The serodiagnosis of Rift Valley fever (RVF) relies on the use of inactivated whole virus based reagents
which present biosafety, financial and operational constraints. There are no vaccines for humans, the
availability of animal vaccines is limited and they have several drawbacks. The aim of this study was to
evaluate a bacterially expressed recombinant RVF virus (RVFV) nucleocapsid protein (recNP) as a safe
immunodiagnostic reagent, and an immunogen in a mouse and host animal model. Several enzyme-linked
immunosorbent assays (ELISAs) were developed in this study, enabling sensitive and specific detection
of antibodies and RVFV antigen in human and animal specimens. The recNP was combined with
different adjuvants and used to immunize mice and sheep subsequently challenged with a virulent wild
type RVFV strain. Depending on the recNP/adjuvant combination, protection against disease in mice
ranged between 17 and 100%, with sterilizing immunity elicited in some experimental groups, compared
to 100% morbidity/mortality and excessive viral replication in adjuvant and PBS control mice.
Immunization with recNP combined with Alhydrogel, an adjuvant that biases immunity towards Th2
humoral immunity, that yielded 100% protection, induced an earlier and stronger type I interferon
response in mice after challenge, compared to repression of the same gene in adjuvant and PBS control
mice. There was massive activation of pro-inflammatory responses and genes with pro-apoptotic effects
in the livers of control mice at the acute phase of infection, accompanied by high viral replication,
possibly contributing to the pathology of the liver. There was also evidence of activation and repression
of several genes involved in activation of B- and T-cell immunity in control mice, some indicating
possible immune evasion by the challenge virus. Immunization of sheep with the same recNP/adjuvant
combinations were, however, not able to decrease replication of challenge virus. The recNP based
ELISAs are an important addition to and improvement of the currently available serodiagnostic tests for
RVF. The mechanism by which recNP immunization protects mice from developing severe disease
during the acute phase of infection is now better understood, but the mechanism for earlier clearance of
the virus needs further investigation
