) Metastatic cancer cells, like trophoblasts of the developing placenta, are invasive and must escape immune surveillance to survive. Complement has long been thought to playa significant role in the tumor surveillance mechanism. Two small integrin-binding glycoproteins expressed by trophoblasts, osteopontin (OPN, ETA-1) and bone sialoprotein (BSP), have been shown to be strongly up-regulated by many tumors with the latter being a positive indicator of the invasive potential of some tumors. We have recently found that circulating solution phase BSP and OPN are complexed with complement factor H and that this interaction blocks their subsequent binding to integrins. We have also shown that recombinant OPN and BSP can protect murine erythroleukemia cells from attack by complement at the membrane phase. Our hypothesis is that the expression of OPN and BSP in tumor cells provides a selective advantage for survival via (a) initial binding to aV beta 3 integrins or CD44 on the cell surface, (b) sequestration of factor H to the cell surface and factor H-mediated dampening of complement mediated cell lysis. This hypothesis will be tested through further characterization of the nature and specificity of the BSP/OPN binding interaction with factor H. Specifically, the structural regions involved in recognition and binding will be determined through site directed mutagenesis of both receptor and ligand using an adenovirus expression system and blocking peptides and antibodies. The ability of BSP and OPN to specifically protect breast cancer cells from complement will initially be tested in vitro using cell lines. A long term goal is to test the model in murine systems (both normal and BSP and OPN knock outs, with chemically induced tumors and implanted tumor cells). This basic model mechanism for tumor cell evasion of host humoral surveillance provides novel insight into developing new diagnostic procedures as well as the potential for new therapeutic regimens involving subversion of tumor cell cloaking from the immune/complement system.
