Poultry Ii Gene Cloning, Expression And Function Of A Preliminary Study

The invariant chain (Ii) is a non-polymorphic type II transmembrane protein and plays a central role in regulating the expression and function of class II major histocompatibility complex (MHC) molecules. The biosynthesis of distinct forms of the invariant chain (Ii) protein from a unique gene as the result of differential splicing patterns has been observed in humans and mice. However, there have been no reports on the existence of Ii isoforms in avian species. In the present study, we identified two chicken Ii cDNAs by RT-PCR and RACE, and examined the Ii gene copy number, mRNA expression and protein expression by Southern blotting, Northern blotting and immunofluorescence confocal microscopy, respectively. One of the Ii cDNAs, named Ii-1, was 1,151 bp in length, and had an open reading frame (ORF) of 672 nucleotides, in agreement with a previously identified chicken Ii sequence; the other, named Ii-2, was 1,337 bp long and had an ORF of 861 nucleotides. The Ii-1 and Ii-2 isoforms are similar to each other where they overlap, and the sequence similarity between them is 99% at either the nucleotide or amino acid level. However, they differ in the presence or absence of a Tg domain. Southern blotting confirmed that these cDNAs were derived from a single copy gene. Northern blotting performed with total RNA from various tissues of 6-week-old chickens revealed high levels of Ii-1 and Ii-2 mRNA expression in the spleen and bursa of Fabricius, and low levels of Ii-1 expression in the thymus, heart and liver, while Ii-2 was not expressed in these tissues. High levels of expression of both Ii isoforms were detected in the spleen and bursa of Fabricius during late embryogenesis. Immunofluorescence staining showed that Ii proteins were expressed in the cell membranes of the splenocytes. These data suggest that chicken Ii exists in two isoforms resulting from alternative splicing, and is strongly expressed in the major immune organs.With a pair of degenerate primers, we further identified a duck invariant chain (Ii)) cDNA, named duck Ii-1, by RT-PCR and RACE. It was 1,190 bp in length and contained a 41 bp 5' untranslated region, a 669 bp open reading frame and a 480 bp 3' untranslated region. An alternative transcript encoding a thyroglobulin...