Immunoglobulin (Ig) D in Labeo rohita is widely expressed and differentially modulated in viral, bacterial and parasitic antigenic challenges.

Immunoglobulins (Igs) play critical roles in protecting host against diverse pathogenic invasion and diseases. Among all Ig isotypes, IgD is the most recently-evolved and enigmatic molecule detected in all vertebrates species except birds. In South-East Asia, Labeo rohita (rohu) is the leading candidate fish species for freshwater aquaculture, and this article describes about IgD gene expression in rohu following viral, bacterial and parasitic antigenic challenges. The partial cDNA (761bp) of Labeo rohita-IgD (LrIgD) was cloned and submitted in the GenBank with the accession no KT883581. Phylogenetically, LrIgD was closely related to grass carp IgD. Analysis of LrIgD gene expression in juveniles by quantitative real-time PCR (qRT-PCR) assay revealed gradual increase in IgD expression with the advancement of time. In the healthy rohu fingerlings, LrIgD expression occurred predominantly in kidney followed by liver and spleen. In response to rhabdoviral antigenic stimulation, LrIgD expression was significantly enhanced in all tested tissues. In bacterial (Aeromonas hydrophila) infection, transcripts of LrIgD increased more dramatically in liver followed by kidney and gill. In parasitic (Argulus) infection, most significant expression of IgD was noted in the skin, followed by kidney, liver, spleen and gill. These results collectively suggest the key role of IgD in the immune response of rohu during viral, bacterial and parasitic infections.