The lupus susceptibility gene Pbx1 regulates the balance between follicular helper t cell and regulatory t cell differentiation

Pbx1 controls chromatin accessibility to a large number of genes and is entirely conserved between mice and humans. The Pbx1-d dominant-negative isoform is more frequent in CD4⁺ T cells from lupus patients than from healthy controls. Pbx1-d is associated with the production of autoreactive T cells in mice carrying the Sle1a1 lupus-susceptibility locus. Transgenic (Tg) expression of Pbx1-d in CD4⁺ T cells reproduced the phenotypes of Sle1a1 mice, with increased inflammatory functions of CD4⁺ T cells and impaired Foxp3⁺ regulatory T cell (Treg) homeostasis. Pbx1-d-Tg expression also expanded the number of follicular helper T cells (TFHs) in a cell-intrinsic and Ag-specific manner, which was enhanced in recall responses and resulted in Th1-biased Abs. Moreover, Pbx1-d-Tg CD4⁺ T cells upregulated the expression of miR-10a, miR-21, and miR-155, which were implicated in Treg and follicular helper T cell homeostasis. Our results suggest that Pbx1-d impacts lupus development by regulating effector T cell differentiation and promoting TFHs at the expense of Tregs. In addition, our results identify Pbx1 as a novel regulator of CD4⁺ T cell effector function.