Characterization of Innate Immune Genes of Catfish: CXC Chemokines and Toll-Like Receptors
Type of DegreeDissertation
Fisheries and Allied Aquacultures
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Innate immunity is known as the first line of defense against pathogenic invasions. Fish, as lower vertebrates, are believed to maintain their healthy status mostly by relying on their innate immunity. Of the many types of genes involved in innate immunity, chemokines and toll-like receptors (TLRs) are known to play critical roles during early stages of microbial infections. Chemokines represent a superfamily of small molecular cytokines involved in recruitment, activation and adhesion of a variety of leukocyte types to inflammatory foci, while TLRs have been elucidated to serve as receptors for specific pathogen-associated molecular patterns (PAMPs). In order to better understand catfish immune responses, I have characterized five genes involved in the innate immunity including CXCL2, CXCL10, CXCL12, CXCL14 and TLR2. While the chemokine genes shared high levels of sequence identities and structural features with their counterparts from other species, the catfish (Ictalurus sp.) TLR2 is an intronless gene, dissimilar to those from fugu (Takifugu rubripes), flounder (Paralichthys olivaceus), and human (Homo sapiens), but similar to that of zebrafish (Danio rerio). Analysis of expression of these genes provided functional hints. While CXCL10 was highly inducible upon an infection with Gram-negative Edwardsiella ictaluri, causative agent of enteric septicemia of catfish (ESC), CXCL2, CXCL12, and CXCL14 were constitutively expressed, suggesting their homeostatic functions in addition to serving as chemokines. The catfish TLR2 was also respond to Gram-negative ESC bacteria, although a major role of TLR2 response is for Gram-positive bacterium. This work also mapped the chemokines and the TLR2 to BAC clones, setting the foundation for mapping them to the catfish physical map. Various copy numbers seemed to exist in the catfish genome. CXCL2, CXCL12, and CXCL14 exist as a single copy gene in the catfish genome; TLR2 appeared to have two genomic copies, whereas CXCL10 appeared to have a multi-gene family with at least four copies arranged in a tandem fashion. While this research for the first time identified the chemokine and TLR genes in catfish, functional studies are required to answer many of the interesting questions raised by this work.