Dendritic Cell Editing by Activated Natural Killer Cells Results in a More Protective Cancer-Specific Immune Response Barbara Morandi1., Lorenzo Mortara2., Laura Chiossone3, Roberto S. Accolla4, Maria Cristina Mingari1,5, Lorenzo Moretta3, Alessandro Moretta1, Guido Ferlazzo6* 1 Department of Experimental Medicine, University of Genoa, Genoa, Italy, 2 Department of Biotechnology and Life Sciences, University of Insubria, Varese, Italy, 3 Istituto G.Gaslini, Genoa, Italy, 4 Department of Surgical and Morphological Sciences, University of Insubria, Varese, Italy, 5 Istituto Di Ricovero e Cura a Carattere Scientifico A.O.U. San Martino-IST-Istituto Nazionale per la Ricerca sul Cancro, Genoa, Italy, 6 Department of Human Pathology, University of Messina, Messina, Italy Abstract Over the last decade, several studies have extensively reported that activated natural killer (NK) cells can kill autologous immature dendritic cells (DCs) in vitro, whereas they spare fully activated DCs. This led to the proposal that activated NK cells might select a more immunogenic subset of DCs during a protective immune response. However, there is no demonstration that autologous DC killing by NK cells is an event occurring in vivo and, consequently, the functional relevance of this killing remains elusive. Here we report that a significant decrease of CD11c+ DCs was observed in draining lymph nodes of mice inoculated with MHC-devoid cells as NK cell targets able to induce NK cell activation. This in vivo DC editing by NK cells was perforin-dependent and it was functionally relevant, since residual lymph node DCs displayed an improved capability to induce T cell proliferation. In addition, in a model of anti-cancer vaccination, the administration of MHC-devoid cells together with tumor cells increased the number of tumor-specific CTLs and resulted in a significant increase in survival of mice upon challenge with a lethal dose of tumor cells. Depletion of NK cells or the use of perforin knockout mice strongly decreased the tumor-specific CTL expansion and its protective role against tumor cell challenge. As a whole, our data support the hypothesis that NK cell-mediated DC killing takes place in vivo and is able to promote expansion of cancer-specific CTLs. Our results also indicate that cancer vaccines could be improved by strategies aimed at activating NK cells. Citation: Morandi B, Mortara L, Chiossone L, Accolla RS, Mingari MC, et al. (2012) Dendritic Cell Editing by Activated Natural Killer Cells Results in a More Protective Cancer-Specific Immune Response. PLoS ONE 7(6): e39170. doi:10.1371/journal.pone.0039170 Editor: Francesco Dieli, University of Palermo, Italy Received March 19, 2012; Accepted May 16, 2012; Published June 19, 2012 Copyright: 2012 Morandi 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: This work has been supported by Associazione Italiana Ricerca sul Cancro http://www.airc.it) IG5624 and IG11650 to GF and IG 8862 to RSA; by Ministero Italiano della Salute - Programma Strategico Ricerca Oncologica (http://www.ministerosalute.it/) to GF; by Fondazione Cariplo 2008–2230 (www. fondazionecariplo.it) and by Ministero Italiano dell’ Istruzione,Universita’,Ricerca, - Progetto Rilevanza Nazionale (http://prin.miur.it/) 2008-WXF7KK to RSA. 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: guido.ferlazzo@unime.it . These authors contributed equally to this work. Introduction Natural Killer (NK) cells, which were originally identified as lymphoid cells capable of lysing a number of tumor cell lines in the absence of previous stimulation in vivo or in vitro [1–3], are now appreciated as multifunctional innate immune cells [4–6]. Their activation is guided by a balance of signals given by different groups of activating [7] and inhibitory receptors [8–11], the latter recognizing MHC class I molecules on target cells. Recently, the key role for a cooperative dialogue between DCs and natural killer (NK) cells in triggering the immune response has emerged [12–17]. It has been shown that their interaction results in a bi-directional activation and in the development of a Th1 and CTL mediated response [18–21]. In humans, at least in vitro, this cross-talk also results in the lysis of immature DCs, while mature DCs are protected [13,15]. The activating receptor NKp30 and DNAM-1 are critical receptors for DC lysis, while resistance to lysis is mediated by up-regulation of MHC class I molecules during DC maturation [15,22–24]. On the basis of these in vitro findings, it has been proposed that the killing of immature DCs by NK cells should promote the survival of the most immunogenic DCs (i.e. mature DCs), favoring initiation of an efficient and protective immune response [25–27]. In vivo, DC/NK-cell interactions might occur in lymphoid organs as well as in non lymphoid tissues [28]. Adoptively transferred, ex vivo generated, immature DCs have been reported to be rapidly eliminated by NK cells via TNF-related apoptosis- inducing ligand (TRAIL) [29,30]. Similarly, it has been shown that the transplantation of alloreactive NK cells can suppress T-cell- mediated graft-versus-host disease by eliminating host DCs [31,32]. During chronic viral infections, an aberrant DC susceptibility to NK cell-mediated lysis resulted in an accumula- tion of poorly immunogenic DCs in lymph nodes, causing progressive immune dysfunction [33]. On the other hand, DC lysis by NK cells could also negatively regulate the duration of virus-specific T cell responses in vivo by limiting exposure of T cells to infected antigen-presenting cells [34]. PLoS ONE | www.plosone.org 1 June 2012 | Volume 7 | Issue 6 | e39170