One of the primary characteristics of Type 1 Diabetes (T1D) is the presence of autoantibodies directed to self-antigens expressed in the pancreatic islets, specifically those associated with the insulin secretory machinery. Although T1D is well characterized as a T-cell mediated disease, a mechanistic role of islet autoantibodies generated during the progression of T1D have not been elucidated. The tyrosine phosphatase-like protein (IA-2) is a major neuroendocrine autoantigen associated with the progression of T1D. We previously identified a new candidate biomarker within the extracellular domain of IA-2 (IA-2ec) in humans, which is associated to rapid acceleration of T1D onset compared to conventional IA-2 antibody biomarkers of T1D. Little is known about the clonality and the V region structures of IA-2 autoantibodies, an important consideration given their diagnostic and predictive value. Thus, our objective is to perform de novo sequencing to determine the clonality and V region structures of human autoantibodies directed against the islet autoantigen IA-2. This method can be applied to multiple serum samples and has been validated by the discovery of a clonotypic autoantibody specific for an immunodominant determinant of target antigens of a number of autoimmune diseases including systemic lupus erythematosus (SLE) rheumatoid arthritis (RA). In particular, we aim to characterize the IA-2 autoantibody sequence in the sera of the well-characterized population of T1D patients of the nPOD network in an effort to determine if these autoantibodies express a highly restricted B-cell clonotype repertoire which may be shared (public) across different disease phenotypes. We will then identify unique V region peptides to interrogate human sera for IA-2 by a targeted mass spectrometry (MS) protocol. Mass spectrometry-based detection of specific autoantibody motifs may provide a powerful new tool for analysis of humoral autoimmunity in T1D.