Abstract
Pattern recognition receptors (PRRs) are essential for recognition of conserved molecular constituents found on infectious microbes, and Toll-like receptors (TLRs) are a critical component of the PRR repertoire. mRNA expression of the avian-specific TLR 15 has recently been shown to be increased in response to multiple Salmonella serovars, suggesting that the ligand for TLR 15 is some component of Salmonella. The objective of this study was to determine whether mRNA expression of TLR 15 was Salmonella-specific or could be induced by exposure to other species of bacteria commonly found in the chicken. We also wanted to determine whether TLR 15 mRNA expression could be induced by exposure to known TLR ligands that are components of Salmonella. To test this hypothesis, we stimulated chicken heterophils with live Salmonella enterica serovar Enteritidis (SE), heat-killed or formalin-killed SE, an avian isolate of Escherichia coli, and an avian isolate of Enterococcus gallinarum. TLR ligands used as stimuli included LPS, flagellin, and CpG. Gene expression was measured using real-time quantitative RT-PCR. We found that mRNA expression of TLR 15 was significantly increased (P < 0.05) in response to stimulation with all of the bacteria tested. There were no significant differences in stimulation between live and dead bacteria nor between species of bacteria. Across bacteria tested, the average increase in TLR 15 expression was 8-fold greater (range, 6.5 to 9.1) than in unstimulated heterophils. In contrast, stimulation of heterophils with the TLR ligands LPS, flagellin, and CpG induced no significant increase in expression of TLR 15 mRNA. These data demonstrate that TLR 15 recognizes some component of both Gram-positive and Gram-negative bacteria, but that LPS, flagellin, and CpG are likely not the components of these bacteria that are recognized.
