Inductive expression of toll-like receptor 5 (TLR5) and associated downstream signaling molecules following ligand exposure and bacterial infection in the Indian major carp, mrigal (Cirrhinus mrigala)

Toll-like receptors (TLRs) are one of the key components of innate immunity. Among various types of TLRs, TLR5 is involved in recognizing bacterial flagellin and after binding, it triggers myeloid differentiation primary response gene 88 (MyD88)-dependent signaling pathway to induce pro-inflammatory cytokines. In this report, we analyzed the expression profile of TLR5 and its associated downstream signaling molecules like MyD88 and tumor necrosis factor (TNF) receptor-associated factor (TRAF) 6 in the Indian major carp (IMC), mrigal (Cirrhinus mrigala) which is highly commercially important fish species in the Indian subcontinent. Ontogeny analysis of TLR5, MyD88 and TRAF6 revealed constitutive expression of these genes in all embryonic developmental stages, and highlighted the importance of embryonic innate immune defense system in fish. Tissue specific expression analysis of these genes by quantitative real-time PCR (qRT-PCR) revealed their wide distribution in various organs and tissues; highest expression of TLR5 and MyD88 was in liver and TRAF6 was in kidney. Modulation of TLR5, MyD88 and TRAF6 gene expression, and the induction of interleukin (IL)-8 and TNF-α were analyzed in various organs by qRT-PCR following flagellin stimulation, and Aeromonas hydrophila and Edwardsiella tarda infection. In the treated fish, majority of the tested tissues exhibited significant induction of these genes, although with varied intensity among the tissues and with the types of treatments. Among the examined tissues, a significant relationship of TLR5 induction, MyD88 and TRAF6 up-regulation, and enhanced expression of IL-8 and TNF-α gene transcripts was observed in the blood and intestine of both flagellin stimulated and bacteria infected fish. These findings may indicate the involvement of TLR5 in inducing IL-8 and TNF-α, and suggest the important role of TLR5 in augmenting innate immunity in fish in response to pathogenic invasion. This study will enrich the information in understanding the innate immune mechanism in fish and may be helpful in developing preventive measures against infectious diseases in fish.

► TLR5 (membrane form) and MyD88 cDNAs were partially cloned in the Indian major carp, mrigal (Cirrhinus mrigala).
► TLR5, MyD88 and TRAF6 genes expression were analyzed during ontogenesis, showing their expression in all developmental stages.
► TLR5, MyD88 and TRAF6 genes were widely expressed in various organs suggesting their role in immunosurveillance to the host.
► Bacterial flagellin and motile bacterial infection induced TLR5-signaling cascades.
► Activation of TLR5 was associated with inductive expression of immunoregulatory cytokines, IL-8 and TNF-α.