Administration of lipopolysaccharide increases specific and non-specific immune parameters and survival in carp (Cyprinus carpio) infected with Aeromonas hydrophila

Relative percent survival (RPS) and immunomodulating effect of lipopolysaccharide (LPS) were studied in Cyprinus carpio against the bacterial pathogen, Aeromonas hydrophila. A virulent strain of A. hydrophila was isolated from previously infected fish. LPS was extracted from the virulent A. hydrophila and characterized by SDS-PAGE analysis. Different concentrations of LPS (10, 50 and 100 µg/fish, 15, 75 and 150 µg ml− 1, 0.1, 0.25 and 0.5%) were administered to test animals on days 1, 7 and 14 through intraperitoneal injection (i.p.), bathing and oral routes, respectively. Control and test animals were challenged with LD50 concentration of A. hydrophila by i.p. on day 16, mortality determined and RPS calculated. Intraperitoneal injection of 50 and 100 µg of LPS/fish and 150 µg of LPS ml− 1 through bathing significantly enhanced the RPS; oral administration did not influence the RPS rate. Test animals injected with 100 µg of LPS showed significant increase in total leucocyte count and an increase in the population of monocytes and neutrophils. Superoxide anion production by kidney macrophages and secondary immune response against A. hydrophila following vaccination were also elevated. RT-PCR and northern blot analysis of interleukin-1 mRNA showed elevated expression in kidney macrophages on day 16 in fish injected with 50 and 100 µg of LPS/fish, which presumably aided efficient killing of the bacterial pathogen. However, oral administration of LPS did not protect the animals from A. hydophila infection. Classical and alternative complement pathways were not affected by LPS administration by any of the three routes. LPS administration through injection and bathing effectively stimulates the non-specific cellular as well as secondary immune response and offers protection against A. hydrophila infection in carps.