Host genetic variation in susceptibility to Punta Toro virus

Abstract

Infection of small laboratory animals by Punta Toro virus (PTV), family Bunyaviridae, genus Phlebovirus, is a model for the study of the human pathogen Rift Valley fever virus (RVFV). We have identified inbred mouse strains with significant differences in host response to the Adames strain of PTV. Nine inbred strains of mice representing major branches in the Mus musculus phylogeny were inoculated subcutaneously with a high dose of PTV in survival experiments. Two inbred strains of mice, NZW/LacJ and 129S1/SvImJ, died ~4 days after PTV infection, whereas 7 other strains survived the challenge and showed no clinical signs of disease. Histologically, 129S1/SvImJ mice showed massive hepatocellular necrosis and had additional lesions in lung, brain, and spleen, whereas NZW/LacJ mice had mild piecemeal hepatocellular necrosis. PTV viral loads in the livers of infected mice were determined by reverse transcriptase quantitative PCR. Inbred mice from strains that showed clinical signs and succumbed to PTV infection had higher liver viral loads than did mice of resistant strains. Hybrid F₁ mice were generated by crossing susceptible 129S1 and resistant FVB/N mice and tested for susceptibility. The hybrid F₁ mice showed significantly higher viral loads in the liver than the resistant parental FVB/N mice, suggesting that susceptibility is dominant. These findings will enable an unbiased genetic approach to identify host genes mediating susceptibility to PTV.

Figure 1

Survival of PTV-infected mice. Male mice (3–5 weeks old) of the indicated strains were inoculated s.c. with 10⁶ PFU PTV; n=5 mice for all groups. Mice were euthanized when moribund or at the conclusion of the experiment.

Figure 2

Virus loads in susceptible and resistant mouse strains. Mice of the indicated strains were inoculated s.c. with 10² PFU PTV and euthanized 3 dpi. RT-qPCR was performed on RNAs prepared from liver homogenates. Analysis of each sample was done in separate wells for PTV primers and GAPDH primers; quantities of RNA were determined with standard curves for PTV and GAPDH; and copies of PTV RNA were normalized to GAPDH for each sample. 129S1 and FVB mice were males. F1 designates (129S1 × FVB)F₁ mice (23 males, 13 females). The means and 95% confidence intervals are shown; n, number of mice per group. Kruskal-Wallis ANOVA was performed with a Dunn’s multiple comparison posttest. *, P < 0.05; ***, P < 0.001. There was no statistical difference between F₁ males and females.

Figure 3

Histopathological lesions induced by PTV infection. Mice were inoculated s.c. with 10⁶ PFU of PTV and organs were harvested 3 dpi. Livers were stained with hematoxylin and eosin. Mock 129S1: S, normal sinusoid, H, normal hepatocyte. Infected NZW: focal hepatocellular degeneration and hemorrhage; I, infiltration of inflammatory cells. Infected 129S1: N, necrosis. Size bar is 25 µm.