Methods: Purified human NK cells and monocytes were co-cultured with MSCs. The function of NK cells was assessed using 51Cr release assay, and the release of IFN-g and TNF-a by multiplex cytokine array or specific ELISAs. Cell death was determined by either Propidium iodide alone or by staining with Annexin V and PI. The function of MSCs was assessed using secretion of VEGF, bFGF and IL-6.
Results: Monocytes increased NFkB activity and augmented the survival and function of MSCs. Increased function of MSCs by monocytes is evident by significant increase in VEGF, and IL-6 secretion by MSCs. NK cells mediated significant lysis of MSCs as evident by 51Cr release assay and by increased release of IFN-g in the co-cultures of NK cells and MSCs. Addition of monocytes to MSCs rendered these cells resistant to NK cell cytotoxicity even though they had also increased NK cell survival and function. Increased survival of NK cells and MSCs by monocytes coincided with an increased secretion of IFN-g by the NK cells.
Conclusion: These studies established that monocytes increase the resistance of MSCs against NK cell cytotoxicity. Thus, the role of monocytes is to increase the overall survival and function of all the cells resulting in the orchestrated amplification of the immune system and the release of cytokines important in maturation and differentiation of MSCs. Therefore, identification and characterization of the subset of monocytes with highest capacity to induce protection against NK cell mediated cytotoxicity would be the key to augment survival of MSCs in the setting of allogeneic and autologoues transplantation for a number of dental and medical applications.