MURINE MONOCLONAL-ANTIBODY RECOGNIZING A 90-KDA CELL-SURFACE DETERMINANT SELECTIVELY LOST BY MULTI-DRUG-RESISTANT VARIANTS OF CEM CELLS

We describe a murine IgG, monoclonal antibody (MAb56), specific for a cell‐surface protein structure (MC56 determinant) expressed by the human CEM cell line. A large band of approximately 90 kDa was identified as the main specific component of the MC56 determinant. Such a 90‐kDa protein is significantly associated with the drug‐sensitive phenotype, its expression being progressively reduced quantitatively in multi‐drug‐resistant (MDR) variants of CEM cells, according to the extent of drug resistance. In addition, the MC56 determinant is expressed de novo in drug‐sensitive revertant cell lines derived from MDR cells and unreactive with the MAb56. The MAb56 shows a high affinity towards the immunizing drug‐sensitive CEM cell line (Ka =1.86 × 109 L/mole) while not binding to MDR cell variants. The expression of the MC56 molecule on a variety of human cells and tissues makes such a cellular determinant a candidate as a marker for studying the MDR phenomenon both in vivo and in vitro. Copyright © 1990 Wiley‐Liss, Inc., A Wiley Company