Generation of a long-lasting, protective, and neutralizing antibody response to the ranavirus FV3 by the frog Xenopus * Gregory D. Maniero a , Heidi Morales b , Jennifer Gantress b , Jacques Robert b, * a Department of Biology, Stonehill College, Easton, MA 02356, USA b Department of Microbiology and Immunology, University of Rochester Medical Center, Rochester, NY 14642, USA Received 16 June 2005; revised 7 September 2005; accepted 7 September 2005 Available online 4 October 2005 Abstract Xenopus serves as an experimental model to evaluate the contribution of adaptive immunity in host susceptibility to emerging ranaviral diseases that may contribute to amphibian population declines. It has been previously shown that following a secondary infection with the ranavirus frog virus 3 (FV3), adult Xenopus more rapidly clear FV3 and generate specific anti-FV3 IgY antibodies. We have further evaluated the potency and persistence of the Xenopus antibody response against FV3. Frogs inoculated with FV3 (without adjuvant) up to 15 months after priming produce specific, thymus-dependent anti-FV3 IgY antibodies detectable from 10 days to 8 weeks post-infection. These antisera from boosted frogs are neutralizing in vitro and provide partial passive protection to susceptible larvae when they are injected a few minutes before FV3 inoculation. These results with Xenopus suggest that other anuran amphibians are likely to develop effective long-lasting protective humoral immunity after an initial viral exposure. q 2005 Elsevier Ltd. All rights reserved. Keywords: Xenopus laevis; Amphibians; Humoral immunity; Antibody; IgY. FV3 1. Introduction Ranaviruses (family Iridoviridae) infect a wide range of vertebrate species, including fish and both anuran (frogs and toads) and urodele (salamanders) amphibians, and are thought to contribute to the world-wide decline of amphibian populations [rev. in 1]. Frog Virus 3 (FV3), a large (120–200 nm) DNA virus, was first isolated from the North American leopard frog Rana pipiens; FV3 or FV3-like viruses are now found in diverse amphibian species throughout the world [1–4]. Indeed, ranaviruses have been causally implicated in several amphibian die- offs world-wide [5,6], and the prevalence of ranaviral diseases affecting amphibian populations’ world wide is increasing [1,4,7; http://lsvl.la.asu.edu/irceb/amphi- bians/]. Whereas several species of amphibians are highly susceptible to ranavirus infections, Xenopus laevis is relatively resistant [8]. Specifically, infection of adult outbred Xenopus by i.p. injection of FV3 is mildly pathogenic (only w10–20% of adults infected with 10 7 pfu die within a month) unless the host’s immune system has been compromised experimentally by sub-lethal irradiation or CD8 T cell depletion [9]. In contrast, Xenopus larvae are highly susceptible [8]. Interestingly, specific anti-FV3 IgY antibody cannot be detected following a primary infection of adults by an Developmental and Comparative Immunology 30 (2006) 649–657 www.elsevier.com/locate/devcompimm 0145-305X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.dci.2005.09.007 * This research was supported by grants MCB-0136536, MCB- 0445509 and IRCEB- DEB-0213851 from the NSF and R24-AI- 059830 from the NIH; GM and HM received support from NIH T32- AI 07285. * Corresponding author. Tel.: C1 585 275 1722; fax: C1 585 473 9573. E-mail address: robert@mail.rochester.edu (J. Robert).