Curcumin Retains Anti-Inflammatory Effects Despite IDO Inhibition-Associated Neutrophilic Shift in OVA-Induced Rat Model

Allergic asthma is characterized by Th2-driven airway inflammation. Indoleamine 2,3-dioxygenase (IDO) maintains immune tolerance, but its role in modulating the anti-inflammatory actions of Curcumin (CN) is unclear. This study investigated whether CN retains efficacy in a rat model of Ovalbumin (OVA)-induced airway inflammation under pharmacological IDO inhibition. Rats were sensitized and challenged with OVA and treated with methylprednisolone (MP, 15 mg/kg), CN (200 mg/kg), the IDO inhibitor 1-MT (70 mg/kg), or CN+1-MT. Assessments included systemic and pulmonary leukocyte profile, delayed-type hypersensitivity (DTH), OVA-specific IgE, bronchoalveolar lavage fluid (BALF) inflammatory cells and nitric oxide (NO) levels, lung wet/dry weight ratio, gene expression of tumor necrosis factor alpha (TNF-α), interleukin (IL)-4, IL-6, Cxcr2 and transcription factor Nfkb1 and lung histopathology. CN alone or combined with 1-MT normalized leukocyte counts, suppressed eosinophilic and neutrophilic infiltration, restored BALF NO levels, reduced DTH response and OVA-specific IgE, decreased pulmonary edema, and preserved pulmonary vascular integrity. CN significantly attenuated TNF-α, IL-4, IL-6, and Cxcr2 gene expression despite pharmacological IDO inhibition. Histopathology revealed reduced inflammatory infiltration and maintained alveolar structure. CN maintains anti-inflammatory, immunomodulatory and tissue-protective efficacy in allergic airway inflammation despite pharmacological IDO inhibition. These findings call for future research on CN as a potential adjunctive therapy method for inflammatory airway diseases. However, to elucidate the underlying processes, more research on IDO activity and downstream immunometabolic pathways is needed.