Wc. Vooijs et al., EFFICACY AND TOXICITY OF PLASMA-CELL-REACTIVE MONOCLONAL-ANTIBODIES B-B2 AND B-B4 AND THEIR IMMUNOTOXINS, Cancer immunology and immunotherapy, 42(6), 1996, pp. 319-328
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.