TGM6, a helminth secretory product, mimics TGF-β binding to TβRII to antagonize TGF-β signaling in fibroblasts

Abstract

The murine helminth parasite Heligmosomoides polygyrus expresses a family of proteins structurally related to TGF-β Mimic 1 (TGM1), a secreted five domain protein that activates the TGF-β pathway and converts naïve T lymphocytes to immunosuppressive Tregs. TGM1 signals through the TGF-β type I and type II receptors, TβRI and TβRII, with domains 1-2 and 3 binding TβRI and TβRII, respectively, and domains 4-5 binding CD44, a co-receptor abundant on T cells. TGM6 is a homologue of TGM1 that is co-expressed with TGM1, but lacks domains 1 and 2. Herein, we show that TGM6 binds TβRII through domain 3, but does not bind TβRI, or other type I or type II receptors of the TGF-β family. In TGF-β reporter assays in fibroblasts, TGM6, but not truncated TGM6 lacking domains 4 and 5, potently inhibits TGF-β- and TGM1-induced signaling, consistent with its ability to bind TβRII but not TβRI or other receptors of the TGF-β family. However, TGM6 does not bind CD44 and is unable to inhibit TGF-β and TGM1 signaling in T cells. To understand how TGM6 binds TβRII, the X-ray crystal structure of the TGM6 domain 3 bound to TβRII was determined at 1.4 Å. This showed that TGM6 domain 3 binds TβRII through an interface remarkably similar to the TGF-β:TβRII interface. These results suggest that TGM6 has adapted its domain structure and sequence to mimic TGF-β binding to TβRII and function as a potent TGF-β and TGM1 antagonist in fibroblasts. The coexpression of TGM6, along with the immunosuppressive TGMs that activate the TGF-β pathway, may prevent tissue damage caused by the parasite as it progresses through its life cycle from the intestinal lumen to submucosal tissues and back again.