Oncologic Trogocytosis Figure 6. Intercellular transfer or ‘‘oncologic trogocytosis’’. A and B. Flow cytometry of OVCAR3 cells or Hospicells co-cultured with Cell TrackerTM Orange CMTMR-labelled cells for 3 minutes or 3 hours. A. The Hospicells were labelled with the membranous lipophilic dye PKH67B. The OVCAR3 cells were labelled with PKH67. The value given in the plot is the mean PKH67 fluorescence intensity. C. Confocal microscopy of PKH67- stained hospicells (green) and CMTMR-labelled OVCAR3 cells (red) after 3 minutes incubation. D. Uptake of PKH67 by OVCAR3 cells is observed at 3 hours indicating membrane transfer without cytoplasmic transfer. doi:10.1371/journal.pone.0003894.g006 staining. Figure 8A displays the different level of Hospicells staining. As displayed in Figure 8B TMA from chemoresistant patients had significantly higher density of Hospicells compared to chemosensitive patients. Demographic characteristics of the patients included in the study are displayed in Figure 8C. Discussion There are many evidences that preferential invasion of the peritoneum by ovarian cancer is not only due to anatomic proximity but also to an array of molecular signals secreted by cancer cells and stromal cells predisposing the peritoneum to invasion. Indeed the role of molecule-dependant cell-cell interac- tions (integrin b-1, CD44), secreted factors (interleukin 6 and 8, VEGF, FGF2, TGF-b) and matrix metalloproteinases, has been illustrated in several reports [26–30]. However one of the most important partners of cancer cells in the peritoneum are mesothelial cells of the peritoneal surface. As described in the introduction, few studies are addressing the role of these cells in ovarian cancer. In our study, in order to avoid the bias associated to the heterogeneity of peritoneal sampling, we isolated cancer cell aggregates. The hypothesis of our work was that stromal cells among these aggregates would display a high specificity regarding their interactions with ovarian cancer cells. In this study we have isolated stromal cells not previously described that act as host cells. These cells represent a new subset of stromal cells that do not express any of the common markers for other cell types. The co-expression of the cell surface markers CD9, CD10, CD29, CD146 and CD 166 has not been linked to any specific lineage. However they have already been described as part of the complex phenotype of the mesenchymal stem cells in particular the adipose tissue derived stromal cells also called stromal vascular fraction [31]. The Hospicells might therefore represent a differentiated stromal subset of the mesenchymal stem PLoS ONE | www.plosone.org 7 December 2008 | Volume 3 | Issue 12 | e3894