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Growth and survival of breast tumor cells is well known to require angiogenesis, i.e., new blood vessel formation by proliferating endothelial cells. Expression of endoglin (CD105), a co-receptor of the TGF-beta receptor complex, is markedly enhanced on proliferating endothelial cells within the breast tumor microenvironments. Although CD105 is known to play an important role in regulating the survival of proliferating endothelial cells, its role as a target for DNA vaccines has not been extensively investigated. Thus, we established experimental systems to examine 1) the extent of anti-angiogenic/anti-tumor effects achieved with an oral DNA vaccine encoding murine CD105 carried by double attenuated (Dam-, AroA-) Salmonella typhimurium to secondary lymphoid organs, i.e., Peyer’s Patches, and 2) determine whether the knock-down of CD105 on highly metastatic murine 4T1 breast tumor cells achieved by specific siRNA affects their invasive and migratory characteristics. We demonstrate here that the breast carcinoma cell line, 4T1 strongly expresses endoglin on the cell surface, whereas another breast carcinoma cell line, D2F2 does not. However, even though the D2F2 cell line does not express endoglin, we found that in vivo proliferating endothelial cells within the D2F2 tumor microenvironment strongly express it. In fact, BALB/C mice immunized with a DNA vaccine encoding endoglin and co-expressing secretory chemokine CCL21 effectively suppressed D2F2 pulmonary metastases when compared to mice vaccinated with either endoglin or CCL21. In addition, D2F2 target cells stably transfected with endoglin were more sensitive to CTL killing than non-expressors. These results suggest that a DNA vaccine encoding both endoglin and secretory CCL21 is effective in inducing T-cell mediated immune responses against proliferating endothelial cells. In summary, we validated murine endoglin as an effective target for a DNA-based vaccine encoding it which markedly suppresses breast cancer metastases in a prophylactic setting.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]