Submitted to: Journal of Immunology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/6/2006
Publication Date: 8/15/2006
Citation: Stafford, J.L., Wilson, M., Nayak, D., Quiniou, S., Clem, L.W., Miller, N.W., Bengten, E. 2006. Identification and characterization of a FcR homolog in an ectothermic vertebrate, the channel catfish (Ictalurus punctatus). Journal of Immunology. 177(4):2505-2517. Interpretive Summary: The knowledge of fish immune system is limited. This study describes the identification of an immune molecule (Fc receptor homolog) in catfish only known in higher vertebrates so far. This molecule was identified by DNA sequencing and shown to bind to immunoglobulin (antibodies) molecules as in mammals by protein assays. This information will help us comprehend how the immune system clear pathogens.
Technical Abstract: A Fc receptor homolog (IpFcRI)3, representing the first such receptor from an ectothermic vertebrate, has been identified in the channel catfish (Ictalurus punctatus). Mining of the catfish expressed sequence tag (EST) databases using mammalian FcR sequences for CD16, CD32, and CD64 resulted in the identification of a teleost Ig-binding receptor. IpFcRI is encoded by a single copy gene containing three immunoglobulin C2-like domains, but lacking a transmembrane segment and cytoplasmic tail. The encoded Ig domains of IpFcRI are phylogenetically and structurally related to mammalian FcRs and the presence of a putative Fc-binding region appears to be conserved. IpFcRI-related genomic sequences are also present in both pufferfish and rainbow trout, indicating the likely presence of a soluble FcR in other fish species. Northern blot and qualitative PCR analyses demonstrated that IpFcRI is primarily expressed in IgM negative leukocytes derived from the lymphoid kidney tissues and PBL. Significantly lower levels of IpFcRI expression were detected in catfish clonal leukocyte cell lines. Using the native leader, IpFcRI was secreted when transfected into insect cells and importantly the native IpFcRI glycoprotein was detected in catfish plasma using a polyclonal antibody (pAb). Recombinant IpFcRI binds catfish IgM as assessed by both co-immunoprecipation and cell transfection studies and it is presumed that it functions as a secreted FcR akin to the soluble FcRs found in mammals. The identification of an FcR homolog in an ectothermic vertebrate is an important first step towards understanding the evolutionary history and functional importance of vertebrate Ig-binding receptors.