Summary Ehrlichia chaffeensis is an obligate intracellular bacterium that causes human monocytic ehrlichiosis. Ehrlichiae have a biphasic developmental cycle consisting of dense-cored cells (DCs) and reticulate cells (RCs). Isolated DCs are more stress resistant and infectious than RCs. Here, we report that a response regulator, CtrA was upregulated in human monocytes at the late growth stage when DCs develop. E. chaffeensis CtrA bound to the promoters of late-stage transcribed genes: ctrA, ompA (peptidoglycan-associated lipoprotein), bolA (stress-induced morphogen), and surE (stationary phase survival protein), which contain CtrA-binding motifs, and transactivated ompA, surE, and bolA promoter-lacZ fusions in Escherichia coli. OmpA was predominantly expressed in DCs. E. chaffeensis binding to and subsequent infection of monocytes were inhibited by anti-OmpA IgG. E. chaffeensis BolA bound to the promoters of genes encoding outer surface proteins TRP120 and ECH_1038, which were expressed in DCs, and transactivated trp120 and ECH_1038 promoter-lacZ fusions. E. chaffeensis bolA complemented a stress-sensitive E. coli bolA mutant. E. coli expressing E. chaffeensis surE exhibited increased resistance to osmotic stress. Our results suggest that E. chaffeensis CtrA plays a role in coordinating development of the stress resistance for passage from the present to the next host cells through its regulon.

Ehrlichia chaffeensis, an obligatory intracellular rickettsial pathogen, enters and replicates in monocytes/macrophages and several non-phagocytic cells. E. chaffeensis entry into mammalian cells is essential not only for causing the emerging zoonosis, human monocytic ehrlichiosis, but also for its survival. It remains unclear if E. chaffeensis has evolved a specific surface protein that functions as an ‘invasin’ to mediate its entry. We report a novel entry triggering protein of Ehrlichia, EtpE that functions as an invasin. EtpE is an outer membrane protein and an antibody against EtpE (the C-terminal fragment, EtpE-C) greatly inhibited E. chaffeensis binding, entry and infection of both phagocytes and non-phagocytes. EtpE-C-immunization of mice significantly inhibited E. chaffeensis infection. EtpE-C-coated latex beads, used to investigate whether EtpE-C can mediate cell invasion, entered both phagocytes and non-phagocytes and the entry was blocked by compounds that block E. chaffeensis entry. None of these compounds blocked uptake of non-coated beads by phagocytes. Yeast two-hybrid screening revealed that DNase X, a glycosylphosphatidyl inositol-anchored mammalian cell-surface protein binds EtpE-C. This was confirmed by far-Western blotting, affinity pull-down, co-immunoprecipitation, immunofluorescence labeling, and live-cell image analysis. EtpE-C-coated beads entered bone marrow-derived macrophages (BMDMs) from wild-type mice, whereas they neither bound nor entered BMDMs from DNase X-/- mice. Antibody against DNase X or DNase X knock-down by small interfering RNA impaired E. chaffeensis binding, entry, and infection. E. chaffeensis entry and infection rates of BMDMs from DNase X-/- mice and bacterial load in the peripheral blood in experimentally infected DNase X-/- mice, were significantly lower than those from wild-type mice. Thus this obligatory intracellular pathogen evolved a unique protein EtpE that binds DNase X to enter and infect eukaryotic cells. This study is the first to demonstrate the invasin and its mammalian receptor, and their in vivo relevance in any ehrlichial species.

SET domain genes have been identified in numbers of bacterial genomes based on similarity to SET domains of eukaryotic histone methyltransferases. Herein, a Chlamydophila pneumoniae SET domain gene was clarified to be coincidently expressed with hctA and hctB genes encoding chlamydial histone H1-like proteins, Hc1 and Hc2, respectively. The SET domain protein (cpnSET) is localized in chlamydial cells and interacts with Hc1 and Hc2 through the C-terminal SET domain. As expected from conservation of catalytic sites in cpnSET, it functions as a protein methyltransferase to murine histone H3 and Hc1. However, little is known about protein methylation in the molecular pathogenesis of chlamydial infection. cpnSET may play an important role in chlamydial cell maturation due to modification of chlamydial histone H1-like proteins. INTRODUCTIONChlamydophila pneumoniae is an obligatory intracellular eubacterium that causes acute respiratory diseases and may be involved in chronic inflammatory processes, such as atherosclerosis (Rosenfeld et al., 2000), asthma (Hahn et al., 1991) and Alzheimer's disease (Itzhaki et al., 2004). Persistence of chlamydial infection has been thought to be important for chronic diseases and has been characterized using model animals and activation stimuli such as cytokines and antibiotics (Beatty et al., 1993;Belland et al., 2003;Malinverni et al., 1995;Mehta et al., 1998). However, molecular-level relationships between chronic disease progression and persistent infection are not yet clear.Chlamydiae exhibit a unique life cycle in which they alternate morphologies between elementary bodies (EBs) and reticulate bodies (RBs). EBs are transcriptionally inactive electron-dense particles that are internalized into host cells by inducing phagocytosis. EB differentiation into RBs occurs with the development of phagosomes into inclusions. Transcriptionally active RBs multiply by binary fission with nutrients acquired from the host cell. At the end of the developmental cycle, RBs are converted into EBs and released from host cells for the next infection. Besides the developmental cycle, during persistent infection caused by exposure to interferon gamma (IFN-c) or antibiotics, RBs differentiate into aberrantly large and non-multiplying RBs (Belland et al., 2003). However, little is known about the switching mechanism whereby vegetative RBs convert into infectious EBs or aberrant RBs. Understanding this molecular system should be helpful for the prevention of persistent chlamydial infection.Two eukaryotic histone H1-like proteins of chlamydiae, Hc1 and Hc2, are present mainly in EBs, where those proteins bind DNA and promote genomic DNA condensation (Barry et al., 1992;Hackstadt et al., 1991;Perara et al., 1992;Tao et al., 1991). Recently a small regulatory RNA gene was identified as a suppressor of the lethal phenotype of hctA overexpression in Escherichia coli and it was shown to negatively regulate Hc1 synthesis at an early stage of infection (Grieshaber et al., 2006 HEp-2 cells (ATCC CCL-23) were used ...