TL1A represents a member of the TNF/TNFR super-family specifically implicated in the control of T-cell activation events that affect both central tolerance and T-helper responses triggered during pathology. In this respect, TL1A enhances costimulatory events that regulate T-cell activation. Our studies build on these prior findings and suggest that TL1A signaling in naive and preactivated effector cells has the ability to suppress the initial commitment of naive T cells into Th17 cells but can serve to maintain the effector characteristics of preconditioned Th17 cells. These data suggest that TL1A-driven T-cell responses are based on mechanistic differences between the signaling events required for the initial differentiation, maintenance, and proliferation of effector cells.
Prior investigations have shown that TL1A is associated with the expansion of T-helper subsets and can synergize with IL-12 and IL-18 to affect development of γIFN-producing T cells (3
). This finding led to the view that TL1A, signaling via
its receptor DR3, complements the expansion of Th1 cells (1
). More recent investigations of TL1A/DR3 signaling in experimental models of encephalitis have shown that DR3−/−
mice and mice lacking TL1A display reduced disease severity, which is associated with a reduced number of CD4+
T cells, including Th17 cells, in the CNS (7
). As proposed by Pappu et al.
), this finding suggests that TL1A might promote the development of Th17 responses, which promote disease pathology. However, it is unclear whether this impaired T-cell infiltrate arises due to a defect in T-cell trafficking, survival, or maintenance of effector function. Further proinflammatory roles for TL1A have also been described in Th2-mediated Ova-induced lung disease (7
), suggesting that TL1A might have a more universal role in regulating effector outcomes. Based on models of colitis (1
) and rheumatoid arthritis (6
), it is likely that TL1A predominantly bolsters development of Th1 and Th17 responses. However, it is currently unclear how TL1A coordinates these effector functions. In this respect, we now report that TL1A/DR3 signaling in newly challenged naive T cells inhibits Th17 (based on IL-17A and IL-17F measurements) and Tc17 development; the level of TCR activation influences the ability of TL1A to drive T-cell proliferation; TL1A control of IL-17A expression is independent of its ability to govern proliferative responses; and TL1A can maintain the effector characteristics of precommitted Th17 cells. These data indicate that TL1A might differentially affect the outcome of T-cell responses elicited by naive and activated/memory T-cell subsets.
Previously, T-cell expression of receptors for TNF superfamily members has been linked with T-cell survival, memory expansion, and costimulatory events (36
). The ability of TL1A to elicit T-cell responses relates to an activation-induced regulation of DR3 expression. Specifically, DR3 is expressed universally by activated T cells; however, a composite analysis of the CD4+
population show that DR3 is highly expressed by human IL-17-secreting T cells. A similar expression pattern has also been observed in murine T cells with TL1A predominantly associated with the activation of CD4+
memory subsets (2
). An activation-induced control of T-cell DR3 expression would therefore suggest that signaling via
this receptor might occur during antigen presentation by dendritic cells. Indeed, CD11chi
dendritic cells display TL1A as a membrane-bound ligand, which could be presented to DR3+
T cells (2
). A primary function of DR3 signaling might therefore be to steer the nature of the T-cell response following TCR activation. In this regard, we noted that TL1A dose-dependently inhibited the differentiation of Th17 cells from naive T cells and an increased number of Th17 cells was observed in draining lymph nodes isolated from DR3−/−
mice. An inhibitory role for TL1A in Th17 differentiation is somewhat unexpected, considering the proinflammatory nature of its outcomes in T-cell-mediated inflammatory disease (7
). This finding might point to another level of complexity for TL1A/DR3 signaling in governing T-cell responses. In addition to these centrally regulated immunological processes, TL1A/DR3 is also likely to influence inflammatory events at sites of disease activity. Endothelial cells, for example, secrete TL1A, while gene analysis of DR3 expression in human osteoblasts and synovial cells from rheumatoid arthritis patients suggest a more localized role for TL1A in degenerative and inflamma-tory arthritis (11
). When compared with WT littermates, mice deficient in TL1A or DR3 display an overall reduction in the number of effector T cells at sites of inflammation (2
). This provides a potential link between TL1A/DR3 and the regulation of T-cell recruitment or the retention of activated cells at sites of inflammation. Our results show that TL1A signaling in preactivated T cells supported maintenance of IL-17A expression. This finding might explain the apparent paradox between the inhibitory action of TL1A in Th17 differentiation and the protection provided by TL1A/DR3-signaling-deficient mice in experimental disease models where Th17 cells have been implicated in the pathology. In this regard, the ability of TL1A to inhibit Th17 differentiation might be secondary to its proinflammatory role in the local maintenance of Th17-effector function. This observation builds on studies that demonstrate the importance of TGFβ and the role of IL-6 trans-signaling in maintaining a Th17 phenotype at inflammatory lesions (41
). Consistent with our study, none of these Th17 “maintenance factors” skew or influence the proportion of γIFN-secreting T cells, which suggests a selective activity on Th17 cells.
The negative regulation of Th17 cells by TL1A implies that DR3 activation antagonizes the commitment signals governing development of this population from naive T cells. The TL1A control of Th17 differentiation appears to be independent from its capacity to regulate proliferative responses. Consistent with previous reports, TL1A only promoted T-cell proliferation in suboptimal conditions of TCR activation (2
). This condition, however, was not associated with an induction of IL-17A. Instead, optimal TCR engagement was required for maximal IL-17A secretion. In this respect, TL1A was found to block Th17 generation but not T-cell proliferation. These findings point to an inverse relationship between TL1A/DR3 signaling and TCR activation that differentially affects T-cell proliferation vs.
Th17 commitment. Coordination of these events could relate to the known TL1A regulation of T-cell IL-2 expression, which acts as a negative regulator of Th17 generation and a survival factor for regulatory T cells (29
). Although an anti-IL-2 blocking antibody enhanced IL-6/TGFβ and IL-6/TGFβ/IL-23 differentiation of Th17 cells, the negative regulation provided by TL1A appeared independent of IL-2 activity. Similar to other cytokines of the TNF superfamily, TL1A primarily triggers NF-κB and MAP-kinase signaling events (3
). Although it is unclear how these signaling cascades might feed into Th17 differentiation pathways, activation of DR3 by TL1A affects the expression of T-cell activating cytokines, including γIFN, which triggers STAT1 mediated responses (1
). Factors that enhance STAT1 activity in T cells are characterized inhibitors of Th17 differentiation, and αIFN, βIFN, γIFN, IL-27, and retinoic acid all inhibit STAT3-driven Th17 generation (13
). Utilizing cytokine-deficient mouse strains and anticytokine neutralizing antibodies, studies excluded a role for STAT1 in the TL1A-driven suppression of Th17 commitment. The mostly likely explanation for TL1A control of Th17 development is therefore a potential signaling interplay between a signaling event associated with the TNFRSF and the latent transcription factor STAT3, which is activated by IL-6, IL-21, and IL-23 to steer Th17 commitment. In this respect, CD4+
T cells lacking the molecular adaptor TNF receptor-associated factor-6 (TRAF-6) display increased Th17 differentiation (45
). Although TRAF6 plays an intrinsic role in the maintenance of peripheral tolerance and anergy induction, studies have not examined activation of TRAF6 by TL1A. Our data further emphasize that while STAT3 activity is integral to Th17 differentiation, it is insufficient to maintain the Th17 phenotype in the absence of TCR stimulation. In this regard, we noted that TL1A stimulation of precommitted Th17 cells helped to retain IL-17A expression.
Given an ability of TL1A to differentially control Th17 responses in naive vs.
activated T cells, it is conceivable that the signals required for the long-term maintenance of Th17 cells are distinct from those affecting differentiation. One possible explanation could relate to a TL1A modulation of pro- or antiapoptotic regulators, including members of the Bcl or IAP families. Indeed, activation-induced changes in T-cell X-linked inhibitor of apoptosis (XIAP) expression have been associated with the development of EAE, which is considered a Th17-driven model (46
). Notably, DR3−/−
mice are resistant to EAE pathology (7
). However these activities might need to be placed in the context of STAT3 activation, which drives Th17 differentiation but also rescues T cells from entering apoptosis (47
). An alternative hypothesis would suggest a modification in T-cell signaling capacity following activation. When compared to resting T cells, activated memory T cells display altered IL-6 signaling, which is attributable to both an activation-induced loss of IL-6R and dramatically impaired STAT1, but not STAT3, activity (41
). Our findings echo these findings and suggest that activation-induced expression of DR3 contributes to an alteration in TL1A responsiveness. Although the molecular basis for this regulation is unclear, the observed relationship between TL1A activity and TCR signaling remains an intriguing aspect of this study. Collectively, these results support the view that TL1A activities depend on the activation status or activation history of the T-cell population.