A Study to Assess the Safety and Tolerability of Different Doses of AG019 Administered Alone or in Combination With Teplizumab in Participants With Recent-onset Diagnosed Type 1 Diabetes (T1D)

Inclusion Criteria: * Male or non-pregnant, non-lactating females, 18 - 40 years of age (both inclusive) or 12-17 years of age (both inclusive) * Diagnosis of diabetes according to the American Diabetes Association (ADA) recommended criteria * Evidence of auto-antibodies to at least 1 β-cell autoantigen * Stimulated C-peptide measured during 4h Mixed Meal tolerance Test (MMTT) \> 0.2 nmol/L * The first administration of AG019 should occur no later than 150 days post diagnosis of diabetes * Body weight ≥ 33kg * Written informed consent obtained and documented (participant, parent, guardian as applicable) Exclusion Criteria: * Previous history of serious cytokine release syndrome to teplizumab or other humanized anti-CD3 monoclonal antibodies with no or minimal capacity to bind Fc receptors. (Participants enrolled in the second phase of the trial in either Combination Cohort 1 or Combination Cohort 2, only) * Use of immunosuppressive or immunomodulatory therapies, including systemic steroids within 1 month prior to randomization * Participation in another investigational drug trial within 12 weeks prior to the first study drug intake and during participation in this study * History of recurrent infections, other autoimmune diseases, cardiac disease, malignancy, or any other (chronic) medical condition which, in the investigator's opinion, could compromise participant safety * Documented history of human immunodeficiency virus (HIV), Hepatitis Virus Type C (HCV), Hepatitis Virus Type B (HBV) infection * Evidence of active infection with Epstein-Barr Virus (EBV) or cytomegalovirus (CMV) * Evidence of active or latent tuberculosis (TB) * Administration of anti-CD3 antibody in past year * Current therapy with any other anti-diabetic agents other than insulin (MDI, CSII or analogue). Current or planned therapy with experimental (i.e., unapproved) insulin. Patients on therapy for type 2 diabetes (e.g. metformin) should stop their therapy in order to be eligible for study participation. * Use of medications known to influence glucose tolerance * Daily use of non-steroidal anti-inflammatory agents * Compromised GI mucosal integrity or motility, not attributable to T1D (i.e., recent diarrhea, gluten sensitive enteropathy, inflammatory bowel disease, irritable bowel syndrome), or current use of medications known to influence GI motility * Positive result of SARS-Cov2 PCR test at screening or within 3 days before randomization