Attenuated Francisella asiatica iglC mutant induces protective immunity to francisellosis in tilapia

Francisella asiatica is a Gram-negative, facultative intracellular bacteria that causes fish francisellosis. Fish francisellosis is a severe sub-acute to chronic granulomatous disease with high mortalities and high infectivity rates in cultured and wild fish. To date, there is no approved vaccine for this widespread emergent disease. The goal of this study was to characterize the efficacy of a defined F. asiatica mutant (Delta iglC) as a live attenuated vaccine against subsequent immersion challenge with the wild-type (WT) organism. In previous work, the Delta iglC was found to be attenuated upon intraperitoneal injection and immersion challenges. In vitro, the Delta iglC exhibited reduced growth in tilapia head-kidney derived macrophages, and was significantly attenuated (p <0.001) as demonstrated by cytopathogenic and apoptosis assays. In this study, the Delta iglC was tested to determine its ability to protect tilapia against challenge with high doses (lethal dose 80) of WT bacteria. Naive tilapia vaccinated by immersion with a suspension of the Delta iglC and subsequently challenged with WT F. asiatica were protected (90% mean percent survival) from the lethal challenges. F. asiatica-specific antibodies produced in response to immunization with the Delta iglC were subsequently found to protect naive tilapia against high-dose F. asiatica challenge in passive immunization experiments. Significant protection (p < 0.001) was obtained when fish were passively immunized and challenged with 10(4) and 10(6) CFU/fish of WT F. asiatica; but not when challenged with 10(6) CFU/fish. This is the first report of a defined live attenuated strain providing protection against F. asiatica in fish. (C) 2010 Elsevier Ltd. All rights reserved.