Ilheus and Saint Louis encephalitis viruses elicit cross-protection against a lethal Rocio virus challenge in mice

Abstract

Rocio virus (ROCV) was the causative agent of an unprecedented outbreak of encephalitis during the 1970s in the Vale do Ribeira, Sao Paulo State, in the Southeast region of Brazil. Surprisingly, no further cases of ROCV infection were identified after this outbreak; however, serological surveys have suggested the circulation of ROCV among humans and animals in different regions of Brazil. Cross-protective immunity among flaviviruses is well documented; consequently, immunity induced by infections with other flaviviruses endemic to Brazil could potentially be responsible for the lack of ROCV infections. Herein, we evaluated the cross-protection mediated by other flaviviruses against ROCV infection using an experimental C57BL/6 mouse model. Cross-protection against ROCV infection was observed when animals had prior exposure to Ilheus virus or Saint Louis encephalitis virus, suggesting that cross-reactive anti-flavivirus antibodies may limit ROCV disease outbreaks.

Fig 1. Susceptibility of Balb/c and C57BL/6 mice to ROCV infection.

Mice were infected with different doses of ROCV (n = 10 mice per group). Survival rate and body weight loss for C57BL/6 (A and B) and for Balb/c (C and D). Comparison of mortality rate between C57BL/6 and Balb/c (Fig 1E). Survival rate and body weight loss were measured for up to 21 days post-infection. Statistically significant differences in B and D: ****p<0.0001 was determined by one-way ANOVA with Dunnett's multiple comparisons test, in E: *p<0.01 by Log-rank (Mantel-Cox) test.

Fig 2. Antibody responses following infection with different flaviviruses.

(A) The level of homotypic antibodies (IgG) of a group of 10 animals was compared with the corresponding level of a negative control group (n = 10 mock-infected animals). (B) The level of neutralizing antibodies in C57BL6 mice infected with ROCV (n = 10) and (C) different flaviviruses circulating in Brazil.

Fig 3. Evaluation of cross-protective immunity against ROCV after prior infections with different flaviviruses.

Immunization and challenge regime (A). Mice (n = 40 per group) were infected twice with the different flaviviruses known to circulate in Brazil, and were then challenged (n = 20 per group) with 2.76x10⁷ PFU of ROCV and survival rates (B and E), clinical scores (C and F) and body weight loss (D and G) were determined up to 21 days post-infection. Statistically significant differences in B and E: *p<0.01, **p<0.001 and ****p<0.0001 was determined by Log-rank (Mantel-Cox) test. In F: ****p<0.0001 was analyzed by a student t-test. In D-G *p<0.01, **p<0.001 and ****p<0.0001 was determined by One-way ANOVA with Dunnett's multiple comparisons test. All statistically significant differences were with group control (Mock).

Fig 4. Evaluation of in vivo cross-protective immunity against ROCV after a single infection with ILHV and SLEV.

Immunization and challenge regime (A). Mice (n = 20 per group) were infected once with ILHV or SLEV or ROCV and were then challenged with 2.76x10⁷ PFU of ROCV. Survival rates (B), clinical scores (C) and body weight loss (D) were then determined up to 21 days post-infection. Statistically significant differences in A: **p<0.001, ***p<0.0001 and ****p<0.0001 was determined by Log-rank (Mantel-Cox) test. In B-C: ****p<0.0001 was determined by One-way ANOVA with Dunnett's multiple comparisons test.