Decreased frequency and inflammatory change of FoxP3⁺ regulatory T cells in immunopathogenesis of human acute graft-versus-host disease

Background/Aims: Acute graft-versus-host disease (GvHD) is a severe complication of allogeneic stem cell transplantation, characterized by immune-mediated tissue damage primarily affecting the skin, liver, and gastrointestinal tract. Regulatory T (Treg) cells play a critical role in maintaining immune homeostasis. However, the pathogenic roles of changes in Treg cell number and function on acute GvHD remain poorly understood. This study aimed to investigate the quantitative and qualitative changes in Treg cells and their clinical and pathogenic implication of acute GvHD. Methods: A total of 62 patients who underwent allogeneic stem cell transplantation at a tertiary institution from 2019 to 2024 were enrolled. Peripheral blood mononuclear cells were isolated and analyzed by multicolor flow cytometry. Treg cell subsets and cytokine production were assessed after T-cell receptor stimulation. Serum levels of inflammatory cytokines were measured using cytometric bead array, and Treg cell suppressive function was evaluated through co-culture experiments. Results: Patients with acute GvHD showed a decreased frequency of circulating Treg cells, with a notable increase in the CD45RA^- FoxP3^lo pro-inflammatory subset. Treg cells produced inflammatory cytokines including TNF-α upon stimulation and exhibited reduced suppressive activity. The frequency of TNF-α⁺ Treg cells correlated with the clinical severity of acute GvHD. Elevated serum levels of IL-6 and IL-21 were associated with the inflammatory conversion of Treg cells. Conclusions: During human acute GvHD, frequencies of circulating Treg cells are significantly decreased. Inflammatory change of Treg cells, represented by TNF-α production and reduced suppressive capacity, contributes to the immunopathogenesis of acute GvHD.