Regulation of the Expression of GARP/Latent TGF-b1 Complexes on Mouse T Cells and Their Role in Regulatory T Cell and Th17 Differentiation

GARP/LRRC32 was defined as a marker of activated human regulatory T cells (Tregs) that is responsible for surface localization of latent TGF-b1. We find that GARP and latent TGF-b1 are also found on mouse Tregs activated via TCR stimulation; however, in contrast to human Tregs, GARP is also expressed at a low level on resting Tregs. The expression of GARP can be upregulated on mouse Tregs by IL-2 or IL-4 exposure in the absence of TCR signaling. GARP is expressed at a low level on Tregs within the thymus, and Treg precursors from the thymus concomitantly express GARP and Foxp3 upon exposure to IL-2. The expression of GARP is independent of TGF-b1 and TGF-b1 loading into GARP and is independent of furin-mediated processing of pro–TGF-b1 to latent TGF-b1. Specific deletion of GARP in CD4 + T cells results in lack of expression of latent TGF-b1 on activated Tregs. GARP-deficient Tregs develop normally, are present in normal numbers in peripheral tissues, and are fully competent suppressors of the activation of conventional T cells in vitro. Activated Tregs expressing GARP/latent TGF-b1 complexes are potent inducers of Th17 differentiation in the presence of exogenous IL-6 and inducers of Treg in the presence of IL-2. Induction of both Th17-producing cells and Tregs is caused preferentially by Tregs expressing the latent TGF-b1/GARP complex on their cell surface rather than by secreted latent TGF-b1. T he three mammalian TGF-b genes encode a translation product consisting of an N-terminal propeptide (termed latency-associated peptide [LAP]) and bioactive TGF-b. This product (referred to in this article as pro–TGF-b) is cleaved intracellularly by furin, and LAP remains noncovalently associated with TGF-b to form the small latent complex. In most cells, the small latent complex is covalently attached to latent TGF-b– binding proteins (LTBPs) prior to secretion. Activated Foxp3 + T regulatory cells (Tregs) express a distinct LTBP termed GARP/ LRRC32 (1) that is required for surface expression of latent TGF-b1 on human Tregs, as well as platelets (2–4). Recombinant latent TGF-b1 was found to bind directly to GARP by both covalent and noncovalent interactions, and GARP was critical for tethering latent TGF-b1 to the cell surface. GARP was also shown to out-compete LTBP for binding to latent TGF-b1 (5). Latent TGF-b does not have biological activity, and the release of active TGF-b from LAP is a critical regulatory step for TGF-b function and signaling. Active TGF-b can be released from …