Teplizumab vs ATG for Stage 3 Type 1 Diabetes

Inclusion Criteria: * Willing to provide informed consent or have a parent or legal guardians provide informed consent when the participant is \<18 years of age. * Aged ≥4 to \<35 years * A history of at least two or more diabetes-related biochemical autoantibodies (mIAA, GADA, ICA, IA-2A, ZnT8A) present on the same sample. In the absence of other antibodies, ICA and GADA positivity alone will not suffice for eligibility in this trial. * Participants must meet ADA stage 2 T1D glycemic criteria\* by TrialNet testing within 100 days of the baseline visit. \*The ADA definition of stage 2 T1D is characterized by glucose intolerance or dysglycemia in the presence of two or more islet autoantibodies, impaired fasting glucose (≥ 100mg/dL), impaired glucose tolerance (2-hour post 75g glucose load ≥ 140mg/dL), high glucose levels at intermediate time points on OGTT (30, 60, 90 min timepoints of ≥ 200 mg/dL), and/or HbA1c between 5.7% and 6.4% or ≥ 10% increase in HbA1c within a two year window, with the most recent HbA1c value obtained within 100 days of the baseline visit. * Participants, regardless of serostatus, must meet all of the following: * Be EBV and CMV PCR negative prior to randomization * Be EBV and CMV PCR negative within 2 weeks prior to the baseline visit * Have no signs or symptoms of a CMV or EBV-compatible illness lasting longer than 7 days within 30 days prior to the baseline visit. * Be at least 8 weeks from last live immunization at the time of the baseline visit. * Be willing to forgo vaccines (other than non-live influenza and COVID-19) during the 3 months after study drug treatment period and forgo live vaccines for 12 months after study drug treatment period. * Must meet TrialNet eligibility minimum immunization recommendations found in Appendix A of the manual of operations (MOO). * With the exception of stage 2 T1D, participants must be healthy, as defined by absence of any other untreated diagnoses that the investigator deems to be a potential confounder. * If a female participant with reproductive potential, willing to avoid pregnancy (abstinence or adequate contraceptive method) through the completion of the study infusions and up to 3 months after study drug administration and undergo pregnancy testing prior to each study visit. * Must be residing or have accommodations within 1 hour of the infusion site during study drug infusions and must be within 1 hour of a medical care facility for 1 day after completion of infusions. * Participants must live in a location with rapid access to emergency medical services. Exclusion Criteria: * Immunodeficiency or clinically significant chronic lymphopenia: (Leukopenia (\<3,000 leukocytes/μL), neutropenia (\<1,500 neutrophils/μL), lymphopenia (\<1000 lymphocytes/μL), thrombocytopenia (\<150,000 platelets/μL). * Hemoglobin less than 13 g/dL for adult men and less than 11.5g/dL for adult females and less than 11 g/dL for participants under age 18. * Active signs or symptoms of acute or chronic infection at the time of the baseline visit including SARS-Cov-2. * Uncontrolled autoimmune thyroid disease and/or celiac disease (participants must be well controlled for the previous 6 months). * Evidence of prior or current tuberculosis infection through any one or more of the following: 1. A history of latent or active TB 2. Signs and/or symptoms of TB 3. Recent close contact with a person with known or suspected active TB unless appropriate prophylaxis for TB was given 4. A history of a chest X-ray consistent with active TB or old, inactive TB, or interferon gamma release assay IGRA (QuantiFERON) test 5. A history of a positive purified protein derivative (PPD) skin test result (\>10 mm induration), or positive/repeatedly indeterminate on an interferon-gamma release assay (IGRA; e.g., QuantiFERON-TB test). * Currently pregnant or lactating or anticipate getting pregnant within the study period. * Require use of other immunosuppressive agents including chronic use of oral or intravenous injectable steroids. * Evidence of current or past HIV or Hepatitis B or current Hepatitis C infection. * Any complicating medical issues or abnormal clinical laboratory results that may interfere with study conduct, or cause increased risk to include pre-existing cardiac disease, COPD, sickle cell disease, Down syndrome, adrenal insufficiency, neurological disease, or blood count abnormalities. * A history of malignancies other than of skin. * Evidence of liver dysfunction with AST or ALT ≥ 2 times the upper limit of the reference range. * Evidence of renal dysfunction with creatinine ≥ 1.5 times the upper limit of the reference range. * Increased bilirubin ≥ 2 times (total) or ≥ 1.5 times (direct) the normal limit (Participants with documentation of Gilbert's Disease permitted). * Vaccination with a live vaccine within the last 8 weeks or killed/inactivated vaccine within the last 2 weeks of the baseline visit. * Current or ongoing use of non-insulin pharmaceuticals that affect glycemic control within 14 days of screening. * Prior treatment with Teplizumab or ATG (either in a previous clinical trial or clinically). * Has previously participated in a clinical trial for diabetes prevention and received active study agent within 6 months of treatment. * Known allergy to rabbits or rabbit derived products. * Prior adverse reactions to heparin. * Any condition that in the investigator's opinion may adversely affect study participation. * Any screening/baseline laboratory result not otherwise stated out of normal reference range and/or medical history that may increase the risk of the participant's participation in this trial. * Previously diagnosed with Stage 3 TID according to ADA criteria.