1. ¹Microvascular Research Laboratories, Department of Physiology, University of Bristol, Bristol, UK
2. ²Department of Pathology, Frenchay Hospital, Bristol, UK
3. ³Department of Cardiac and Vascular Sciences, Dermatology Unit, St George's Hospital Medical School, London, UK
4. ⁴Department of Plastic Surgery, Frenchay Hospital, Bristol, UK
5. ⁵Department of Physiology, St George's Hospital Medical School, London, UK

Correspondence: Dr D Bates, Microvascular Research Laboratories, Department of Physiology, University of Bristol, Preclinical Veterinary School, Southwell Street, Bristol BS2 8EJ, UK. E-mail: dave.bates@bris.ac.uk

⁶These authors contributed equally to this work.

Received 23 May 2006; Revised 29 September 2006; Accepted 6 October 2006; Published online 27 November 2006.

Abstract

The mechanisms that cause tumors such as melanomas to metastasize into peripheral lymphatic capillaries are poorly defined. Non-mutually-exclusive mechanisms are lymphatic endothelial cell (LEC) chemotaxis and proliferation in response to tumor cells (chemotaxis-lymphangiogenesis hypothesis) or LECs may secrete chemotactic agents that attract cancer cells (chemotactic metastasis hypothesis). Using migration assays, we found evidence supporting both hypotheses. Conditioned medium (CM) from metastatic malignant melanoma (MMM) cell lines attracted LEC migration, consistent with the lymphangiogenesis hypothesis. Conversely, CM from mixed endothelial cells or LECs, but not blood endothelial cells, attracted MMM cells but not non-metastatic melanoma cells, consistent with the chemotactic metastasis hypothesis. MMM cell lines expressed CCR7 receptors for the lymphatic chemokine CCL21 and CCL21 neutralizing antibodies prevented MMM chemotaxis in vitro. To test for chemotactic metastasis in vivo tumor cells were xenotransplanted into nude mice 1 cm from an injected LEC depot. Two different MMM grew directionally towards the LECs, whereas non-metastatic melanomas did not. These observations support the hypothesis that MMM cells grow towards regions of high LEC density owing to chemotactic LEC secretions, including CCL21. This chemotactic metastasis may contribute to the close association between metastasizing tumor cells and peri-tumor lymphatic density and promote lymphatic invasion.