Persisting Rickettsia typhi Causes Fatal Central Nervous System Inflammation

Abstract

bRickettsioses are emerging febrile diseases caused by obligate intracellular bacteria belonging to the family Rickettsiaceae. Rickettsia typhi belongs to the typhus group (TG) of this family and is the causative agent of endemic typhus, a disease that can be fatal. In the present study, we analyzed the course of R. typhi infection in C57BL/6 RAG1 ؊/؊ mice. Although these mice lack adaptive immunity, they developed only mild and temporary symptoms of disease and survived R. typhi infection for a long period of time. To our surprise, 3 to 4 months after infection, C57BL/6 RAG1 ؊/؊ mice suddenly developed lethal neurological disorders. Analysis of these mice at the time of death revealed high bacterial loads, predominantly in the brain. This was accompanied by a massive expansion of microglia and by neuronal cell death. Furthermore, high numbers of infiltrating CD11b ؉ macrophages were detectable in the brain. In contrast to the microglia, these cells harbored R. typhi and showed an inflammatory phenotype, as indicated by inducible nitric oxide synthase (iNOS) expression, which was not observed in the periphery. Having shown that R. typhi persists in immunocompromised mice, we finally asked whether the bacteria are also able to persist in resistant C57BL/6 and BALB/c wild-type mice. Indeed, R. typhi could be recultivated from lung, spleen, and brain tissues from both strains even up to 1 year after infection. This is the first report demonstrating persistence and reappearance of R. typhi, mainly restricted to the central nervous system in immunocompromised mice.

CD4+ T Cells Are as Protective as CD8+ T Cells against Rickettsia typhi Infection by Activating Macrophage Bactericidal Activity

“…Bacteria were centrifuged at 4300×g for 5 min at room temperature and frozen in FCS with 7.5% DMSO in liquid nitrogen in Cryo.S tubes (Greiner Bio-One, Frickenhausen, Germany). Thawed bacterial stocks were centrifuged at 6200×g for 5 min at room temperature, washed twice with PBS and analyzed for bacterial content by quantitative real-time PCR (qPCR) and immunofocus assay as described previously [48] to determine spot forming units (sfu).…”

“…typhi , bacteria in cell cultures and organs from infected mice was performed by amplification of a 137 bp fragment of the PrsA gene as previously described [48]. …”

“…typhi because we recently showed that R . typhi persists in mice [48]. Moreover, in mice it has been shown that R .…”

“…conorii at least for 20 days [61] which is also true for R . typhi [48], suggesting that innate immune mechanisms can control the bacteria at least for a certain period of time. In C57BL/6 RAG1 -/- mice, however, R .…”

“…typhi , re-appears several months after infection and then grows predominantly in the brain [48]. Animals develop massive CNS inflammation accompanied by neuronal cell loss and succumb to neurological disorders [48]. These findings clearly demonstrate the need of adaptive immunity for the control of persisting R .…”