mesothelial cells from ovarian cancer patients were able to stimulate the clonogenic growth of ovarian tumor cells [15,16]. Mesothelial cells undergo morphologic changes in cancerous ascites, cirrhotic ascites and peritonitis. Indeed, before tumoral peritoneal implan- tation, mesothelial cells have been reported to become hemispheric and exfoliate into the peritoneal cavity [17]. In particular it has been demonstrated that the morphology of mesothelial cells obtained from patients with EOC is different from those obtained from non– cancer-bearing individuals. The mesothelial cells from patients with EOC displayed also a growth advantage suggesting an activated state [9]. Finally by using DNA microarray analysis, Wang et al. demonstrated that peritoneal and subjacent stroma of patients with EOC were transcriptionaly different from that of patients with benign ovarian disorders [9]. The genes that were differentially regulated were implicated in different important processes such as cell adherence, growth and invasion. As illustrated above there is a strong support that the interaction between EOC and its surrounding microenvironment is a primordial step in the development and progression of metastatic EOC. Evidences in the literature are suggesting a ‘‘cross-talk’’ between cancer cells and peritoneal stromal cells. Beyond the development of a peritoneal disease the occurrence of chemore- sistant tumoral clones during chemotherapy remains a major issue in ovarian cancer. It has not yet been clearly illustrated whether mesothelial cells of the peritoneum play an active role in the phenomenon of chemoresistance. Therefore we hypothesized that in the peritoneal Oncologic Trogocytosis cavity mesothelial cells could act as privileged partners of ovarian cancer cells. We isolated non-previously described stromal cells closely associated to cancer cells. These cells conferred chemore- sistance to ovarian cancer cell. The molecular mechanism underlying chemoresistance acquisition is described. Results Isolation and characterization of mesothelial cells Mesothelial cells are known to exfoliate at the beginning of the metastatic process. This suggests that ovarian cancer-specific mesothelial cells could interact with epithelial ovarian cancer cell aggregates (EOCA). We thus isolated EOCA from ascitic fluid from non-previously treated patients with stage IIIc ovarian cancer (Figure 1A, B). Demographic characteristics of these patients are described in Table 1. The EOCA, which were initially strongly aggregated, gradually dissociated as from 10 to 15 days of culture (Figure 1C), enabling us to clone the constitutive mesothelial cells (Hospicells) by limiting dilution and enrichment. As demonstrated in Figure 1C only some stromal cells had the ability to interact with the ovarian cancer aggregates. Cloned Hospicells are large cells with a unique morphology. They have long, thin pseudopods forming a kind of net (Figure 1D) and an active cytoskeleton (Figure 1E, F) able to capture bound cells. Further analysis using electron microscopy studies have shown that EOCA are aggregates of EOC cells and Hospicells (Figure 2A). The Hospicells interact with the EOC cells via their Figure 1. Isolation of Hospicells. A. Cellular aggregates from ascitic fluid of patients with ovarian carcinosis in suspension (640). B. Cellular aggregates on culture plates (640). C. Gradual disaggregation of strongly interacting stromal cells (‘‘Hospicells’’) (thick arrow) and cancer cells (thin arrow) at day 10 days of culture (620). D. Fluorescence microscopy (actin immunostaining) showing the ‘‘hammock-like’’ active cytoskeleton of freshly isolated Hospicells (660). E. Lamellipodal formation at the extremity of the Hospicells containing active actin fibers. F. Details of a Hospicell’s fillopode demonstrating numerous ‘‘comb-like filaments’’. doi:10.1371/journal.pone.0003894.g001 PLoS ONE | www.plosone.org 2 December 2008 | Volume 3 | Issue 12 | e3894