Strategies to Rescue Mesenchymal Stem Cells (MSCs) and Dental Pulp Stem Cells (DPSCs) from NK Cell Mediated Cytotoxicity Anahid Jewett1*, Aida Arasteh 1 , Han-Ching Tseng 1 , Armin Behel1, Hobie Arasteh 1 , Wendy Yang 1 , Nicholas A. Cacalano 2 , Avina Paranjpe1 1 Division of Oral Biology and Medicine, The Jane and Jerry Weintraub Center for Reconstructive Biotechnology, UCLA School of Dentistry and Medicine, University of California Los Angeles, Los Angeles, California, United States of America, 2 Department of Radiation Oncology, The Jonsson Comprehensive Cancer Center (JCCC), UCLA School of Dentistry and Medicine, University of California Los Angeles, Los Angeles, California, United States of America Abstract Background: The aim of this paper is to study the function of allogeneic and autologous NK cells against Dental Pulp Stem Cells (DPSCs) and Mesenchymal Stem Cells (MSCs) and to determine the function of NK cells in a three way interaction with monocytes and stem cells. Methodology/Principal Findings: We demonstrate here that freshly isolated untreated or IL-2 treated NK cells are potent inducers of cell death in DPSCs and MSCs, and that anti-CD16 antibody which induces functional split anergy and apoptosis in NK cells inhibits NK cell mediated lysis of DPSCs and MSCs. Monocytes co-cultured with either DPSCs or MSCs decrease lysis of stem cells by untreated or IL-2 treated NK cells. Monocytes also prevent NK cell apoptosis thereby raising the overall survival and function of NK cells, DPSCs or MSCs. Both total population of monocytes and those depleted of CD16+ subsets were able to prevent NK cell mediated lysis of MSCs and DPSCs, and to trigger an increased secretion of IFN-c by IL-2 treated NK cells. Protection of stem cells from NK cell mediated lysis was also seen when monocytes were sorted out from stem cells before they were added to NK cells. However, this effect was not specific to monocytes since the addition of T and B cells to stem cells also protected stem cells from NK cell mediated lysis. NK cells were found to lyse monocytes, as well as T and B cells. Conclusion/Significance: By increasing the release of IFN-c and decreasing the cytotoxic function of NK cells monocytes are able to shield stem cells from killing by the NK cells, resulting in an increased protection and differentiation of stem cells. More importantly studies reported in this paper indicate that anti-CD16 antibody can be used to prevent NK cell induced rejection of stem cells. Citation: Jewett A, Arasteh A, Tseng H-C, Behel A, Arasteh H, et al. (2010) Strategies to Rescue Mesenchymal Stem Cells (MSCs) and Dental Pulp Stem Cells (DPSCs) from NK Cell Mediated Cytotoxicity. PLoS ONE 5(3): e9874. doi:10.1371/journal.pone.0009874 Editor: Jacques Zimmer, Centre de Recherche Public de la Sante´ (CRP-Sante´), Luxembourg Received February 15, 2010; Accepted February 25, 2010; Published March 31, 2010 Copyright: 2010 Jewett et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: Funding was received from the University of California Los Angeles Academic Senate grant. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: ajewett@ucla.edu Introduction Previous reports demonstrated significant immunomodulatory effect of Mesenchymal Stem Cells (MSCs) on different immune effector subsets such as cytotoxic T cells and Natural Killer (NK) cells [1,2,3,4,5]. Several reports have also indicated the immuno- suppressive nature of MSCs on NK cells [4,6,7]. Moreover, activated NK cells were shown to lyse MSCs significantly. In particular, both allogeneic and autologous MSCs were shown to be targets of NK cell mediated lysis [8,9]. Therefore, on one hand the MSCs were shown to be immunosuppressive and on the other they were found to be sensitive to lysis by the NK cells. Thus, these diverse results prompted us to evaluate the function of NK cells against two different types of stem cells, namely DPSCs and MSCs. Although some information is known about the function of NK cells against MSCs, no studies have been performed thus far to determine the effect of NK cells on DPSCs. DPSCs were shown to form colonies in vitro, and they were capable of osteogenic, chondrogenic and adipogenic differentiation [10]. In order to find ways to protect stem cells from lysis by the NK cells we need to first determine the magnitude and the mechanisms by which NK cells lyse stem cells and second to implement strategies based on those findings to protect stem cells from NK cell lysis. Many diverse mechanisms were hypothesized to mediate the immunosuppressive effect of MSCs. TGFb and Hepatocyte Growth Factor (HGF) were both shown to be the mediator of T cell suppression by MSCs [11]. Leukemia Inhibitory Factor induced in co-cultures of MSCs and lymphocytes was shown to mediate suppression of T cell proliferation [12]. IFN-c induced by T cells increased B7H1 inhibitory co-stimulatory receptors on MSCs and resulted in the suppression of T cells [13]. Immunosuppressive function of MSCs is elicited by the combina- PLoS ONE | www.plosone.org 1 March 2010 | Volume 5 | Issue 3 | e9874