PTPRN and type 1 diabetes mellitus: The antigenicity of citrullinated beta-cell antigens that bind to HLA-A2 has been demonstrated by in vitro activation of CD8+ T-cell clones, expanded from peripheral blood of HLA-A2+ T1D subjects, when cultured with citrullinated peptides derived from IA-2, GAD65 and IGRP (13).