A42: Effect of mesenchymal stem cells on the microenvironment and functional morphology of sarcoma M-1

Abstract

The mechanisms of mesenchymal stem cells (MSC) action on malignant neoplasms attract much attention nowadays. Studies relating to the MSC effect on tumors are contradictory and indicate that MSC has both stimulating and suppressive actions on experimental neoplasms. We have shown previously that a single MSC administration to tumor-bearing animals has an oncomodulating effect on the growth of tumors. Short-term stimulation of the growth of tumor nodes initially after the MSC injection was usually followed by subsequent deceleration of their growth. Here, we have studied the effects of allogeneic MSC on sarcoma M-1 in rats. On day 11 after the sarcoma M-1 implantation, 1.5 million MSC grown from the bone marrow cell population of Wistar rats was administered into the tail vein of outbred rats from the experimental group. On days 17 and 30 of the tumors growth, 10 rats from each group were used to study the sarcoma M-1. To analyze the MSC distribution and localization in the tumor parenchyma, on days 11 and 13, five rats in the study were intravenously infused 2 million MSC, labeled in vitro by bromodeoxyuridine (BrdU). The techniques for studying of the tumor reaction on the systemic MSC transplantation included immunostaining for PCNA, BrdU and PECAM-1, in conjunction with computerized analysis of the microscopic images. One day after transplantation, the BrdU-labeled MSC localized in the perivascular areas of angiogenesis on the periphery of tumor nodes with the diameter of more than 6 mm, containing foci of spontaneous necrosis. Sometimes, labeled cells could be seen near the vessels situated deep in the tumor parenchyma. On day 3, cells with the low-intense BrdU immunostaining were found only rarely and mostly perivascularly. These cells were visualized in the region of pericytes’ localization, and the low BrdU-immunostaining intensity of their nuclei indicated the label dilution effect. Six days after the MSC administration, some local areas of the connective tissue enlargements on the periphery of tumor nodes were observed with a distinct vascular ingrowth into the tumor parenchyma. In the same areas, the marginal region of tumor nodes contained a considerable number of neutrophils and lymphocytes. The PCNA staining revealed foci of increased proliferative activity of tumor cells in these areas, as well as intensive proliferation of fibroblasts and vascular endothelium, which indicates the increase of angiogenesis and stroma formation. After the MSC administration the content of parenchyma with PCNA-positive nuclei significantly increased, whereas the volume fraction of the necrosis regions decreased by more than 1.5 times. On day 30 of the sarcoma M-1 growth, tumors in the experimental group rats were surrounded by layers of connective tissue. The peritumoral area was infiltrated by numerous lymphocytes and macrophages. In the terminal period of the sarcoma growth, the most quantitative parameters for tumors in the experimental group did not significantly differ from the data obtained in the control group. Only the rate of tumor cell apoptosis in animals with transplanted MSC was statistically higher than that in control rats. Reviewing the literature regarding the influence of MSC on the malignant growth revealed that this problem still remains quite unclear and disputable. One of the contradictions is their ability to have opposite effects on the repopulation activity of tumor cells. The complex interactions between MSC, tumor microenvironment and neoplastic cells seem to be crucial for the outcome of the oncological process development. Further detailed studies of the mechanisms of the cellular therapy using MSC on carcinogenesis are necessary to generate new insights into this area.