Rift Valley Fever Virus (RVFV; genus Phlebovirus, family Bunyaviridae) is an important zoonotic pathogen transmitted by mosquitoes that causes large outbreaks among ruminants and humans in Africa and the Arabian Peninsula. Human patients typically develop an acute febrile illness, which may be followed by fatal hemorrhagic fever, encephalitis or ocular disease (1;2).  RVFV is known to be sensitive to the interferons (IFNs) (3;4), and an antibody response is critical for protection against the virus (5;6).  In a rodent model, wild type RVFV infection causes acute death with fatal hepatic disease at 2 to 3 days post-infection (dpi), whereas infection with a mutant RVFV strain lacking NSm protein expression results in delayed death with fatal neurological disease as well as hepatic failure (7).

 

This screen is designed to identify genes required for the control of RVFV infection in vivo.  Since wild type RVFV is categorized as an enhanced animal biosafety level 3 (ABSL-3) containment agent, a mutagen-attenuated recombinant strain (arMP-12) that can be handled in ABSL-2 containment is used here (8).  Preliminary experiments indicate that C57BL/6J mice injected intraperitoneally with 1x10⁵ pfu of arMP-12 control the infection with no signs of illness, but Ifnar^-/- and Stat1^{domino/domino} mice succumb to death with high viremia and viral loads in the liver and spleen within 3 dpi (Figure 1). Mutagenized G3 mice that display severe sickness or death after intraperitoneal challenge with 1x10⁵ pfu of arMP-12 are identified as potential mutants in this screen. The Ex Vivo Macrophage Screen for Control of Viral Infection is also employed to discover components important for the control of RVFV infection, but utilizes thioglycolate-elicited peritoneal macrophages, thus saving the animal from infection-induced death.

 

Dulbecco’s modified eagle medium (DMEM) (GIBCO Invitrogen, Carlsbad, CA)

10% (v/v) heat-inactivated fetal bovine serum (FBS) (Atlanta Biologicals, Lawrenceville, GA)

100 IU/ml of penicillin and 100 μg/ml of streptomycin (GIBCO Invitrogen, Carlsbad, CA)

 

Hank’s Balanced Salt Solution (HBSS) (HyClone, Logan, UT)

2% (v/v) FBS

100 IU/ml of penicillin and 100 μg/ml of streptomycin

 

HBSS

5% (v/v) FBS

200 IU/ml of penicillin and 200 μg/ml of streptomycin

 

Modified Eagle Medium (MEM) (GIBCO Invitrogen, Carlsbad, CA)

0.6% (w/v) tragacanth gum (MP Biomedicals, Solon, OH)

2.5% (v/v) FBS

5% (w/v) tryptose phosphate broth (TBP) (BD Biosciences, San Jose, CA)

 

Phosphate-buffered saline (PBS)

10% (w/w) formaldehyde

 

distilled H₂O

0.25% (w/v) crystal violet

2.5% (v/v) ethanol in distilled water

1. Infect VeroE6 cells with a seed stock of arMP-12.

2. Collect culture supernatant at 2 to 3 dpi when most cells show severe cytopathic effect (CPE).

3. Centrifuge supernatant at 1500g for 5 min to remove cell debris.

4. Store clear supernatant at -80C° until use for titration and infection.

 

5. Prepare confluent Vero E6 cells in 24- or 6-well plates. For each virus dilution, plan to infect triplicate wells.

6. Serially dilute (from 1:10 to 1:10⁷) RVFV stock in Vero E6 medium.

7. Infect Vero E6 cells in triplicate wells with each virus dilution. (Use 100 μl for each well of a 24-well plate; 400 μl for each well of a 6-well plate).  Incubate at 37^oC/5%CO₂ in a humidified incubator for 1 hour.

8. Wash infected cells with PBS three times.

9. Add plaque assay medium to each well and incubate at 37^oC/5%CO₂ in a humidified incubator for 4 days.

10. At 4 dpi, remove plaque assay medium and fix cells with cell fixation solution for at least 10 minutes at room temperature (RT).

11. Remove cell fixation solution and stain cells with cell staining solution for 5 to 10 min.

12. Remove cell staining solution and wash cells with water.

13. Count plaques and calculate titers according to dilution factors.

 

14. Prepare the virus inoculum (5x10⁵ pfu/ml) by diluting the titrated virus stock with virus stock diluent.

15. Challenge each G3 mouse intraperitoneally with 200 μl of the inoculum (1x10⁵ pfu/mouse). Save leftover inoculum for re-titration.

16. Check mice daily for sickness and death up to 2 to 3 weeks.

17. Once affected mice are identified, collect blood, liver, spleen and brain from the mice.

18. Make 10% (w/v) tissue homogenates in tissue diluent and determine virus titers as described in RVFV titration (steps 5-13).

19. Check for RVFV susceptibility in siblings of affected mice.

20. Fix and map mutations using general genetic mapping protocol.

   1. Schmaljohn, C. S., and Nichol, S. T. (2007) Bunyaviruses, in Fields Virology (B. N. Fields, D. M. Knipe, P. M. Howley, and D. E. Griffin, Eds.) pp 1741-1789, Lippincott Williams and Wilkins, Philadephia, PA.

   2. Balkhy, H. H., and Memish, Z. A. (2003) Rift Valley Fever: An Uninvited Zoonosis in the Arabian Peninsula. Int. J. Antimicrob. Agents. 21, 153-157.

   3. Morrill, J. C., Czarniecki, C. W., and Peters, C. J. (1991) Recombinant Human Interferon-Gamma Modulates Rift Valley Fever Virus Infection in the Rhesus Monkey. J. Interferon Res. 11, 297-304.

   4. Morrill, J. C., Jennings, G. B., Cosgriff, T. M., Gibbs, P. H., and Peters, C. J. (1989) Prevention of Rift Valley Fever in Rhesus Monkeys with Interferon-Alpha. Rev. Infect. Dis. 11 Suppl 4, S815-S825.

   5. Pittman, P. R., Liu, C. T., Cannon, T. L., Makuch, R. S., Mangiafico, J. A., Gibbs, P. H., and Peters, C. J. (1999) Immunogenicity of an Inactivated Rift Valley Fever Vaccine in Humans: A 12-Year Experience. Vaccine. 18, 181-189.

   6. Peters, C. J., Reynolds, J. A., Slone, T. W., Jones, D. E., and Stephen, E. L. (1986) Prophylaxis of Rift Valley Fever with Antiviral Drugs, Immune Serum, an Interferon Inducer, and a Macrophage Activator. Antiviral Res. 6, 285-297.

   7. Bird, B. H., Albarino, C. G., and Nichol, S. T. (2007) Rift Valley Fever Virus Lacking NSm Proteins Retains High Virulence in Vivo and may Provide a Model of Human Delayed Onset Neurologic Disease. Virology. 362, 10-15.