A TL1A signature in perianal fistulising Crohn's that anti-TNF does not touch

Design

single-cell RNA sequencing of rectal biopsies with functional validation in explants and cell culture

Population

31 patients with Crohn's disease, with and without perianal fistulising disease

Primary outcome

cellular and transcriptomic signature specific to perianal fistulising disease

Effect

TL1A-driven signature via LTa1b2 and interleukin 22, independent of luminal inflammation and of TNF signalling

Perianal fistulising disease affects about a fifth of patients with Crohn's disease and its aetiology has never been explained, which is part of why it responds so poorly to therapy aimed at luminal inflammation. This study took rectal biopsies from 31 patients with Crohn's disease, with and without perianal fistulising disease, and applied single-cell RNA sequencing, then tested the findings functionally in peripheral T cells, tissue explants, primary fibroblasts and epithelial monolayers.

The rectal mucosa of patients with fistulising disease carried cellular and transcriptomic alterations specific to that phenotype and independent of luminal inflammation — that is, present whether or not the rectum looked inflamed. The pattern appeared to be driven by TL1A activation in CD4-positive T cells. Downstream, the authors identified lymphotoxin beta, as the heterotrimer LTa1b2, acting alongside interleukin 22 to induce the signature in fibroblasts and epithelium respectively: more fibroblasts, induction of matrix-degrading enzymes, transcriptomic rewiring of lamina propria fibroblasts.

The finding with therapeutic teeth is that this induction occurs independently of TNF signalling and remains active under anti-TNF therapy. That is a mechanistic account of why perianal disease persists in patients whose luminal disease is controlled, and it is arriving as anti-TL1A antibodies move through late-phase trials in inflammatory bowel disease. This is laboratory work in 31 patients, not a trial — but it is the kind of result that determines which trial gets designed next.

• Record perianal disease as a distinct phenotype in the notes, not as a severity marker of luminal disease
• Do not assume luminal remission predicts perianal response
• Watch the anti-TL1A programmes in inflammatory bowel disease for perianal-specific endpoints
• Continue current management — nothing here changes treatment today
• Note the signature was present independently of visible rectal inflammation

Why it matters

It gives a molecular reason why perianal disease persists when luminal Crohn's disease is controlled.

Don't overread it

Single-cell sequencing and cell culture in 31 patients — no patient was treated with anything on the basis of this.

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