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Title: Expression of the avian-specific toll-like receptor 15 in chicken heterophils is mediated by Gram-negative and Gram-postive bacteria, but not TLR agonists

item Nerren, Jessica
item He, Louis
item Genovese, Kenneth - Ken
item Kogut, Michael - Mike

Submitted to: Veterinary Immunology and Immunopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/22/2010
Publication Date: 7/1/2010
Citation: Nerren, J.R., He, H., Genovese, K.J., Kogut, M.H. 2010. Expression of the avian-specific toll-like receptor 15 in chicken heterophils is mediated by Gram-negative and Gram-postive bacteria, but not TLR agonists. Veterinary Immunology and Immunopathology. 136:151-156.

Interpretive Summary: Host immune cells recognize unique components of bacteria using structures located on their surface called pattern recognition receptors (PRRs). These PRRs then send signals to the brain of the cell, the nucleus, which causes the cell to respond in a defensive manner. Toll-like receptors (TLRs) are a family of PRR and have been shown to play an important role in the immune system's defense against bacteria, fungi, and viruses. A new TLR that is specific to the chicken has been identified and appears to recognize Salmonella. However, it is unknown what component of Salmonella it recognizes, whether or not it recognizes any other bacteria, and whether or not it recognizes only bacteria found in chickens. In the present study, immune cells from chickens were stimulated with living and dead Salmonella enteritidis (SE), Escherichia coli, Enterococcus gallinarum, and Rhodococcus equi, a bacteria that causes disease in horses but has never been found in chickens. Also included as stimuli were important purified components of bacteria that are known to be recognized by other TLRs. After stimulation, levels of mRNA (a molecule that is translated into protein) for TLR15 were measured. Following stimulation with living and dead SE, E. coli, and E. gallinarum, the immune cells responded by increasing their production of mRNA for TLR15. In contrast, the immune cells did not increase their production of mRNA for TLR15 following stimulation with R. equi or the purified components of bacteria. Results from this study suggest that the chicken-specific TLR15 recognizes an unknown component of numerous bacteria that are found in chickens, and that it may recognize only bacteria found in chickens.

Technical Abstract: Pattern recognition receptors (PRRs) are essential for recognition of conserved molecular constituents found on infectious microbes. Toll-like receptors (TLRs) are a critical component of the PRR repertoire in both mammalian and avian species. While most mammalian TLRs have been well characterized, less is known about their avian counterparts. In particular, very little is known about the chicken-specific TLR15, for which there is no known mammalian orthologue. We recently demonstrated that this receptor is differentially expressed between Salmonella-susceptible and resistant chickens, indicating a potential role in the innate immune response of chickens to infection with Salmonella. The aim of the present study was to gain better insight into the nature of the ligand for TLR15 by characterizing gene expression patterns of TLR15 in response to various stimuli. We measured changes in mRNA expression by heterophils, the primary avian granulocyte, in response to the bacterial-derived TLR agonists LPS, flagellin, CpG oligodeoxynucleotides, lipotechoic acid (LTA), peptidoglycan (PGN), Pam3CSK4 (PAM), stimulation with live, heat-killed, and formalin-killed Salmonella enterica serovar Enteritidis (SE), chicken isolates of Escherichia coli and Enterococcus gallinarum, and the equine-specific pathogen Rhodococcus equi. Expression of TLR15 increased significantly in response to stimulation with heat-killed and formalin-killed SE, as well as to chicken isolates of E. coli and E. gallinarum, but was unaffected by stimulation with known TLR agonists and R. equi. Overall, these observations demonstrate that the individual TLR agonists are not the ligand for TLR15, and that it recognizes a unique, non-secreted, heat-stabile component of both Gram-negative and Gram-positive bacteria commonly found in and/or capable of causing disease in chickens.