Melanoma Cell Extravasation Under Flow Conditions is Modulated by Leukocytes and Endogenously Produced Interleukin 8

Attachment of tumor cells to the endothelium (EC) under flow conditions is critical for the migration of tumor cells out of the vascular system to establish metastases. Innate immune system processes can potentially promote tumor progression through inflammation dependant mechanisms. White blood cells, neutrophils (PMN) in particular, are being studied to better understand how the host immune system affects cancer cell adhesion and subsequent migration and metastasis. Melanoma cell interaction with the EC is distinct from PMN-EC adhesion in the circulation. We found PMN increased melanoma cell extravasation, which involved initial PMN tethering on the EC, subsequent PMN capture of melanoma cells and maintaining close proximity to the EC. LFA-1 (CD11a/CD18 integrin) influenced the capture phase of PMN binding to both melanoma cells and the endothelium, while Mac-1 (CD11b/CD18 integrin) affected prolonged PMN-melanoma aggregation. Blocking E-selectin or ICAM-1 (intercellular adhesion molecule) on the endothelium or ICAM-1 on the melanoma surface reduced PMN-facilitated melanoma extravasation. Results indicated a novel finding that PMN-facilitated melanoma cell arrest on the EC could be modulated by endogenously produced interleukin-8 (IL-8). Functional blocking of the IL-8 receptors (CXCR1 and CXCR2) on PMN, or neutralizing soluble IL-8 in cell suspensions, significantly decreased the level of Mac-1 up-regulation on PMN while communicating with melanoma cells and reduced melanoma extravasation. These results provide new evidence for the complex role of hemodynamic forces, secreted chemokines, and PMN-melanoma adhesion in the recruitment of metastatic cancer cells to the endothelium in the microcirculation, which are significant in fostering new approaches to cancer treatment through anti-inflammatory therapeutics.