This is a two-part Phase 2a study compromising a 12-week, double-blind, placebo-controlled, treatment period followed by a 100-week, open label, treatment period to evaluate short-term efficacy, and the short- and long-term safety of MEDI2070 in subjects with moderate to severe, active CD who have failed or are intolerant to anti-TNFα therapy as determined by the investigator. Approximately 120 subjects will be randomized in a 1:1 ratio to initially receive a fixed IV dose of MEDI2070 or placebo on Week 0(Day1) and Week 4 (Day 29) during the 12-week, double-blind, placebo-controlled, treatment period. At the completion of the double-blind, placebo-controlled, treatment period (Week 12), subjects will have the option to enter a 100-week, open-label, treatment period where they will receive open-label MEDI2070 (SC) Q4W (Week 12 through Week 112). Subjects will be followed for safety at 3 visits over 28 weeks after their last dose of IP. Subjects will also be contacted by phone 36 weeks after their last dose of IP for safety.