Background/Purpose: Regulatory T cells (Tregs) contribute to immune tolerance and play a pivotal role in the prevention of autoimmune diseases such as rheumatoid arthritis (RA). In active RA, the suppressive function of Tregs is markedly reduced. Recently, a membrane-associated molecule, glycoprotein A repetitions predominant (GARP), has been identified to be specifically expressed on Tregs in response to activation. While the function of GARP is not completely understood, diminution of GARP expression attenuates the suppressive capacity of Tregs. Therefore, we hypothesized that diminished expression of GARP on the surface of Tregs in RA patients might be responsible for their reduced suppressive function. To provide insight, we investigated in detail the molecular mechanisms of GARP transcription and analyzed GARP expression on Tregs from RA patients.

Methods: To delineate transcriptional mechanisms of GARP expression, human GARP promoter sequences and several CNS regions were cloned into reporter vectors. Their transcriptional activity in response to different stimuli was assessed. To analyze the chromatin configuration state in primary Tregs, chromatin immunoprecipitation of GARP promoters and CNS regions in activated and non-activated Tregs was performed. In order to investigate GARP expression, CD25⁺ and CD25^– CD4 T cells were purified from peripheral blood, and GARP expression was compared between eight RA patients with early active disease (disease duration 3.0±2.4 months, DAS28 5.2±1.2) and an age- and gender-matched healthy cohort.

Results: Two different transcript variants under the control of different promoters (denoted as promoter 1 and 2) are encoded by the GARP gene. Whereas GARP promoter 1 activity was dependent on TCR stimulation and the presence of Forkhead box protein 3 (Foxp3), the master transcription factor of Tregs, Foxp3 and retinoic acid synergistically induced transcription from GARP promoter 2 in a concentration dependent manner. Histone modifications in both promoter regions and in an upstream located CNS changed towards a more accessible chromatin configuration upon T cell activation. When GARP expression was analyzed in primary T cells, only CD25⁺ Foxp3-expressing CD4 T cells upregulated GARP, confirming Treg specificity of GARP expression and its Foxp3 dependency. The upregulation of GARP expression upon stimulation was however less pronounced on Tregs from RA patients as compared to healthy controls.

Conclusion: Thus, GARP expression is attenuated in RA patients. As transcriptional activity of the GARP gene is initiated by coordinate action of TCR stimulation, Foxp3 and retinoic acid on two promoters and one CNS region, alterations of these control mechanisms in RA might cause diminished GARP expression and subsequently might result in the reduced suppressive capacity of Tregs in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/transcriptional-regulation-of-garp-expression/