Targeting of cancer cells with monoclonal antibodies specific for C3b(i)

Purpose: Te goal of this research is to determine the feasibility of an imm unotherapeutic approach based on the use of monoclonal antibodies (mAb) to target complement activation fragments on opsonized cancer cells. methods: We investigated whether treatment of LNCaP and C4-2 human prostate cancer c ell lines with normal human serum would allow for deposition of sufficient amounts of the complement-activation protein C3b and its fragments [collect ively referred to as C3b(i)] such that these proteins could serve as cancer -cell-associated antigens for targeting by mAb. Radioimmunoassays, flow cyt ometry, and magnetic purging with specific immunomagnetic beads were used f or the analyses. Results: In vitro opsonization of human prostate cancer ce lls with normal human serum resulted in deposition of C3b(i) in sufficient quantity (approx. 1000,000 molecules/cell) for the cells to be targeted in a variety of protocols. We found that Cr-51-labeled and C3b(i)-opsonized ca ncer cells could be specifically purged at high efficiency (95%-99%) using anti-C3b(i) mAb covalently coupled to magnetic beads. Flow-cytometry experi ments indicated that most normal white cells were not removed under similar conditions. Opsonization of cancer cells with sera from men with prostate cancer led to lower levels of cell-associated IgM and, subsequently, lower amounts of C3b(i) deposited than in normal subjects. Prototype experiments suggested that this deficiency could be corrected by addition of IgM from n ormal donor plasma. Conclusion: mAb directed against complement-activation products may provide new opportunities to deliver diagnostic and therapeuti c agents selectively to cancer cells and tumor deposits. These opportunitie s may include ex vivo purging of C3b(i)-opsonized cancer cells prior to aut ologous bone marrow or stem cell transplantation.