Toll-like receptors (TLRs) in aquatic animals: Signaling pathways, expressions and immune responses

• Pattern recognition receptors (PRR) in aquatic animals play major role in innate immune response.
• TLRs form early barriers against infectious disease by eliciting first line of innate immune defense.
• TLR signaling cascades leads to the induction of pro-inflammatory cytokines such as TNF, IL-1β, IL-12, IL-8 or IL-6.
• The immune modulations from the different repertoire of TLR are important for therapeutic strategies in aquaculture.
• Supplementation with TLR activators that stimulate dendritic cells can be effective in designing vaccines against aquatic diseases.

The innate system's recognition of non-self and danger signals is mediated by a limited number of germ-line encoded pattern recognition receptors (PRRs) that recognize pathogen associated molecular patterns (PAMPs). Toll-like receptors (TLRs) are single, non-catalytic, membrane-spanning PRRs present in invertebrates and vertebrates. They act by specifically recognizing PAMPs of a variety of microbes and activate signaling cascades to induce innate immunity. A large number of TLRs have been identified in various aquatic animals of phyla Cnidaria, Annelida, Mollusca, Arthropoda, Echinodermata and Chordata. TLRs of aquatic and warm-blooded higher animals exhibit some distinctive features due to their diverse evolutionary lineages. However, majority of them share conserve signaling pathways in pathogen recognition and innate immunity. Functional analysis of novel TLRs in aquatic animals is very important in understanding the comparative immunology between warm-blooded and aquatic animals. In additions to innate immunity, recent reports have highlighted the additional roles of TLRs in adaptive immunity. Therefore, vaccines against many critical diseases of aquatic animals may be made more effective by supplementing TLR activators which will stimulate dendritic cells. This article describes updated information of TLRs in aquatic animals and their structural and functional relationship with warm-blooded animals.