Modulation of immunity to tuberculosis through vitamin A-dependent nuclear receptors

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Project Summary Tuberculosis (TB), the leading cause of death due to an infectious disease, is faced by a number of challenges, one of which is our inability to predict disease outcome after exposure. The limited knowledge on risk factors for TB progression and the mechanisms by which these promote susceptibility limits the ability to develop new prevention and treatment approaches. Recently, our collaborators have identified moderate, subclinical vitamin A deficiency as a substantial risk factor for developing active TB, carrying a 10-fold higher risk. The substantial TB risk carried by deficiency in vitamin A highlights the need to understand the mechanisms by which this molecule contributes to protection, particularly in the context of malnutrition among TB-affected communities. Vitamin A functions through nuclear receptors, which are ligand-activated transcription factors. Other nuclear receptors beyond vitamin A receptors have been shown to regulate immune function and contribute to response to M. tuberculosis infection. In particular, PPARγ, a nuclear receptor with obligatory association with vitamin A receptors is enriched in macrophages and influences the immune response during TB. Because we have shown that rosiglitazone, an activator of PPARγ, improves outcome of tuberculosis in guinea pigs, we hypothesize that that availability of vitamin A and PPAR ligands together influence the transcriptional outcome during TB and consequently, the ability to control infection. The goal of this research is to better understand the contribution of nuclear receptor-ligand interaction, and the impact of impaired vitamin A availability on immune response, control of bacterial growth, and TB disease outcome. These will be investigated through Aims using bone marrow- derived macrophage cell models and a guinea pig model of vitamin A deficiency, both developed in our laboratory. We will first determine the contribution of ligand activation of nuclear receptor heterodimers to transcriptional and microbicidal response to infection with M. tuberculosis in bone marrow macrophages. This approach uses a combination of validated knock-down techniques targeting vitamin A (RAR, RXR) and PPARγ receptors, and the vitamin A converting enzyme RALDH, alongside ligand agonists, all-trans retinoic acid (RAR), 9-cis retinoic acid (RXR), and thiazolidenones (PPARγ). This will be followed by the use of our recently developed guinea pig model of dietary vitamin A deficiency to determine the impact of vitamin A and PPARγ signaling on TB immunity. Guinea pigs with sufficient or severely deficient vitamin A status will be treated with or without rosiglitazone, a specific PPARγ agonist, based on our previous assessment of this drug in guinea pigs and immune response and disease outcome measured. Upon completing these experiments, we will have determined the impact of vitamin A and nuclear receptor control over response to infection and consequently, TB disease outcome. These results will guide development of new host-directed therapies to minimize risk and improve outcome of exposure in TB-endemic regions.