Abstract
Using quantitative PCR (QPCR), the relative transcriptional levels of five toll‐like receptors (TLR2, TLR3, TLR5, TLR20a and TLR21) were studied in the channel catfish, Ictalurus punctatus (Rafinesque), under uninfected and acutely infected conditions [1‐, 2‐, 4‐, 6‐, 12‐, 24‐, 36‐ and 48‐h post‐injection (hpi)]. Under uninfected conditions, the transcriptional levels of the five TLRs were significantly lower than that of 18S rRNA ( P 0.001). QPCR results also revealed that the transcriptional levels of TLR20a and TLR5 were higher than those of TLR2, TLR3 or TLR21. The transcriptional level of TLR3 was significantly lower than that of the other four TLRs ( P 0.001). However, when channel catfish were acutely infected by Edwardsiella ictaluri through intraperitoneal injection, the transcriptional levels of TLRs increased significantly ( P 0.005) at 6 hpi. Among the five TLRs studied, the transcriptional levels of TLR3, TLR5 and TLR21 were never significantly lower than under uninfected conditions ( P = 0.16, 0.27 and 0.19, respectively), suggesting these three TLRs might play important roles in host defence against infection by E. ictaluri . The amount of E. ictaluri in the anterior kidney increased at 12 and 24 hpi but decreased at 36 and 48 hpi. Our results suggest that TLRs are important components in the immune system in the channel catfish, and their rapid transcriptional upregulation (within 6 hpi) in response to acute E. ictaluri infection might be important for survival from enteric septicaemia of catfish.