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Scavenger receptors (SRs) play crucial roles in innate immunity by acting as pattern recognition receptors. Although SRs are widely documented in mammals, data on their occurrence and functions in ancient vertebrates are limited. In this study, we report, to our knowledge, the first cloning and functional characterization of an SR molecule from teleost fish (Tetraodon nigroviridis). This SR (7/tSR) was identified as a homolog to mammalian scavenger receptor class A member 5 with the conserved structure of a class A SR. 7nSR contained multidomains in a type II transmembrane receptor, including an SR cysteine-rich domain, two coiled-coil collagenous domains, a transmmebrane domain, and a short N-terminal intracellular region with an unexpected TNFR-associated factor 2-binding consensus motif similar to that in human MSR molecules. Phylogenetic analysis suggested that TnSR may be an ancient member of class A SRs resulting from the close relationship between scavenger receptor class A member 5 and macro-phage SR in vertebrates associated with the subtle differences in TnSR structure. Subcellular localization analysis showed that TnSR was a cell membrane receptor with homotrimer forms involved in the recognition and internalization of LPS from surface membranes into lysosomes. Functionally, TnSR expression was dramatically induced by LPS stimulation. TnSR served as a neg-ative regulator in LPS-induced NF-KB activation by the competitive recruitment of TNFR-associated factor 2 from the TNF-ɑ signaling pathway. To our knowledge, this is the first report showing that SR plays an inhibitory role in LPS-elicited inflammation by cross-talking with the TNF-ɑ inflammatory pathway. These findings contribute to a better understanding of the biological and evolutionary history of the SR family. [ABSTRACT FROM AUTHOR]