Immunoglobulin (Ig) heavy chain gene locus and immune responses upon parasitic, bacterial and fungal infection in loach, Misgurnus anguillicaudatus

Teleost fish are the most primitive bony vertebrates that contain immunoglobulin (Ig). Although teleost Ig is known to be important during tetrapod evolution and comparative immunology, little is known about the genomic organization of the immunoglobulin heavy-chain (IgH) locus. Here, three Ig isotype classes, IgM, IgD and IgT, were firstly identified in dojo loach (Misgurnus anguillicaudatus), and the IgH locus covering τ, μ and δ genes was also illustrated. Variable (V) gene segments lie upstream of two tandem diversity (D), joining (J) and constant (C) clusters and the genomic organization of the IgH locus presented as Vn-Dn-Jn-Cτ-Dn-Jn-Cμ-Cδ, similar to some other teleost fish. However, unlike some other teleost fish, ten VH, ten D and nine J genes were observed in this locus, which suggest teleost Igs might be conserved and diverse. Thus, it would be interesting to determine how Igs divide among themselves in immune response to different antigens. To address this hypothesis, we have developed three models by bath infection with parasitic, bacterial and fungal pathogens, respectively. We found that IgM, IgD and IgT were highly upregulated in the head kidney and spleen after infection with Ichthyophthirius multifiliis (Ich), suggesting that the three Igs might participate in the systemic immune responses to Ich. Moreover, the high expression of IgT in mucosal tissue, such as skin or gills, appeared after being infected with three different pathogens infection, respectively, in which the expression of IgT increased more rapidly in response to Ich infection. Interestingly, the expression of IgD showed a higher increase in spleen and head kidney being challenged with fungi, suggesting that IgD might play an important role in antifungal infection.