Some patients with granular lymphocyte-proliferative disorders (GLPD) have been reported to have an aggressive clinical course with a poor prognosis and to be refractory to chemotherapy. In this study, expression of multidrug resistance P-glycoprotein on peripheral blood mononuclear cells (PBMC) of 11 patients with GLPD was examined by staining with MRK 16, a monoclonal antibody that binds to an external epitope of P-glycoprotein, and with the dye rhodamine, a known substance to be excreted from the cells through P-glycoprotein. Among those tested, the PBMC of six of eight patients with T-cell-type GLPD as well as those of three of three patients with natural killer cell- type GLPD expressed P-glycoprotein significantly. Furthermore, PBMC of two of two patients were also poorly stained with the dye rhodamine, and the treatment of PBMC with either verapamil or quinidine, multidrug resistance-reversing agents, led to their increased staining, suggesting that these PBMC actively release chemotherapeutic agents.

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