HETEROGENEITY IN P-GLYCOPROTEIN (MULTIDRUG-RESISTANCE) ACTIVITY AMONGMURINE PERIPHERAL T-CELLS - CORRELATION WITH SURFACE PHENOTYPE AND EFFECTOR FUNCTION

P-glycoprotein (P-gly) is the transmembrane efflux pump responsible fo r multidrug resistance in tumor cells. Functional P-gly activity can b e conveniently assessed microfluorometrically using the fluorescent dy e rhodamine 123 (Rh123), which is an artificial substrate for the P-gl y transporter. Here we assess P-gly activity in subsets of mouse perip heral Tlymphocytes using the Rh123 efflux assay. Our data indicate tha t virtually all CD8(+) cells extrude Rh123 efficiently, whereas only a subset of CD4(+) cells exhibit P-gly activity. Correlation of P-gly a ctivity in CD4(+) cells with the expression of a panel of surface mark ers revealed that cells bearing an ''activated/memory'' phenotype (CD4 5RB(-), CD44(hi), CD 62L(-), CD25(+), CD69(+)) were exclusively found in the fraction that can extrude Rh123. In contrast ''naive'' phenotyp e CD4(+) cells (CD45RB(+), CD44(lo), CD62L(+), CD25(-), CD69(-)) could be further subdivided into two major subsets based on P-gly activity. In functional studies of sorted cell populations the Rh123-extruding subset of ''naive'' CD4(+) cells proliferated more strongly and secret ed higher levels of interleukin (IL)-2 than its Rh123-retaining counte rpart when activated by a variety of polyclonal stimuli. Furthermore, this subset produced detectable levels of interferon (IFN)-gamma upon stimulation but no IL-4 or IL-10. As expected, the Rh123-retaining ''n aive'' subset produced only IL-2 after stimulation,whereas the ''memor y'' subset produced IFN-gamma, IL-4 and IL-10 in addition to low level s of IL-2. Collectively, our data indicate that P-gly activity is a no vel parameter that can be used to distinguish a subset of ''preactivat ed'' CD4(+) cells that would be considered as naive on the basis of th eir surface phenotype.