TL1A inhibition in IBD: tulisokibart vs anti-TNF

The TL1A/DR3 axis: four pathogenic mechanisms in IBD

TL1A (TNFSF15) is a member of the TNF superfamily that drives intestinal inflammation through at least four distinct, parallel mechanisms — T-cell costimulation, innate immune activation, direct fibroblast stimulation, and disruption of the epithelial-microbiome interface. This breadth of action distinguishes it from single-pathway targets and explains the growing interest from the IBD drug development community. According to research published in Nature-affiliated journals, genetic polymorphisms in TNFSF15 are strongly associated with IBD risk, particularly in Asian populations, and TL1A expression in inflamed intestinal mucosa correlates with disease severity and fibrotic complications.

TL1A exists in both membrane-bound and soluble forms and signals primarily through its cognate receptor death receptor 3 (DR3, TNFRSF25), expressed predominantly on activated lymphocytes, innate lymphoid cells, and stromal cells. A decoy receptor, DcR3, provides negative regulation. This receptor architecture is entirely separate from the TNF/TNFR1/TNFR2 axis targeted by infliximab, adalimumab, and related agents.

1. Universal T-cell costimulation

TL1A acts as a potent costimulator of effector T cells in synergy with IL-12 (for Th1 responses) and IL-23 (for Th17 responses). It enhances production of IFN-γ, IL-17A, IL-9, and IL-22 from CD4+ T cells, and is particularly effective on mucosal CCR9+ T cells and CD4+CD161+ T cells enriched in the gut. This breadth of T-cell costimulation — spanning Th1, Th2, Th9, Th17, Treg, and ILC3 subsets — gives TL1A blockade a multi-pronged immunomodulatory profile that no single cytokine neutralisation strategy can replicate.

2. Innate immune and ILC activation

TL1A activates group 3 innate lymphoid cells (ILC3s), promoting GM-CSF production, which subsequently activates neutrophils and contributes to tissue damage. Critically, TL1A can directly induce pro-inflammatory cytokines — including TNF-α — from CD161+ T cells independent of TNF signalling. This TNF-independent inflammatory loop is a key reason why ongoing TL1A-driven pathology can persist even in patients receiving anti-TNF therapy.

3. Direct fibrogenic activity — the unique differentiator

DR3 is expressed on intestinal fibroblasts and myofibroblasts. TL1A directly activates these cells, increasing collagen I and III deposition and promoting fibroblast proliferation and migration via the TGF-β1/Smad3 pathway. Transgenic mice overexpressing TL1A spontaneously develop intestinal fibrosis. Stricture formation in Crohn's disease — a major driver of surgical intervention — is a complication not addressed by any currently approved biologic. This direct fibrogenic activity is arguably the most clinically important mechanistic differentiator for TL1A inhibition.

4. Epithelial and microbiome disruption

TL1A overexpression induces Paneth cell dysfunction and morphological abnormalities, and alters gut microbiome composition by enriching short-chain fatty acid (SCFA)-producing bacteria. Acetate accumulation promotes ileal inflammation in TL1A transgenic models. This creates a feed-forward loop: dysbiosis drives TL1A production, which causes further Paneth cell dysfunction and microbiome disruption — a cycle that purely immunological interventions may not fully interrupt.

How TL1A inhibition differs from anti-TNF and anti-integrin therapies

Anti-TNF and anti-integrin biologics each address one dimension of IBD pathology — cytokine excess or lymphocyte trafficking — while leaving fibrosis, T-cell costimulation, and ILC-mediated innate immunity largely untouched. TL1A inhibition acts on a distinct receptor system that intersects all four of these dimensions, which is why its mechanistic profile looks so different on paper and increasingly in clinical data.

Anti-TNF: powerful but incomplete

Anti-TNF agents (infliximab, adalimumab, golimumab, certolizumab) neutralise soluble and membrane-bound TNF-α, blocking TNFR1 and TNFR2 signalling and suppressing NF-κB activation. They remain the most widely used advanced therapies in IBD, endorsed by bodies including WHO and major gastroenterology societies. However, primary non-response affects 20–30% of patients, and 30–40% lose response within one year, often due to immunogenicity. Anti-TNF therapy has no direct anti-fibrotic effect, and a substantial proportion of IBD inflammation is driven by TNF-independent pathways.

Anti-integrin: gut-selective but slow and modest in CD

Vedolizumab (anti-α4β7 integrin) blocks lymphocyte trafficking to gut tissue by preventing α4β7 integrin binding to MAdCAM-1 on intestinal endothelium. Its gut-selective mechanism reduces systemic immunosuppression, a genuine safety advantage. However, clinical benefit may take 10–14 weeks to emerge, efficacy in Crohn's disease is modest compared to ulcerative colitis, and response rates decline in patients previously exposed to anti-TNF agents. Like anti-TNF agents, vedolizumab has no anti-fibrotic activity and does not neutralise inflammatory mediators already present in tissue.

The table below summarises the mechanistic differentiation across the three therapeutic classes based on published evidence.

"TL1A can directly induce pro-inflammatory cytokines — including TNF-α — from CD161+ T cells independent of TNF signalling, meaning TL1A-driven inflammation persists even when anti-TNF therapy is present."

Clinical evidence: tulisokibart and RVT-3101 trial results

Two anti-TL1A antibodies have generated Phase 2 clinical data in IBD: tulisokibart (MK-7240, developed by Merck) and RVT-3101 (PF-06480605, developed by Telavant/Roivant). Both have demonstrated statistically significant efficacy in ulcerative colitis, with Crohn's disease programmes underway. The ARTEMIS-UC and TUSCANY-2 trials represent the most complete dataset currently available for the TL1A inhibitor class.

Tulisokibart: ARTEMIS-UC Phase 2 results

The ARTEMIS-UC trial, published in the New England Journal of Medicine in 2024, was a randomised, double-blind, placebo-controlled study in patients with moderate-to-severe ulcerative colitis who had corticosteroid dependence or failure of conventional and/or advanced therapies. Patients received IV tulisokibart 1,000 mg on day 1, then 500 mg at weeks 2, 6, and 10. At the primary endpoint of week 12, clinical remission was achieved by 26.5% of tulisokibart-treated patients versus 1.5% on placebo — a difference of 25.0 percentage points (95% CI 13.9–36.6%, P<0.001). In the biomarker-positive subpopulation, remission was 31.6% versus 10.8% (difference 20.8%, P=0.02). Adverse events were comparable between groups, with most events mild-to-moderate in severity. Merck presented long-term maintenance data at UEG Week 2024, and Phase 3 trials — ATLAS-UC and ARES-CD — are now underway, representing the first Phase 3 studies of an anti-TL1A antibody in IBD.

RVT-3101: TUSCANY-2 and the TAHOE Crohn's trial

The TUSCANY-2 Phase 2b trial in ulcerative colitis demonstrated a notable durability signal: clinical remission improved from 29% at week 14 (induction) to 36% at week 56 (maintenance). Endoscopic improvement reached 50% at week 56, compared to 36% at week 14. In biomarker-positive patients, clinical remission was 43% and endoscopic improvement 64% at week 56 — figures that compare favourably with maintenance data for established biologics. The TAHOE Phase 2 trial is evaluating RVT-3101 in Crohn's disease, with two monthly doses being assessed and clinical remission per CDAI at week 14 as the primary endpoint. The safety profile across trials has been well tolerated with no significant increase in serious infections.

Potential to address anti-TNF non-responders

The mechanistic case for TL1A inhibition in anti-TNF non-responders rests on four independent lines of evidence, each pointing to the same conclusion: TL1A-driven inflammation continues operating in patients where TNF blockade is insufficient. This is not a theoretical overlap — it is a parallel inflammatory circuit that anti-TNF therapy is structurally unable to interrupt.

Four mechanistic arguments

First, TL1A drives inflammatory pathways that do not require TNF signalling. Studies show TL1A can directly induce pro-inflammatory cytokines from T cells even when anti-TNF therapy is present. Second, TL1A acts upstream of TNF in some contexts — it can induce TNF production from immune cells, meaning TL1A blockade may provide broader anti-inflammatory effects than blocking TNF alone. Third, anti-TNF therapies do not prevent or reverse intestinal fibrosis, a major cause of treatment failure and surgical intervention in Crohn's disease; TL1A inhibition directly targets fibroblast activation and collagen deposition. Fourth, as a fully human monoclonal antibody targeting a different epitope, anti-TL1A agents would not be affected by anti-drug antibodies developed against anti-TNF therapies — there is no cross-immunogenicity.

Clinical and preclinical support

The ARTEMIS-UC trial enrolled patients with failure of conventional and/or advanced therapies, including prior biologic exposure, and demonstrated efficacy regardless of prior treatment history. Animal models show TL1A blockade effective even in severe, established colitis, and DR3-deficient mice show reduced inflammation in models not responsive to TNF blockade. Critically, TL1A levels remain elevated in IBD patients despite anti-TNF therapy — a direct signal of ongoing TL1A-mediated pathology that persists through TNF blockade.

For historical context, research published by NIH-affiliated investigators and summarised in systematic reviews shows that switching from infliximab to adalimumab after secondary failure achieves remission in 30–45% of patients, and vedolizumab response rates decline compared to bio-naïve patients after anti-TNF failure. Ustekinumab maintained efficacy in Crohn's disease after anti-TNF failure, suggesting that alternative mechanisms do benefit this population — a precedent that supports optimism for TL1A inhibition.

What evidence is still needed

Definitive proof of superiority in anti-TNF failures awaits prespecified subgroup analyses from the ongoing Phase 3 trials (ATLAS-UC and ARES-CD) stratified by prior biologic exposure, head-to-head comparisons with other biologics in bio-experienced populations, and real-world effectiveness data post-approval. The mechanistic rationale is strong; the clinical confirmation is still being generated.

Safety profile and treatment positioning

Phase 2 trial data for both tulisokibart and RVT-3101 show adverse event profiles comparable to placebo, with most events mild-to-moderate in severity and no significant increase in serious infections — a concern that has historically limited the use of systemic immunosuppression in IBD. The favorable tolerability supports chronic administration, which is necessary for a disease requiring long-term management.

Theoretical safety advantages

DR3 expression is particularly high in intestinal lymphocytes, providing some degree of tissue selectivity analogous to — though less pronounced than — vedolizumab's gut-selective mechanism. Preclinical data suggest TL1A blockade reduces pathogenic inflammation while leaving protective immune responses intact. As fully human monoclonal antibodies, anti-TL1A agents are expected to carry low immunogenicity risk compared to chimeric or humanised anti-TNF agents. Longer-term monitoring will be required for infection risk with prolonged immunomodulation, malignancy surveillance, and impact on vaccine responses — standard requirements for all biologics, as outlined by regulatory authorities including the EMA and FDA.

Where TL1A inhibitors may fit in the treatment algorithm

Based on current evidence, TL1A inhibitors have potential utility in at least three clinical scenarios. As a first-line biologic option, given efficacy comparable to or exceeding anti-TNF in early trials, they may be considered particularly in patients at high risk for fibrotic complications. As a second-line therapy after anti-TNF failure, the non-overlapping pathway and absence of cross-immunogenicity make TL1A inhibition an attractive alternative to switching between anti-TNF agents. And as a potential disease-modifying therapy in Crohn's disease with stricturing phenotype, the direct anti-fibrotic mechanism addresses an indication not adequately served by any currently approved biologic. Companion diagnostics may further enable biomarker-guided patient selection, consistent with the precision medicine approaches being developed across the broader IBD field, as tracked by organisations including WIPO in their annual innovation metrics for the pharmaceutical sector.

Limitations and knowledge gaps that remain

The mechanistic and early clinical case for TL1A inhibition is compelling, but the evidence base has material gaps that should temper conclusions about its ultimate place in IBD treatment. Acknowledging these gaps is essential for accurate positioning of the therapeutic class.

• No head-to-head data: No direct comparisons with anti-TNF or anti-integrin therapies in controlled trials have been conducted.
• Subgroup analyses pending: Detailed efficacy data in bio-experienced patients, particularly anti-TNF failures, have not been fully published from completed trials.
• Long-term efficacy unknown: Durability of response beyond one year requires Phase 3 long-term extension data.
• Crohn's disease data immature: CD trials are ongoing; efficacy in stricturing and fistulising phenotypes is not yet established in humans.
• Fibrosis endpoints not yet met: While mechanistically promising, clinical trials have not yet demonstrated prevention or reversal of established intestinal fibrosis in humans.
• Real-world effectiveness: Post-marketing surveillance will be needed to confirm that trial efficacy translates to diverse real-world populations.

Future research priorities include biomarker validation to confirm predictive value for patient selection, evaluation of combination strategies with other IBD therapies, incorporation of cross-sectional imaging and fibrosis biomarkers as trial endpoints, and extended follow-up for rare adverse events and durability of remission.

"TL1A inhibition has strong mechanistic rationale and encouraging early clinical data — but definitive evidence of superiority in anti-TNF failures awaits completion of Phase 3 trials with prespecified subgroup analyses."