The CD8α gene in duck (Anatidae): cloning, characterization, and expression during viral infection

Cluster of differentiation 8 alpha (CD8 alpha) is critical for cell-mediated immune defense and T-cell development. Although CD8 alpha sequences have been reported for several species, very little is known about CD8 alpha in ducks. To elucidate the mechanisms involved in the innate and adaptive immune responses of ducks, we cloned CD8 alpha coding sequences from domestic, Muscovy, Mallard, and Spotbill ducks using reverse transcription polymerase chain reaction (RT-PCR). Each sequence consisted of 714 nucleotides and encoded a signal peptide, an IgV-like domain, a stalk region, a transmembrane region, and a cytoplasmic tail. We identified 58 nucleotide differences and 37 amino acid differences among the four types of duck; of these, 53 nucleotide and 33 amino acid differences were between Muscovy ducks and the other duck species. The CD8 alpha cDNA sequence from domestic duck consisted of a 61-nucleotide 50 untranslated region (UTR), a 714-nucleotide open reading frame, and an 849-nucleotide 30 UTR. Multiple sequence alignments showed that the amino acid sequence of CD8 alpha is conserved in vertebrates. RT-PCR revealed that expression of CD8 alpha mRNA of domestic ducks was highest in the thymus and very low in the kidney, cerebrum, cerebellum, and muscle. Immunohistochemical analyses detected CD8 alpha on the splenic corpuscle and periarterial lymphatic sheath of the spleen. CD8 alpha mRNA in domestic ducklings was initially up-regulated, and then down-regulated, in the thymus, spleen, and liver after treatment with duck hepatitis virus type I (DHV-1) or the immunostimulant polyriboinosinic polyribocytidylic acid (poly I:C).