Tumors polarize their microenvironment to escape immune-mediated destruction and to promote their growth. Macrophages respond to microenvironmental signals and can be activated toward a classical (M1) or an alternative (M2) phenotype. Tumor-associated macrophages (TAM) resemble to M2 macrophages with a phenotype characterized by the overexpression of IL10, scavenger receptor SR-A, CD68 and MR phenotype but hampered levels of IL-12. M2, in contrast to M1 macrophages, promote TH2 immune responses as well as tissue remodelling and M2 infiltration in solid tumors has been shown to significantly correlate with poor prognosis. Recent evidences suggest that macrophage differentiation towards TAM involves a “chemical conversation” via exchange of unknown soluble extracellular mediators between tumor cells and macrophages. In addition, previous work showed that the cross-linking mannose receptor (MR) with an anti-MR mAb could cause dendritic cell differentiation into APCs promoting T-cell anergy.

We hypothesized that MR expressed by macrophages could be bound by tumor-released GPI-anchored glycoproteins through the GPI-anchor mannose residues freed by cleavage. We also hypothesized that MR binding by GPI-anchor could trigger macrophage polarization toward TAMs. To test our hypotheses, we established an in vitro model system of cell co-culture in transwell allowing chemical exchanges between human monocyte-derived macrophages and human ovarian cancer cell line Ovcar3 that sheds mesothelin, a cancer biomarker and a GPI-anchored glycoprotein. Macrophage phenotype changes were monitored by flow cytometry and qRT-PCR. We also isolated three novel recombinant antibodies (scFv) against MR by magnetic and flow sortings from a novel yeast-display human scFv library. We describe here for the first time that tumor-released mesothelin bound to macrophage cell surface in presence of calcium, and that mesothelin binding to macrophages could be blocked by the anti-MR scFv. Furthermore, macrophage phenotype switch toward M2 during coculture with tumor cells could be prevented by one of the anti-MR scFv. Finally, we demonstrated that M1 phenotype could be rescued during coculture with tumor cells with one of the anti-MR scFv. To further characterize the role of MR in vivo, we have implanted mannose receptor knock-out mice intraperitoneally (IP) with luminescent-labeled ovarian cancer cells that overexpress mesothelin. We are now monitoring the tumor growth by in vivo imaging.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3809.