Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), is one of the world's most prevalent bacterial pathogen, infecting roughly one third of the human population and causing over 1 million deaths a year. More than 90% of healthy individuals infected by Mtb develop asymptomatic latent TB infection (LTBI) wherein the immune system drives the bacteria into a state of dormancy, from which they may emerge when immunity is compromised, establishing active disease. The ability of the immune system to control infection is greatly affected by nutrition, in particular by iron (Fe) intake and host Fe homeostasis. However, while adequate Fe intake is important for normal immune function, the relationship between host Fe status and Mtb infection is complicated by the fact that Fe is also necessary for Mtb growth and virulence. Indeed, several studies have shown that in anemic patients with LTBI Fe supplementation can trigger Mtb reactivation. Additionally, studies in the lab of our collaborator, Dr. Marcela Rodriguez, have shown that in vitro Fe deprivation drives Mtb cells into a state of dormancy, from which they emerge upon reintroduction of Fe to the medium. Together, these observations suggest that Fe supplementation to LTBI individuals may trigger reactivation of disease with renewed Mtb growth from a state of dormancy. This is a matter of concern because the World Health Organization has implemented a strategy of Fe supplementation to improve general health in developing countries with endemic TB. Therefore, it is essential to gain a comprehensive understanding of the effects of Fe status on host immunity during Mtb infection, with a particular emphasis on LTBI establishment and reactivation, ideally by using an animal model of Fe-supplementation during Mtb infection. We have developed a rabbit model that recapitulates key features of human Mtb infection, including maturation/evolution of lung granulomas, spontaneous establishment of LTBI and reactivation of LTBI upon immune suppression. In this application, we propose to use this rabbit model system to study the effects of Fe supplementation during LTBI. We propose two Specific Aims:
in Aim 1 we will study the impact of host Fe supplementation on the early protective immunity against Mtb infection, while in Aim 2 we will investigate the effect of Fe supplementation on the establishment/reactivation of LTBI. In both Aims, the effects of a Fe supplementation regimen on Mtb infection will be assayed by a combination of bacteriological and immunological assays that are well established in our lab. Results obtained in these studies will reveal critical insights into the role of Fe homeostasis during Mtb infection, as well as guide future studies of the effects of micronutrients on the host immunity to infection and help design a better public health policy addressing Fe supplementation therapy to anemic patients in TB-endemic countries.
Iron supplementation has been implemented by the World Health Organization in developing/TB-endemic countries as a part of public health measure to treat anemia. This proposal explores the impact of this iron-supplementation therapy on the establishment and reactivation of latent tuberculosis.
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