Lymphatic endothelium-specific hyaluronan receptor LYVE-1 is expressed by stabilin-1+, F4/80+, CD11b+ macrophages in malignant tumours and wound healing tissue in vivo and in bone marrow cultures in vitro: implications for the assessment of lymphangiogenesis

Lymphangiogenesis is a novel prognostic parameter for several cancers that is preferentially quantified by immunohistochemistry of the lymphatic endothelium‐specific hyaluronan receptor LYVE‐1. Recently, the specificity of LYVE‐1 was challenged by serendipitous observations of LYVE‐1 expression in rare tissue macrophages. As expression of the hyaluronan receptor‐like molecule stabilin‐1 is shared by sinusoidal endothelium and macrophages, a thorough analysis of LYVE‐1 expression was performed using macrophage‐specific markers in vivo and in vitro. In murine tumour models and excisional wound healing, LYVE‐1 expression occurred in a subset of CD11b+, F4/80+ tissue macrophages that preferentially co‐expressed stabilin‐1. Upon comparison of single‐ and double‐labelling immunofluorescence, it became apparent that LYVE‐1+ macrophages mimic sprouting and collapsed lymphatic vessels. In vitro, LYVE‐1 expression was induced in 25–40% of murine bone marrow‐derived macrophages upon exposure to B16F1 melanoma‐conditioned medium and IL‐4/dexamethasone. By FACS analysis, 11.5% of bone marrow‐derived macrophages were LYVE‐1+, stabilin‐1+ double‐positive, while 9.9% were LYVE‐1+, stabilin‐1− and 33.5% were LYVE‐1−, stabilin‐1+. Northern and western analyses confirmed expression of LYVE‐1 mRNA and protein in bone marrow‐derived macrophages. In the light of the current debate about true endothelial trans‐differentiation versus endothelial mimicry of monocytes/macrophages, LYVE‐1+, stabilin‐1+ non‐continuous endothelial‐like macrophages will require further developmental and functional analyses. In conclusion, the findings imply that LYVE‐1 staining must be supplemented by double labelling with macrophage markers in order to differentiate clearly between LYVE‐1+ lymphatics and LYVE‐1+ tumour‐infiltrating macrophages. This improved approach will help to clarify the prognostic significance of lymphangiogenesis in malignant tumours. Copyright © 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.