Class A scavenger receptors mediate extracellular dsRNA sensing, leading to downstream antiviral gene expression in a novel American toad cell line, BufoTad

Dev Comp Immunol. 2019 Mar;92:140-149. doi: 10.1016/j.dci.2018.11.012. Epub 2018 Nov 16.


Viral double-stranded (ds)RNA is a potent pathogen-associated molecular pattern (PAMP), capable of inducing a strong antiviral state within the cell, protecting the cell from virus infection. In mammals and fish, sensing extracellular dsRNA is mediated by cell-surface class A scavenger receptors (SR-As). Currently, very little is known about SR-As in amphibians, including: sequence, expression patterns and function. To this end, SR-A expression and function was studied in a novel American toad (Anaxyrus americanus) tadpole cell line called BufoTad. BufoTad was derived from a whole tadpole. The cell line exhibits a cobblestone morphology and expresses abundant levels of transcripts for cytokeratin 19, vimentin, claudin 3, chemokine receptor CXCR4, and SR-AI, one of the five members of the SR-A family, collectively suggesting that BufoTad could be endothelial-like. BufoTad cells bound acetylated LDL, whereas the Xenopus laevis kidney epithelial A6 cell line did not, suggesting functional SR-A activity in BufoTad cells. Additionally, three SR-A competitive ligands (DxSO4, fucoidan, poly inosine (pI)) completely blocked AcLDL binding in BufoTad cells, whereas their three corresponding non-competitive ligands (ChSO4, fetuin, poly cytosine (pC)) did not. A commercial dsRNA, poly IC, induced robust expression of an Mx-like gene transcript, a possible antiviral protein in BufoTad cells. Employing the same SR-A ligand blocking assay used for AcLDL blocked dsRNA-induced ISG expression. This study is the first demonstration that amphibian SR-As have functional ligand binding activities in a live biological cellular model and that sensing extracellular dsRNA in amphibian cells leads to antiviral gene expression that is mediated by class A scavenger receptors.

Keywords: AcLDL; American toad; Amphibian; Class A scavenger receptor; Mx; dsRNA.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amphibian Proteins / genetics
  • Amphibian Proteins / metabolism*
  • Animals
  • Antiviral Agents / metabolism
  • Anura / physiology*
  • Cell Line
  • Extracellular Space
  • Immunity, Innate
  • Ligands
  • Pathogen-Associated Molecular Pattern Molecules / immunology*
  • Protein Binding
  • RNA, Double-Stranded / genetics
  • RNA, Double-Stranded / immunology*
  • RNA, Viral / genetics
  • RNA, Viral / immunology*
  • Scavenger Receptors, Class A / genetics
  • Scavenger Receptors, Class A / metabolism*
  • Signal Transduction


  • Amphibian Proteins
  • Antiviral Agents
  • Ligands
  • Pathogen-Associated Molecular Pattern Molecules
  • RNA, Double-Stranded
  • RNA, Viral
  • Scavenger Receptors, Class A