Inflammation-Induced Reactivation of the Ranavirus Frog Virus 3 in Asymptomatic Xenopus laevis

Jacques Robert,Leon Grayfer,Francisco De Jesús Andino,Eva-Stina Edholm,Brian Ward,Michael Klymkowsky

doi:10.1371/journal.pone.0112904

Abstract

Natural infections of ectothermic vertebrates by ranaviruses (RV, family Iridoviridae) are rapidly increasing, with an alarming expansion of RV tropism and resulting die-offs of numerous animal populations. Notably, infection studies of the amphibian Xenopus laevis with the ranavirus Frog Virus 3 (FV3) have revealed that although the adult frog immune system is efficient at controlling RV infections, residual quiescent virus can be detected in mononuclear phagocytes of otherwise asymptomatic animals following the resolution of RV infections. It is noteworthy that macrophage-lineage cells are now believed to be a critical element in the RV infection strategy. In the present work, we report that inflammation induced by peritoneal injection of heat-killed bacteria in asymptomatic frogs one month after infection with FV3 resulted in viral reactivation including detectable viral DNA and viral gene expression in otherwise asymptomatic frogs. FV3 reactivation was most prominently detected in kidneys and in peritoneal HAM56+ mononuclear phagocytes. Notably, unlike adult frogs that typically clear primary FV3 infections, a proportion of the animals succumbed to the reactivated FV3 infection, indicating that previous exposure does not provide protection against subsequent reactivation in these animals.

Highlights

Infections and die-offs caused by ranaviruses (RVs, family Iridoviridae) are increasing in prevalence concomitant with an unprecedented and alarming rise in the numbers of susceptible host species [1,2]
Inefficiency of peritoneal macrophages to produce infectious virus Given our previous [14,15] and present (Fig. 6) observations indicating that Frog Virus 3 (FV3) infection of macrophages is different from that of epithelial cells such as in the kidney, we investigated the ability of peritoneal leukocytes (PLs) to support in vitro viral replication and to produce infectious virus
Studies using FV3 inoculations of rats as a hepatitis model have underlined the preferential tropism of FV3 for liver macrophages (Kupffer cells, [30])

Summary

Introduction

Infections and die-offs caused by ranaviruses (RVs, family Iridoviridae) are increasing in prevalence concomitant with an unprecedented and alarming rise in the numbers of susceptible host species (including amphibians, bony fishes and reptiles) [1,2]. The remarkable ability of RVs to cross species barriers of numerous ectothermic vertebrates, suggests that these pathogens possess potent immune evasion mechanisms [3]. Some ectothermic vertebrate species are highly susceptible to RVs, others are more resistant and may serve as asymptomatic carriers that disseminate infectious virus. While RV infections and host pathogen interactions are increasingly documented for a variety of amphibian species, there is still very little known about mechanisms of ranaviral pathogenicity and host immune defenses to these large complex DNA viruses, whose genomes encode some 100 genes [4,5,6]. While our studies have shown the critical roles of CD8 T cells and antibody responses in controlling FV3 infections, our findings underscore a prominent role for macrophage-lineage cells both in the defense against RVs and as contributors to their immune evasion and possibly persistence [9]

Methods

Results

Conclusion