interferon response [31]. The RVFV M segment non-structural protein (NSm) was implicated in the pathogenesis of RVF by acting as an anti-apoptotic protein [32]. Do Valle et al. [33] utilizing microarray and quantitative PCR showed that a specific strain of mice, BALB/cByJ, was more resistant to RVFV infection when compared to a wild mouse strain, MBT/Pas. The study analyzed the expression of genes involved in the innate immune response by infecting mouse embryonic fibroblasts (MEF) isolated from both strains with RVFV in vitro, with results indicating a more significant type I IFN response in the BALB/cByJ MEFs. The activation of adaptive immunity after RVFV infection at gene expression level in the main target organ for RVFV replication, the liver, as well as in the spleen which plays a very important role in host immunity, has not been investigated. A better understanding of the activation of memory humoral and cellular immune responses in these organs might aid the development of improved RVFV vaccines and identification of genes and their products as useful targets for development of gene therapy and antivirals. The role that anti-NP responses play in the protection of vaccinated individuals against RVFV infection is also not clearly understood. The results from this study suggest that an earlier type I interferon response in recNP/Alhydrogel immunized mice contribute to decreased challenge virus replication, whereas Protective Anti-RVFV NP Responses in Mice upregulation of genes with the ability to result in immunopathol- ogy, combined with uncontrolled challenge virus replication, in non-immunized mice probably contributed to liver damage and morbidity, and downregulation of humoral and cellular immunity in the spleen possibly contributed to immune evasion. Results Immune response elicited by recNP/Alhydrogel immunization The activation of either Th-1 cellular immunity, indicated by IgG2A subclass expression, or Th-2 humoral immunity, indicated by IgG1 subclass expression, was evaluated by testing serial bleeds after recNP/Alhydrogel immunization. Strong total IgG and IgG1 subclass responses were detectable by day 7 after the first immunization, with strong responses still detectable on the day before the booster inoculation (day 12) and RVFV challenge (day 27 after the booster)(IgG: average = 72.2 PP, standard devia- tion = 5.5 PP, OD405 nm = 0.79; IgG1: average = 46.7 PP, standard deviation = 5.3 PP, OD405 nm = 0.47 on day 12 after the initial immunization; IgG: average = 129.9 PP, standard deviation = 4.7 PP, OD405 nm = 1.42; IgG1: average = 131.3 PP, standard devia- tion = 12.0 PP, OD405 nm = 1.31 on day 27 after the booster, Figure 1). The IgG2A subclass response (IgG2A: average = 16.8 PP, standard deviation = 4.5 PP, OD405 nm = 0.16) was weak Figure 1. Anti-RVFV recNP response in recNP/Alhydrogel immunized mice. The total IgG (–N–), IgG1 subclass (—) and IgG2A subclass (–D–) responses are shown, with error bars indicating the standard deviations from the mean PP values of three biological replicates per time point. doi:10.1371/journal.pone.0025027.g001 PLoS ONE | www.plosone.org 2 September 2011 | Volume 6 | Issue 9 | e25027