Regulatory T Cells Decreased during Recovery from Mild COVID-19

Depending on the intensity and duration of SARS-CoV-2 infection, the host immune response plays a significant role in immunological protection. Here, we studied the regulatory T-cell (Treg) response in relation to kinetic change and cytokine production in patients with mild COVID-19. Nineteen SARS-CoV-2-positive patients were recruited, and blood was collected at four time points, i.e., seven days after admission, after discharge, and one and three months after recovery. CD3(+)CD4(+)CD25(+)CD127(low) was marked as the Treg population, with IL-10 and TGF-β used to study cytokine-producing Tregs. IFN-γ-producing CD8(+) T cells were observed for an effector response. The Treg percentage in patients with mild COVID-19 increased during hospitalization compared to during the recovery period. Peripheral blood mononuclear cells (PBMCs) were quantified, and the T-cell response was characterized by re-stimulation with S1 and N peptides. IL-10 and TGF-β were produced by CD25(+)CD127(low) T cells during the active infection phase, especially with N peptide stimulation. Compared to N peptide stimulation, S1 peptide stimulation provided superior IFN-γ-secreting CD8(+) T-cell responses. Our results suggest that while IFN-γ(+)CD8(+) T cells confer antiviral immunity, cytokine-producing Tregs may have a substantial role in regulating inflammatory responses in mild SARS-CoV-2 infection. Novel vaccine development may also consider enhancing T-cell repertoires.