Nicotinamide enhances Treg differentiation by promoting Foxp3 acetylation in immune thrombocytopenia

Imbalanced nicotinamide adenine dinucleotide (NAD⁺) homeostasis has been reported in multiple autoimmune diseases and supplementation with NAD⁺ precursors has consistently demonstrated positive therapeutic benefits for these conditions. Immune thrombocytopenia (ITP) is an acquired autoimmune disease, in which the decreased number and impaired function of regulatory T cells (Tregs) contribute to the main pathogenesis. Here we found NAD⁺ level was decreased in the plasma and CD4⁺ T cells of ITP patients. Supplementation with NAD⁺ precursor nicotinamide (NAM), but not nicotinamide mononucleotide (NMN), increased Treg frequency and ameliorated thrombocytopenia in an ITP murine model. Moreover, whilst both NAM and NMN restored cytosolic NAD⁺ level in the CD4⁺ T cells from ITP patients, only NAM promoted Treg differentiation. Mechanistically, Sirtuin1 (Sirt1), a major consumer of NAD⁺, was highly expressed in the CD4⁺ T cells of ITP patients, potentially contributing to the low level of NAD⁺. NAM, which could act as Sirt1 inhibitor, promoted Foxp3 acetylation and stability in induced Tregs derived from naïve CD4⁺ T cells of ITP patients. These findings suggest that NAM holds promise as a novel therapeutic strategy for restoring immune balance in ITP.