Marrow stromal cells (MSC) have several unique properties, which make them well suited both for regenerative medicine and gene delivery. These include the ease of isolation and the ability to be considerably expanded in culture without losing engraftment capacity. Furthermore, MSC have been reported to reduce local inflammation, blunt immune response, and counteract the chemotactic signals released to recruit immune cells to the site of injury/inflammation.
Nevertheless, controversy still remains whether these cells, upon transplantation, would be able to be recognized in an allogeneic setting, lessening their therapeutic potential when compared with their autologous counterpart. Since MSC express negligible amounts of HLA-II, but display variable levels of HLA-I on their surface, and harbor several ligands to activating NK cell receptors, it is likely that MSC can become a target of NK and CTL. We genetically engineered MSC to express the HCMV proteins US2, US3, US6, and US11, since they were shown to reduce HLA-I surface levels on somatic cells. Moreover, this reduction in HLA-I levels prevented CTL recognition thereby preventing activation and killing of infected cells during a normal HCMV infection. We demonstrated that US6 and US11 were the most efficient in downregulating expression of HLA-I and therefore reducing MSC immunogenicity, as demonstrated by the correlation between HLA-I expression and the decrease in human PBMNC proliferation using MLR assays. We also investigated the role of the HCMV US proteins US 2, 3, 6 and 11 in protecting MSC from complement lysis, since the complement system, plays an essential role in the inflammatory process, and serves as a critical bridge between the innate and adaptive arms of the immune response. MSC express soluble factor H, the complement regulatory proteins CD46, CD55, and CD59, that enable MSC inhibit activation of the complement system to a certain extent. Still, in the presence of inflammation and complement system activation, these innate mechanisms of protection are insufficient to prevent cellular damage and death. Expression of US2 HCMV protein on MSC had the ability to increase expression of CD46, CD55, and CD59 surface molecules, and functional assays demonstrated a significant reduction in complement lysis in MSC expressing US2 protein, when compared to the control.
In conclusion, we are able to genetically engineer MSC to have an enhanced survival advantage in the presence of an exacerbated inflammatory microenvironment and/or in an allogeneic transplantation setting, as a result of decreased rejection by the immune system. This should extend their survival time and thereby enhance their therapeutic potential.