H₂O₂ can mediate the antiproliferative effect of IL4I1. CD45RO⁺ and CD45RA⁺ T cells were incubated through a 0.4-μm transwell for 3 hours with mock beads, hIL4I1 beads, or E481A-hIL4I1 beads (A) or in the presence of increasing concentrations of H₂O₂ (B) or phenylpyruvate (C) and analyzed for ³H-thymidine incorporation. Results are expressed as the average cpm of quadruplicates ± SD. (B-C) CD45RO⁺ (○) and CD45RA⁺ (●). A representative experiment (of 4) is shown. *P < .05. (D) Proliferation was assessed on T cells incubated with 50 μM H₂O₂ or with 10 μL mock beads or hIL4I1 beads through a 0.4-μm transwell for 3 hours with or without preincubation with 1000 U/mL catalase. Results are expressed as the average cpm of sextuplicate samples ± SD. A representative experiment is shown. *P = .04; **P < .005.