Purpose: Our recent study revealed that CD55-high population in breast cancer cell line was resistant to apoptosis and formed colonies in vitro more efficiently than CD55-low population. The present study was conducted to examine whether CD55-high population in breast cancer cell line possesses higher tumorigenic potential in vivo and presence of CD55-high cells in breast cancer affects clinicopathologic behavior of patients.

Experimental Design: CD55-high and CD55-low population was sorted from breast cancer cell line, injected into immunodeficient mice, and the resultant tumor volume was measured. CD55 expression was immunohistochemically examined in clinical samples from 74 cases with breast cancers, and cases with >1% of tumor cells showing high level of CD55 expression were categorized as CD55 high.

Results: The xenotransplanted tumor volume derived from CD55-high population was significantly larger than that from CD55-low population. Fifty (67.6%) of 74 cases of breast cancer were CD55-high. A significant correlation was observed between CD55-high character and relapse rate (P < 0.001). Univariate analysis showed that tumor size (P = 0.005) and CD55 expression (P = 0.005) were unfavorable prognostic factors. Multivariate analysis revealed that the tumor size (P = 0.013) and CD55 expression (P = 0.011) were independent prognostic factors.

Conclusions: CD55 play an important role in tumorigenesis of breast cancer, and presence of small population of cells with strong CD55 expression would be sufficient to predict poor prognosis of patients.