Immune evasion strategies of ranaviruses and innate immune responses to these emerging pathogens

Abstract

Ranaviruses (RV, Iridoviridae) are large double-stranded DNA viruses that infect fish, amphibians and reptiles. For ecological and commercial reasons, considerable attention has been drawn to the increasing prevalence of ranaviral infections of wild populations and in aquacultural settings. Importantly, RVs appear to be capable of crossing species barriers of numerous poikilotherms, suggesting that these pathogens possess a broad host range and potent immune evasion mechanisms. Indeed, while some of the 95-100 predicted ranavirus genes encode putative evasion proteins (e.g., vIFα, vCARD), roughly two-thirds of them do not share significant sequence identity with known viral or eukaryotic genes. Accordingly, the investigation of ranaviral virulence and immune evasion strategies is promising for elucidating potential antiviral targets. In this regard, recombination-based technologies are being employed to knock out gene candidates in the best-characterized RV member, Frog Virus (FV3). Concurrently, by using animal infection models with extensively characterized immune systems, such as the African clawed frog, Xenopus laevis, it is becoming evident that components of innate immunity are at the forefront of virus-host interactions. For example, cells of the macrophage lineage represent important combatants of RV infections while themselves serving as targets for viral infection, maintenance and possibly dissemination. This review focuses on the recent advances in the understanding of the RV immune evasion strategies with emphasis on the roles of the innate immune system in ranaviral infections.

Keywords: FV3; Iridovirus; anti-viral; cytokines; frog virus 3; immune-evasion; inflammation; innate immunity; macrophage; ranavirus.

Figure 1

Electron micrographs of peritoneal macrophages from FV3-infected Xenopus laevis adults. Peritoneal leukocytes were collected from frogs 2 days subsequent to infection with 5 × 10⁶ PFU of FV3, processed and visualized under a Hitachi 7650 TEM. (A) Mononucleated macrophage-like cells with an intracellular pool of FV3 virions (arrow, scale bar: 2 μm). (B) A magnified image of the intracellular macrophage pool of FV3 virions (scale bar: 200 nm). (C) A macrophage-like cell shedding viral particles (arrow, scale bar: 2 μm). (D) A magnified image of a virus-shedding macrophage (scale bar: 0.5 μm).

Figure 2

Comparison of Xenopus laevis tadpole and adult peritoneal macrophages. Peritoneal leukocytes were collected from frogs 3 days after elicitation with heat-killed Escherichia coli. Cells were seeded in and allowed to adhere to bottoms of 24 well plates for 3 hours. Non-adherent cells were removed, the adherent cells washed and cultured for further 4 days. Cells were cytospun onto glass slides and Giemsa stained according to standard protocol. Upper panels represent peritoneal macrophages collected from tadpoles and lower panels represent peritoneal macrophages derived from adult frogs.

Figure 3

Comparison of Xenopus laevis tadpole and adult peritoneal macrophages infected by FV3. Peritoneal leukocytes were collected from one adult (A) or from 10 pooled pre-metamorphic tadpoles (B and C) 2 days after FV3 infection by intraperitoneal injection. Cells were fixed, stained with a rabbit anti-53R (green) and DNA dye Hoechst-33258 (blue), then mounted in anti-fade medium and visualized with a Leica DMIRB inverted fluorescence microscope.