The fight for iron at mitochondria: implications for respiratory infection and disease

Abstract

Biological systems have co-opted iron to exploit its electron donor capabilities, fuelling essential enzymatic reactions. Mitochondria are the primary sites of iron storage and utilisation in mammalian cells, generating heme and iron-sulfur clusters which fuel macrophage immunometabolism. As such, mitochondrial iron must be exquisitely regulated to supply enough iron for anabolic processes whilst curtailing oxidative damage driven by excessive overload. Numerous pathogens hijack host mitochondria for essential nutrients, but whether changes in mitochondrial iron levels dictate macrophage function in response to infection is unknown. Mitochondrial iron overload has been implicated in the lung disease chronic obstructive pulmonary disease (COPD). Acute respiratory infections, like with Streptococcus pneumoniae, are a significant cause of mortality in COPD, yet whether mitochondrial iron is increased in lung alveolar macrophages (AMs), and how in turn this would alter outcomes of the host-pathogen fight for iron, is unclear. We hypothesised that excess AM mitochondrial iron is a bacterial target for iron piracy which in turn renders individuals with COPD more susceptible to infection.

In this thesis, this hypothesis was tested through in silico data mining approaches and experimentally through in vitro models of live infection (S. pneumoniae), genetic and experimental murine models, and human AMs isolated via bronchoscopy and pharmacologically manipulated ex vivo. Together, these approaches revealed alterations in AM mitochondrial iron in human COPD and experimental murine models, and in S. pneumoniae infection. Murine AMs subjected to experimental COPD may have higher total, mitochondrial and heme iron, with elevated expression of ferritin and the transferrin receptor. These �COPD� macrophages show an increased bacterial burden and blunted pro-inflammatory cytokine secretion following S. pneumoniae infection, confirmed in human AMs isolated from smokers. Subcellular compartment-specific iron loading and depletion suggested this increased bacterial burden may associate with mitochondrial iron loading in murine and human AMs. A novel murine model of genetic mitoferrin 2 deficiency specific to AMs exhibited decreased inflammatory cell infiltration following S. pneumoniae infection, associated with decreased weight loss and bacterial dissemination.

This work represents the most in-depth characterisation of mitochondrial iron in the macrophage response to infection to date, including the generation of the first transgenic strain of mice with targeted loss of a mitochondrial iron importer in an immune cell population. Our results suggest mitochondrial iron as a candidate mediator of nutritional immunity in the lung and present mitochondrial iron chelation as a potentially druggable target in both COPD and other diseases characterised by recurrent acute infections.