Immune responses to live and inactivated Nocardia seriolae and protective effect of recombinant interferon gamma (rIFN γ) against nocardiosis in ginbuna crucian carp, Carassius auratus langsdorfii

• Ginbuna crucian carp were immunised with live and inactivated Nocardia seriolae.
• CD8α+T cells increased significantly within 3 days post immunisation in both groups.
• Surface Ig M+ cells were also increased in both immunised groups.
• Antibody level was highest at 15th day post immunisation in both groups.
• Recombinant interferon gamma (at 10 μg/fish) protected ginbuna against N. seriolae.

Looking into the fact that substantial mortality and morbidity is associated with intracellular Gram +ve bacterium, Nocardia seriolae infection, an effective vaccine against this pathogen is necessary to control the significant losses in aquaculture practices. Therefore, an attempt was made to evaluate the effect of live (sub-lethal) and inactivated (antigenic form) N. seriolae on cellular and humoral immunity in ginbuna crucian carp, Carassius auratus langsdorfii as well as the therapeutic potency of recombinant interferon gamma (rIFN γ) against N. seriolae infection. Effect of live and inactivated N. seriolae immunisation on the proliferation of CD4+ T cells, CD8α+ T cells and surface Ig M+ cells in peripheral blood leucocytes, spleen, head kidney and trunk kidney of ginbuna was studied after 1st, 3rd, 7th, 15th and 30th day post immunisation. The percentage of CD8α+ T cells in spleen and head kidney of ginbuna was significantly higher at 3rd day post immunisation. Similarly, surface Ig M+ cells level was found to increase in both live and inactivated N. seriolae immunised groups. On the contrary, high percentage of CD4+ T cells was observed in live N. seriolae immunised group in both the head and trunk kidneys at 30th day post immunisation. The humoral immune response to live and inactivated N. seriolae immunised ginbuna showed high antibody titre at 15th day post immunisation but the level declined subsequently in both the immunised groups. On challenge with virulent N. seriolae (1.2 × 108 CFU/ml), the relative percent survival was 62.5 and 75 in live and inactivated N. seriolae immunised groups, respectively. Furthermore, we have also studied the therapeutic potency of rIFN γ and found the possible involvement of IFN γ in resistance mechanism in fish. Administration of rIFN γ into ginbuna (at 10 μg/fish) one day before challenge study was found to protect ginbuna. The relative percent survival of ginbuna was 43.75 and 60 when challenged with 2 different doses of N. seriolae i.e., 1.2 × 108 CFU/ml and 5 × 107 CFU/ml, respectively. In summary, this study indicates that both forms of N. seriolae immunisation as well as rIFN γ indeed elicit an effective protective immunity which will help in designing suitable vaccine and/or adjunct therapy against N. seriolae infection in fish.