Response of toll-like receptors, lysozyme, and IGF-I in back-cross hybrid (F1 male (blue × channel) × female channel) catfish challenged with virulent Edwardsiella ictaluri

Responses of toll-like receptors (TLR3 and TLR5), lysozyme, and insulin-like growth factor-I (IGF-I) to experimental challenge with virulent Edwardsiella ictaluri were measured in back-cross hybrid (F1 male (blue × channel) × female channel) catfish. The resistance levels to E. ictaluri and host response mechanisms of back-cross hybrids are unknown. Fish were challenged with virulent E. ictaluri and sampled pre-challenge, 2 h and 2, 5, 8, 14, and 21 days post-challenge. Levels of mRNA expression of two toll-like receptors (TLR3 and TLR5) in liver, kidney, spleen, and stomach, plasma lysozyme activity, and circulating IGF-I levels were measured at each timepoint. Throughout challenge, TLR3 was expressed at higher levels than TLR5 in liver (P = 0.0011) and kidney (P = 0.0007) whereas TLR5 was more highly expressed than TLR3 in stomach (P = 0.0032). TLR3 was upregulated in comparison to non-exposed controls in liver (P = 0.0015) and stomach (P < 0.0001) on day 14 and TLR5 was upregulated in liver (P = 0.0175) on days 2 through 8. Plasma lysozyme activity peaked on day 5 (P < 0.001) and IGF-I levels significantly decreased on days 2 through 14 (P < 0.0001). TLR expression patterns suggest that both TLR3 and TLR5 may play a role in host response to bacterial challenge. Plasma lysozyme activity also increased and circulating IGF-I decreased in response to the presence of the pathogen.