Secretory IgD has an evolutionarily conserved role in respiratory mucosal defense (160.18)

Recently, we discovered secreted IgD in rainbow trout, which is produced by a novel exon-splicing mechanism. Molecular and cellular analysis revealed that secIgD plasma cells were distributed within the primary and secondary immune tissues. The concentration of serum IgD (2-80µg/ml) was ~400x lower than serum IgM (800-9000µg/ml). Two dominant secIgD heavy chain isoforms were identified in serum of trout which were associated with antibody light chains to form 240 or 370 kDa IgD-monomers. Antigen-specific secIgD was produced in response to vaccination with either TI or TD antigens. Interestingly, the distribution of antigen-specific IgD plasma cells within mucosal tissues revealed that they were preferentially located in the gills of rainbow trout. Analysis of gill mucosal secretions revealed that secIgD was the dominant antibody isotype, which contrasts the profile observed in serum. However, our data suggests that the number of IgD plasma cells in the gills of rainbow trout could not account for the elevated IgD concentration observed in the gill mucus. We hypothesize that serum IgD is preferentially transported across the gill epithelium to neutralize pathogens at this important site. The receptor responsible for trans-epithelial transport of IgD has proved elusive. However, the analogous distribution of IgD plasma cells and secreted IgD in the respiratory mucosa of trout and mammals would suggest that IgD provides an evolutionarily conserved function within this tissue.