Intestinal flagellin drives multisystem inflammation through TLR5-IL-15-ARA axis.

Two current US guidelines govern Crohn's disease and should be read together. ACG 2025 (Lichtenstein et al.) remains the reference for diagnosis, mild-to-moderate disease, perianal, postoperative and abscess management. DIAGNOSIS: fecal calprotectin (>50-100 μg/g) separates inflammatory from noninflammatory disease of the colon. Workup is ileocolonoscopy with biopsies, including uninvolved mucosa, plus small-bowel imaging. CTE and MRE are comparable; prefer MRE in patients <35 y and when serial exams are likely. Intestinal ultrasound is an ungraded adjunct. Do routine CRC surveillance in Crohn's colitis. MILD-TO-MODERATE: recommend against oral mesalamine; sulfasalazine only for symptomatic mild colonic CD; ileal-release budesonide 9 mg for induction in ileocecal disease but not for maintenance. MODERATE-TO-SEVERE (AGA Living Guideline, Scott et al., Gastroenterology 2025;169(7):1397-1448, 16 recommendations: 1 strong, 9 conditional, 6 knowledge gaps; updated quarterly, adult outpatients only): RECOMMEND infliximab, adalimumab, ustekinumab, risankizumab, mirikizumab, guselkumab or upadacitinib over no treatment; SUGGEST certolizumab pegol or vedolizumab over no treatment. Advanced-therapy-naive: suggest higher-efficacy agents (infliximab, adalimumab, vedolizumab, ustekinumab, risankizumab, mirikizumab, guselkumab) over lower-efficacy ones (certolizumab pegol, upadacitinib). Prior advanced-therapy (especially anti-TNF) exposure: suggest higher- or intermediate-efficacy agents (adalimumab, risankizumab, guselkumab, upadacitinib; ustekinumab, mirikizumab) over lower-efficacy ones (vedolizumab, certolizumab pegol). Suggest against thiopurine monotherapy for induction; suggest it for maintenance of remission. Suggest SC/IM methotrexate monotherapy; suggest against oral methotrexate. In thiopurine-naive patients starting infliximab, suggest infliximab + thiopurine over infliximab alone. No recommendation on IFX + MTX, adalimumab + immunomodulator, or non-TNF biologic + immunomodulator. After ≥6 months of steroid-free remission on anti-TNF + immunomodulator, suggest withdrawing the immunomodulator and suggest against withdrawing the anti-TNF. SUGGEST upfront advanced therapy over step-up therapy (corticosteroids and/or immunomodulator first). No recommendation on treating to endoscopic vs symptomatic remission. The ACG 2025 moderate-to-severe recommendations (anti-TNF, IFX + thiopurine combination and IL-12/23/IL-23 agents, each strong/moderate; upadacitinib after anti-TNF exposure; steroids for short-term induction only; TPMT/NUDT15 testing before thiopurines) still stand alongside AGA. ACG perianal (infliximab first; drain abscesses and place setons before escalating), postoperative (endoscopy at 6-12 months; anti-TNF strong, vedolizumab conditional in high-risk patients) and intra-abdominal abscess (>2 cm: antibiotics plus drainage, hold immunosuppression) positions are unchanged. THERAPEUTIC ENDOSCOPY (AGA Clinical Practice Update, CGH 2026; details below come from AGA news coverage, not the full text, and are ungraded advice): get cross-sectional imaging before the procedure. Endoscopic balloon dilation is first-line for short strictures (about ≤4-5 cm) with few risk factors (fistula, prestenotic dilation >5 cm, high CRP, prior anti-TNF). Longer or multiple strictures, or ≥3 risk factors, favor surgery.