Blocking a novel 55 kDa melanoma-associated cell surface antigen inhibits the development of spontaneous metastases and interactions with frozen lung section.

We recently identified a novel 55-kDa cell-cell adhesion protein (p55) whose expression is upregulated in primary melanomas in the transition from radial growth phase to vertical growth phase. However, the functional role of p55 in various steps of the metastatic process had not been investigated. We provide evidence that subcutaneous injection of metastatic melanoma variant T1P26 in immunosuppressed newborn rats rapidly caused spontaneous metastatic lung lesions that could be readily detected by histochemical analysis with the anti-p55 monoclonal antibody (MAb) LY1. Subsequently, we were able to demonstrate that multiple subcutaneous injections of the LY1 MAb starting on the same day after tumor cell inoculation of T1P26 cells specifically blocked the formation of spontaneous lung metastases, yet had no effects on primary tumor growth, suggesting a critical role of p55 in the earlier steps of the intravasation process. To study later stages in spontaneous metastasis, we investigated the role of p55 in organ-specific cell adhesion of tumor cells in vitro. We showed that the T1P26 variant attached preferentially to lung frozen sections compared with other organs, reflecting the pattern of organ involvement of metastasis in vivo and that LY1 significantly blocked this interaction. However, no significant differences in attachment to lung sections were observed between the parental melanoma cell line M(4)Beu and its derived variant, although cellular topography analysis indicated a preferential attachment of a T1P26 variant on specific compartments of the lungs such as the perialveolar components, the endothelium and the vessel lumen of pulmonary venules. Attachment of the T1P26 variant to lung sections is not due to alterations of tumor cell adherence to basement membrane matrix by the LY1 MAb, suggesting that p55 is involved in cellular adhesion with cellular elements of the lung. p55 could represent a new functional constituent that contributes to the metastatic spread of melanoma cells by promoting the intravasation process and subsequent specific interactions between tumor cells and the target lung organ. Copyright 2002 Wiley-Liss, Inc.