Uptake of inhalable microparticles affects defence responses of macrophages infected with Mycobacterium tuberculosis H37Ra

Objectives: To investigate whether inhalable microparticles containing two anti-tuberculosis agents, isoniazid and rifampicin, evoke host-defence strategies in macrophages in addition to targeting the incorporated drugs. Methods: Microparticles were prepared by spray-drying a homogeneous solution of drugs and poly (lactic acid) (PLA; apparent viscosity 1.1 cP). Four parts PLA and three parts rifampicin were dissolved in dichloromethane. One part isoniazid was dissolved in methanol. The two solutions were mixed in the ratio 22:3 at which none of the solutes precipitated. These were administered as 'nose-only' inhalations to mice or exposed to cultured J774 mouse macrophages. Targeting to lung macrophages was investigated by transmission electron microscopy. Reactive oxygen species (ROS) were estimated by a cytochrome c assay and flow cytometry. Reactive nitrogen intermediates (RNI) were assayed using Griess reagent. Cytokines in culture supernatants were estimated by ELISA. Results: Treatment with inhalable microparticles targeted lung macrophages in vivo and induced intense Golgi activity in the vicinity of microparticle-containing phagosomes. Microparticles induced a respiratory burst involving NADPH oxidase and enhanced NO production by infected macrophages. Microparticle-induced NADPH oxidase activation required optimal calcium ions. Microparticles efficiently induced tumour necrosis factor-alpha (TNF-alpha) secretion by macrophages recovered from infected mice. Conclusions: Microparticle phagocytosis induces responses in infected murine macrophages that are indicative of activation of innate bactericidal mechanisms, and are inimical to bacterial survival. It is likely that such responses augment straightforward drug action on the bacterium and contribute to the unexpectedly high efficacy of microparticles in experimental tuberculosis.