Antiviral immunity in amphibians.
Although a variety of virus species can infect amphibians, diseases caused by ranaviruses ([RVs]; Iridoviridae) have become prominent, and are a major concern for biodiversity, agriculture and international trade. The relatively recent and rapid increase in prevalence of RV infections, the wide range of host species infected by RVs, the variability in host resistance among population of the same species and among different developmental stages, all suggest an important involvement of the amphibian immune system. Nevertheless, the roles of the immune system in the etiology of viral diseases in amphibians are still poorly investigated. We review here the current knowledge of antiviral immunity in amphibians, focusing on model species such as the frog Xenopus and the salamander (Ambystoma tigrinum), and on recent progress in generating tools to better understand how host immune defenses control RV infections, pathogenicity, and transmission.
PubMed ID: 22163335
PMC ID: PMC3230842
Grant support: 2 R24 AL 059830-06 PHS HHS
Genes referenced: cd40 myh4 myh6 tnf
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|Figure 1. Slow clearance of FV3 DNA in X. tropicalis. FV3 DNA detected by PCR (35 cycles) using primers specific for the major capsid protein (MCP) on genomic DNA purified from various tissues of outbred X. tropicalis adults that were infected with FV3 by i.p. injection of 1 × 106 PFU for 2, 6, 13, 27 and 60 days (2 individuals per time point).|
|Figure 2. Schematic view of Xenopus adult immune response kinetics in infected kidneys. During both primary and secondary FV3 infections, MHC class II+ innate immune cell effectors (leukocytes) rapidly accumulate in the kidneys (violet line), the main site of infection, and pro-inflammatory genes (e.g., TNF-α, IL-1β) are induced. This is followed by an adaptive CD8 T cell response and infiltration (green line) that peak at 6 dpi during a primary infection. During a second FV3 infection, CD8 T cell response and infiltration peak 3 days earlier, which suggests T cell memory. However, the lower number of infiltrated CD8 T cells (5 time less) suggests that anti-FV3 antibodies (blue line) and B cell memory are playing a prominent role during secondary infection resulting in a faster viral clearance (red line).|
|Figure 3. Schematic view of the complex role of macrophages in Xenopus host defenses against RV. As innate immune cell effectors, macrophage can acquire viral antigen by direct infection, pinocytosis of opsonized viruses or phagocytosis of infected cells, as well as release cytokines, chemokines and toxins that contribute to limiting the infection. As an adaptive immune cell effector, macrophages process and present viral antigens through MHC class I and class II pathways, up-regulated co-stimulatory molecules (B7, CD40) and activate anti-RV CD8 and CD4 T cells. Finally, macrophage can harbor quiescent RV in asymptomatic frogs.|
|Figure 4. Immunofluorescence microscopy of baby hamster kidney cells (BHK, left) and Xenopus PLs infected in vitro for 2 days with FV3 (0.3 MOI). Cells were cytocentrifuged on microscope slides, fixed with formaldehyde, permeabilized with ethanol, incubated with a rabbit anti-53R and FITC-conjugated donkey anti-rabbit Abs (Green); then stained with the DNA dye Hoechst-33258 (Blue) mounted in anti-fade medium and visualized with a Leica DMIRB inverted fluorescence microscope. Note the large viral assembly sites in BHK cells that contain large amount of viral DNA stained Hoechst-33258 and anti-53R Ab (arrows). In contrast, anti-53R staining is weaker in PLs, and no assembly sites are detected.|