Targeting TL1A and DR3: the new frontier of anti-cytokine therapy in IBD

Abstract

TNF-like cytokine 1A (TL1A) and its functional receptor, death-domain receptor 3 (DR3), are members of the TNF and TNFR superfamilies, respectively, with recognised roles in regulating innate and adaptive immune responses; additional existence of a decoy receptor, DcR3, indicates a tightly regulated cytokine system. The significance of TL1A:DR3 signalling in the pathogenesis of inflammatory bowel disease (IBD) is supported by several converging lines of evidence. Herein, we aim to provide a comprehensive understanding of what is currently known regarding the TL1A/DR3 system in the context of IBD. TL1A and DR3 are expressed by cellular subsets with important roles for the initiation and maintenance of intestinal inflammation, serving as potent universal costimulators of effector immune responses, indicating their participation in the pathogenesis of IBD. Recent evidence also supports a homoeostatic role for TL1A:DR3 via regulation of Tregs and innate lymphoid cells. TL1A and DR3 are also expressed by stromal cells and may contribute to inflammation-induced or inflammation-independent intestinal fibrogenesis. Finally, discovery of genetic polymorphisms with functional consequences may allow for patient stratification, including differential responses to TL1A-targeted therapeutics. In conclusion, TL1A:DR3 signalling plays a central and multifaceted role in the immunological pathways that underlie intestinal inflammation, such as that observed in IBD. Such evidence provides the foundation for developing pharmaceutical approaches targeting this ligand-receptor pair in IBD.

Keywords: CROHN'S DISEASE; CYTOKINES; ULCERATIVE COLITIS.

Figure 1.. Timeline of anti-TL1A/DR3 therapeutic development.

TL1A was described for the first time in 2002, as a member of the TNF superfamily of proteins, shown to bind to two previously identified receptors: DR3, associated with functional signaling, and DcR3, which acts as a decoy/inhibitory receptor. A strong link with IBD was supported by expression studies showing significant upregulation of the TL1A/DR3/DcR3 system in inflammatory lesions from patients with CD and UC, and the discovery of IBD risk-modifying polymorphisms in the respective genes. Subsequent research in experimental systems delineated the immunological properties of TL1A:DR3 signaling, whereas animal models provided mechanistic in vivo evidence to support the important pathogenetic role of this cytokine system in several murine strains with intestinal inflammation, including the prominent anti-inflammatory effects of anti-TL1A and anti-DR3 therapies. These pre-clinical data laid the foundation that initiated the development and clinical testing of various monoclonal antibodies targeting TL1A in patients with CD and UC. Abbreviations: CD, Crohn’s disease; DR3, death-domain receptor; DcR3, decoy receptor 3; IBD, inflammatory bowel disease; TL1A, TNF-like cytokine 1A; TNF, tumor necrosis factor; UC, ulcerative colitis. Created with Biorender.com.

Figure 2.. Characteristics of the TL1A/DR3/DcR3 system.

TL1A is a member of the TNF superfamily of proteins encoded by TNFSF15. TL1A is a type II transmembrane protein containing cytoplasmic, transmembrane, and extracellular domains. Like other TNF-family members, TL1A exists as a stable trimer and as a membrane-bound (mTL1A) isoform on the cell’s surface. A soluble, fully functional isoform may also be encountered (sTL1A), produced by enzymatic cleavage of mTL1A by matrix-metalloproteinases (TACE). The functional receptor for TL1A is DR3, a member of the TNFR superfamily (TNFRSF25). DR3 is a type I protein with extracellular, transmembrane, and cytoplasmic domains. The intracellular region of DR3 contains the characteristic configuration of a death domain, which signals via TRADD. Upon TL1A binding, downstream signaling may take one of two distinct pathways, depending on the cellular and molecular context of ligand engagement. In the first scenario, the adaptor proteins, TRAF and RIPK1, are recruited to the membrane complex, leading to activation of effector kinases, PI3K and MAPKs, and transcription factor NF-kB. This pathway generates proliferative and pro-inflammatory signals in DR3-bearing activated immunocytes. In the second scenario, TRADD dissociates from the death domain of DR3 and binds in the cytoplasm to FADD, leading to activation of caspase-8 and −3. The outcome in this case is the induction of apoptosis with programmed cell death. Several splice variants have been described for TNFRSF25, which encodes for either transmembrane or soluble isoforms, which may have important implications for cell functionality and response to TL1A. TL1A is also capable of binding to decoy receptor DcR3 (TNFRSF6B), which competes with DR3 for ligand engagement, thus inhibiting the functional effects of TL1A. DcR3 only exists as a soluble protein, as it lacks a transmembrane domain. In contrast to DR3, which binds TL1A only, DcR3 also associates with FasL/TNFSF6 and LIGHT/TNFSF14 and prohibits transmission of signals generated by the association of these ligands with their respective functional receptors. Abbreviations: DR3, death-domain receptor; FADD, Fas-associated protein with death domain; FasL, Fas ligand; LIGHT, receptor homologous to lymphotoxin that exhibits inducible expression, competes with HSV glycoprotein D for the HVEM, and is expressed by T lymphocytes; MAPK, mitogen-activated protein kinase; NF-kB, nuclear factor kappa B; PI3K, Phosphoinositide 3-kinase; RIPK1, receptor-interacting serine/threonine-protein kinase 1; TACE, tumor necrosis factor-alpha converting enzyme; TL1A, TNF-like cytokine 1A; TNF, tumor necrosis factor; TNFSF, TNF superfamily of proteins; TNFRSF, TNF receptor superfamily of proteins; TRADD, TNF receptor 1-associated death domain protein; TRAF, TNF receptor associated factors. Created with Biorender.com.

Figure 3.. TL1A:DR3 as master regulators of homeostatic and inflammatory mucosal pathways.

TL1A:DR3 interactions contribute to several homeostatic pathways at the intestinal mucosa. Intact TL1A:DR3 signaling appears to be necessary for the uneventful recovery from an acute insult to the intestinal wall via several different mechanisms. Upon such triggers, TL1A-expressing cells, such as DCs and CX3CR1⁺ mononuclear phagocytes, interact with DR3-expressing homeostatic cell populations to re-instate a healthy mucosal status. Those TL1A-mediated responses include enhancement of proliferation of Tregs with increased regulatory cytokine secretion and maintenance of suppressor function, secretion of the epithelial barrier-protecting cytokine, IL-22, by gut-resident RORγt⁺ group 3 ILCs and direct effects on the epithelial compartment itself. Such effects are highly influenced by microbiota-derived signals, which are shown to regulate the expression of TL1A on monocytes and DCs. In addition, TL1A:DR3 engagement in monocytes is necessary for optimal bacterial uptake and intracellular bacterial clearance, indicating the existence of locally organized and tightly regulated homeostatic responses. The establishment of intestinal inflammation is associated with high mucosal upregulation of both TL1A and DR3, which act as powerful amplifiers of inflammatory pathways. Activated lymphocytes express DR3 and respond to APC-derived TL1A, with the net result of a universal costimulatory stimulation of effector adaptive immunity pathways. Teff and their corresponding cytokines act as pivotal pathogenetic modules, which perpetuate mucosal inflammation in IBD. Pathogenic ILCs are equally affected and contribute to the generation of a pro-inflammatory mucosal milieu, enriched with IL-13/IL-5 secreted by ILC2s and IFNγ by ILC1s. Inflammatory chemotactic factors are also elevated and may facilitate the trafficking of inflammatory cells towards the intestinal mucosa, thus creating a self-amplifying loop between cell recruitment and inflammatory mediator production. Overall, current evidence supports the concept that TL1A:DR3 act on complex cellular and molecular networks, the net effect which dictates the balance between homeostasis and inflammation at the intestinal mucosa. Created with Biorender.com.

Figure 4.. TL1A and DR3 are critically positioned on cell populations with key roles during immunological responses.

At steady-state, TL1A is not constitutively expressed, but is significantly induced by inflammatory and bacterial signals. TL1A avidly localizes on APCs, although also detected in macrophages, lymphocytes and plasma cells, as well as endothelial and stromal cells (inner circle). On the other hand, DR3, the functional receptor for TL1A, is expressed by activated lymphocytic populations, including all Teff lineages (Th1, Th2, Th9, Th17), Tregs, NK and NK-T cells and all subsets of ILCs (2^nd and 3^rd circles).This broad cellular distribution of DR3 is translated into remarkable functional potential of TL1A:DR3 signaling that is mediated via the enhancement of proliferation of and amplification of cytokine production by responding lymphocytes and ILCs. Overall, TL1A:DR3 act as a universal co-stimulatory system that amplifies TCR or cytokine-provided signals, particularly under sub-optimal stimulatory conditions (4^th circle). The localization of DR3 on cell populations of the innate immunity also suggests participation in homeostatic mucosal pathways. Finally, avid localization and regulation of TL1A and DR3 on stromal cells, including SEMFs, have triggered research regarding their participation in the pathogenesis of conditions that are characterized by inflammation-fibrosis phenotypes. Experimental data from animal models and translational data from patients strongly indicate that the immunological properties attributed to the TL1A:DR3 system are highly relevant for several immune-mediated and inflammatory conditions that affect not only the intestinal mucosa but may also underlie such diverse diseases as allergic asthma, inflammatory arthritides, scleroderma and psoriasis (outer circle). Abbreviations: APCs, antigen presenting cells; DR3, death-domain receptor; ILCs, innate lymphoid cells; NK, natural killer; SEMFs, subepithelial myofibroblasts; TCR, T cell receptor; T-eff, effector T cells; Th, T helper; TL1A, TNF-like cytokine 1A; Tregs, T regulatory cells. Created with Biorender.com.

Figure 5.. TL1A:DR3-dependent pathways in intestinal fibrosis.

Following mucosal damage, local inflammatory pathways are generated in response to bacterial- or tissue-injury related triggers. Resident dendritic cells and macrophages are critical cell populations during this process and constitute avid local providers of inflammatory cytokines, such as IL-1 and TNF. Such phenomena also initiate tissue repair mechanisms and activate stromal cells, including SEMFs. Activated SEMFs upregulate the expression of TL1A in response to pro-inflammatory signals that dominate the mucosal immunological milieu. Further input arises from epithelial cells that, during inflammatory conditions, contribute to the pool of TL1A-inducing mediators, and also express DR3, thus potentiating epithelial:mesenchymal interactions. Interestingly, SEMFs also express DR3, which makes them responsive to autocrine stimulation, with initial TL1A upregulation and secondary DR3 signaling after ligand:receptor binding. This leads to collagen production and pro-fibrotic effects that are downregulated after effective anti-TL1A blockade in both clinical and experimental settings. This cellular complexity of ligand/receptor distribution and inflammatory regulation point to the existence of local amplification loops for SEMFs that are initiated via paracrine inflammatory signals and then self-perpetuate via autocrine pathways. In addition, TL1A is also part of the local inflammatory response and may also affect fibrogenesis indirectly via the stimulation of pro-fibrotic cytokine production by DR3-expressing cells. Notably, genetic of inflammatory-induced mucosal overexpression of TL1A is universally associated with muscular hypertrophy and fibrostenosis, alongside mucosal elevations of IL-13, IL-17A, and TGFβ1, which are amongst the most prominent pro-fibrotic factors. Abbreviations: DR3, death-domain receptor; IL-1, interleukin-1; FADD, Fas-associated protein with death domain; SEMFs, subepithelial myofibroblasts; TL1A, TNF-like cytokine 1A; TNF, tumor necrosis factor; TNFSF, TNF superfamily of proteins. Created with Biorender.com.

Figure 6.. Future perspectives of TL1A:DR3 research.

At the moment, TL1A is considered the only ligand that associates with DR3, leading to functional signaling with critical involvement in a plethora of cellular and molecular pathways. Several lines of evidence, however, have questioned the exclusivity of this engagement. DR3- and TL1A-deficiency do not produce identical spontaneous, or induced, immunophenotypes and affect differently the kinetics and outcomes of experimental inflammation. In addition, the existence of additional ligands for DR3 has already been shown under certain conditions. DR3 that is expressed on HT29 IECs associates with E-selectin and Atsttrin, a progranulin-derived molecule, and can bind to DR3, in addition to TNFR1 and 2. The validation and discovery of additional DR3 ligands will definitely test the intriguing possibility that targeting DR3 may provide an alternative, and potentially more effective, anti-inflammatory strategy than anti-TL1A approaches, and justify the development and testing of DR3-targeted therapies for human immune-mediated conditions. Created with Biorender.com.