Subjects who complete 26 weeks of blinded treatment in HZNP-ACT-301, (NCT02415127) will be eligible to enter this 6-month study. All subjects who choose to participate will receive ACTIMMUNE three times a week (TIW) for 26 weeks. In order to maintain the study blind in HZNP-ACT-301, all subjects in this open-label extension study will undergo ACTIMMUNE titration, regardless if they received ACTIMMUNE or placebo in HZNP-ACT-301.The Week 26 Visit from HZNP-ACT-301 will serve as the Baseline Visit (Day 1) for this study. The study drug dose is planned to be escalated on a weekly basis over the first 4 weeks of treatment (from 10 µg/m2 to 100 µg/m2), however, the dose may be reduced, interrupted, or held based on tolerability. During the treatment period, additional clinic visits are scheduled at Weeks 4, 13, and 26; in between clinic visits, subjects (and/or caregivers) will be monitored via emails/phone calls on a weekly basis until subjects reach their maximum tolerated dose and on a monthly basis thereafter.