SECRETORY IMMUNE-SYSTEM OF THE DUCK (ANAS-PLATYRHYNCHOS) - IDENTIFICATION AND EXPRESSION OF THE GENES ENCODING IGA AND IGM HEAVY-CHAINS

IgA has not previously been identified in waterfowl. Studies instead r evealed physical and antigenic similarities between duck bile immunogl obulin (Ig) and serum IgM. Here, a differential screening approach was used to clone, from a duck spleen library, the cDNA encoding the heav y (H) chains of IgM and the Ig, identified here as IgA, occurring in d uck secretions. Phylogenetic comparisons of inferred amino acid sequen ces of entire H chain constant (C) regions and of individual domains r evealed that the duck mu chain was closest to chicken mu (54 % overall identity), and duck alpha was closest to chicken alpha (50 % identity ). Comparison of the mu and alpha C regions revealed areas of up to 65 % amino acid similarity within the C4 domains, accounting for the pre viously noted antigenic overlap of duck IgM and IgA. Messages for alph a and mu were detected in duck lymphoid organs but the alpha message w as most abundant in the respiratory, alimentary and reproductive tract s. The alpha message first appeared around 14 days of age and reached adult levels of expression only at 35-50 days. The results indicate th at the duck has a mucosal immune system which utilizes IgA; however, t he delayed expression and secretion of duck IgA explains the susceptib ility of ducklings to mucosal pathogens. Since the waterfowl are among the most primitive extant birds, the recognition of IgA in the duck s upports the conclusion that IgA occurs throughout the class Aves and a lso existed in the common ancestors of birds and mammals.