The goal of this Phase 2 study is to assess safety and efficacy of frexalimab to preserve -cell function in participants with newly diagnosed type 1 diabetes (T1D). In Part A, an exploratory part, adults (ages 18-35) will be treated with either high dose frexalimab, or placebo (inactive study drug). In Part B, adolescents and young adults (ages 12-21) will be treated with 3 dose levels of frexalimab or placebo.