Expression of mdr1 and mrp in the normal B‐cell homologue of B‐cell chronic lymphocytic leukaemia

B‐cell chronic lymphocytic leukaemia (CLL) cells commonly express the multidrug resistance phenotype. The aim of this study was to establish whether the normal homologue in B‐cell ontogeny of B‐CLL also expressed the multidrug resistance (mdr) phenotype. Human tonsillar lymphocytes were sorted to yield two B‐cell subsets based on the expression of CD19, CD5 and CD10. The normal homologue was represented by a population of B cells that was CD19 positive, CD10 negative and weakly expressed CD5. Based upon functional analysis and the detection of mdr1 mRNA by semi‐quantitative PCR, these cells expressed the mdr phenotype. In contrast, functional multidrug resistance could not be demonstrated in CD19‐positive CD10‐positive cells with strong expression of CD5, nor could mdr1 mRNA be found in these cells. MRP was variably expressed in both B‐cell subsets with no discernable differences in the pattern of expression. We conclude that normal B cells with a phenotype resembling that of B‐CLL cells express the multidrug resistance phenotype.