Endotoxin recognition: in fish or not in fish?

Area of Science:

• Immunology
• Comparative Biology
• Molecular Biology

Background:

• Pathogen recognition receptors (PRRs), including Toll-like receptors (TLRs), initiate host-pathogen interactions.
• Lipopolysaccharide (LPS) from Gram-negative bacteria activates the immune system via TLR4, crucial for mammalian response and endotoxic shock.
• Fish exhibit resistance to endotoxic shock, contrasting with mammals.

Purpose of the Study:

• To investigate the mechanism of lipopolysaccharide (LPS) recognition and signaling in fish.
• To determine the role of Toll-like receptor 4 (TLR4) in fish immune responses to LPS.
• To explore alternative receptors involved in fish leukocyte activation by LPS.

Main Methods:

• In vitro studies on trout mononuclear phagocytes.
• Analysis of LPS-induced gene expression.
• In silico mining of piscine genomic and EST databases.
• Comparative analysis of TLR signaling elements between fish and mammals.

Main Results:

• Fish leukocytes demonstrated significantly lower sensitivity to LPS activation compared to mammals.
• LPS failed to induce antiviral gene expression in trout mononuclear phagocytes, an event downstream of TLR4 in mammals.
• Genomic analysis revealed fish possess major TLR signaling elements but lack specific mammalian TLR4-LPS recognition/signaling molecules.

Conclusions:

• The findings question the existence of functional Toll-like receptor 4 (TLR4)-mediated cellular responses to LPS in fish.
• Alternative receptors, potentially beta-2 integrins, may mediate LPS activation of piscine leukocytes.
• This suggests a divergent immune response to LPS in fish compared to mammals.