CD25(+) FOXP3(+) CD4(+) regulatory T cells (Tregs) are induced by transforming growth factor β (TGFβ) and further expanded by retinoic acid (RA). We have previously shown that this process was defective in T cells from lupus-prone mice expressing the novel isoform of the Pbx1 gene, Pbx1-d. This study tested the hypothesis that CD4(+) T cells from systemic lupus erythematosus (SLE) patients exhibited similar defects in Treg induction in response to TGFβ and RA, and that PBX1-d expression is associated with this defect.
Peripheral blood mononuclear cells (PBMCs) were collected from 142 SLE patients and 83 healthy controls (HCs). The frequency of total, memory and naïve CD4(+) T cells was measured by flow cytometry on fresh cells. PBX1 isoform expression in purified CD4(+) T cells was determined by reverse transcription polymerase chain reaction (RT-PCR). PBMCs were stimulated for three days with anti-CD3 and anti-CD28 in the presence or absence of TGFβ and RA. The expression of CD25 and FOXP3 on CD4(+) T cells was then determined by flow cytometry. In vitro suppression assays were performed with sorted CD25(+) and CD25(-) FOXP3(+) T cells. CD4(+) T cell subsets or their expansion were compared between patients and HCs with two-tailed Mann-Whitney tests and correlations between the frequencies of two subsets were tested with Spearman tests.
The percentage of CD25(-) FOXP3(+) CD4(+) (CD25(-) Tregs) T cells was greater in SLE patients than in HCs, but these cells, contrary to their matched CD25(+) counterparts, did not show a suppressive activity. RA-expansion of TGFβ-induced CD25(+) Tregs was significantly lower in SLE patients than in HCs, although SLE Tregs expanded significantly more than HCs in response to either RA or TGFβ alone. Defective responses were also observed for the SLE CD25(-) Tregs and CD25(+) FOXP3(-) activated CD4(+) T cells as compared to controls. PBX1-d expression did not affect Treg induction, but it significantly reduced the expansion of CD25- Tregs and prevented the reduction of the activated CD25(+) FOXP3(-) CD4(+) T cell subset by the combination of TGFβ and RA.
We demonstrated that the induction of Tregs by TGFβ and RA was defective in SLE patients and that PBX1-d expression in CD4(+) T cells is associated with an impaired regulation of FOXP3 and CD25 by TGFβ and RA on these cells. These results suggest an impaired integration of the TGFβ and RA signals in SLE T cells and implicate the PBX1 gene in this process.