Molecular characterization and tissue-specific expression of invariant chain in the muscovy duck (Cairina moschata).

The invariant chain (Ii) plays an important role as a chaperone for MHC II maturation and facilitates antigen presentation in vertebrates. We cloned, characterized and made a homology analysis of healthy adult muscovy duck Ii (MDIi), from a poultry farm in the suburban district of Hefei city in China, by rapid amplification of cDNA ends (RACE)-PCR and by measuring expression of the MDIi gene in various tissues by real-time quantitative PCR. A full-length cDNA sequence of MDIi was obtained, 1188-bp long, encoding a 222-amino acid protein. A comparison of the amino acid sequence of Ii between muscovy duck and other birds showed high similarity (66.3-95.3%). Characteristic functional domains found in Ii of other species, such as cytoplasmic domain, transmembrane domain, class II-associated Ii-derived peptide (CLIP) and trimerization domain, were identified in MDIi. Although all functional domains of Ii were found to be highly conserved, small differences in the CLIP sequence occur among the various species. Expression of MDIi was detected in all tissues at different levels. A higher expression level was found in the spleen, intestinal mucosa and the bursa stipe (bursa of Fabricius stipe) than other tissues. This tissue-specific expression suggests that MDIi plays an essential role in all tissues and differential expression may be a function of the innate structures and essential functions of these tissues.