Tuberculosis is a major cause of mortality throughout the world with an estimated 8-9 million new cases of tuberculosis in 2000 and close to 2 million deaths in the same year. Although one third of the world's population is believed to be infected with the Mycobacterium tuberculosis bacillus, only about 10% of those infected will go on to develop disease. Risk factors for disease progression include immune suppression, chronic disease, nutritional status and genetics; despite the identification of these factors, the cellular mechanisms which lead to this risk are not well understood. M. tuberculosis is an intracellular pathogen which resides in macrophages. Multiple lines of evidence suggest that access to host iron is essential for mycobacterial growth. These include clinical studies of the effect of iron supplementation on outcomes of tuberculosis infection, autopsy studies and studies in animal and cell models. These findings suggest that syndromes that result in high levels of iron in macrophages may lead to increased growth of M. tuberculosis and clinical progression of disease in those infected. Recently, the metabolic pathways leading to macrophage iron overload have been elucidated; an iron transport protein, ferroportin 1 (Fpn1), is believed to export iron from macrophages and a recently described hepatic peptide, hepcidin, has been shown to bind to Fpn1, leading to its internalization and degradation. We hypothesize that genetic variants of Fpn1 which lead to iron overloading of macrophages may increase mycobacterial growth in these cells. Similarly, conditions that lead to high levels of hepcidin expression are also hypothesized to result in increased cellular susceptibility to M. tuberculosis infection. To investigate this, we propose to measure mycobacterial growth in J774 cells that overexpress Fpn1 and in J774 cells that have been stimulated by hepcidin. We also propose to establish clinically relevant Fpn1 mutants generated by site directed mutagenesis. We will then characterize the expression, membrane trafficking and function of these mutants and use retroviral transduction to establish stable macrophage cell lines that express Fpn1 mutants and measure mycobacterial growth in these cells. The elucidation of the role of host iron status on the growth of M. tuberculosis may help identify those at high risk for disease progression and lead to therapeutic interventions aimed at reducing macrophage iron stores. ? ?
|Johnson, Erin E; Sandgren, Andreas; Cherayil, Bobby J et al. (2010) Role of ferroportin in macrophage-mediated immunity. Infect Immun 78:5099-106|