been studied. CD146, the melanoma cell adhesion molecule (M- CAM), is a cell adhesion molecule currently used as a marker for endothelial cell lineage. Its function is still poorly understood, but evidence points to it being part of the endothelial junction associated with the actin cytoskeleton. It is expressed, activated human T cells, endothelial progenitors such as angioblasts and mesenchymal stem cells, and strongly expressed on blood vessel endothelium and smooth muscle [44]. In Ovarian cancer, M- CAM is a marker of early relapse and poorer outcome in EOC. In particular, M-CAM expression identifies a subgroup of front-line therapy-responding patients who undergo dramatic relapses [45]. Mesenchymal stem cells expressing this marker seem to be associated with perivascular cells surrounding the blood vessels [46]. The morphology of these cells is perfectly adapted to their role as they have many pseudopodia that allow large contacts with cancer cells. As we observed a specific and intense adhesion between the two cell types, we hypothesized that the Hospicells would play a role in ovarian cancer cell physiology. Intercellular communication can lead to acquisition of complex phenotypes. Several mechanisms can occur during inter-cellular communication such as molecules transferring through gap- junctions, coupling through ligand receptor interactions and finally transfer of membranes proteins leading to new-cell surface proteins conferring new properties to the accepting cells [20,36– 38]. Levchenko et al. have described intercellular transfer of functional P-glycoprotein among several tumor cell lines [47]. This inter-cellular protein transfer was able to induce chemoresistance in previously sensitive cells. As one of the major challenges in ovarian cancer treatment remains chemo-resistance, we investi- gated the role of Hospicells in the survival of cancer cells treated with different chemotherapeutic agents. We were able to describe the ability of a subset of normal cells to confer chemo-resistance to ovarian cancer cells through inter-cellular contact. Two findings were important in the chemoresistance assay performed: the specificity of the Hospicells to confer chemoresistance compared to other cell types used as controls and the importance of the inter- cellular adherence. All together these two findings suggest that Hospicells can be a crucial key in the chemoresistance phenom- enon, and their presence might have consequences on patients’ prognoses. Moreover the necessity of the contact between the two cells rules out the role of secreted factors and confirms the role of the peritoneal cells ’’per se’’ in the occurrence of ovarian tumor chemoresistance. Several mechanisms can be responsible for chemoresistance acquisition. The role of the MDR proteins in particular in ovarian carcinoma has been widely illustrated in the literature [24,25]. We were able to demonstrate the presence of MDR proteins on Hospicells from several different patients with a variable expression between the patients. However the expression of the functional P-gp and LRP was constant. The ability of Verapamil to revert the chemoresistance in the co-culture also confirmed the role of the MDR proteins. The MDR proteins of interest in this study were P-glycoprotein (P-gp, MDR1, ABC B1), a membrane glycoprotein. Its resistance phenotype is similar and includes anthracyclines (doxorubicin), vinca-alkaloids, epidophyloxins and taxanes and it has also been related to carboplatin resistance [18,19,24,25]. Several mechanisms can underlie the occurrence of chemore- sistance after inter-cellular contact. However the specificity, the crucial role of intercellular interaction, and the morphology of these interactions displayed by electronic microscopic analysis suggested the occurrence of ‘‘oncologic synapses’’ as compared to ‘‘immunological synapses’’. Indeed lymphoid cells (effectors) will Oncologic Trogocytosis initiate their interactions with surveyed cells (targets) by setting synapses that enable their surface receptors to interact with ligand, to concentrate activation signals, and finally to deliver effectors functions [48,49]. The lymphocyte cell surface in this contact area makes small bridges with the target cell and captures patches of its membrane on its own cell surface, an active process referred to as trogocytosis [48]. Using the same experimental designs, we describe ‘‘trogocytosis’’ between a cancer cell (as an effectors) and a normal host cell (as a target) with a transfer of MDR proteins from the host cells to the cancer cells. Trogocytosis enables a better survey by the immune cells in the classical ‘‘immunological setting’’. The failure of such mechanisms can play a role in immuno-evasion. Similarly in the ‘‘oncologic setting’’ trogocytosis might help cancer cells acquire new properties spontaneously or under selective pressure (initiation of metastasis, chemotherapy). The correlation of Hospicells’ infiltrate with the patients’ response to neo-adjuvant chemotherapy displayed in the prelim- inary clinical study might lead to the prediction of spontaneous chemoresistance in patients and to optimisation of the chemo- therapy regimen. One of the limitation of our TMA study is the small number of patients included, however to avoid bias linked to the heterogeneity of ovarian tumors and management protocols we have selected only patients treated by neo-adjuvant chemo- therapy. Moreover chemosensitivity was rigorously defined as the absence of any active tumoral infiltrate at the time of final surgery. These findings have to be confirmed in a larger independent set of patients to raise clinical relevance. The role of the host as major actor in controlling neoplasic disease was illustrated in several reports underlying the role of the immunologic activation in patients’ prognoses. Galon et al. demonstrated that a sign of an immune response within colorectal cancers was associated with the absence of pathological evidence of early metastatic invasion and with prolonged survival [50]. These data, as well as our present data on acquired chemoresis- tance of ovarian cancer cells by MDR protein transfer from Hospicells, suggest that the host has a key role in the development or control of neoplastic disease. The determination of cellular mechanisms and pathways leading to intercellular recognition and ‘‘oncologic trogocytosis’’ with Hospicells could lead to the identification of novel therapeutic approaches targeting intercel- lular interactions. Materials and Methods Cell cultures Mesothelial cells interacting with ovarian carcinoma cells were isolated from ascitis of non-previously treated patients with stage III ovarian cancer undergoing ascitis evacuation for clinical discomfort following routine protocols approved by the IRB of the Hospital Hoˆtel-Dieu, Paris. As ascitis evacuation is part of the routine management of patients in the medical oncology department of Hoˆ tel-Dieu only oral consent was obtained from the patients. The ascitis were then de-identified and addressed to the research laboratory. As the laboratory and hospital have two different locations there was no way to link the ascitis to the patients’ file. Five different patients’ ascitis were used in this study; ascitis fluids were centrifuged at 800 rpm for 1 minute. Lympho- cytes and erythrocytes were separated from cancer cell aggregates using a Ficoll procedure; dilution method was used to select ovarian cancer cells aggregated on mesothelial cells. The aggregates were separated using trypsin (Mediatech, Inc). Using serial dilution and enrichment, mesothelial cells (Hospicells) interacting specifically with the ovarian cancer aggregates were PLoS ONE | www.plosone.org 10 December 2008 | Volume 3 | Issue 12 | e3894