via influence of these, or similar, paracrine factors on immune cells, especially on macrophages. Several studies, however, confirm the ability of MSCs to differentiate into hepatocytes, which leads to quite a different explanation for the positive influence of MSCs on liver repair.
In our experiments we found that intrasplenic allogeneic transplantation of the umbilical cord-derived multipotent stromal cells stimulated hepatocyte proliferation and organ mass recovery after subtotal resection. These effects may result from positive paracrine influence of the transplanted cells on polarization of the liver resident macrophages to M2 phenotype.
The MSCs were obtained from the intervascular tissue of umbilical cords, dissected from rat fetuses, by the explant culture technique. The vital labeling of MSCs with РКН26 was carried out on the 3rd passage. The subtotal resection was performed on male Sprague-Dawley rats. The experimental group animals received a transplant 106 MSCs infused into the spleen. Hepatocyte proliferation was assessed by counting of either mitotic figures or Ki67-positive cells in microscopic images. MSC differentiation was assessed with antibodies to hepatocyte-specific marker cytokeratin 18 (CK18), cholangiocyte-specific protein CK19, smooth muscle cell-specific protein α-SMA, the endothelial cell marker CD31, or the active fibroblast marker FAPα. Total macrophages of the liver were selectively stained in cryosections incubated with anti-CD68 antibodies (1:100, Abcam), while the M2a and M2c macrophage populations were selectively stained with anti-CD206 antibodies. Expression of interleukin and growth factor genes was evaluated with PCR-RT.
Intrasplenic allogeneic transplantation of the umbilical cord-derived multipotent stromal cells stimulates reparative processes within the residual liver tissue after subtotal resection (removal of 80% of the organ mass), as indicated by increased rates of hepatocyte proliferation and accelerated organ mass recovery. These effects may result from paracrine influence of the transplanted cells on the resident macrophage population of the liver. The transplantation favors polarization of macrophages to M2 phenotype (the M2-polarized macrophages specifically express CD206; they are known to suppress inflammation and support tissue repair). No differentiation of the transplanted cells into any of the liver cell types have been observed in the study.
In this study we observed a stimulating effect of the umbilical cord-derived MSCs on liver regeneration after subtotal resection in rats, manifested as increased survival of the operated animals, increased rates of liver mass recovery, and increased proliferation activity of hepatocytes. We found no direct evidence for the paracrine effect of MSCs on liver regeneration after the subtotal liver resection in rats. However, the paracrine mechanism of the therapeutic activity of transplanted MSC is indirectly indicated by a decrease in the total number of CD68 + macrophages and an increase in the proportion of M2 pro-repair macrophages in the regenerating liver as compared to animals in which the transplantation was only mimicked.
Exact molecular mechanisms of the paracrine effect of MSC transplantation on hepatocyte proliferation and liver macrophage behavior in the aftermath of subtotal liver resection remain unclear. Further studies in this direction are very desirable, since any possibility of controlling hepatocyte proliferation and/or liver macrophage polarization could be of great therapeutic importance, especially in severe liver injuries.