Expression Of Scavenger Receptor-A On Catfish Cytotoxic Cells     Harjeet Kaur ,  Liliana Jaso-Friedmann, Donald L. Evans.   College of Veterinary Medicine, University of Georgia, Athens, GA     Nonspecific cytotoxic cells (NCC) are the teleost equivalent of mammalian NK cells.  In the present study the possible role of NCC in mediating innate anti-bacterial responses was examined by determining if these cells expressed membrane receptors for bacterial DNA.  Among the oligodeoxynucleotide (ODN) sequences examined for binding and cellular activation was a unique 20-mer poly-guanosine nucleotide (dG20).  dG20 specifically bound to NCC.  Although dG20 binding to NCC was saturable at nanogram concentrations, “cold” ligand competition experiments using homologous as well as CpG and GpC containing ODNs demonstrated possible multiple receptor specificities for bDNA.  To examine the repertoire of receptors on NCC for ODNs, experiments were done to determine whether these cells expressed Scavenger Receptors (SR).  In mammals, these receptors bind poly-guanosine containing ODNs in the form of G-tetrads.  A subset (23%) of purified NCC from catfish expressed SR-A determined by staining with an anti-human SR-A (Type I) monoclonal (mab).  Cold competition binding with two other types of ligands known to bind (mammalian) SR-A (i.e. polyvinyl sulfate and dextran sulfate) decreased anti-SR-A binding to NCC by 40%.  Thus, similar to mammalian cells, teleost SR-A also has promiscuous ligand specificities.  Other receptors than those recognized by anti-SR-A mab bound dG20 because binding competition experiments with this mab only produced approximately 40% inhibition of dG20 binding.  These data demonstrated that NCC express a heterogenous population of DNA receptors. Recognition of bDNA by NCC may comprise one pathway of innate (anti-bacterial) immunity in teleosts.