EFFICACY AND TOXICITY OF PLASMA-CELL-REACTIVE MONOCLONAL-ANTIBODIES B-B2 AND B-B4 AND THEIR IMMUNOTOXINS

Immunotherapy based on the delivery of toxic agents to the tumor site using monoclonal antibodies (mAb) may be a promising modality in the t reatment of hematological malignancies. In the selection of mAb, both for ex vivo but even more for in vivo therapy, not only their reactivi ty to the neoplastic cells should be considered, but also reactivity t o other body constituents. Here we describe the screening of two human plasma-cell-reactive mAb B-B2 and B-B4, which may be used for immunot herapy of multiple myeloma. Cross-reactivity of B-B2 and B-B4 was dete rmined by immunohistochemistry on a series of tissues. This revealed f or both B-B2 and B-B4 a strong staining of epithelial cells in various organs, e.g. lung, liver, skin, kidney and gut, while only a weak and diffuse staining was seen with endothelial cells. In bone marrow reac tivity was only found with plasma cells and not with hemopoietic precu rsors (CD34(+) cells). Immunotoxins from B-B2 and B-B4 were constructe d by coupling them to the plant-derived ribosome-inactivating protein saporin. Both B-B2 and B-B4 immunotoxins appeared to be efficient in s pecific inhibition of protein synthesis in plasma cell lines (IC50 res pectively 1 nM and 0.1 nm). The immunotoxins were also tested on epith elial cell line A431, on liver cell line HepG2 and on human umbilical vein endothelial cells. The epithelial cell line A431 was reactive wit h both B-B2 and B-B4, but was only inhibited by B-B4 immunotoxin. Cell with both mAb, but was not immunotoxin. The endothelial cells showed no reactivity with B-B2 and B-B4 and were not inhibited by either immu notoxin. Bone marrow treated with B-B2 and B-B4 immunotoxin did not sh ow a decrease in colonies of hemopoietic precursor cells. Incubation o f multiple-myeloma-derived bone marrow with these immunotoxin resulted in a clear decrease of the number of plasma cells. From these data we conclude that B-B2 and B-B4 immunotoxin can be used for ex vivo bone marrow purging. Discrepancies were found between immunohistochemistry, binding assays and cytotoxicity assays with the mAb and the immunotox in, which underlines the necessity for these various assays as a precl inical screening.