Purging peripheral blood progenitor cell grafts from lymphoma cells: quantitative comparison of immunomagnetic CD34+ selection systems.

Autologous peripheral blood progenitor cell (PBPC) transplantation is increasingly being used for treatment of indolent lymphomas. Since involvement of bone marrow and peripheral blood is frequent and methods to reduce the lymphoma cell load of PBPC grafts are thus highly desirable, we have studied purging of PBPC comparing two immunomagnetic CD34+ selection systems (VarioMACS, Miltenyi Biotech; Bergisch Gladbach, Germany, and Isolex50 System, Baxter; Irvine, CA). Samples of freshly collected mobilized PBPCs were contaminated with BALM-3 or KARPAS422 lymphoma cells that had been labeled with the fluorescent DNA stain Hoechst 33342. The mixture was subjected to separation with the two devices and the resulting "CD34+" fractions were screened for lymphoma cells by limiting dilution using fluorescence microscopy and by polymerase chain reaction amplification of t(14;18) or CDRIII-rearrangements. Both devices yielded comparable purities (MACS 97% [87%-99%]; Isolex 97% [84%-99%]) and recoveries of CD34+ cells (MACS 56% [30%-81%]; Isolex 45% [24%-63%]). The overall depletion of lymphoma cells was 3.9 log (2.6-5.9), however, residual contaminating cells were seen in every single experiment. The purging efficacy was dependent on the type of contaminating lymphoma cell (BALM-3: 4.4 log [3.7-4.8]; KARPAS422: 3.2 log [2.6-4.2]; p = 0.018), whereas the type of selection system used or the percentage of CD34+ cells in the starting material had no influence. We conclude that excellent purification of CD34+ cells leading to a vigorous depletion of lymphoma cells can be achieved with both CD34+ selection systems investigated. However, the efficacy of purging may greatly differ between individual lymphomas, and complete eradication of contaminating cells from PBPC grafts may rarely be achieved with CD34+ selection alone.