Suppressing autoimmunity by TGF-β: not just through T(reg) cells.

Transforming growth factor-β (TGF-β) is a pleiotropic factor. It regulates many aspects of T-cell biology including thymocyte development, T cell differentiation and effector T cell function.1 Previous work has shown that the loss of TGF-β signaling in the T-cell lineage leads to severe lymphoproliferative autoimmunity.2,3 Until now, it has been unclear to what extent the autoimmunity observed in those mice is driven by TGF–unresponsive conventional T cells, as TGF- is also crucial for the maintenance and function of regulatory T cells (Treg). A recent study published in Nature Immunology revealed a novel role of TGF-β signaling in non-regulatory naive T cells. It was found that TGF-β controls lymphopenia-driven T-cell proliferation in neonatal mice, thereby preventing the development of autoimmunity.4 Zhang and Bevan made this discovery by comparing two animal models in which type II TGF-β receptor (TGF-βRII) was ablated in T cells at different developmental stages. In the previous study, researchers specifically deleted TGF-βRII using Cre recombinase driven by the promoter of CD4 (Tgfbr2f/f CD4-Cre). In this model system, TGF-βRII is deleted in T cells by the time they emerge from the thymus. As a result, these mice show severe autoimmunity and early mortality (3–5 weeks). The autoimmune phenotype observed in these mice cannot be rescued by transferring wild-type Treg cells or by reconstituting them with mixed bone marrow from wild-type and Tgfbr2f/f CD4-Cre mice.2,3 Together, these findings show that the onset of autoimmunity in the Tgfbr2f/f CD4-Cre model is at least partially due to the self-reactivity of TGF-β unresponsive conventional T cells. Zhang and Bevan studied a different TGF-βRII conditional deletion model, in which the expression of Cre is driven by the distal promoter of the kinase Lck (Tgfbr2f/f dLck-Cre). In this model, the T cell-specific deletion of TGF-βRII begins in mature T cells and is much slower. Although TGF-βRII is almost completely deleted in T cells in adult mice (7–9 weeks), the expression of TGF-βRII is unaffected in neonates (day 4). The delayed deletion of TGF-βRII led to a different phenotype: Tgfbr2f/f dLck-Cre showed no signs of autoimmune lesions and survived for up to a year.3,4 Using this model, Zhang and Bevan examined why the expression of TGF-βRII on T cells during neonatal development protects the host from autoimmunity later in life. In neonatal mice, the lymphopenic environment allows naive T cells to undergo proliferation driven by weak engagement of the T-cell receptor (TCR) with self ligands and homeostatic cytokines interleukin 7 (IL-7) and IL-15.5 Zhang and Bevan found that TGF-β signaling in naive T cells was essential to restrain this lymphopenia-driven proliferation in neonatal animals. TGF-βRII-deficient naive T cells (from Tgfbr2f/f dLck-Cre mice) proliferated rapidly upon transfer to Rag1−/− or sublethally irradiated wild-type hosts, demonstrating the critical role of TGF-β in regulating homeostatic proliferation in both chronic and acute lymphopenic conditions, respectively. The lack of TGF-β signaling also resulted in an effector phenotype characterized by the increased expression of KLRG1, granzyme B and interferon-γ. The hyperproliferative effector phenotype of TGF-βRII-deficient T cells in Rag1−/− recipients was associated with autoimmunity, as demonstrated by T-cell organ infiltration and weight loss. The enhanced proliferative capacity of TGF-β-unresponsive T cells does not appear to be caused by commensal bacteria. Commensal flora generates antigens and inflammatory signals, which contribute to rapid T-cell expansion and autoimmunity in the host.6 Loss of commensal bacteria upon antibiotic treatment significantly inhibits the lymphopenia-driven proliferation of wild-type T cells; but fails to control the hyperproliferation of TGF-βRII-deficient T cells. Pro-inflammatory cytokines, such as interferon-γ, IL-6, tumor-necrosis factor, IL-1β and IL-18, did not enhance the proliferation of TGF-β-unresponsive T cells, and interferon-α and IL-12 even reduced the proliferative advantage of TGF-βRII-deficient T cells over wild-type T cells. These results suggest that TGF-β signaling prevents T-cell proliferation induced by weak TCR signals, but does not affect proliferation induced by foreign antigens or pro-inflammatory cytokines. The most important finding of this study is related to the role of TGF-β signaling in controlling the T-cell response to tonic ‘tickling'.1 During thymocyte development, negative selection deletes T cells with high affinity to self-antigens. Meanwhile, the mature T cells leaving the thymus retain a low affinity to self-ligands. The engagement of TCR by self-ligands in the periphery maintains the viability of naive T cells; however, under certain conditions, unwanted responses and expansion triggered by self-antigens may cause autoimmune disease. Zhang and Bevan showed that TGF-β signaling strongly inhibited T-cell responses to low affinity ligands. When stimulated with a low concentration (1 ng/ml) of T4 ligand (a weak agonist peptide), OT-I T cells proliferated slowly in the presence of TGF-β1, and neutralization of TGF-β greatly enhanced the proliferation rate of these cells. In contrast, OT-I T cells showed comparable proliferation in the presence or absence of TGF-β signaling when stimulated with a high concentration of a strong agonist (N4). The preferential inhibition of naive T-cell responses against low affinity ligands by TGF-β may be a crucial protective mechanism against autoimmunity (Figure 1). Figure 1 TGF-β signaling in naive T cells inhibits T-cell responses against low affinity ligands, but not high affinity antigens. When T cells are stimulated with high-affinity antigens (foreign antigens), TGF-β signaling has limited effect on ... Despite these findings, this study still may not fully explain the severe autoimmune phenotype observed in the Tgfbr2f/f CD4-Cre model. First, the extent to which the loss of Treg cells in the periphery contributes to the onset of autoimmunity remains unresolved. Unfortunately, TGF-βRII is not deleted in most peripheral Treg cells in the Tgfbr2f/f dLck-Cre model. Although the transfer experiments suggest that factors other than Treg cells caused the autoimmunity observed in the Tgfbr2f/f CD4-Cre model, it is possible that the loss of Treg cells enhances the severity of the disease. Second, because CD4-Cre is expressed before thymic negative selection, the loss of TGF-β signaling may alter the T-cell repertoire. Finally, it should also be noted that the development of NKT cells is abrogated in Tgfbr2f/f CD4-Cre mice.7 Whether this population plays a role in preventing autoimmunity, however, remains unknown. In summary, Zhang and Bevan's work demonstrates an inhibitory role for TGF-β signaling in naive T cells during homeostatic proliferation in lymphopenic conditions. Moreover, the dampening effect of TGF-β signaling on T-cell activation in response to weak but not strong TCR stimuli provides valuable insight to understand thymocyte development, T cell maintenance and peripheral tolerance.