SUMMARY:It is still unclear which membrane-bound regulatory proteins (mCRP) are important in vivo to protect tumor cells from complement-mediated damage. To address this question, the expression levels of CD46, CD55, and CD59 were measured semi-quantitatively in situ on renal cell carcinomas and compared with the expression level and cellular distribution of these mCRP in proximal tubuli within each patient (n ϭ 31). It was also determined whether the expression of mCRP on tumor cells is associated with deposition of C3d and C5b-9. CD46 expression was decreased on tumor cells; in contrast, CD55 was expressed on tumor cells (12 out of 31 samples), while it was not detected on proximal tubular epithelial cells (PTEC). Also, expression of CD59 on tumor cells was increased as compared with its expression on PTEC. Furthermore, the localization on the cell surface of mCRP as observed on PTEC was altered on tumor cells. Because expression of mCRP may limit a complement-mediated anti-tumor response, we determined whether complement deposition was associated with the expression level of CD46, CD55, and CD59. The presence of C3d on tumor cells was associated with a low expression level of CD46 (p Ͻ 0.02). The expression level of CD46 was also associated with a low tumor stage (p Ͻ 0.04). The results suggest that in vivo CD46 plays a role in the protection of human renal tumor cells from complement-mediated injury. (Lab Invest 2000, 80:335-344).

Tumor cells may inhibit the induction of a complement-mediated inflammatory response through overexpression of membrane-bound regulators of complement activation. Therefore, it is of interest to determine the most efficient approach to block these membrane-bound complement regulators on tumor cells. In the present study, we first generated a bispecific mAb directed against both CD55, using the functional blocking mAb MBC1, and the highly expressed HLA class I molecule as a model for a tumor-associated Ag, using the mAb W6/32. Tumor cells opsonized with bispecific mAb W6/32*MBC1, then exposed to complement and subsequently stained for C3 deposition, were assessed by flow cytometric analysis. We found that opsonization with W6/32*MBC1 resulted in a 92% enhancement of C3 deposition on renal tumor cells as compared with opsonization with W6/32 alone and a 17% enhancement of the C3 deposition as compared with incubation with a mixture of both parental mAb. Based on these results, we developed a bispecific mAb recognizing both CD55 and the relatively low expressed renal tumor-associated Ag G250. Increasing concentrations of the bispecific mAb G250*MBC1 resulted in a 25 to 400% increase in C3 deposition on renal tumor cells as compared with C3 deposition in the presence of the parental mAb G250 alone. G250*MBC1 enhanced C3 deposition by 21% in comparison with a mixture of both parentals. Furthermore, opsonization of tumor cells with G250*MBC1 rendered these cells more sensitive to complement-mediated lysis. In conclusion, the bispecific mAb G250*MBC1 induces deposition of C3 and tumor cell lysis more efficiently than G250 alone.