Subjects are being asked to volunteer for this research study because they have been diagnosed with Dermatomyositis (DM). This study will test the safety and effectiveness of the investigational new drug, IMO-8400. Subjects will receive a subcutaneous injection of the study drug or placebo once a week for up to 24 weeks during the study. Subjects will complete a total of 27 visits over the course of 32 weeks. After the baseline visit, subjects will have the option of having a visiting nurse (who has been trained in the protocol and approved by the Sponsor) conduct the intervening weekly study visits 2-25 outside of the clinic (e.g., at your home or workplace) rather than coming in to clinic for injections.