Activation of human peripheral blood T cells does not lead to increased P-glycoprotein expression

The expression of P-glycoprotein (Pgp) on normal human lymphocytes, and its drug exclusion capacity, implies that Pgp might be involved in cytokine se cretion. We used two-color flow cytometry to detect simultaneously Pgp expr ession and IL-2 accumulation in resting and mitogen-activated human lymphoc ytes. Among resting lymphocytes from five healthy donors less than 1% were Pgp(+) as determined by reactivity with the anti-Pgp monoclonal antibody (m Ab) 4E3. The percentage of Pgp(+) lymphocytes increased to 3% after 24 hr o f mitogenic stimulation that induced maximal production of cytoplasmic IL-2 . The percentage of lymphocytes that coexpressed membrane Pgp and cytoplasm ic IL-2 accounted for <10% of the total IL-2 producing lymphocytes. Finally , mitogen-induced cytoplasmic IL-2 accumulation was enhanced by stimulation in the presence of monensin but not the Pgp functional inhibitor verapamil . Because mAb 4E3 detected lower than expected numbers of Pgp+ lymphocytes, we compared the binding of mAbs MRK16 and 4E3 concomitant with doxorubicin (DOX)-uptake by K562 and R7 tumor cells and purified CD8(+) lymphocytes. T he MRK16 mAb was found to be sensitive but not very specific (30%). In cont rast, the sensitivity of 4E3 was equivalent to MRK16 (98%) and was highly s pecific (98.5%). There was also a positive association between DOX efflux a nd the level of Pgp expression as detected by 4E3 but not MRK16. Thus, huma n T cells do not markedly up-regulate their expression of functional Pgp mo lecules as detected by mAb 4E3 following activation, suggesting that Pgp do es not play a major role in IL-2 secretion by activated T cells.