Immune function of bitter taste receptors in human macrophages

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PROJECT SUMMARY T2R bitter taste receptors are G-protein coupled receptors (GPCRs) originally identified on the tongue, but which also serve diverse roles in other tissues. “Extraoral” (outside the tongue) taste receptors are likely important in airway infection, asthma, pre-term labor, and even cancer. In airway epithelial cells, T2Rs 4, 14, 16, and 38 recognize quorum-sensing molecules produced by gram-negative bacteria, including the opportunistic pathogen Pseudomonas aeruginosa. Stimulation of these receptors modifies production of antibacterial molecules, including nitric oxide and defensins. Some polymorphisms reducing the functionality of T2Rs increase patient susceptibility to upper respiratory infection and chronic rhinosinusitis. Targeting T2Rs as therapies requires better elucidation of their expression and downstream effects. This must be studied in primary human cells, as mouse T2Rs differ in number and agonists that activate them. We identified several T2R receptors in primary human monocytes and macrophages. Macrophages are important innate immune cells that phagocytose and kill bacteria as well as secrete pro- or anti- inflammatory cytokines to modify immune responses. Stimulation of these receptors activates calcium release and nitric oxide production that acutely (within 5-15 min) enhances phagocytosis of bacteria. Preliminary data suggest that T2Rs may be targets for stimulation of innate immune responses to kill bacteria without the use of conventional antibiotics, reducing pressures for antibiotic resistance. This may be especially useful in upper respiratory diseases. Acute and chronic rhinosinusitis account for >20% of all antibiotic prescriptions in the US, and thus are important drives of antibiotic resistant micro-organisms. Chronic rhinosinusitis and associated nasal polyps also cause dysosmia, hyposmia, and/or anosmia in many patients. We will further define the signaling pathway that increases macrophage phagocytosis and test how it affects bacterial killing (Aim 1). As bacteria such as P. aeruginosa can invade cells and live as intracellular pathogens to evade further immune detection, we will test how T2Rs affect macrophage bactericidal activity. This work will also be among the first to use live cell imaging and fluorescent biosensors to study macrophage GPCR signaling in real time. Next (Aim 2), we will examine how T2R activation reduces inflammatory responses to toll-like receptors (TLRs). Our preliminary data suggest novel pathways of T2R-TLR crosstalk possibly involving Akt. Finally (Aim 3) we will compare phenotype, T2R expression, and T2R function in macrophages from inflamed chronic rhinosinusitis nasal polyps with naïve serum-derived macrophages from the same patients. We will examine if macrophage infiltration of polyps is altered with T2R polymorphisms or bitter taste perception. We also examine if increased macrophage numbers in nasal polyps affects chronic rhinosinusitis disease phenotypes.