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

Abstract

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.

P‐glycoprotein (PGP) and lung resistance‐related protein (LRP) expression and function in leukaemic blast cells

“…PGP expression was determined using the MRK-16 MoAb, in fixed and saponine permeabilized cells. As mentioned in Methods, the PLP and saponine pretreatment of cells was required because, although MRK-16 is known to be a monoclonal antibody which binds an external epitope of PGP, Cumber et al (1990) and Damiani et al (1993) had previously shown that these pretreatments of samples enabled a higher staining probably due to the unmasking of the MRK-16 binding site. The majority (60-90%) of the preliminarily tested normal bone marrow and peripheral mononuclear leucocytes, and >90% of the cells of non-MDR cell lines, were weakly stained by the MRK-16, by the LRP-56 and, in a lower percentage of cells (20-60%), also by the MRPm6.…”

Differential recognition of mdr1a and mdr1b gene products in multidrug resistant mouse tumour cell lines by different monoclonal antibodies

“…Its epitope encompasses at least two of six predicted extracellular peptide loops and so it has been suggested that the sequences involved which are separated by about 625 amino acids in the linear structure must be spatially situated in close proximity in the native protein (Georges et al, 1991). There are also reports that sialation may mask detection of the epitope (Cumber et al, 1990). Less is known about JSB-1.…”

Comparative study of multidrug resistance evaluated by means of the quantitative immunohistochemical detection of p‐glycoprotein and the functional release of rhodamine 123

“…However, false negative results may be obtained with these latter antibodies due to the concealment of some epitopes by other glycoproteins [31]. On the other hand, the digital image analysis provides the possibility to detect a small easily identifiable P-GP population of cells.…”