Charles L. Larson

Dot/Icm Type IVB Secretion System Requirements for Coxiella burnetii Growth in Human Macrophages

ABSTRACT Central to Q fever pathogenesis is replication of the causative agent, Coxiella burnetii, within a phagolysosome-like parasitophorous vacuole (PV) in mononuclear phagocytes. C. burnetii modulates PV biogenesis and other host cell functions, such as apoptotic signaling, presumably via the activity of proteins delivered to the host cytosol by a Dot/Icm type IVB secretion system (T4BSS). In this study, we utilized a C. burnetii strain carrying IcmD inactivated by the Himar1 transposon to investigate the requirements for Dot/Icm function in C. burnetii parasitism of human THP-1 macrophage-like cells. The icmD::Tn mutant failed to secrete characterized T4BSS substrates, a defect that correlated with deficient replication, PV development, and apoptosis protection. Restoration of type IVB secretion and intracellular growth of the icmD::Tn mutant required complementation with icmD, -J, and -B, indicating a polar effect of the transposon insertion on downstream dot/icm genes. Induction of icmDJB expression at 1 day postinfection resulted in C. burnetii replication and PV generation. Collectively, these data prove that T4BSS function is required for productive infection of human macrophages by C. burnetii. However, illustrating the metabolic flexibility of C. burnetti, the icmD::Tn mutant could replicate intracellularly when sequestered in a PV generated by wild-type bacteria, where Dot/Icm function is provided in trans, and within a phenotypically similar PV generated by the protozoan parasite Leishmania amazonensis, where host cells are devoid of Dot/Icm T4BSS effector proteins. IMPORTANCE Coxiella burnetii, the cause of human Q fever, is the only bacterial pathogen known to replicate in a vacuole resembling a phagolysosome. The organism manipulates host macrophages to promote the biogenesis of a vacuolar compartment permissive for growth. By analogy to the well-established cellular microbiology of Legionella pneumophila, the Dot/Icm type IVB secretion system of C. burnetii is implicated as a critical virulence factor in host cell modification that delivers proteins with effector functions directly into the host cell cytosol. Using new genetic tools, we verify that Dot/Icm function is essential for productive infection of human macrophages by C. burnetii. Interestingly, despite the production of homologous secretion systems, L. pneumophila and C. burnetii have strikingly different temporal requirements for Dot/Icm function during their respective infectious cycles. Coxiella burnetii, the cause of human Q fever, is the only bacterial pathogen known to replicate in a vacuole resembling a phagolysosome. The organism manipulates host macrophages to promote the biogenesis of a vacuolar compartment permissive for growth. By analogy to the well-established cellular microbiology of Legionella pneumophila, the Dot/Icm type IVB secretion system of C. burnetii is implicated as a critical virulence factor in host cell modification that delivers proteins with effector functions directly into the host cell cytosol. Using new genetic tools, we verify that Dot/Icm function is essential for productive infection of human macrophages by C. burnetii. Interestingly, despite the production of homologous secretion systems, L. pneumophila and C. burnetii have strikingly different temporal requirements for Dot/Icm function during their respective infectious cycles.

Campylobacter jejuni FlpA binds fibronectin and is required for maximal host cell adherence.

Campylobacter jejuni is one of the most frequent bacterial causes of food-borne gastrointestinal disease in developed countries. Previous work indicates that the binding of C. jejuni to human intestinal cells is crucial for host colonization and disease. Fibronectin (Fn), a major constituent of the extracellular matrix, is a approximately 250-kDa glycoprotein present at regions of cell-to-cell contact in the intestinal epithelium. Fn is composed of three types of repeating units: type I (approximately 45 amino acids), type II (approximately 60 amino acids), and type III (approximately 90 amino acids). The deduced amino acid sequence of C. jejuni flpA (Cj1279c) contains at least three Fn type III domains. Based on the presence of the Fn type III domains, we hypothesized that FlpA contributes to the binding of C. jejuni to human INT 407 epithelial cells and Fn. We assessed the contribution of FlpA in C. jejuni binding to host cells by in vitro adherence assays with a C. jejuni wild-type strain and a C. jejuni flpA mutant and binding of purified FlpA protein to Fn by enzyme-linked immunosorbent assay (ELISA). Adherence assays revealed the binding of the C. jejuni flpA mutant to INT 407 epithelial cells was significantly reduced compared with that for a wild-type strain. In addition, rabbit polyclonal serum generated against FlpA blocked C. jejuni adherence to INT 407 cells in a concentration-dependent manner. Binding of FlpA to Fn was found to be dose dependent and saturable by ELISA, demonstrating the specificity of the interaction. Based on these data, we conclude that FlpA mediates C. jejuni attachment to host epithelial cells via Fn binding.

Two Systems for Targeted Gene Deletion in Coxiella burnetii

ABSTRACT Coxiella burnetii is a ubiquitous zoonotic bacterial pathogen and the cause of human acute Q fever, a disabling influenza-like illness. C. burnetiis former obligate intracellular nature significantly impeded the genetic characterization of putative virulence factors. However, recent host cell-free (axenic) growth of the organism has enabled development of shuttle vector, transposon, and inducible gene expression technologies, with targeted gene inactivation remaining an important challenge. In the present study, we describe two methods for generating targeted gene deletions in C. burnetii that exploit pUC/ColE1 ori-based suicide plasmids encoding sacB for positive selection of mutants. As proof of concept, C. burnetii dotA and dotB, encoding structural components of the type IVB secretion system (T4BSS), were selected for deletion. The first method exploited Cre-lox-mediated recombination. Two suicide plasmids carrying different antibiotic resistance markers and a loxP site were integrated into 5′ and 3′ flanking regions of dotA. Transformation of this strain with a third suicide plasmid encoding Cre recombinase resulted in the deletion of dotA under sucrose counterselection. The second method utilized a loop-in/loop-out strategy to delete dotA and dotB. A single suicide plasmid was first integrated into 5′ or 3′ target gene flanking regions. Resolution of the plasmid cointegrant by a second crossover event under sucrose counterselection resulted in gene deletion that was confirmed by PCR and Southern blot. ΔdotA and ΔdotB mutants failed to secrete T4BSS substrates and to productively infect host cells. The repertoire of C. burnetii genetic tools now allows ready fulfillment of molecular Kochs postulates for suspected virulence genes.

Coxiella burnetii effector protein subverts clathrin- mediated vesicular trafficking for pathogen vacuole biogenesis

Significance The vesicular trafficking pathways required for generation of the phagolysosome-like vacuole occupied by Coxiella burnetii are poorly defined, and no pathogen effectors of vesicular trafficking are known. Here, we reveal an important role for clathrin-mediated vesicular trafficking in Coxiella vacuole formation and identify a type 4B secretion system effector protein [Coxiella vacuolar protein A (CvpA)] that engages this pathway. C. burnetii CvpA traffics through the endocytic recycling compartment, and endocytic sorting motifs within CvpA bind the clathrin adaptor complex AP2. Mutation of cvpA, or depletion of AP2 or clathrin, significantly restricts Coxiella replication. Thus, our results reveal a effector–clathrin interaction that benefits pathogen replication. Successful macrophage colonization by Coxiella burnetii, the cause of human Q fever, requires pathogen-directed biogenesis of a large, growth-permissive parasitophorous vacuole (PV) with phagolysosomal characteristics. The vesicular trafficking pathways co-opted by C. burnetii for PV development are poorly defined; however, it is predicted that effector proteins delivered to the cytosol by a defective in organelle trafficking/intracellular multiplication (Dot/Icm) type 4B secretion system are required for membrane recruitment. Here, we describe involvement of clathrin-mediated vesicular trafficking in PV generation and the engagement of this pathway by the C. burnetii type 4B secretion system substrate Coxiella vacuolar protein A (CvpA). CvpA contains multiple dileucine [DERQ]XXXL[LI] and tyrosine (YXXΦ)-based endocytic sorting motifs like those recognized by the clathrin adaptor protein (AP) complexes AP1, AP2, and AP3. A C. burnetii ΔcvpA mutant exhibited significant defects in replication and PV development, confirming the importance of CvpA in infection. Ectopically expressed mCherry-CvpA localized to tubular and vesicular domains of pericentrosomal recycling endosomes positive for Rab11 and transferrin receptor, and CvpA membrane interactions were lost upon mutation of endocytic sorting motifs. Consistent with CvpA engagement of the endocytic recycling system, ectopic expression reduced uptake of transferrin. In pull-down assays, peptides containing CvpA-sorting motifs and full-length CvpA interacted with AP2 subunits and clathrin heavy chain. Furthermore, depletion of AP2 or clathrin by siRNA treatment significantly inhibited C. burnetii replication. Thus, our results reveal the importance of clathrin-coated vesicle trafficking in C. burnetii infection and define a role for CvpA in subverting these transport mechanisms.

Identification of Campylobacter jejuni Proteins Recognized by Maternal Antibodies of Chickens

ABSTRACT Campylobacter jejuni is one of the leading bacterial causes of food-borne gastroenteritis. Infection with C. jejuni is frequently acquired through the consumption of undercooked poultry or foods cross-contaminated with raw poultry. Given the importance of poultry as a reservoir for Campylobacter organisms, investigators have performed studies to understand the protective role of maternal antibodies in the ecology of Campylobacter colonization of poultry. In a previous study, chicks with maternal antibodies generated against the S3B strain of C. jejuni provided protection against Campylobacter colonization (O. Sahin, N. Luo, S. Huang, and Q. Zhang, Appl. Environ. Microbiol. 69:5372-5379, 2003). We obtained serum samples, collectively referred to as the C. jejuni S3B-SPF sera, from the previous study. These sera were determined to contain maternal antibodies that reacted against C. jejuni whole-cell lysates as judged by enzyme-linked immunosorbent assay. The antigens recognized by the C. jejuni S3B-SPF antibodies were identified by immunoblot analysis, coupled with mass spectrometry, of C. jejuni outer membrane protein extracts. This approach led to the identification of C. jejuni proteins recognized by the maternal antibodies, including the flagellin proteins and CadF adhesin. In vitro assays revealed that the C. jejuni S3B-SPF sera retarded the motility of the C. jejuni S3B homologous strain but did not retard the motility of a heterologous strain of C. jejuni (81-176). This finding provides a possible mechanism explaining why maternal antibodies confer enhanced protection against challenge with a homologous strain compared to a heterologous strain. Collectively, this study provides a list of C. jejuni proteins against which protective antibodies are generated in hens and passed to chicks.

Campylobacter jejuni Strains Compete for Colonization in Broiler Chicks

ABSTRACT Campylobacter jejuni isolates possess multiple adhesive proteins termed adhesins, which promote the organisms attachment to epithelial cells. Based on the proposal that one or more adhesins are shared among C. jejuni isolates, we hypothesized that C. jejuni strains would compete for intestinal and cecal colonization in broiler chicks. To test this hypothesis, we selected two C. jejuni strains with unique SmaI pulsed-field gel electrophoresis macrorestriction profiles and generated one nalidixic acid-resistant strain (the F38011 Nalr strain) and one streptomycin-resistant strain (the 02-833L Strr strain). In vitro binding assays revealed that the C. jejuni F38011 Nalr and 02-833L Strr strains adhered to LMH chicken hepatocellular carcinoma epithelial cells and that neither strain influenced the binding potential of the other strain at low inoculation doses. However, an increase in the dose of the C. jejuni 02-833L Strr strain relative to that of the C. jejuni F38011 Nalr strain competitively inhibited the binding of the C. jejuni F38011 Nalr strain to LMH cells in a dose-dependent fashion. Similarly, the C. jejuni 02-833L Strr strain was found to significantly reduce the efficiency of intestinal and cecal colonization by the C. jejuni F38011 Nalr strain in broiler chickens. Based on the number of bacteria recovered from the ceca, the maximum number of bacteria that can colonize the digestive tracts of chickens may be limited by host constraints. Collectively, these data support the hypothesis that C. jejuni strains compete for colonization in chicks and suggest that it may be possible to design novel intervention strategies for reducing the level at which C. jejuni colonizes the cecum.

Coxiella burnetii Effector Proteins That Localize to the Parasitophorous Vacuole Membrane Promote Intracellular Replication

ABSTRACT The intracellular bacterial pathogen Coxiella burnetii directs biogenesis of a parasitophorous vacuole (PV) that acquires host endolysosomal components. Formation of a PV that supports C. burnetii replication requires a Dot/Icm type 4B secretion system (T4BSS) that delivers bacterial effector proteins into the host cell cytosol. Thus, a subset of T4BSS effectors are presumed to direct PV biogenesis. Recently, the PV-localized effector protein CvpA was found to promote C. burnetii intracellular growth and PV expansion. We predict additional C. burnetii effectors localize to the PV membrane and regulate eukaryotic vesicle trafficking events that promote pathogen growth. To identify these vacuolar effector proteins, a list of predicted C. burnetii T4BSS substrates was compiled using bioinformatic criteria, such as the presence of eukaryote-like coiled-coil domains. Adenylate cyclase translocation assays revealed 13 proteins were secreted in a Dot/Icm-dependent fashion by C. burnetii during infection of human THP-1 macrophages. Four of the Dot/Icm substrates, termed Coxiella vacuolar protein B (CvpB), CvpC, CvpD, and CvpE, labeled the PV membrane and LAMP1-positive vesicles when ectopically expressed as fluorescently tagged fusion proteins. C. burnetii ΔcvpB, ΔcvpC, ΔcvpD, and ΔcvpE mutants exhibited significant defects in intracellular replication and PV formation. Genetic complementation of the ΔcvpD and ΔcvpE mutants rescued intracellular growth and PV generation, whereas the growth of C. burnetii ΔcvpB and ΔcvpC was rescued upon cohabitation with wild-type bacteria in a common PV. Collectively, these data indicate C. burnetii encodes multiple effector proteins that target the PV membrane and benefit pathogen replication in human macrophages.

Right on Q: genetics begin to unravel Coxiella burnetii host cell interactions

Invasion of macrophages and replication within an acidic and degradative phagolysosome-like vacuole are essential for disease pathogenesis by Coxiella burnetii, the bacterial agent of human Q fever. Previous experimental constraints imposed by the obligate intracellular nature of Coxiella limited knowledge of pathogen strategies that promote infection. Fortunately, new genetic tools facilitated by axenic culture now allow allelic exchange and transposon mutagenesis approaches for virulence gene discovery. Phenotypic screens have illuminated the critical importance of Coxiellas type 4B secretion system in host cell subversion and discovered genes encoding translocated effector proteins that manipulate critical infection events. Here, we highlight the cellular microbiology and genetics of Coxiella and how recent technical advances now make Coxiella a model organism to study macrophage parasitism.

Essential Role for the Response Regulator PmrA in Coxiella burnetii Type 4B Secretion and Colonization of Mammalian Host Cells

Successful host cell colonization by the Q fever pathogen, Coxiella burnetii, requires translocation of effector proteins into the host cytosol by a Dot/Icm type 4B secretion system (T4BSS). In Legionella pneumophila, the two-component system (TCS) PmrAB regulates the Dot/Icm T4BSS and several additional physiological processes associated with pathogenesis. Because PmrA consensus regulatory elements are associated with some dot/icm and substrate genes, a similar role for PmrA in regulation of the C. burnetii T4BSS has been proposed. Here, we constructed a C. burnetii pmrA deletion mutant to directly probe PmrA-mediated gene regulation. Compared to wild-type bacteria, C. burnetii ΔpmrA exhibited severe intracellular growth defects that coincided with failed secretion of effector proteins. Luciferase gene reporter assays demonstrated PmrA-dependent expression of 5 of 7 dot/icm operons and 9 of 11 effector-encoding genes with a predicted upstream PmrA regulatory element. Mutational analysis verified consensus sequence nucleotides required for PmrA-directed transcription. RNA sequencing and whole bacterial cell mass spectrometry of wild-type C. burnetii and the ΔpmrA mutant uncovered new components of the PmrA regulon, including several genes lacking PmrA motifs that encoded Dot/Icm substrates. Collectively, our results indicate that the PmrAB TCS is a critical virulence factor that regulates C. burnetii Dot/Icm secretion. The presence of PmrA-responsive genes lacking PmrA regulatory elements also suggests that the PmrAB TCS controls expression of regulatory systems associated with the production of additional C. burnetii proteins involved in host cell parasitism.

High-Content Imaging Reveals Expansion of the Endosomal Compartment during Coxiella burnetii Parasitophorous Vacuole Maturation

Coxiella burnetii is an obligate intracellular pathogen and the causative agent of human Q fever. Replication of the bacterium within a large parasitophorous vacuole (PV) resembling a host phagolysosome is required for pathogenesis. PV biogenesis is a pathogen driven process that requires engagement of several host cell vesicular trafficking pathways to acquire vacuole components. The goal of this study was to determine if infection by C. burnetii modulates endolysosomal flux to potentially benefit PV formation. HeLa cells, infected with C. burnetii or left uninfected, were incubated with fluorescent transferrin (Tf) for 0–30 min, and the amount of Tf internalized by cells quantitated by high-content imaging. At 3 and 5 days, but not 1 day post-infection, the maximal amounts of fluorescent Tf internalized by infected cells were significantly greater than uninfected cells. The rates of Tf uptake and recycling were the same for infected and uninfected cells; however, residual Tf persisted in EEA.1 positive compartments adjacent to large PV after 30 min of recycling in the absence of labeled Tf. On average, C. burnetii-infected cells contained significantly more CD63-positive endosomes than uninfected cells. In contrast, cells containing large vacuoles generated by Chlamydia trachomatis exhibited increased rates of Tf internalization without increased CD63 expression. Our results suggest that C. burnetii infection expands the endosomal system to increase capacity for endocytic material. Furthermore, this study demonstrates the power of high-content imaging for measurement of cellular responses to infection by intracellular pathogens.