NK cell mediated lysis. In accordance, significant IFN-c secretion could also be obtained in supernatants removed from co-cultures of NK cells with monocytes (Fig. 9A) or T cells (Fig. 9A) or B cells (Fig. 9B). One distinction was in the magnitude of secreted IFN-c by the NK cells treated with IL-2 and anti-CD16 mAb in which those cultured with monocytes secreted higher levels than those cultured with either T cells or B cells (Fig. 9). Monocytes are major inducers of NFkB activity in epithelial cells Purified subpopulations of the immune effectors, namely, highly purified monocytes, Natural Killer cells, PMNs and total populations of peripheral blood mononuclear cells (PBMCs) and peripheral blood lymphocytes (PBLs) were each co-cultured with NFkB reporter transfected HEp2 cells (Fig. 10), and the levels of NFkB activity were determined after 4 hours of incubation. As shown in Fig. 10 monocytes induced the highest increase in NFkB activity whereas all the other populations including NK cells had minimal or no effect on NFkB activity (p,0.05). Discussion Successful transplantation of stem cells in an immunocompe- tent host will eventually depend on the ability of transplanted stem cells to survive during their interaction with the host immune effectors. Although a number of previous studies have NK Cells Kill Stem Cells attributed a lack of immunogenicity to stem cells, others have shown significant lysis of these cells by the cytotoxic immune effectors [4,6,7]. Therefore, detailed analysis of immune cell interaction with stem cells is important, and should provide the basis for designing novel strategies to increase the viability and function of stem cells in different tissue engineering applications. It is therefore, important to determine the potential mechanisms by which stem cells can be protected from the lysis by cytotoxic immune effectors. To study the detailed function of cytotoxic immune effectors against stem cells we purified human NK cells and studied their effect on MSCs and DPSCs. In addition, we determined the interaction of NK cells with other subsets of immune effectors during their interaction with stem cells. In agreement with previous studies we also show in this report that NK cells can target and kill MSCs significantly. Indeed, when compared to K562 lysis, NK cells can also mediate significant lysis of MSCs (Figs. 1 and 2) and DPSCs. Moreover, we demonstrate for the first time that DPSCs are also targets of NK cell mediated killing, and that stem cells in general have exquisite sensitivity to NK cell mediated lysis. In support of this notion, we have recently determined that human embryonic stem cells are the most sensitive stem cells to NK cell mediated cytotoxicity when compared to DPSCs (manuscript in preparation). In addition, autologous DPSCs are as good of a target for NK cell mediated lysis as the allogeneic DPSCs. Figure 9. Total monocytes or T cells or B cells synergize with NK cells and DPSCs to induce IFN-c secretion. DPSCs (16105 cells/well) were co-cultured with highly purified total monocytes (Fig. 9A and 9B) or T cells (Fig. 9A) or B cells (Fig. 9B) at 1:1 DPSCs to monocyte or T cell or B cell ratios for 24–48 hours before untreated or IL-2 (1000 u/ml) pre-treated or anti-CD16mAb (3 mg/ml) pre-treated, or a combination of IL-2 (1000 u/ml) and anti-CD16 mAb (3 mg/ml) pre-treated NK cells at 1:1 DPSCs to NK cells were added. NK cells were pre-treated as indicated for 24– 48 hours before they were added to the co-cultures of monocytes or T cells or B cells and DPSCs. After 24–48 hours of the addition of NK cells the supernatants were removed and subjected to specific ELISAs for IFN-c. One of four representative experiments is shown in this figure. doi:10.1371/journal.pone.0009874.g009 PLoS ONE | www.plosone.org 11 March 2010 | Volume 5 | Issue 3 | e9874