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Blood, 15 March 2007, Vol. 109, No. 6, pp. 2557-2564.
Prepublished online as a Blood First Edition Paper on November 21, 2006; DOI 10.1182/blood-2006-08-042424.
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Submitted August 22, 2006
Accepted November 6, 2006
The growth factor fusion construct containing B-lymphocyte stimulator (BLyS) and the toxin rGel induces apoptosis specifically in BAFF-R positive CLL cells
Ramadevi Nimmanapalli, Mi-Ae Lyu, Min Du, Michael J Keating, Michael Rosenblum, and Varsha Gandhi*
Dept of Experimental Therapeutics, The University of Texas M.D. Anderson Cancer Center, Houston, TX
Dept of Leukemia, The University of Texas M.D. Anderson Cancer Center, Houston, TX
* Corresponding author; email: vgandhi{at}mdanderson.org.
The cytokine B lymphocyte stimulator (BLyS) mediates its effect through cell-surface receptors BAFF-R, TACI and BCMA. BLyS receptors are expressed only on B-cells and not present in other normal cells including normal T-lymphocytes. Chronic lymphocytic leukemia (CLL) is a B-cell disease and CLL lymphocytes express BLyS receptors. Gelonin, a type 1 ribosome-inactivating toxin lacks cell membrane binding domain and hence is non-toxic to intact cells. We generated a construct of recombinant gelonin (rGel) fused to BLyS to specifically target quiescent B-CLL lymphocytes. The construct rGel/BLyS specifically binds and internalizes through BAFF-R into CD19+ve B-CLL lymphocytes and induces apoptosis at nanomolar concentrations. In contrast, rGel alone was not able to internalize in to these leukemic lymphocytes. Mechanistically, the rGel/BLyS construct inhibits protein synthesis with an IC50 of <3 nM compared to >5,000 nM for rGel toxin alone. This rGel/BLyS-mediated decrease in protein synthesis was associated with a decline in short-lived proteins such as MCL-1 and XIAP; the two survival proteins in B-CLL. There was a strong relationship between a decrease in these proteins and the cleavage of PARP, a hall-mark feature of apoptosis. Taken together, these data suggest that the rGel/BLyS fusion toxin may have potential therapeutic efficacy for B-CLL patients.

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