Singh, S.; Smyth, D. J.; Cunningham, K.; Mukundan, A.; Byron, C.-H.; Hinck, C. S.; White, M. P. J.; Ciancia, C.; Wosowska, N.; Sanders, A.; Jin, R.; Lilla, S.; Zanivan, S.; Schoenherr, C.; Inman, G.; van Dinther, M. A. H.; ten Dijke, P.; Hinck, A. P.; Maizels, R. M.

The murine helminth parasite Heligmosomoides polygyrus expresses a family of modular proteins which, replicating the functional activity of the immunomodulatory cytokine TGF-beta, have been named TGM (TGF-beta beta Mimic). Multiple domains bind to different receptors, including TGF-beta receptors TbetaRI (ALK5) and TbetaRII through domains 1-3, and prototypic family member TGM1 binds the cell surface co-receptor CD44 through domains 4-5. This allows TGM1 to induce T lymphocyte Foxp3 expression, characteristic of regulatory (Treg) cells, and to activate a range of TGF-beta-responsive cell types. In contrast, a related protein, TGM4, targets a much more restricted cell repertoire, primarily acting on myeloid cells, with less potent effects on T cells and lacking activity on other TGF-beta-responsive cell types. TGM4 binds avidly to myeloid cells by flow cytometry, and can outcompete TGM1 for cell binding. Analysis of receptor binding in comparison to TGM1 reveals a 10-fold higher affinity than TGM1 for TGFbetaR-I (TbetaRI), but a 100-fold lower affinity for TbetaRII through Domain 3. Consequently, TGM4 is more dependent on co-receptor binding; in addition to CD44, TGM4 also engages CD49d (Itga4) through Domains 1-3, as well as CD206 and Neuropilin-1 through Domains 4 and 5. TGM4 was found to effectively modulate macrophage populations, inhibiting lipopolysaccharide-driven inflammatory cytokine production and boosting interleukin (IL)-4-stimulated responses such as Arginase-1 in vitro and in vivo. These results reveal that the modular nature of TGMs has allowed the fine tuning of the binding affinities of the TbetaR- and co-receptor binding domains to establish cell specificity for TGFbeta-signalling in a manner that cannot be attained by the mammalian cytokine.