 In Part 2 of the base study Week 12 to Week 28, participants in Arm A and Arm B will receive matching placebo to tildrakizumab to maintain blinding at Week 12 and will receive either tildrakizumab 200 mg Arm A or tildrakizumab 100 mg Arm B at Week 16. Participants in Arm A and Arm B will also receive matching placebo to etanercept once weekly through study Week 28. At study Week 12, Arm C participants will be re-randomized to receive their first dose of tildrakizumab 200 mg or tildrakizumab 100 mg, and will receive additional doses of study medication according to their re-randomized treatment assignment at Week 16. Participants in Arm C will also receive matching placebo to etanercept through treatment Week 28. Participants in Arm D will continue with once weekly doses of etanercept through study Week 28 in combination with matching placebo to tildrakizumab. 