To investigate the relative abundance and activities of Th17 cells and natural Treg cells in systemic lupus erythematosus (SLE).

Blood samples were collected from 50 adult patients with SLE. Samples were processed to detect Th17 cells and natural Treg cells by flow cytometry, and related gene expression was assessed by real‐time reverse transcription–polymerase chain reaction. Skin biopsy specimens were collected for histologic assessment. The function of Th17 cells in relation to human umbilical vein endothelial cells (HUVECs) was studied in vitro. Th17 cells were also examined in lupus‐prone MRL/Mp‐lpr/lpr (MRL/lpr) mice.

We demonstrated the presence of Th17 cells among the peripheral blood mononuclear cells (PBMCs) and in the involved organs of patients with active SLE. Both the percentage of circulating Th17 cells and the ability to produce interleukin‐17A (IL‐17A) were increased in samples derived from patients with active SLE. The number of Th17 cells increased during SLE flare, especially in patients with vasculitis, and decreased following certain treatments. We observed that IL‐17A from patients with SLE could induce adhesion molecule messenger RNA expression in HUVECs and adhesion of T cells to HUVECs. An increase in the percentage of Th17 cells was correlated with natural Treg cell depletion during disease flare. Finally, expansion of the Th17 cell population was detected in MRL/lpr mice.

SLE flare might be linked to the expansion of the Th17 cell population and the depletion of natural Treg cell subpopulations. Expansion of the Th17 cell population might be related to a distinct cytokine environment in active SLE. Th17 cells and microenvironmental IL‐17A are involved in vascular inflammation in SLE. Antagonism of Th17 cells by IL‐17A–blocking antibodies should be explored as a treatment of SLE.