Department of Pediatrics, Columbia University, New York, New York, USA.
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First published October 8, 2012 - More info
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The human innate immune response to pathogens is complex, and it has been difficult to establish the contribution of epithelial signaling in the prevention of upper respiratory tract infection. The prevalence of chronic sinusitis in the absence of systemic immune defects indicates that there may be local defects in innate immunity associated with such mucosal infections. In this issue of the JCI, Cohen and colleagues investigate the role of the bitter taste receptors in airway epithelial cells, and find that these are critical to sensing the presence of invading pathogens.
The participation of respiratory mucosal epithelial cells in innate immune defense has been increasingly appreciated. Not only do airway cells express the full complement of pattern recognition receptors and corresponding adaptor proteins to signal the recruitment of professional immune cells in response to perceived infection, they also participate directly in pathogen eradication. Mucociliary clearance is activated in response to bacterial components, and bacterial killing is mediated through epithelial production of NO and antimicrobial peptides. Although major defects in ciliary function (e.g., Kartagener syndrome) are clearly associated with increased respiratory infection rates, more subtle epithelial abnormalities that might be important in susceptibility to common conditions such as chronic sinus infection have not been fully characterized. Mutations in cystic fibrosis transmembrane conductance regulator (CFTR) that do not cause cystic fibrosis have been associated with chronic rhinosinusitis, although the specific pathogenetic mechanisms involved have not been determined (1). Given the complexity of the human innate immune response to pathogens, it has been difficult to establish the contribution of epithelial signaling in the prevention of upper respiratory tract infection. Nonetheless, given the large number of patients with chronic sinusitis, in the absence of any clinically apparent systemic immune defect, it seems likely that there must be local defects in innate immunity associated with such mucosal infections. In this issue of the JCI, Cohen and coworkers explore unexpected players in innate immune defense: the bitter taste receptors (2).
The bitter taste receptor family (T2Rs) comprises over 25 G protein–coupled receptors that activate PLC-β2 and Ca2+ fluxes (3). These proteins recognize a chemically diverse set of bitter-tasting agonists (4). Bitter taste is innate, present in human neonates and in many animals including invertebrates, although it is not universally present in herbivores. The putative function of these receptors is to prevent the consumption of bitter toxins, and there are clear species-specific differences in the relative degrees of bitterness that correlate with various T2R alleles (5). The human genetics of the T2R38 bitter taste receptor has been studied in detail because it confers the ability to perceive the bitter taste of phenylthiocarbamide (PTC) (6). Three nucleotide polymorphisms resulting in 5 haplotypes correlate with the expression of the functional allele of the receptor containing proline, alanine, and valine (PAV/PAV) and a nonfunctional allele containing alanine, valine, and isoleucine (AVI/AVI). The presence of the valine in the third position is suggested to stabilize receptor structure, facilitating activation (7).
The diversity of T2R ligands suggests that functions in addition to bitter taste perception may also be linked to their activation. Among the structures of the agonists for the human bitter taste receptor TAS2R46 are members of the sesquiterpene lactones and similar compounds (4). This group of compounds includes the acylhomoserine lactones (AHLs) produced by P. aeruginosa. These AHLs are secreted by bacteria and function in cell-to-cell signaling or quorum sensing that results in the coordinate activation of bacterial genes, including those involved in biofilm formation (8).
As would be expected, the T2Rs are found on the tongue and in the oral mucosae, but in addition they are prominently expressed in the motile cilia and on the surface of airway epithelial cells. T2R activation on ciliated airway epithelial cells induces Ca2+ fluxes, resulting in the stimulation of ciliary beat frequency (9). The function of these receptors present on the proximal airway cells was unclear but was hypothesized to represent a mechanism for the mechanical clearance of noxious airway particles or volatile molecules. The T2Rs have also been identified on airway smooth muscle cells, where they induce Ca2+ fluxes that induce bronchodilation (10), and the utility of this family of receptors as therapeutic targets for bronchodilation in asthma and chronic obstructive pulmonary disease (COPD) had been suggested (10). In addition, given the ability of these receptors to respond to homoserine lactones, it was also postulated that they could function in innate immunity. However, the biologically relevant agonists for the T2R receptors strategically placed in the respiratory tract were unknown.
In the present work, Cohen and coworkers demonstrate that the expression of T2R38 in human respiratory epithelial cells functions to signal the presence of the P. aeruginosa homoserine lactone (2). Activation of the functional PAV/PAV but not the nonfunctional AVI/AVI receptors results in Ca2+ fluxes, stimulation of ciliary beat frequency, and induction of sufficient epithelial NO production to kill P. aeruginosa. Using material from patients referred for sinus surgery, Cohen and colleagues (2) demonstrate that the TSR38 PAV/PAV genotype, as opposed to nonfunctional genotypes, is highly associated with the absence of either Gram negative or, specifically, P. aeruginosa sinus infection (Figure 1).
Bitter taste receptors in innate immune defense. AHLs produced by P. aeruginosa are sensed by T2R38, activating Ca2+ signaling, leading to increased ciliary beat frequency, NO production, and bacterial killing. This process prevents colonization of the airway. Individuals harboring the inactive AVI allele are at greater risk of infection.
While previous studies hinted at a role for T2Rs in the recognition of bacterial products (9), this study very clearly demonstrates that T2R38 is present not simply to enhance the perception of bitterness or to trigger avoidance of potential toxins, but also to play an important role in host defense. Cohen and coworkers demonstrate the generation of Ca2+ fluxes within 5 seconds of exposure to P. aeruginosa AHLs. The T2R38 receptors thus function in the immediate detection of these compounds, acting well before any consequences of TLR or Nod-like receptor signaling would be detectable. Although AHLs are usually associated with P.aeruginosa within the context of biofilm formation and established infection, the authors demonstrate that AHLs are expressed even in the very early stages of infection, in amounts sufficient to trigger T2R38 signaling. P. aeruginosa mutants lacking AHL production do not trigger T2R38 activation. Thus these receptors function in the prevention of colonization. As colonization of the upper airway generally precedes aspiration of these pathogens and lower respiratory tract infection in susceptible hosts, the T2R receptors are likely be important in the prevention of pneumonia as well as sinusitis.
The protection mediated by T2R signaling is likely to function against a large group of potential Gram-negative pathogens. AHLs are highly conserved molecules used by many Gram-negative bacteria to coordinate gene expression (8). Although P. aeruginosa is the best-studied AHL system and arguably the most relevant human pathogen in this group of opportunists, similar signaling systems are widespread, particularly in the marine Vibrios and Aeromonas species, as well as in common opportunists such as Acinetobacter and Burkholderia species (11, 12). Thus, T2R signaling would be expected to contribute to innate immune defenses against a number of potential pathogens that are associated with respiratory infection.
Given the interest in identifying genes that confer increased risk or resistance to specific diseases, would the identification of a patient’s T2R38 genotype be clinically useful? In those with known increased susceptibility to respiratory infection, such as those with cystic fibrosis or COPD, or even patients in intensive care units expected to require assisted ventilation, Gram-positive as well as Gram-negative infections are frequent, and it would be difficult to imagine a scenario in which the T2R genotype would affect therapy that is usually based on bacterial culture results. However, in selected patient groups, such as those with chronic sinusitis, it should be possible to design a prospective study to determine whether the T2R38 genotype can be used to direct preventative antibiotic therapy.
The author’s laboratory is supported by NIH grants RO1HL079395 and HL73989.
Conflict of interest: The author has declared that no conflict of interest exists.
Citation for this article:J Clin Invest. 122(11):3847–3849. doi:10.1172/JCI66182.
See the related article beginning on page 4145.