Molecular cloning and characterization of Toll-like receptor 3, and inductive expression analysis of type I IFN, Mx and pro-inflammatory cytokines in the Indian carp, rohu (Labeo rohita).

Toll-like receptors (TLRs) are one of the key components of innate or non-specific immunity. Among various types of TLRs, TLR3 recognizes dsRNA, the genetic material or replicative intermediate of many RNA viruses and triggers TIR-domain-containing adapter-inducing interferon-β dependent signalling pathway to induce type I interferon (IFN) and pro-inflammatory cytokines. In this study, we cloned and characterized full-length TLR3 cDNA in rohu (Labeo rohita), that comprised of 2,619 bp nucleotides encoding a putative protein of 873 amino acid with the estimated molecular mass of 98.57 kDa. The constitutive expression of TLR3 gene was detected in all embryonic developmental stages and in various organs/tissues of rohu fingerlings. In vivo tissue specific modulation of TLR3, type I IFN, Mx (myxovirus-resistant protein) and pro-inflammatory cytokines (TNF-α and IL-1β) gene expression were analysed by quantitative real-time PCR following intravenous injection of polyinosinic-polycytidylic acid (poly I:C), a synthetic analogue of viral dsRNA. A significant relationship of TLR3 induction, and type I IFN, Mx, IL-1β and TNF-α gene expression were observed in majority of the treated fish tissues, as compared to their control. Together, these data highlight the important role of TLR3 in recognizing dsRNA, and in augmenting the innate immunity in fish in response to viral infections.