We have previously demonstrated that an anti-CD19 monoclonal antibody (MAb; HD37) inhibits the function of the Pglycoprotein (P-gp) pump in a multidrugresistant (MDR) B-lymphoma cell line, Namalwa/MDR1, and that this effect is not due to the recognition of a cross-reactive epitope on P-gp. In this study, we have used the same cell line to define the mechanisms responsible for the effect of HD37 on the P-gp pump. Using fluorescence resonance energy transfer (FRET), we show that CD19 and P-gp are constitutively associated in cells. In the absence of treatment with anti-CD19, 40% of P-gp molecules expressed by Namalwa/MDR1 cells reside in the low-density lipid (ie, cholesterol-rich) microdomains (lipid rafts). Following treatment of the cells with HD37 and disruption of the interactions between P-gp and CD19, P-gp translocated out of lipid rafts and CD19 translocated into lipid rafts. The effect of chemosensitization on Namalwa/MDR1 cells was specific for CD19; an anti-CD22 MAb had no such effect, although the cells express CD22. These results suggest that anti-CD19 might chemosensitize P-gp ؉ cells by interfering with interactions between CD19 and P-gp, rapidly resulting in the translocation of P-gp into a compartment on the plasma membrane where it is no longer active. IntroductionFollowing chemotherapy, multidrug-resistant (MDR) tumors often emerge; patients with such tumors have a poor prognosis. 1 A variety of agents can prevent MDR but all have side effects. These agents include analogs of drug transporter substrates (verapamil) and inhibitors of adenosine triphosphate (ATP) binding or utilization. 2,3 More recently, monoclonal antibodies (MAbs) against P-glycoprotein (P-gp; MRK16 and UIC2) have been used to inhibit the P-gp transporter. [4][5][6] However, P-gp is expressed on cells in many normal tissues (eg, kidney, brain, gut, and liver), and by targeting MDR tumors, those organs are targeted as well, resulting in potentially undesirable side effects. 7 Therefore, studies designed to find alternative nontoxic strategies to chemosensitize MDR tumors are needed.Our previous studies have shown that an anti-CD19 MAb, HD37, induces cell cycle arrest in human B-lymphoma cell lines in vitro and has significant antitumor activity in severe combined immunodeficient (SCID) mice with human lymphoma xenografts. 8 More recently, we demonstrated that HD37 can chemosensitize a P-gp ϩ lymphoma cell line in vitro and that this effect is due to its ability to inhibit the functional activity of P-gp. 9 P-gp is an ATP-driven pump expressed on many MDR tumors, 2,3 and several studies have suggested that the phosphorylation of P-gp can regulate drug efflux. 10,11 HD37 does not alter ATP levels and does not cross-react with P-gp. 9 P-gp is highly sensitive to its lipid environment and actively mediates cholesterol redistribution in the cell membrane. 12 Forty percent of P-gp resides in the low-density lipid (ie, cholesterolrich) microdomains of the plasma membrane (lipid rafts), 13,14 and this proportion increases when the ce...

The objective of this study was to investigate the ability of the anti-CD20 antibody, Rituximab (RTX), to inhibit the activity of P-glycoprotein (P-gp), and reverse multidrug resistance (MDR) in 2 P-gp/CD20 lymphoma cell lines. We determined whether RTX would chemosensitize the 2 P-gp cell lines in vitro, and inhibit the ability of the cells to efflux Rhodamine 123. One cell line was infected with an MDR1 vector and the other was generated by drug selection. We also determined whether RTX induced P-gp to translocate out of lipid rafts. RTX chemosensitized 2 different MDR cell lines, inhibited the activity of P-gp in both, and induced P-gp to translocate out of lipid rafts in the 1 cell line that was studied in greater detail. In contrast, 3 other anti-CD20 antibodies did not chemosensitize, inhibit the activity of P-gp, or induce it to translocate out of rafts, despite the fact that 1 antibody recognized the same epitope on CD20. Our results suggest that RTX can chemosensitize 2 CD20/P-gp cell lines in vitro by inhibiting the activity of the P-gp pump. The inhibition of P-gp activity correlated with the ability of RTX to induce P-gp to translocate out of lipid rafts. Although the mechanisms by which RTX effects P-gp translocation and activity are not yet known, they are not associated with acid-sphingomyelinase activation in raft microdomains, as described for the antiproliferative activity of RTX.