Type 1 diabetes progression is associated with loss of CD3+CD56+ regulatory T cells that control CD8+ T-cell effector functions. [*Terrazzano G., Bruzzaniti S., Rubino V. co-first]

An unresolved issue in autoimmunity is the lack of surrogate biomarkers of immunological self-tolerance for disease monitoring. Here, we show that peripheral frequency of a regulatory T cell population, characterized by the coexpression of CD3 and CD56 molecules (TR3-56), is reduced in individuals with new-onset type 1 diabetes (T1D). In three independent T1D cohorts, we find that low frequency of circulating TR3-56 cells is associated with reduced beta-cell function and with the presence of diabetic ketoacidosis. Since autoreactive CD8+ T cells mediate disruption of insulin-producing beta cells1,2,3, we demonstrate that TR3-56 cells can suppress CD8+ T cell functions in vitro by reducing the levels of intracellular reactive oxygen species. The suppressive function, phenotype and transcriptional signature of TR3-56 cells are also altered in children with T1D. Together, our findings indicate that TR3-56 cells constitute a regulatory cell population that controls CD8+ effector functions, whose peripheral frequency may represent a traceable biomarker for monitoring immunological self-tolerance in T1D.