Abstract
INTRODUCTION: Isoniazid (INH), a first-line drug for tuberculosis, exerts bactericidal effects through inhibition of mycolic acid synthesis. However, its potential to modulate host immunity remains unclear. Reactive oxygen species (ROS) are critical antimicrobial effectors produced by innate immune cells, and their regulation is essential for effective immune signalling and pathogen clearance, whereas excessive ROS can contribute to inflammation and tissue damage. This duality makes it important to determine whether INH modifies ROS production in innate immune cells, particularly in individuals with latent infection. METHODS: We investigated whether INH affects ROS production in circulating immune cells and plasma cytokines in healthy controls and LTBI individuals before treatment initiation (n = 9 per group). Whole blood was incubated with INH at plasma concentrations observed in humans (2, 4.5, and 10.5 μg/mL). Intracellular ROS production in neutrophils, eosinophils, and monocytes was quantified using flow cytometry with the oxidation-sensitive probe DHR-123, following stimulation with fMLP, Escherichia coli, or PMA. IL-1β, IL-6, IL-8, TNF, IFN-γ, IL-10, and TGF-β1 were measured in INH-exposed unstimulated blood by cytometric bead array. RESULTS: In healthy controls, INH induced a significant reduction in ROS production in monocytes (median values of DHR+ classical monocytes after E. coli stimulation with 0 μg/mL INH were 26.2%, 2 μg/mL INH 19.9% (p < 0.05), 4.5 μg/mL INH 16.2% (p < 0.01), and 10.5 μg/mL INH 16.3% (p < 0.01)). In contrast, INH had no effect on ROS production in LTBI individuals, who overall displayed significantly lower ROS responses to stimulation compared with healthy controls, particularly in E. coli-stimulated eosinophils (p < 0.0001 all data ± INH aggregated) and PMA-stimulated classical monocytes (p < 0.0001 all data ± INH aggregated). INH did not alter cytokine levels in unstimulated blood after 24 h. DISCUSSION: These findings suggest that INH dose-dependently downregulates monocyte ROS production in healthy individuals, whereas LTBI individuals exhibit an diminished ROS response compared to healthy controls that is not further affected by INH. This work provides new insight into the immunomodulatory properties of INH and highlights the need to consider host responses, such as ROS production, in optimizing TB treatment and adjunctive therapy development.