Abstract
TNFRSF25, a member of the TNF receptor superfamily, is a type I transmembrane protein that has a cytoplasmic death domain. TNFRSF25 is expressed primarily by T lymphocytes, including CD4+, CD8+, and NKT cells and is efficiently upregulated after T cell stimulation. The natural ligand for TNFRSF25, TL1A, is a type II transmembrane protein can be subsequently cleaved as a soluble trimeric protein by a member of metalloproteases. Our lab previously reported that TNFRSF25 stimulation using an agonist antibody, 4C12, expands pre-existing CD4+FoxP3+ Tregs in vivo. To determine how the physiological ligand differs from the antibody, we generated the soluble mouse TL1A-Ig fusion protein (TL1A-Ig) that forms a dimer of TL1A-trimers in solution with an apparent m.w. of 528 kDa. In vitro, TL1A-Ig mediated rapid proliferation of Foxp3+Treg and a population of CD4+FoxP3- conventional T cells (Tconv). TL1A-Ig also blocked de novo biogenesis of inducible Tregs (iTregs) and it attenuated the suppressive function of Treg. Treatment with TL1A-Ig in vivo induced the proliferation and activation of memory Tconvs, as well as, the expansion of Tregs such that they became 30-35% of all CD4+ T cells in the peripheral blood within 5 days of treatment. Treg proliferation was dependent upon TCR engagement with MHC class II. Elevated Tregs were maintained for at least 20 days with daily injections of TL1A-Ig. TL1A-Ig-expanded Treg cells expressed high levels of activation/memory markers KLRG1 and CD103 and were highly suppressive ex vivo. TL1A-Ig mediated Treg expansion in vivo was protective against allergic lung inflammation, a mouse model for asthma, by reversing the ratio of Tconv cells to Tregs in the lung and blocking eosinophil exudation into the bronchoaveolar fluid. The potential of TNFRSF25 agonists was also tested in transplant tolerance. TNFRSF25 mediated Treg cell expansion in vivo significantly prolonged allogeneic skin graft survival. Our findings shed light on the role of TNFRSF25 signaling in CD4+ T cell subsets and the role of TL1A/TNFRSF25 in mediating inflammatory responses and resolving inflammation in tissue. Thus, TL1A-Ig fusion proteins are highly active and tightly controllable agents to stimulate Treg proliferation in vivo, and are uniquely able to maintain high-levels of expanded Treg for long periods of time by repeated administration.