OmpA-mediated rickettsial adherence to and invasion of human endothelial cells is dependent upon interaction with α2ß1 integrin.

Rickettsia conorii, a member of the spotted fever group (SFG) of the genus Rickettsia and causative agent of Mediterranean spotted fever, is an obligate intracellular pathogen capable of infecting various mammalian cell types. SFG rickettsiae express two major immunodominant surface cell antigen (Sca) proteins, OmpB (Sca5) and OmpA (Sca0). While OmpB-mediated entry has been characterized, the contribution of OmpA has not been well defined. Here we show OmpA expression in Escherichia coli is sufficient to mediate adherence to and invasion of non-phagocytic human endothelial cells. A recombinant soluble C-terminal OmpA protein domain (954-1735) with predicted structural homology to the Bordetella pertussis pertactin protein binds mammalian cells and perturbs R. conorii invasion by interacting with several mammalian proteins including ß1 integrin. Using functional blocking antibodies, small interfering RNA transfection, and mouse embryonic fibroblast cell lines, we illustrate the contribution of α2ß1 integrin as a mammalian ligand involved in R. conorii invasion of primary endothelial cells. We further demonstrate that OmpA-mediated attachment to mammalian cells is in part dependent on a conserved non-continuous RGD motif present in a predicted C-terminal 'pertactin' domain in OmpA.Our results demonstrate that multiple adhesin-receptor pairs are sufficient in mediating efficient bacterial invasion of R. conorii.

Contribution of NK cells to the innate phase of host protection against an intracellular bacterium targeting systemic endothelium.

We investigated the mechanisms by which natural killer (NK) cells mediate innate host defense against infection with an endothelium-targeting intracellular bacterium, Rickettsia. We found that a robust Rickettsia-induced innate response in resistant mice cleared the bacteria early in the infection and was associated with significantly higher frequencies of splenic interferon (IFN)-γ (+) CD8(+) T cells and cytotoxic NK cells compared with susceptible mice. More importantly, NK cell-deficient Rag(-/-)γc(-/-) animals displayed significantly increased susceptibility to Rickettsia infection compared with NK cell-sufficient Rag(-/-) mice, as evidenced by impaired bacterial clearance, early development of severe thrombosis in the liver, and a decreased serum level of IFN-γ. Furthermore, the lack of NK cells also impaired host resistance of CB-17 scid mice to Rickettsia, similar to what was observed in Rag(-/-)γc(-/-) mice. Interestingly, perforin deficiency in Rag(-/-)Prf1(-/-) mice resulted in greater thrombosis and insignificantly different systemic levels of IFN-γ compared with Rag(-/-) mice, suggesting that perforin, which is mainly produced by NK cells, is involved in the prevention of vascular damage. Together, these findings reveal that NK cells mediate the innate phase of host protection against infection with rickettsiae, most likely via IFN-γ production. Furthermore, NK cells are involved in preventing rickettsial infection-induced endothelial cell damage, possibly via perforin production.

IL-1 Superfamily Member (IL-1A, IL-1B and IL-18) Genetic Variants Influence Susceptibility and Clinical Course of Mediterranean Spotter Fever.

Mediterranean Spotted Fever (MSF) is one of the most common spotted fever Rickettsioses. Most cases of MSF follow a benign course, with a minority of cases being fatal. The severity of the infection depends on bacterial virulence, dose and host factors such as effective immune response and genetic background. Herein, we reported data on typing by competitive allele-specific PCR of functionally relevant polymorphisms of genes coding for MyD88 adapter-like (Mal/TIRAP) protein (rs8177374), interleukin(IL)-1 cluster (IL-1A rs1800587, IL-1B rs16944 and rs1143634) and IL-18 (rs187238), which might be crucial for an efficient immune response. The results enlighten the role that IL-1 gene cluster variants might play in susceptibility against Rickettsia conorii infection. In particular, the IL-1A rs1800587TT genotype was significantly increased in patients alone and combined in a haplotype composed by minor alleles rs1800587T, rs16944A and rs1143634A. This result was confirmed using the decision tree heuristic approach. Using this methodology, IL-1A rs1800587TT genotype was the better discrimination key among MSF patients and controls. In addition, the IL-1 gene cluster SNP genotypes containing minor alleles and IL-18 rs187238G positive genotypes were found as associated with risk of severe complications such as sepsis, septic shock, acute respiratory distress syndrome and coma. In conclusion, these data suggest that the evaluation of IL-1A, IL-1B and IL-18 gene SNPs can add useful information on the clinical course of patients affected by Mediterranean Spotted Fever, even if further confirmatory studies will be necessary.

Global Transcriptomic Profiling of Pulmonary Gene Expression in an Experimental Murine Model of Rickettsia conorii Infection.

Mediterranean spotted fever develops from an infection with Rickettsia conorii, an obligate intracellular, Gram-negative, endotheliotropic, and tick-transmitted bacterial pathogen, and is an acute, febrile illness that can progress to life-threatening complications if not diagnosed and treated early with effective antibiotics. Despite significant morbidity and mortality, little is known about changes in gene expression that determine the host responses during in vivo infection. We have investigated the transcriptional landscape of host lungs as a prominently affected organ system in an established murine model of infection by RNA-sequencing. Ingenuity pathway analysis resulted in the identification of 1332 differentially expressed genes and 292 upstream regulators. Notably, genes encoding for ubiquitin D, aconitate decarboxylase, antimicrobial peptides, calgranulins, cytokines and chemokines, and guanylate binding proteins were highly up-regulated, whereas those involved in hemoglobin biosynthesis and heme homeostasis were significantly down-regulated. Amongst response regulators, nucleotide-binding oligomerization domain-containing protein 2 and killer cell lectin-like receptors were differentially expressed, and gene clustering revealed eukaryotic initiation factor-2, oxidative phosphorylation, and ubiquitination as the predominantly activated biological pathways. Collectively, this first global transcriptomic profiling has identified R. conorii-induced regulation of novel genes and pathways in the host lungs, further in-depth investigation of which will strengthen our understanding of the pathogenesis of human rickettsioses.

Role of TLR4 receptor polymorphisms in Boutonneuse fever.

The genetics of the interaction between host and microbes plays an essential role in the survival of the individual and attainment of longevity. The activation of toll-like receptor (TLR)4 plays a key role in natural and clonotypic immune responses. We evaluated whether TLR4 genotype is a component of genetic background protective versus rickettsiosis and whether this background influences longevity. We genotyped for +896A/G TLR4 polymorphism 78 patients affected with Boutonneuse fever, 78 age-matched controls and 78 advanced age individuals from Sicily. The +869G allele, that attenuates receptor signalling, was significantly overrepresented in patients in comparison with age-matched controls. By analyzing data according to gender, this allele was significantly higher in female patients when compared to advanced age women. Pro-inflammatory responses are programmed to resist fatal infections. So, it is not surprising that the genetic background of people that survive to an advanced age may be protective against infections. However, this seems to occur in women but not in men. In a previous study, the +896G TLR4 allele was overrepresented in advanced age men and underrepresented in men affected by myocardial infarction. Thus, previous and present results tend to agree with the suggestion that men and women may follow different trajectories to reach longevity. For men it might be more important to control atherogenesis, whereas for women it might be more important to control infectious diseases.

Rickettsia massiliae and Rickettsia conorii Israeli Spotted Fever Strain Differentially Regulate Endothelial Cell Responses.

Rickettsiae primarily target microvascular endothelial cells. However, it remains elusive how endothelial cell responses to rickettsiae play a role in the pathogenesis of rickettsial diseases. In the present study, we employed two rickettsial species with high sequence homology but differing virulence to investigate the pathological endothelial cell responses. Rickettsia massiliae is a newly documented human pathogen that causes a mild spotted fever rickettsiosis. The "Israeli spotted fever" strain of R. conorii (ISF) causes severe disease with a mortality rate up to 30% in hospitalized patients. At 48 hours post infection (HPI), R. conorii (ISF) induced a significant elevation of IL-8 and IL-6 while R. massiliae induced a statistically significant elevated amount of MCP-1 at both transcriptional and protein synthesis levels. Strikingly, R. conorii (ISF), but not R. massiliae, caused a significant level of cell death or injury in HMEC-1 cells at 72 HPI, demonstrated by live-dead cell staining, annexin V staining and lactate dehydrogenase release. Monolayers of endothelial cells infected with R. conorii (ISF) showed a statistically significant decrease in electrical resistance across the monolayer compared to both R. massiliae-infected and uninfected cells at 72 HPI, suggesting increased endothelial permeability. Interestingly, pharmacological inhibitors of caspase-1 significantly reduced the release of lactate dehydrogenase by R. conorii (ISF)-infected HMEC-1 cells, which suggests the role of caspase-1 in mediating the death of endothelial cells. Taken together, our data illustrated that a distinct proinflammatory cytokine profile and endothelial dysfunction, as evidenced by endothelial cell death/injury and increased permeability, are associated with the severity of rickettsial diseases.

A complex interaction between Rickettsia conorii and Dickkopf-1--potential role in immune evasion mechanisms in endothelial cells.

The pathophysiological hallmark of spotted fever group rickettsioses comprises vascular inflammation. Based on the emerging importance of the wingless (Wnt) pathways in inflammation and vascular biology, we hypothesized that Dickkopf-1 (DKK-1), as a major modulator of Wnt signaling, could be involved in the pathogenesis in rickettsial infections. Our major findings were: (i) While baseline concentration of DKK-1 in patients with R. conorii infection (n = 32) were not different from levels in controls (n = 24), DKK-1 rose significantly from presentation to first follow-up sample (median 7 days after baseline). (ii) In vitro experiments in human umbilical vein endothelial cells (HUVECs) showed that while heat-inactivated R. conorii enhanced the release of interleukin-6 (IL-6) and IL-8, it down-regulated the release of endothelial-derived DKK-1 in a time- and dose-dependent manner. (iii) Silencing of DKK-1 attenuated the release of IL-6, IL-8 and growth-related oncogene (GRO)α in R. conorii-exposed HUVECs, suggesting inflammatory effects of DKK-1. (iv) Silencing of DKK-1 attenuated the expression of tissue factor and enhanced the expression of thrombomodulin in R. conorii-exposed HUVECs suggesting pro-thrombotic effects of DKK-1. The capacity of R. conorii to down-regulate endothelial-derived DKK-1 and the ability of silencing DKK-1 to attenuate R. conorii-induced inflammation in endothelial cells could potentially reflect a novel mechanism by which R. conorii escapes the immune response at the site of infection.

Relevance of gamma interferon, tumor necrosis factor alpha, and interleukin-10 gene polymorphisms to susceptibility to Mediterranean spotted fever.

The acute phase of Mediterranean spotted fever (MSF) is characterized by dramatic changes in cytokine production patterns, clearly indicating their role in the immunomodulation of the response against the microorganism, and the differences in cytokine production seem to influence the extent and severity of the disease. In this study, the single nucleotide polymorphisms (SNPs) of tumor necrosis factor alpha (TNF-alpha) -308G/A (rs1800629) and interleukin-10 (IL-10) -1087G/A (rs1800896), -824C/T (rs1800871), and -597C/A (rs1800872) and the gamma interferon (IFN-gamma) T/A SNP at position +874 (rs2430561) were typed in 80 Sicilian patients affected by MSF and in 288 control subjects matched for age, gender, and geographic origin. No significant differences in TNF-alpha -308G/A genotype frequencies were observed. The +874TT genotype, associated with an increased production of IFN-gamma, was found to be significantly less frequent in MSF patients than in the control group (odds ratio [OR], 0.18; 95% confidence interval [95% CI], 0.06 to 0.51; P corrected for the number of genotypes [Pc], 0.0021). In addition, when evaluating the IFN-gamma and IL-10 genotype interaction, a significant increase of +874AA/-597CA (OR, 5.31; 95% CI, 2.37 to 11.88; P(c), 0.0027) combined genotypes was observed. In conclusion, our data strongly suggest that finely genetically tuned cytokine production may play a crucial role in the regulation of the immune response against rickettsial infection, therefore influencing the disease outcomes, ranging from nonapparent or subclinical condition to overt or fatal disease.

Intralesional expression of mRNA of interferon- gamma , tumor necrosis factor- alpha , interleukin-10, nitric oxide synthase, indoleamine-2,3-dioxygenase, and RANTES is a major immune effector in Mediterranean spotted fever rickettsiosis.

BACKGROUND: The mechanisms of immunity to Rickettsia conorii that have been elucidated in mouse models have not been evaluated in human tissues. METHODS: In this study, quantitative real-time polymerase chain reaction was used to determine the levels of expression of inflammatory and immune mediators in skin-biopsy samples collected from 23 untreated patients with Mediterranean spotted fever (MSF). RESULTS: In all 23 patients, the levels of intralesional expression of mRNA of tumor necrosis factor (TNF)- alpha , interferon (IFN)- gamma , interleukin (IL)-10, RANTES, and indoleamine-2,3-dioxygenase (IDO), an enzyme involved in limiting rickettsial growth by tryptophan degradation, were higher than those in control subjects; 6 of the 23 patients had high levels of inducible nitric oxide synthase (iNOS), a source of microbicidal nitric oxide. Positive correlations between TNF- alpha , IFN- gamma , iNOS, IDO, and mild/moderate MSF suggest that type 1 polarization plays a protective role. Significantly higher levels of intralesional expression of IL-10 mRNA were inversely correlated with levels of intralesional expression of IFN- gamma mRNA and TNF- alpha mRNA. The mRNA-expression level of the chemokine RANTES was significantly higher in patients with severe MSF. CONCLUSION: Mild/moderate MSF is associated with a strong and balanced intralesional proinflammatory and anti-inflammatory response, with a dominant type 1 immunity, whereas severe MSF is associated with increased expression of chemokine mRNA. Whether these factors are simply correlates of mild and severe MSF or contribute to antirickettsial immunity and pathogenesis remains to be determined.

Plasmids and rickettsial evolution: insight from Rickettsia felis.

BACKGROUND: The genome sequence of Rickettsia felis revealed a number of rickettsial genetic anomalies that likely contribute not only to a large genome size relative to other rickettsiae, but also to phenotypic oddities that have confounded the categorization of R. felis as either typhus group (TG) or spotted fever group (SFG) rickettsiae. Most intriguing was the first report from rickettsiae of a conjugative plasmid (pRF) that contains 68 putative open reading frames, several of which are predicted to encode proteins with high similarity to conjugative machinery in other plasmid-containing bacteria. METHODOLOGY/PRINCIPAL FINDINGS: Using phylogeny estimation, we determined the mode of inheritance of pRF genes relative to conserved rickettsial chromosomal genes. Phylogenies of chromosomal genes were in agreement with other published rickettsial trees. However, phylogenies including pRF genes yielded different topologies and suggest a close relationship between pRF and ancestral group (AG) rickettsiae, including the recently completed genome of R. bellii str. RML369-C. This relatedness is further supported by the distribution of pRF genes across other rickettsiae, as 10 pRF genes (or inactive derivatives) also occur in AG (but not SFG) rickettsiae, with five of these genes characteristic of typical plasmids. Detailed characterization of pRF genes resulted in two novel findings: the identification of oriV and replication termination regions, and the likelihood that a second proposed plasmid, pRFdelta, is an artifact of the original genome assembly. CONCLUSION/SIGNIFICANCE: Altogether, we propose a new rickettsial classification scheme with the addition of a fourth lineage, transitional group (TRG) rickettsiae, that is unique from TG and SFG rickettsiae and harbors genes from possible exchanges with AG rickettsiae via conjugation. We offer insight into the evolution of a plastic plasmid system in rickettsiae, including the role plasmids may have played in the acquirement of virulence traits in pathogenic strains, and the likely origin of plasmids within the rickettsial tree.

Rickettsiae of the spotted-fever group in ixodid ticks from Hungary: identification of a new genotype ('Candidatus Rickettsia kotlanii').

Three common European 'anthrophilic' ticks, Ixodes ricinus, Haemaphysalis concinna and Dermacentor reticulatus, were collected in Hungary and tested, in assays based on nested PCR, for rickettsiae of the spotted-fever group. Low percentages of I. ricinus (2.7%) and H. concinna (1.0%) and a high percentage of D. reticulatus (26.8%) were found to be infected. The rickettsiae in the ticks were then identified, by sequencing of the genes coding for 16S ribosomal RNA (16S rDNA), citrate synthase (gltA) and the rOmpA outer-membrane protein (ompA), as Rickettsia helvetica, Rickettsia monacensis, Rickettsia sp. RpA4, or what is probably a newly recognized Rickettsia species ('Candidatus Rickettsia kotlanii'). These results raise the possibility that rickettsiae other than Rickettsia slovaca are involved in human disease in Hungary. Current knowledge on the distributions of the rickettsiae of the spotted-fever group that are emerging in Europe is also summarized.

Familial cases of boutonneuse fever.

Pairs of cases of Boutonneuse Fever (BF) occurred in three families. The illness appeared almost simultaneously in both members of each family, but was generally more serious in one as judged by clinical and laboratory parameters. The possibility of a "bed rickettsiosis", that is reactivation of rickettsiae by the blood meal obtained from the first individual by the same tick which fed upon the second individual, could be excluded in two of the three pairs of cases. In only one of the case pairs were the individuals sharing the same bed. The differences in severity of symptoms may be related to the different immunological pattern observed in these patients. Previous rickettsial infection may have provided partial immune protection, as is repeatedly reported in the literature. In one couple, the more seriously ill patient had antibodies of the IgM class, suggesting that this was his first exposure to Rickettsia conorii. The less severely ill patient had antibodies of the IgG class only, presumably as the result of re-exposure after previous asymptomatic infection with spotted-fever-group rickettsia.