Background: Autologous transplantation improves survival in multiple myeloma patients, however, most eventually relapse. As an attempt towards improving relapse-free survival, we designed a negative-selection purging strategy, to remove myeloma cells from leukapheresis harvests using MAbs specific for Ags on myeloma cells.
Methods: CD38 is highly expressed on myeloma plasma cells, but expressed at lower levels on normal progenitors and absent on in vivo repopulating cells. We evaluated depletion of CD38-expressing cells, with or without depletion of B-cell Ag-expressing cells. Using myeloma BM or blood cells diluted into allogeneic G-CSF primed leukapheresis cells, bispecific tetrameric Ab complexes that bind dextran iron particles were used to label and retain cells in a magnetic column, StemSep. Depletion efficacy was measured by semi-quantitative allele-specific oligonucleotide (ASO)-PCR amplification of patients' clonotypic IgH gene.
Results: Low (0.2 microg/mL) concentrations of anti-CD38 with CD19 and CD20 complexes depleted approximately 3-5 logs of clonotypic cells, with recovery of approximately 19% of colony-forming cells, approximately 50% primitive progenitors measured by LTCIC and retention of non-obese diabetic /SCID engrafting ability. Scale-up experiments using leukapheresis harvests and 0.5-1 x 10(10) cell capacity columns demonstrated no loss of log depletion of highly positive cells, or recovery of unlabelled cells.
Discussion: These results compare favorably with other purging techniques and allow the retention of most normal BM cells, including T cells, which may be important for immunity. These results support the development of a clinical trial using this strategy for purging myeloma cells.