Characterization of autotransporters of the Surface cell antigen (Sca) family in Rickettsia typhi

Abstract

Members of the genus Rickettsia, a group of obligate intracellular α-proteobacteria, include the causative agents of typhus and spotted fevers. Murine typhus is a reemerging infectious disease with endemic foci established worldwide and little is understood about how its agent, R. typhi, infects its mammalian and insect hosts. Due to the genetic intractability of rickettsiae, few factors have been identified as being required for their pathogenesis; however, it is well established that rickettsial outer membrane proteins A and B (OmpA and OmpB) are required for invasion of mammalian host cells. Both proteins are autotransporters (type V secretion system) that belong to the surface cell antigen (Sca) family in rickettsia. R. typhi, the agent of murine typhus, has five scas including OmpB (Sca5). Considering the importance of OmpB during infection of mammalian hosts, characterization of other Sca family members may yield some insight into the pathogenesis of murine typhus. Each Sca protein was expressed during in vitro infection in mouse fibroblasts and in vivo infections in spleens from infected Sprague-Dawley rats (a model reservoir) and Ctenocephalides felis cat fleas (a common vector). All Scas were detected on the bacterial surface by immunogold electron microscopy. Subsequent investigations focused on Sca2, which is predicted to mediate adhesion, invasion, bacterial aggregation and collagen binding. Heterologous expression of Sca2 mediated autoaggregation and biomass accumulation comparable to that of the Escherichia coli autotransporter involved in diffuse adherence-1. Treatment of rickettsiae with anti-Sca2 serum decreased rickettsial burden in primary human umbilical vein endothelial cells and invasion of mouse fibroblasts. Additionally, E. coli expressing Sca2, which contains a von Willebrand factor type A collagen binding domain, bound collagen types I, III, IV and VI. This finding has implications for subversion of the inflammatory response-associated vasculitis observed in rickettsial infections. Sca proteins may be crucial to the invasion process and/or dampening inflammation associated with rickettsial infection of endothelial cells. The work presented here is evidence that rickettsial surface proteins mediate rickettsial interactions lending credence towards efforts to characterize the remaining Scas of R. typhi.