Indoximod Attenuates Inflammatory Responses in Acetic Acid-Induced Acute Colitis by Modulating Toll-like Receptor 4 (TLR4) Signaling and Proinflammatory Cytokines in Rats

Abstract

Background and Objectives: Acute ulcerative colitis is characterized by excessive mucosal inflammation and epithelial disruption, often driven by dysregulated cytokine and immune signaling. Indoximod (1-methyl-DL-tryptophan), although not a direct enzymatic inhibitor, modulates the indoleamine 2,3-dioxygenase (IDO) pathway and has been reported to exert immunoregulatory effects in various models of inflammation. This study aimed to evaluate the protective effects of Indoximod in an acetic acid-induced colitis model in rats, focusing on histopathological changes and inflammatory mediators. Materials and Methods: Thirty male Wistar albino rats were randomly assigned to three groups (n = 10 per group): Group 1 (Control) received 0.9% saline oral gavage; Group 2(Colitis) received intrarectal 4% acetic acid to induce colitis and were then treated with saline; Group 3 (Colitis + Indoximod) received 4% acetic acid followed by oral gavage administration of Indoximod (30 mg/kg) for 15 consecutive days. Histopathological evaluation of colonic tissues was performed using hematoxylin and eosin (H&E) staining. Colonic expression of Toll-like receptor 4 (TLR4) and plasma levels of tumor necrosis factor-alpha (TNF-α), pentraxin-3 (PTX-3), and platelet-activating factor (PAF) were quantified using enzyme-linked immunosorbent assay (ELISA). Results: Acetic acid-induced colitis significantly increased mucosal damage, TLR4 expression, and circulating levels of TNF-α, PTX-3, and PAF compared with controls (p < 0.001). Indoximod treatment markedly reduced histological injury and significantly suppressed TLR4 and TNF-α levels (p < 0.01), along with partial reductions in PTX-3 (p < 0.05). However, PAF levels remained elevated despite treatment, indicating limited efficacy in PAF-associated pathways. Conclusions: Indoximod exhibited anti-inflammatory effects in this acute colitis model, likely by downregulating key proinflammatory mediators.

Keywords: IDO inhibitor; Indoximod; PAF; TLR4; TNF-α; acetic acid-induced colitis; pentraxin-3.

Figure 1

Schematic illustration of the experimental design. A total of 30 Wistar albino rats were assigned to three groups (n = 10 per group), with colitis induced in 20 rats via intrarectal administration of 4% acetic acid. Rats were then treated with either saline or Indoximod (30 mg/kg/day) by oral gavage for 15 days.

Figure 2

Effects of Indoximod on histopathological damage and inflammatory biomarkers in acetic acid-induced colitis. Box plots represent (A) histopathological score, (B) colonic TLR-4, (C) plasma TNF-α, (D) plasma PTX-3, and (E) plasma PAF levels. Data are expressed as mean ± SEM. Statistical analysis: one-way ANOVA with Tukey’s post hoc test. Significance indicators: *** p < 0.001, ** p < 0.01, * p < 0.05 different from control groups; ### p < 0.001, ## p < 0.01, # p < 0.05 different from colitis group.

Figure 3

Representative H&E-stained colon sections at ×20 and ×40 magnification. (A,B) Control group: normal epithelial structure (arrow) and normal glands (G). (C,D) Colitis group: epithelial disruption (arrow), glandular degeneration (DG), and hemorrhage (h). (E,F) Colon sections from the colitis + Indoximod group display improved mucosal architecture with preserved epithelial lining and regenerated glands (G), indicating partial histological recovery. Scale bar = 50 µm.

Figure 4

Graphical abstract image. Indoximod reduces TLR4-mediated inflammation in acetic acid-induced acute colitis.