Elimination of chemoresistant myeloma clonogenic cells from human bone marrow by monoclonal antibody and complement

We examined the effectiveness of a previously characterized plasma cell-reactive monoclonal antibody (MoAb), MM4, in eliminating multi-drug resistant (MDR1) multiple myeloma (MM) clonogenic colony-forming cells (CCCs). MDR1 sublines with 6-fold (RPMI8226/DOX6) and 40-fold (RPMI 8226/DOX40) resistance to doxorubicin (DOX) were selected from the chemosensitive MM parent line RPMI 8226/S. Both sublines remained reactive with plasma cell MoAbs MM4 and PCA-1, as measured by flow cytometric immunophenotype analysis. MM4 and rabbit complement (C') were cytotoxic to MDR DOX6 (74 +/- 8.5%) and DOX40 (75 +/- 11.3%) cells as well as to chemosensitive 8226/S (80 +/- 5.6%) cells. Treatment with MM4 + C' depleted up to 3 logs of chemosensitive and MDR myeloma CCCs (8226/S: 99.26 +/- 0.52%; DOX6 99.91 +/- 0.08%' DOX40 99.15 +/- 0.55%). In addition, this approach abrogated the selfrenewing capacity of chemoresistant and MDR1 myeloma cell lines, according to doubling time analyses. By comparison, the P-glycoprotein-reactive MoAb MRK-16 and C' was effective in deleting MDR1 CCCs (DOX10: 95.71 +/- 2.51%; DOX40: 99.61 +/- 0.43%) but affected chemosensitive myeloma CCCs only slightly (5.93 +/- 14.52%). When DOX40 cells were mixed with normal bone marrow (BM) in a ratio of 10:90 (MM:BM), treatment with MM4 plus C' deleted MM CCCs (98.80 +/- 0.71%) without affecting the majority of normal BM progenitors. The combination of MM4 and MRK-16 did not enhance MDR myeloma CCC depletion. These observations suggest that MM4 + C' may be useful for depleting MDR as well as chemosensitive myeloma clonogenic cells from human bone marrow.