Abstract
The Toll pathway is essential for inducing an immune response to defend against bacterial invasion in vertebrates and invertebrates. Although Toll receptors and the transcription factor Dorsal were identified in different shrimp, relatively little is known about how the Toll pathway is activated or the function of the pathway in shrimp antibacterial immunity. In this study, three Tolls (Toll1–3) and the Dorsal were identified in Marsupenaeus japonicus. The Toll pathway can be activated by Gram-positive (G+) and Gram-negative (G−) bacterial infection. Unlike Toll binding to Spätzle in Drosophila, shrimp Tolls could directly bind to pathogen-associated molecular patterns from G+ and G− bacteria, resulting in Dorsal translocation into nucleus to regulate the expression of different antibacterial peptides (AMPs) in the clearance of infected bacteria. These findings suggest that shrimp Tolls are pattern recognition receptors and the Toll pathway in shrimp is different from the Drosophila Toll pathway but identical with the mammalian Toll-like receptor pathway in its activation and antibacterial functions.
Highlights
Host defense is known to mainly rely on innate immunity in invertebrates
Activation of the Toll pathway is different between vertebrates and invertebrates, in which the former is directly activated by Toll-like receptors (TLRs) binding to various pathogen-associated molecular patterns (PAMPs) from different pathogens, and the latter is indirectly activated by pathogen infection where Toll receptors bind to the cytokine-like molecule Spätzle but not to PAMPs
The tissue distributions of Toll1–3 in hemocytes, heart, hepatopancreas, gill, stomach, and intestine were analyzed by semiquantitative RT-Polymerase chain reaction (PCR) and the results showed that Toll1–3 were distributed in all tested tissues, but Toll1 exhibited low expression levels in the stomach and intestine (Figure 1B)
Summary
Host defense is known to mainly rely on innate immunity in invertebrates. Studies on the innate immune system of invertebrates have garnered much information regarding the underlying mechanisms of resistance to microbial invasion. The Toll pathway plays important functions in innate immunity against infectious pathogens in vertebrates and invertebrates. TLRs are present in the plasma membrane (TLR1, TLR2, TLR4, TLR5, and TLR6) and endosome (TLR3, TLR7, TLR8, and TLR9) of leukocytes. TLRs in the plasma membrane mainly recognize lipopeptides, lipoteichoic acid, lipopolysaccharide (LPS), or bacterial flagellin and the endosome TLRs mainly recognize different nucleic acid patterns, such as single-stranded RNA, unmethylated CpG motifs that exist in both viral and bacterial DNA [2, 3]
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