Positive and negative selection to reduce tumour contamination in peripheral blood stem cell harvests

Peripheral blood progenitor cells used during high dose treatments for malignancy may be contaminated with tumour cells that could later contribute to recurrence. CD34+ selected harvests still contain tumour cells and an additional negative selection may be capable of reducing this contamination. We have assessed a two-stage technique in which a CD34+ selection is followed by a tumour specific depletion stage using a B cell or breast cancer specific antibody panel. Initial small-scale selections on 11 patients with NHL and breast cancer showed that cell loss was greatest following the CD34+ selection with a median yield of 38.8 per cent (range 17. 2-56.4 per cent). The addition of the depletion stage resulted in a minimal loss of CD34+ cells with a yield for this step of 94.2 per cent (range 77.5-99.3 per cent). Clinical scale selections were performed on seven patients with CLL and a median of 2.8x10(6)/kg CD34+ cells (range 1.5-6.1x10(6)/kg) were collected. Cell recovery was 53.3 per cent following CD34+ selection and 76.9 per cent following the tumour specific depletion stage, resulting in a final product containing a median of 1.0x10(6)/kg CD34+ cells (range 0. 55-2.0x10(6)/kg). All unmanipulated harvests were heavily contaminated with tumour cells (median contamination 10.2 per cent, range 2.0-83.1 per cent) as measured by flow cytometry and a median 4.7 log (range 3-5 log) tumour cell purge was produced following two-stage selection. Six of the patients have received cells manipulated in this way with median engraftment times of neutrophils>0.5x10(9)/l=16 days (range 13-20 days) and platelets>20x10(9)/l=16.5 days (range 11-42 days). At a median follow-up of 25 months, these transplanted patients remain well and in molecular complete remission.