GARP : how it controls TGF-b1 production in human regulatory T cells, and how its expression is regulated at transcriptional and post-transcriptional levels

Regulatory T lymphocytes (Tregs) are a subset of CD4+ T cells specialised in the inhibition of immune responses. Tregs are required for the maintenance of self-tolerance, but in tumours they play a detrimental role by suppressing T lymphocytes directed against cancer cells. Although suppression mechanisms are not fully elucidated yet, Tregs inhibit other immune cells at least in part through the production of active TGF-β1. Most hematopoietic cells, including Tregs, secrete TGF-β1 under a latent form, in which the mature cytokine is kept inactive by association with the so-called Latency-Associated Peptide (LAP). Stimulation of the T Cell Receptor (TCR) induces expression of transmembrane protein GARP on the surface of Tregs, but not other T lymphocytes. GARP binds and presents latent TGF-β1, and allows release of mature TGF-β1 from the LAP close to the Treg surface. GARP may therefore play an important role in Treg suppression through control of active TGF-β1 production. The objectives of our work were twofold. First, we sought to determine whether GARP, in addition to activating latent TGF-β1, also controls earlier steps of the TGF-β1 production pathway in human Tregs. We found that GARP favours the cleavage of the pro-TGF-β1 precursor, and increases secretion of latent TGF-β1. We also found that stimulated Tregs secrete latent TGF-β1 as complexes containing GARP. GARP/TGF-β1 complexes represent a novel form of secreted TGF-β1, with as yet unknown biological function. Second, we sought to identify transcriptional and/or post-transcriptional mechanisms that regulate GARP expression, and hence TGF-β1 production, in human T cells. We showed that GARP levels are regulated post-transcriptionally by miR-142-3p, miR-181a, b, c, d and miR-185. High expression of these miRNAs in non-regulatory T cells may prevent appearance of the GARP protein even in the presence of high GARP mRNA levels. In addition, we found that GARP expression is inhibited at the transcriptional level by TGF-β1 signals. Considering previous data and our own, we propose the following model. TCR stimulation of human Tregs induces GARP expression. This increases latent TGF-β1 production, induces secretion of GARP/TGF-β1 complexes, presentation of latent TGF-β1 at the Treg surface, and production of active TGF-β1. Autocrine TGF-β1 signals, in turn, repress transcription of the GARP gene. Downregulation of GARP by autocrine TGF-β1 in Tregs represents a negative feedback mechanism resulting in the return to a resting state, characterised by the absence of GARP, reduced latent TGF-β1 secretion and shut down of active TGF-β1 production.(BIFA - Sciences biomédicales et pharmaceutiques) -- UCL, 201