Homeodomain protein Pbx1 regulates regulatory T cell development, stability and suppressive function

Abstract Pre-B cell leukemia transcription factor 1 (Pbx1) regulates multiple processes in various cell types. The Pbx1 gene is located in the Sle1a1 lupus susceptibility locus in the NZM2410 lupus-prone mouse model. The Pbx1-d dominant negative isoform lacking the DNA binding domain is more frequent in the CD4+ T cells from NZM240 mice as well as lupus patients as compared to healthy controls. In addition, the transgenic expression of Pbx1-d in CD4+ T cells reproduced the phenotypes of Sle1a1 mice, with impaired Foxp3+ Treg cell homeostasis as well as increased inflammatory CD4+ T cells. To study the role of Pbx1 in Treg cells, we generated Foxp3-specific Pbx1 knock-out mice (Foxp3-Pbx1 KO), which were bred to CD4-Pbx1-d-Tg mice to generate Pbx1-d Tg-KO mice that have Treg cells solely expressing Pbx1-d. Foxp3-Pbx1 KO mice showed reduced numbers of Treg cells with impaired suppressive activity. The numbers of natural Treg (nTrg) cells are decreased in thymus but not in spleen of Foxp3-Pbx1 KO mice, while the number of induced Treg (iTreg) cells are increased in the thymus and decreased in the periphery, suggesting that Pbx1 regulates Treg cell development and stability. The results were enhanced in Pbx1-d Tg-KO mice compared with Foxp3-Pbx1 KO mice, leading to autoimmune phenotypes, suggesting that the expression of Pbx1 functional isoform is necessary for the maintenance and function of Treg cells. RNAseq analyses with Pbx1-KO Treg cells indicated that Pbx1 regulates cell cycle, apoptosis and migration. Overall, our results suggest that Pbx1 regulates Treg cell homeostasis, stability and their suppressive functions and that Pbx1-d promotes lupus development by decreasing Treg cell development and functions. Supported by a grant from the NIH (R01 AI045050)