Simple SummaryHepatitis C is an infectious disease caused by the hepatitis C virus (HCV). Chronic viral hepatitis C is asymptomatic for many years and can lead to the development of liver cirrhosis/cancer. A vaccine is needed to eliminate hepatitis C; however, numerous attempts to create vaccines have so far been unsuccessful. The aim of this work was to investigate the ability of genetically modified human mesenchymal stem cells (mMSCs) expressing the HCV NS5A protein to induce a cellular immune response to HCV. Sixteen stem cell lines of a various origin were tested, and MSCs obtained from the dental pulp were selected which most effectively expressed the HCV protein. The triple immunization of mice with mMSCs showed an increase in the antigen-specific lymphocyte proliferation, the greater production of antiviral cytokine interferon-gamma, as well as an increase in the number of CD4+ memory T cells compared with the immunization with the NS5A gene. Thus, these results, which were obtained for the first time, show that human mMSCs can be a basis for the development of an effective vaccine against hepatitis C that triggers a strong T cell response and prevents the transition of acute hepatitis C to the chronic form due to the rapid activation of memory cells.AbstractHepatitis C virus (HCV) is one of the basic culprits behind chronic liver disease, which may result in cirrhosis and hepatocarcinoma. In spite of the extensive research conducted, a vaccine against HCV has not been yet created. We have obtained human mesenchymal stem cells (hMSCs) and used them for expressing the HCV NS5A protein as a model vaccination platform. Sixteen hMSC lines of a different origin were transfected with the pcNS5A-GFP plasmid to obtain genetically modified MSCs (mMSCs). The highest efficiency was obtained by the transfection of dental pulp MSCs. C57BL/6 mice were immunized intravenously with mMSCs, and the immune response was compared with the response to the pcNS5A-GFP plasmid, which was injected intramuscularly. It was shown that the antigen-specific lymphocyte proliferation and the number of IFN-γ-synthesizing cells were two to three times higher after the mMSC immunization compared to the DNA immunization. In addition, mMSCs induced more CD4+ memory T cells and an increase in the CD4+/CD8+ ratio. The results suggest that the immunostimulatory effect of mMSCs is associated with the switch of MSCs to the pro-inflammatory phenotype and a decrease in the proportion of myeloid derived suppressor cells. Thus, the possibility of using human mMSCs for the creation of a vaccine against HCV has been shown for the first time.