Ricin binding and protein synthesis inhibition in human hematopoietic cell
lines
JE Leonard, CD Grothaus and R Taetle
UC San Diego Cancer Center, La Jolla, CA 92037.
Previous studies showed that human blood cells exhibited varying
sensitivities to ricin. To investigate the basis for these differences,
ricin binding to human hematopoietic cell lines was assessed and correlated
with in vitro ricin sensitivities. Resistant mutants were also isolated and
characterized. Ricin binding to CEM cells was rapid, time-dependent, and
blocked by unlabeled ricin, but not albumin; ricin binding approached
saturation at 3 mumol/L. Scatchard analyses showed multiple classes of
binding sites, with maximum and minimum Kd values estimated at 1.5 x 10(-8)
mol/L and 2.5 x 10(-7) mol/L. At 4 degrees C, membrane-bound ricin
dissociated slowly from the cell surface in the presence of unlabeled
ricin, but greater than 95% of the surface-bound ricin was removed with 0.1
mol/L lactose. At 37 degrees C, ricin dissociated from the cell surface
with biphasic kinetics. Ricin uptake at 37 degrees C increased linearly for
15 to 30 minutes and plateaued at levels representing 12% to 29% of the
amount of ricin bound at 4 degrees C, depending on the cell line. Ricin
binding at 4 degrees C varied two- to fivefold among hematopoietic cell
lines and was reduced approximately tenfold by incubation with lactose.
When compared with parent CEM cells, ricin-resistant CEM variants showed a
greater than 95% reduction in ricin binding and showed no detectable
binding with lactose added. However, these cells were as sensitive as
parent CEM cells to an anti-T-cell ricin immunoconjugate. For all cells
examined, there was a close correlation (r = +.9) between ricin bound per
cell and in vitro ricin sensitivity. Human hematopoietic cells show widely
varying ricin binding, indicating major differences in the carbohydrate
content or structure of surface glycoproteins and glycolipids. These
variations are probably the major determinant of nonspecific toxicity of
ricin immunoconjugates.
Volume 72,
Issue 4,
pp. 1357-1363,
10/01/1988
Copyright © 1988 by The American Society of Hematology
