Formalin inactivated Junin virus: immunogenicity and protection assays

The aim of this study was to determine if Junin virus inactivated with formalin (FA) was immunogenic and able to elicit a protective response in the guinea pig. The XJ-Clone 3 strain of Junin virus grown in Vero cells was exposed to FA at 0 degrees C. The following inactivated antigens were prepared: A1, 0.1% FA for 50 hr; A2, 0.1% FA for 50 hr followed by concentration with polyethylene glycol (PEG); B1, 0.05% FA for 70 hr; B2, 0.05% FA for 70 hr plus PEG concentration; C, 0.1% FA for 50 hr followed by ultracentrifugation and purification by sucrose gradient. No residual infectivity was detected in any inactivated antigen either after two passages in newborn mice or by coculture with Vero cells. After immunization, high-neutralizing and low-immunofluorescent antibody titers wer obtained in adult mice and guinea pigs, thus showing that antigenicity was preserved. However, in spite of the presence of neutralizing antibodies, guinea pigs gained no protection against challenge with the highly pathogenic XJ strain. These results suggest that antibody amount and/or quality may be inadequate; alternatively, mechanisms other than humoral immunity, such as cellular response, not elicited by inactivated antigens may be essential for protection against Junin virus.