The study is a 2-arm, multicenter, 1:1 randomized, placebo controlled clinical trial. All subjects will receive close monitoring for development of AGT or T1DM. Subjects will receive Abatacept or placebo and close monitoring for development of AGT or T1DM. To assess the safety, efficacy, and mode of action of Abatacept to prevent AGT and T1DM. The primary objective is to determine whether intervention with Abatacept will prevent or delay the development of AGT in at-risk autoantibody positive non-diabetic relatives of patients with T1DM. Secondary outcomes include: the effect of Abatacept on the incidence of T1DM; analyses of C-peptide and other measures from the OGTT; safety and tolerability; and mechanistic outcomes.