Expression of the multiple drug resistance gene (mdr-1) and epitope masking in chronic lymphatic leukaemia.

Resistance to cytotoxic agents is a common clinical problem in the treatment of chronic lymphatic leukaemia (CLL). The multidrug resistant (MDR) phenotype characterized by increased levels of a specific cell membrane p-glycoprotein, confers cross resistance to a wide range of structurally dissimilar antineoplastic drugs. We have studied the expression of this p-glycoprotein in chronic lymphatic leukaemia measured by immunofluorescence using a monoclonal antibody MRK 16 by flow cytometry. Initial results showed that only 12% of lymphocyte samples from CLL patients showed increased p-glycoprotein, conflicting with a previous observation that 53% of CLL patients had an increased level of mdr-1 mRNA. Treatment of the cells with neuraminidase to remove sialic acid residues increased the proportion of patients showing increased p-glycoprotein to 52%. This suggest that in a subset of CLL patients post translational modification of the protein occurs masking the epitope recognized by MRK 16. Abnormal sialylation patterns associated with malignancy are a well-recognized phenomenon.