Elimination of leukemia cells from human bone marrow using floating beads.

Abstract

A new in vitro immunophysical method of removing leukemia or lymphoma cells from autologous bone marrow is described. This new technique makes use of low-density polypropylene beads (density: 0.91) coated with a monoclonal antibody anti-CALLA (antibody ALB2). To ascertain its ability to selectively remove human B/pre-B hematopoietic cells, this technique was applied to normal human bone marrow cell suspensions contaminated with 1-5% of tumor cells. Samples were incubated with the floating beads at 4 degrees C on a rotating wheel for 60 min, followed by a 10-min decantation period, after which the beads bearing the tumor cells floated on the surface, whereas unbound normal marrow cells remained in suspension and were easily recovered free of beads. To demonstrate the feasibility of our method, 2 types of assays were carried out, one using target cell radiolabeled with 111indium, and the other a clonogenic assay. The first assays were to calibrate the different parameters (cellular density, quantity of beads, incubation time) with tumor cell lines: Namalwa (CALLA+) and Molt 4 (CALLA-). These 2 cells lines being able to clone, it is hard to envisage clonogenic assays. In this case, it is very hazardous to evaluate correctly the remaining clonogenic units of Namalwa cells. It is why radiolabelling assays were used for these first experiments. The second assays were to study a model close to the clinical setting and to control the safety of the beads on normal bone marrow cells. In this case, the mixture experiments in which only Namalwa cells were able to clone were evaluated with clonogenic assays, which are more sensitive than radiolabeling assays. A 3- to 4-log reduction of tumor load was achieved with 1-step treatment, and an average of 5-log depletion was obtained by repeating the process twice, as ascertained by the clonogenic assay. Viability, average recovery of nucleated cells, and stem cells potential following the purge were excellent.