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1.  Neutrophils Mediate Immunopathology and Negatively Regulate Protective Immune Responses during Fatal Bacterial Infection-Induced Toxic Shock 
Infection and Immunity  2013;81(5):1751-1763.
Ehrlichia chaffeensis is an obligate intracellular bacterium that infects primarily monocytes and macrophages and causes potentially fatal human monocytic ehrlichiosis (HME) that mimics toxic-shock-like syndrome in immunocompetent hosts. Early recruitment of neutrophils to the sites of infection is critical for the control of bacterial infection and inflammatory responses. We recently observed rapid and sustained neutrophil recruitment at a primary site of infection (peritoneum) following lethal murine ehrlichial infection compared to innocuous ehrlichial infection. We examined here the contribution of neutrophils to protective immunity or immunopathology during infection with monocytic Ehrlichia. Unexpectedly, depletion of neutrophils from lethally infected mice enhanced bacterial elimination, decreased immune-mediated pathology, and prolonged survival. Furthermore, compared to lethally infected sham controls, neutrophil depletion in infected mice resulted in amelioration of pathogenic responses, as evidenced by a decreased number of tumor necrosis factor alpha (TNF-α)-producing CD8+ T cells, which is known to mediate immunopathology and toxic shock in a murine model of fatal ehrlichiosis. Although neutrophil depletion did not influence the number of CD4+ Th1 cells and NKT cells producing gamma interferon (IFN-γ), it increased the ratio of IFN-γ- to IL-10-producing NKT cells as well as the ratio of IFN-γ to interleukin 10 (IL-10) transcripts in the liver. This may ameliorate the net suppressive effect of IL-10 on IFN-γ-mediated activation of infected macrophages and thus may account for the enhanced bacterial elimination. Finally, transcriptional analysis of gene expression in the liver indicated that neutrophils contribute to overproduction of cytokines and chemokines during fatal ehrlichiosis. In conclusion, these results revealed an unexpected role of neutrophils in supporting bacterial replication indirectly and promoting immunopathology during severe infection with an intracellular bacterium.
PMCID: PMC3647993  PMID: 23478316
2.  The Effects of PspC on Complement-Mediated Immunity to Streptococcus pneumoniae Vary with Strain Background and Capsular Serotype▿  
Infection and Immunity  2009;78(1):283-292.
Streptococcus pneumoniae may evade complement activity by binding of factor H (FH), a negative regulator of the alternative pathway, to the surface protein PspC. However, existing data on the effects of FH binding to PspC on complement activity are conflicting, and there is also considerable allelic variation in PspC structure between S. pneumoniae strains that may influence PspC-dependent effects on complement. We have investigated interactions with complement for several S. pneumoniae strains in which the gene encoding PspC has been deleted. The degree of FH binding varied between strains and was entirely dependent on PspC for seven strains. Data obtained with TIGR4 strains expressing different capsular serotypes suggest that FH binding is affected by capsular serotype. Results of immunoblot analysis for C3 degradation products and iC3b deposition assays suggested that FH bound to PspC retained functional activity, but loss of PspC had strikingly varied effects on C3b/iC3b deposition on S. pneumoniae, with large increases on serotype 4, 6A, 6B, and 9V strains but only small increases or even decreases on serotype 2, 3, 17, and 23F strains. Repeating C3b/iC3b assays with TIGR4 strains expressing different capsular serotypes suggested that differences in the effect of PspC on C3b/iC3b deposition were largely independent of capsular serotype and depend on strain background. However, data obtained from infection in complement-deficient mice demonstrated that differences between strains in the effects of PspC on complement surprisingly did not influence the development of septicemia.
PMCID: PMC2798213  PMID: 19884335
3.  Persistent Infection Contributes to Heterologous Protective Immunity against Fatal Ehrlichiosis▿  
Infection and Immunity  2009;77(12):5682-5689.
Human monocytotropic ehrlichiosis (HME), an emerging and often life-threatening tick-transmitted disease, is caused by the obligately intracellular bacterium Ehrlichia chaffeensis. HME is modeled in C57BL/6 mice using Ehrlichia muris, which causes persistent infection, and Ixodes ovatus Ehrlichia (IOE), which is either acutely lethal or sublethal depending on the dose and route of inoculation. A persistent primary E. muris infection, but not a sublethal IOE infection, protects mice against an ordinarily lethal secondary IOE challenge. In the present study, we determined the role of persistent infection in maintenance of protective memory immune responses. E. muris-infected mice were treated with doxycycline or left untreated and then challenged with an ordinarily lethal dose of IOE. Compared to E. muris-primed mice treated with doxycycline, untreated mice persistently infected with E. muris had significantly greater numbers of antigen-specific gamma interferon-producing splenic memory T cells, significant expansion of CD4+ CD25+ T regulatory cells, and production of transforming growth factor β1 in the spleen. Importantly, E. muris-primed mice treated with doxycycline showed significantly greater susceptibility to challenge infection with IOE compared to untreated mice persistently infected with E. muris. The study indicated that persistent ehrlichial infection contributes to heterologous protection by stimulating the maintenance of memory T-cell responses.
PMCID: PMC2786466  PMID: 19805532
4.  CD4+ CD25+ Foxp3− T-Regulatory Cells Produce both Gamma Interferon and Interleukin-10 during Acute Severe Murine Spotted Fever Rickettsiosis▿ †  
Infection and Immunity  2009;77(9):3838-3849.
Spotted fever group rickettsiae cause life-threatening human infections worldwide. Until now, the immune regulatory mechanisms involved in fatal rickettsial infection have been unknown. C3H/HeN mice infected with 3 × 105 PFU of Rickettsia conorii developed an acute progressive disease, and all mice succumbed to this infection. A sublethal infection induced protective immunity, and mice survived. Compared to splenic T cells from sublethally infected mice, splenic T cells from lethally infected mice produced significantly lower levels of interleukin-2 (IL-2) and gamma interferon (IFN-γ) and a higher level of IL-10, but not of IL-4 or transforming growth factor β, and there was markedly suppressed CD4+ T-cell proliferation in response to antigen-specific stimulation with R. conorii. Furthermore, lethal infection induced significant expansion of CD4+ CD25+ Foxp3− T cells in infected organs compared to the levels in naïve and sublethally infected mice. In a lethal infection, splenic CD4+ CD25+ Foxp3− T cells, which were CTLA-4high T-bet+ and secreted both IFN-γ and IL-10, suppressed the proliferation of and IL-2 production by splenic CD4+ CD25− Foxp3− T cells in vitro. Interestingly, depletion of CD25+ T cells in vivo did not change the disease progression, but it increased the bacterial load in the lung and liver, significantly reduced the number of IFN-γ-producing Th1 cells in the spleen, and increased the serum levels of IFN-γ. These results suggested that CD4+ CD25+ T cells generated in acute murine spotted fever rickettsiosis are Th1-cell-related adaptive T-regulatory cells, which substantially contribute to suppressing the systemic immune response, possibly by a mechanism involving IL-10 and/or cytotoxic T-lymphocyte antigen 4.
PMCID: PMC2738046  PMID: 19564386
5.  Protective Heterologous Immunity against Fatal Ehrlichiosis and Lack of Protection following Homologous Challenge▿  
Infection and Immunity  2008;76(5):1920-1930.
The roles of antibodies and memory T cells in protection against virulent Ehrlichia have not been completely investigated. In this study, we addressed these issues by using murine models of mild and fatal ehrlichiosis caused by related monocytotropic Ehrlichia strains. Mice were primed with either Ehrlichia muris or closely related virulent ehrlichiae transmitted by Ixodes ovatus (IOE) ticks given intraperitoneally or intradermally. All groups were reinfected intraperitoneally, 30 days later, with a lethal high dose of IOE. Priming with E. muris, but not IOE, induced strong CD4+ and CD8+ memory type 1 T-cell responses, Ehrlichia-specific immunoglobulin G (IgG) antibodies, and persistent infection. Compared to IOE-primed mice, subsequent lethal IOE challenge of E. muris-primed mice, resulted in (i) 100% protection against lethal infection, (ii) strong Ehrlichia-specific secondary gamma interferon (IFN-γ)-producing effector/effector memory CD4+ and CD8+ T-cell responses, (iii) enhanced secondary anti-ehrlichial antibody response, (iv) accelerated bacterial clearance, and (v) the formation of granulomas in the liver and lung. E. muris-primed mice challenged with IOE had lower levels of serum interleukin-1α (IL-1α), IL-6, and IL-10 compared to unprimed mice challenged with IOE. Interestingly, the fatal secondary response in IOE-primed mice correlated with (i) decline in the Ehrlichia-specific CD4+ and CD8+ type 1 responses, (ii) marked hepatic apoptosis and necrosis, and (iii) substantial bacterial clearance, suggesting that fatal secondary response is due to immune-mediated tissue damage. In conclusion, protection against fatal ehrlichial infection correlates with strong expansion of IFN-γ-producing CD4+ and CD8+ effector memory type 1 T cells, which appear to be maintained in the presence of IgG antibodies and persistent infection.
PMCID: PMC2346691  PMID: 18285501
6.  Regulatory Roles of CD1d-Restricted NKT Cells in the Induction of Toxic Shock-Like Syndrome in an Animal Model of Fatal Ehrlichiosis▿  
Infection and Immunity  2008;76(4):1434-1444.
CD1d-restricted NKT cells are key players in host defense against various microbial infections. Using a murine model of fatal ehrlichiosis, we investigated the role of CD1d-restricted NKT cells in induction of toxic shock-like syndrome caused by gram-negative, lipopolysaccharide-lacking, monocytotropic Ehrlichia. Our previous studies showed that intraperitoneal infection of wild-type (WT) mice with virulent Ehrlichia (Ixodes ovatus Ehrlichia [IOE]) results in CD8+ T-cell-mediated fatal toxic shock-like syndrome marked by apoptosis of CD4+ T cells, a weak CD4+ Th1 response, overproduction of tumor necrosis factor alpha and interleukin-10, and severe liver injury. Although CD1d−/− mice succumbed to high-dose IOE infection similar to WT mice, they did not develop signs of toxic shock, as shown by elevated bacterial burdens, low serum levels of tumor necrosis factor, normal serum levels of liver enzymes, and the presence of few apoptotic hepatic cells. An absence of NKT cells restored the percentages and absolute numbers of CD4+ and CD8+ T cells and CD11b+ cells in the spleen compared to WT mice and was also associated with decreased expression of Fas on splenic CD4+ lymphocytes and granzyme B in hepatic CD8+ lymphocytes. Furthermore, our data show that NKT cells promote apoptosis of macrophages and up-regulation of the costimulatory molecule CD40 on antigen-presenting cells, including dendritic cells, B cells, and macrophages, which may contribute to the induction of pathogenic T-cell responses. In conclusion, our data suggest that NKT cells mediate Ehrlichia-induced T-cell-mediated toxic shock-like syndrome, most likely via cognate and noncognate interactions with antigen-presenting cells.
PMCID: PMC2292873  PMID: 18212072
7.  Relative Importance of T-Cell Subsets in Monocytotropic Ehrlichiosis: a Novel Effector Mechanism Involved in Ehrlichia-Induced Immunopathology in Murine Ehrlichiosis▿  
Infection and Immunity  2007;75(9):4608-4620.
Infection with gram-negative monocytotropic Ehrlichia strains results in a fatal toxic shock-like syndrome characterized by a decreased number of Ehrlichia-specific CD4+ Th1 cells, the expansion of tumor necrosis factor alpha (TNF-α)-producing CD8+ T cells, and the systemic overproduction of interleukin-10 (IL-10) and TNF-α. Here, we investigated the role of CD4+ and CD8+ T cells in immunity to Ehrlichia and the pathogenesis of fatal ehrlichiosis caused by infection with low- and high-dose (103 and 105 bacterial genomes/mouse, respectively) ehrlichial inocula. The CD4+ T-cell-deficient mice showed exacerbated susceptibility to a lethal high- or low-dose infection and harbored higher bacterial numbers than did wild-type (WT) mice. Interestingly, the CD8+ T-cell-deficient mice were resistant to a low dose but succumbed to a high dose of Ehrlichia. The absence of CD8+ T cells abrogated TNF-α and IL-10 production, reduced tissue injury and bacterial burden, restored splenic CD4+ T-cell numbers, and increased the frequency of Ehrlichia-specific CD4+ Th1 cells in comparison to infected WT mice. Although fatal disease is perforin independent, our data suggested that perforin played a critical role in controlling bacterial burden and mediating liver injury. Similar to WT mice, mortality of infected perforin-deficient mice was associated with CD4+ T-cell apoptosis and a high serum concentration of IL-10. Depletion of IL-10 restored the number of CD4+ and CD8+ T cells in infected WT mice. Our data demonstrate a novel mechanism of immunopathology in which CD8+ T cells mediate Ehrlichia-induced toxic shock, which is associated with IL-10 overproduction and CD4+ T-cell apoptosis.
PMCID: PMC1951155  PMID: 17562770
8.  Differential Interaction of Dendritic Cells with Rickettsia conorii: Impact on Host Susceptibility to Murine Spotted Fever Rickettsiosis▿  
Infection and Immunity  2007;75(6):3112-3123.
Spotted fever group rickettsioses are emerging and reemerging infectious diseases, some of which are life-threatening. In order to understand how dendritic cells (DCs) contribute to the host resistance or susceptibility to rickettsial diseases, we first characterized the in vitro interaction of rickettsiae with bone marrow-derived DCs (BMDCs) from resistant C57BL/6 (B6) and susceptible C3H/HeN (C3H) mice. In contrast to the exclusively cytosolic localization within endothelial cells, rickettsiae efficiently entered and localized in both phagosomes and cytosol of BMDCs from both mouse strains. Rickettsia conorii-infected BMDCs from resistant mice harbored higher bacterial loads compared to C3H mice. R. conorii infection induced maturation of BMDCs from both mouse strains as judged by upregulated expression of classical major histocompatibility complex (MHC) and costimulatory molecules. Compared to C3H counterparts, B6 BMDCs exhibited higher expression levels of MHC class II and higher interleukin-12 (IL-12) p40 production upon rickettsial infection and were more potent in priming naïve CD4+ T cells to produce gamma interferon. In vitro DC infection and T-cell priming studies suggested a delayed CD4+ T-cell activation and suppressed Th1/Th2 cell development in C3H mice. The suppressive CD4+ T-cell responses seen in C3H mice were associated with a high frequency of Foxp3+ T regulatory cells promoted by syngeneic R. conorii-infected BMDCs in the presence of IL-2. These data suggest that rickettsiae can target DCs to stimulate a protective type 1 response in resistant hosts but suppressive adaptive immunity in susceptible hosts.
PMCID: PMC1932850  PMID: 17403875
9.  An Intradermal Environment Promotes a Protective Type-1 Response against Lethal Systemic Monocytotropic Ehrlichial Infection  
Infection and Immunity  2006;74(8):4856-4864.
Immune responses against monocytotropic ehrlichiosis during infection with a strain of Ehrlichia from Ixodes ovatus (IOE) were evaluated using a model that closely reproduces the pathology and immunity associated with tick-transmitted human monocytotropic ehrlichiosis. C57BL/6 mice were inoculated intradermally or intraperitoneally with high-dose highly virulent IOE or intraperitoneally with mildly virulent Ehrlichia muris. Intradermal (i.d.) infection with IOE established mild, self-limited disease associated with minimal hepatic apoptosis, and all mice survived past 30 days. Intraperitoneal (i.p.) infection with IOE resulted in acute, severe toxic shock-like syndrome and severe multifocal hepatic apoptosis and necrosis, and all mice succumbed to disease. Compared to i.p. infection with IOE, intradermally infected mice had a 100- to 1,000-fold lower bacterial load in the spleen with limited dissemination. Compared to mice infected intraperitoneally with IOE, i.d. infection stimulated a stronger protective type-1 cell-mediated response on day 7 of infection, characterized by increased percentages of both CD4+ and CD8+ splenic T cells, generation of a greater number of IOE-specific, gamma interferon-producing CD4+ Th1 cells, and higher levels of tumor necrosis factor (TNF-α) in the spleen but lower concentrations of serum TNF-α and interleukin-10. These data suggest that under the conditions of natural route of challenge (i.e., i.d. inoculation), the immune response has the capacity to confer complete protection against monocytotropic ehrlichiosis, which is associated with a strong cell-mediated type-1 response and decreased systemic production of pro- and anti-inflammatory cytokines.
PMCID: PMC1539596  PMID: 16861674
10.  Role of Tumor Necrosis Factor Alpha (TNF-α) and Interleukin-10 in the Pathogenesis of Severe Murine Monocytotropic Ehrlichiosis: Increased Resistance of TNF Receptor p55- and p75-Deficient Mice to Fatal Ehrlichial Infection  
Infection and Immunity  2006;74(3):1846-1856.
Intraperitoneal (i.p.) infection with a high dose of a highly virulent Ehrlichia strain (IOE) results in a toxic shock-like syndrome characterized by severe liver injury and systemic overproduction of tumor necrosis factor alpha (TNF-α) by CD8+ T cells. We examined the role of TNF-α and TNF receptors in high-dose-IOE-induced shock/liver injury. TNF receptor (TNFR) I/II−/− mice lacking both the p55 and p75 receptors for this cytokine were more resistant to IOE-induced liver injury than their wild-type background controls. TNFR I/II−/− mice survived longer, dying between 15 and 18 days, with evidence of mild liver necrosis/apoptosis. In contrast, wild-type mice were not rescued from the lethal effect of IOE by TNF-α neutralization. TNF-α-depleted mice developed severe liver injury and succumbed to disease between days 9 and 11 postinfection, similar to sham-treated, infected wild-type mice. Although IFN-γ production in the spleens of IOE-infected TNFR I/II−/− and TNF-α-depleted mice was higher than that detected in wild-type controls, these mice had higher bacterial burdens than infected controls. Following high-dose IOE challenge, TNFR I/II−/− and TNF-α-depleted mice have an early increase in IL-10 levels in sera and spleens, which was produced mainly by adherent spleen cells. In contrast, a late burst of interleukin-10 (IL-10) was observed in control mice. Nonadherent spleen cells were the major source of IL-10 in IOE-infected wild-type mice. We conclude that TNFR I/II and TNF-α participate in Ehrlichia-induced shock and host defense by regulating liver injury and controlling ehrlichial burden. Our data suggest that fatal ehrlichiosis could be a multistep process, where TNF-α is not solely responsible for mortality.
PMCID: PMC1418656  PMID: 16495559
11.  Role for Complement in Development of Helicobacter-Induced Gastritis in Interleukin-10-Deficient Mice 
Infection and Immunity  2003;71(12):7140-7148.
The mechanisms by which the immune response can eradicate gastric Helicobacter infection are unknown. We hypothesized that Helicobacter-induced activation of the complement system could promote both inflammation and eradication of Helicobacter from the stomach. In vitro studies demonstrated that Helicobacter felis activates complement in normal mouse serum but not in serum from Rag2−/− mice, indicating that H. felis activates complement through the classical pathway. Next, we infected complement-depleted wild-type control and interleukin-10-deficient (IL-10−/−) mice with H. felis. Helicobacter infection of wild-type mice elicited a mild, focal gastritis and did not alter serum complement levels. Infection of IL-10−/− mice with H. felis elicited severe gastritis. After the initial colonization, the IL-10−/− mice completely cleared Helicobacter from the stomach by day 8. In contrast to wild-type mice, H. felis-infected IL-10−/− mice had a marked increase in serum complement levels. Complement depletion of wild-type mice did not affect the intensity of gastric inflammation or the extent of Helicobacter colonization compared to that for the wild-type control mice. In contrast, complement depletion of Helicobacter-infected IL-10−/− mice decreased the severity of gastritis, decreased the Helicobacter-induced infiltration of neutrophils into the stomach, and delayed the clearance of bacteria. In vitro studies of stimulated splenocytes and neutrophils from IL-10−/− mice produced a twofold increase in complement production compared to that for wild-type mice. Pretreatment with IL-10 inhibited this increase. These studies identify a role for complement in the local immune response to gastric Helicobacter in IL-10−/− mice and suggest a role for IL-10 in the regulation of complement production.
PMCID: PMC308887  PMID: 14638805
12.  Helicobacter pylori Outer Membrane Vesicles Modulate Proliferation and Interleukin-8 Production by Gastric Epithelial Cells  
Infection and Immunity  2003;71(10):5670-5675.
Helicobacter pylori infection, which is always associated with gastritis, can progress to ulceration or malignancy. The diversity in clinical outcomes is partly attributed to the expression of virulence factors and adhesins by H. pylori. However, H. pylori may not have to adhere to the epithelium to cause gastritis. We hypothesize that outer membrane vesicles (OMV), which are constantly shed from the surface of H. pylori, play a role as independent activators of host cell responses. In this study, we found that low doses of OMV from cag PAI+ toxigenic and cag PAI− nontoxigenic strains increased proliferation of AGS gastric epithelial cells. At higher doses, we detected growth arrest, increased toxicity, and interleukin-8 (IL-8) production. The only strain differences detected were vacuolation with the toxigenic strain and higher levels of IL-8 production with OMV from the cag PAI− nontoxigenic strain. In summary, we suggest that constitutively shed OMV play a role in promoting the low-grade gastritis associated with H. pylori infection.
PMCID: PMC201067  PMID: 14500487
13.  Pathogenic Potential of Environmental Vibrio cholerae Strains Carrying Genetic Variants of the Toxin-Coregulated Pilus Pathogenicity Island  
Infection and Immunity  2003;71(2):1020-1025.
The major virulence factors of toxigenic Vibrio cholerae are cholera toxin (CT), which is encoded by a lysogenic bacteriophage (CTXΦ), and toxin-coregulated pilus (TCP), an essential colonization factor which is also the receptor for CTXΦ. The genes for the biosynthesis of TCP are part of a larger genetic element known as the TCP pathogenicity island. To assess their pathogenic potential, we analyzed environmental strains of V. cholerae carrying genetic variants of the TCP pathogenicity island for colonization of infant mice, susceptibility to CTXΦ, and diarrheagenicity in adult rabbits. Analysis of 14 environmental strains, including 3 strains carrying a new allele of the tcpA gene, 9 strains carrying a new allele of the toxT gene, and 2 strains carrying conventional tcpA and toxT genes, showed that all strains colonized infant mice with various efficiencies in competition with a control El Tor biotype strain of V. cholerae O1. Five of the 14 strains were susceptible to CTXΦ, and these transductants produced CT and caused diarrhea in adult rabbits. These results suggested that the new alleles of the tcpA and toxT genes found in environmental strains of V. cholerae encode biologically active gene products. Detection of functional homologs of the TCP island genes in environmental strains may have implications for understanding the origin and evolution of virulence genes of V. cholerae.
PMCID: PMC145385  PMID: 12540588
15.  Molecular cloning, characterization, and expression in Escherichia coli of iron superoxide dismutase cDNA from Leishmania donovani chagasi. 
Infection and Immunity  1994;62(2):657-664.
A cDNA corresponding to superoxide dismutase (SOD; EC was isolated from a Leishmania donovani chagasi (L. d. chagasi) promastigote cDNA library, using PCR with a set of primers derived from conserved amino acids of manganese SODs (MnSODs) and iron SODs (FeSODs). Comparison of the deduced amino acid sequences with previously reported SOD amino acid sequences revealed that the L. d. chagasi 585-bp open reading frame had considerable homology with FeSODs and MnSODs. The highest homology was shared with prokaryotic FeSODs. The coding region of L. d. chagasi SOD cDNA has been expressed in fusion with glutathione-S-transferase, using an Escherichia coli mutant, QC779, lacking both MnSOD and FeSOD genes (sodA and sodB). Staining of native polyacrylamide gels for SOD activity of Leishmania crude lysate and the recombinant SOD revealed that both had SOD activity that was inactivated by 5 mM hydrogen peroxide but not by 2 mM potassium cyanide, which is indicative of FeSOD. The recombinant enzyme also protected E. coli mutant QC779 from paraquat toxicity. This indicated that the glutathione-S-transferase peptide does not interfere with the in vivo and in vitro activities of the recombinant SOD. Cross-species hybridization showed that FeSOD is highly conserved in the Leishmania genus. Interestingly, the hybridization pattern of the FeSOD gene(s) coincided with other classification schemes that divide Leishmania species into complexes. The cloning of FeSOD cDNA may contribute to the understanding of the role of SODs in Leishmania pathogenesis.
PMCID: PMC186154  PMID: 8300222

Results 1-15 (15)