Background/Purpose: Regulatory T cells (Tregs) represent the fundamental T-cell capable of promoting self-tolerance and balancing excessive inflammation. Quantitative and qualitative Treg deficiencies have been characterized in several systemic rheumatic diseases. Previously we found that the catalytic activity of protein phosphatase 2A (PP2A) in murine Tregs is increased at levels significantly higher than in conventional T cells. Furthermore, mice in which Tregs lack PP2A develop multi-organ inflammation and severe autoimmunity. PP2A-deficient Tregs displayed reduced suppressive function in vitro. This study investigates the underlying molecular mechanism whereby Tregs lose lineage stability, suppressive function and convert to an effector phenotype. 

Methods: We constructed a Foxp3^YFP–crePpp2r1a^flox/flox mouse that lacked PP2A only in Tregs and could be tracked by virtue of co-expressing yellow fluorescent protein (YFP). Age and sex-matched Foxp3^YFP–cre were used as controls. Foxp3 expressing Tregs were isolated from Foxp3^YFP–crePpp2r1a^flox/flox mice by flow cytometry cell sorting (CD4⁺YFP⁺). The adoptive T-cell transfer colitis model was used to determine the suppressive function of Tregs with and without PP2A deficiency in vivo. One million Tregs were adoptively transferred into Rag1 knockout recipients either alone or at a 1:1 ratio with conventional CD4⁺ T-cells. Five weeks later colon, small bowel, spleen and lymph nodes were collected for both histologic and flow cytometry analysis. Standard ELISA was applied to detect cytokines.

Results: Tregs (CD4⁺YFP⁺) isolated from twenty week old Foxp3^YFP–crePpp2r1a^flox/flox mice demonstrated decreased levels of intracellular Foxp3 and IL-10 by flow cytometry. Furthermore, PP2A-deficient Tregs acquired an inflammatory phenotype by producing IL-17. When PP2A-deficient Tregs were co-transferred with effector T cells into Rag1-deficient mice, they lost weight and developed inflammatory colitis. This demonstrated that the transferred Tregs lost their suppressive function in vivo. In fact, co-transfer of PP2A-deficient Tregs with effector T cells accelerated weight loss and worsened colitis compared to transfer of effector T cells alone. Elevated levels of IL-17 in the sera were detected by ELISA compared to sera obtained from the group of mice in which wild type Tregs and effector T cells had been co-transferred.

Conclusion: PP2A is essential for Treg function and maintenance of stable Foxp3 expression. Tregs deficient in PP2A produce less IL-10 and more IL-17. The dependence of Tregs on the presence of PP2A points to approaches to restore and empower Teg function in autoimmune disease. 

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/regulatory-t-cells-deficient-in-protein-phosphatase-2a-lose-suppressive-function-and-convert-to-an-effector-phenotype/