TITLE : "Tissue Eosinophil Counts as Predictors of Infliximab Response in Pediatric Inflammatory Bowel Disease: A Retrospective Nested Case Control Study"

Background: Infliximab is an effective therapy for pediatric inflammatory bowel disease (IBD), but a substantial proportion of patients experience primary non-response or relapse. Tissue eosinophilia has been implicated in treatment resistance, yet its predictive value in pediatric populations remains unclear. Methods: We conducted a retrospective nested case-control study including 80 pediatric IBD patients treated with infliximab at CHU Sainte-Justine between 2014 and 2023 Relapse cases (n = 42) were defined by an increase in the short Pediatric Crohn's Disease Activity Index (sPCDAI) over 10, and compared with non-relapse controls (n = 38). Tissue eosinophil counts were quantified from diagnostic biopsies by blinded reviewers. Primary non-response was defined as a lack of clinical improvement before treatment change. Logistic regression and receiver operating characteristic (ROC) analyses were performed to assess associations between eosinophil counts and treatment outcomes. Results: 80 patients were included in the study. Median age at diagnosis was 14.4 years [Q1-Q3 (first quartile - third quartile) 12.35 - 16.00]. 40 patients had a diagnosis of ulcerative colitis (UC, 50%) and the other half had a diagnosis of Crohn's disease (CD). Baseline eosinophil counts were not associated with subsequent relapse (odds ratio [OR] 1.02, 95% CI 0.98-1.06, p = 0.31), and ROC analysis demonstrated poor discrimination (AUC 0.57). In contrast, higher eosinophil counts were associated with primary non-response in ulcerative colitis patients (OR 1.08, 95% CI 1.01-1.16, p = 0.03). A cutoff of a median of equal or over 36 eosinophils per high-power field among all segments yielded 85% sensitivity and 80% specificity for predicting primary non-response (AUC 0.82). Conclusion: Tissue eosinophil counts do not predict relapse in pediatric IBD patients treated with infliximab. However, elevated counts may identify ulcerative colitis patients at risk of primary non-response, suggesting a role for histological stratification in guiding the selection of biologics. Prospective validation in larger cohorts is warranted.