The goal of this study is to identify a safe dosing regimen for siplizumab in participants with type 1 diabetes. Siplizumab is a drug used to help treat recent onset type 1 diabetes mellitus (T1DM). This study is interested in participants aged 18-45 years with T1DM, who are within 18 months of receiving their diagnosis. Participants will be randomized (selected at random by the study team) to one of four possible treatment groups. All groups will receive weekly siplizumab doses administered subcutaneously (injected into the body through the skin) for a total of 12 weeks.