The Brucella abortus general stress response (GSR) system regulates activity of the alternative
sigma factor, σ(E1), which controls transcription of approximately 100 genes and is required for persistence in a BALB/c mouse
chronic infection model. We evaluated the host response to
infection by a B. abortus strain lacking σ(E1) (ΔrpoE1), and identified pathological and immunological features that distinguish ΔrpoE1-infected mice from wild-type (WT), and that correspond with clearance of ΔrpoE1 from the host. ΔrpoE1
infection was indistinguishable from WT in terms of splenic bacterial burden,
inflammation and histopathology up to 6weeks post-
infection. However, Brucella-specific serum
IgG levels in ΔrpoE1-infected mice were 5 times higher than WT by 4weeks post-
infection, and remained significantly higher throughout the course of a 12-week
infection. Total
IgG and Brucella-specific
IgG levels peaked strongly in ΔrpoE1-infected mice at 6weeks, which correlated with reduced
splenomegaly and bacterial burden relative to WT-infected mice. Given the difference in immune response to
infection with wild-type and ΔrpoE1, we tested whether ΔrpoE1 confers protective immunity to wild-type challenge. Mice immunized with ΔrpoE1 completely resisted WT
infection and had significantly higher serum titers of Brucella-specific
IgG,
IgG2a and IFN-γ after WT challenge relative to age-matched naïve mice. We conclude that immunization of BALB/c mice with the B. abortus GSR pathway mutant, ΔrpoE1, elicits an adaptive immune response that confers significant protective immunity against WT
infection.