FIG. 2.

Virulence of spike variants. Suckling mice were infected intranasally with the indicated viruses and nursed by dams actively immunized with infectious JHM.IA (A) or passively immunized with a cocktail of two anti-JHM neutralizing antibodies (B) as described in Materials and Methods. All pups were monitored daily for signs consistent with acute encephalitis (hunching, ruffled fur, lethargy). The fraction surviving at each day p.i. is shown. Each virus was used to infect five to eight mice. Symbols: ○, JHM.IA; •, JHM.SD; □, rJHM.IA; ▪, rJHM.SD; ▴, rJIA-S.T61A; ◊, rJIA-S.I154T; ⧫, rJIA-S.S310G; ×, rJIA-S.L539F. Note that only JHM.IA, JHM.SD, rJHM.SD, rJHM.IA, and rJIA-S.S310G are analyzed in panel A.