Characterization of IκBα, Rab21 and Rac2 as Innate Immune Genes during Infection with Aeromonas hydrophila and Cyprinid herpesvirus 2 in Crucian Carp Carassius auratus gibelio

IκBα (NF-κB inhibitor), Rab21 (Rab GTPase), and Rac2 (small GTP binding protein) play important roles in innate immunity. Previously, they were identified as differentially expressed immune-related genes in crucian carp Carassius auratus gibelio infected with Cyprinid herpesvirus 2 (CyHV-2) through suppression subtractive hybridization. Here, full-length IκBα, Rab21 and Rac2 cDNAs cloned from crucian carp showed high homologies with transcripts in other bony fish. These genes, usually expressed at low levels in all the examined crucian carp tissues, were significantly upregulated in kidneys from 6-24 h post-infection with CyHV-2 (P < 0.01) and from 6-72 h post-infection with A. hydrophila (P < 0.01). Thus, IκBα, Rab21 and Rac2 were involved in innate immunity to infection with CyHV-2 and A. hydrophila. The expression levels of IκBα and Rab21 in survived fish with viral infection were significantly higher than those in susceptible fish that had died from infection, but similar levels of Rac2 were observed in samples of surviving fish and moribund fish. These data suggested that crucian carp IκBα, Rab21, and Rac2 should share the same innate immune response to pathogen challenge with their mammalian homologues, and the differential expression level of IκBα and Rab21 between acute and chronic infection marked these two immune genes as markers for different infection outcomes.