Inhibition of endogenous 5-HT synthesis impairs T-cell expression of CD25. T cells from saline- or PCPA-treated mice were incubated for 48 hours with plate-bound anti-CD3 mAb (5 μg/mL). (A) Expression of CD25 and CD4 by T cells analyzed by flow cytometry. (B) Absolute number of CD25⁺ CD4⁺ T cells expressed as mean (percentage of control) ± 1 SE from triplicate assays (n = 4, *P < .001).