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David H Walker

Publications and source records attributed to David H Walker.

18 recordsLinked to original sources
PubMed2003

Cross-protection between distantly related spotted fever group rickettsiae.

Two excellent C3H/HeN mouse models of spotted fever rickettsioses caused by the distantly related organisms, Rickettsia conorii and Rickettsia australis, were utilized to evaluate the possibility of the stimulation of broad cross-protective immunity. Sublethal infection stimulated complete immunity, that is absence of disease, after challenge with a dose of the heterologous Rickettsia that uniformly killed naïve mice. In contrast, heterologous immune sera did not protect mice against a lethal dose (two LD50) of rickettsiae in the mouse toxicity neutralization assay, the standard method for evaluation of rickettsial vaccine potency. These observations suggest that development of a broadly protective vaccine against spotted fever group (SFG) rickettsiae is feasible, and the results indicate that mouse toxicity neutralization is an inappropriate method for evaluation of candidate vaccines.

Animals↗
PubMed2003

Pathogenic mechanisms of diseases caused by Rickettsia.

The specter of bioterrorism employing genetically engineered Rickettsia resistant to all antibiotics should reawaken the world's desire to elucidate the pathogenesis of typhus and spotted fever rickettsioses in a search for mechanisms vulnerable to interdiction. The pathogenetic sequence includes rickettsial entry into the dermis, hematogenous dissemination to vascular endothelial cells (most critically in brain and lungs), increased vascular permeability, edema, and immunity mediated by NK cells, IFN-gamma, TNF-alpha, RANTES, antibodies, and cytotoxic T lymphocytes. Silverman has demonstrated the role of reactive oxygen species (ROS) produced by R. rickettsii-infected endothelial cells in peroxidative damage to cell membranes in vitro, and Heinzen has described actin-based rickettsial intracellular mobility and intercellular spread. At this point the availability of sequences of rickettsial genomes and excellent animal models of rickettsioses have yielded insufficient progress towards the identification of rickettsial virulence factors and knowledge of the importance of injury mediated by ROS, phospholipase A(2), protease(s) or other mechanisms in vivo. Attention to the rickettsiosis-associated procoagulant state led to determination that hemostatic mechanisms largely prevent major hemorrhage without disseminated intravascular coagulation or thrombosis-mediated ischemia. Particularly lacking is knowledge of early events in vivo at the portal of entry in skin (or lung), of the effects of the inoculum medium (arthropod saliva or feces), mediators produced by infected endothelium under conditions of flow and of the contributions in vivo of immune effectors to pathology, of the role of apoptosis in rickettsial infection, and of the endothelial cell alterations that account for increased vascular permeability. The host cell receptor for the Rickettsia ligand and the mechanism of rickettsial escape from the phagosome need to be elucidated.

Chronic Disease↗
PubMed2003

Rickettsiosis of the genus Rickettsia in South America.

In South America, human cases of infection by the genus Rickettsia have been described in several countries in the last twenty years. The role of international organizations, such as the Centers for Disease Control and Prevention in Atlanta, Georgia, USA and the World Health Organization Collaborating Center for Tropical Diseases at the University of Texas Medical Branch at Galveston, Texas, USA, was very important in the last twenty years for the development of surveillance systems and for the increase in notification of rickettsial diseases by the countries of South America. We hope that the next goal will be prevention and control of rickettsial diseases in the countries of South America, as well as maintaining the programs developed during the last twenty years, so that a good health system and improved social conditions will be possible.

Animals↗
PubMed2003

Novel Immunoreactive glycoprotein orthologs of Ehrlichia spp.

We previously identified a strongly immunoreactive 43 kDa protein (p43) of Ehrlichia canis. As an immunodiagnostic antigen, the p43 had a 96% accuracy as compared with IFA and provided species-specific diagnosis of E. canis infections. Further investigation has revealed that the E. canis p43 represents the N-terminal portion of the largest immunoreactive protein described in Ehrlichia spp. with a predicted molecular mass of 153 kDa. Analysis of the recombinant N-terminal region (p43) of the p153 by protein gel electrophoresis demonstrated a larger than predicted molecular mass (approximately 30%), and presence of carbohydrate glycans, indicating that the p153 is a glycoprotein. A BLASTn search was performed on the E. chaffeensis genome sequence (95%), and the gene encoding the p153 ortholog was identified in E. chaffeensis. The E. canis p153 (4,263 bp) and E. chaffeensis p156 (4,389 bp) genes had similar chromosomal locations, downstream of the homologous (approximately 87%) deoxyguanosine-triphosphate triphosphohydrolase genes, and homologous (approximately 90%) intergenic sequences preceding the open reading frames. Nucleic acid sequence homology (52%) observed between the glycoprotein genes supported previous findings with regard to genetic divergence of the p43 gene fragment, and the p153 and p156 proteins had amino acid similarity of 32%. A native E. canis protein with a molecular mass of 200 kDa reacted with antisera produced against the N-terminal region (p43) of the p153, suggesting that the native protein was posttranslationally modified. Similarly, recombinant constructs of E. chaffeensis p156 migrated larger than predicted (approximately 200 kDa), and carbohydrate was detected on the recombinant proteins. The chromosomal location, amino acid homology, and biophysical properties support the conclusion that the p153 and p156 glycoproteins (designated gp200s) are species-specific immunoreactive orthologs.

Amino Acid Sequence↗
PubMed2003

Clinical manifestations, epidemiology, and laboratory diagnosis of human monocytotropic ehrlichiosis in a commercial laboratory setting.

Clinical, epidemiological, and laboratory diagnostic issues of human monocytotropic ehrlichiosis (HME) were investigated in a retrospective case study conducted at a national reference laboratory (Focus Technologies, formerly MRL Reference Laboratory), and at the University of Texas Medical Branch at Galveston, Texas, during 1997 and 1998. Standard questionnaires were sent to physicians for each laboratory-diagnosed patient 2 days to 2 weeks after immunofluorescent antibody assay results were available. Among the 41 cases for which data were obtained, 32 (78%) were definite cases of HME, and 9 (22%) were probable cases of HME. Tick bite or exposure to ticks was recorded in more than 97% of cases. The most prominent clinical findings were fever, abdominal tenderness, and regional lymphadenopathy. There was an association between age and severity of illness. The main laboratory findings included leukopenia, thrombocytopenia, and elevated aspartate aminotransferase and alanine aminotransferase. Clinical and laboratory findings were nonspecific and were not good predictors of the severity of illness. The 90% of patients who received doxycycline treatment underwent rapid clinical improvement with a favorable outcome. The usual duration of effective treatment with doxycycline was 7 to 10 days. This retrospective study is unique because it was based in a commercial reference laboratory setting that receives specimens from different geographic locations. The clinical and laboratory information from 41 patients provides insight into the epidemiological, clinical, and laboratory characteristics of HME.

Adolescent↗
PubMed2003

Kinetics of antibody response to Ehrlichia canis immunoreactive proteins.

Immunoreactive proteins of Ehrlichia canis and Ehrlichia chaffeensis that have been characterized include a family of 28-kDa major outer membrane proteins (p28) and two large antigenically divergent surface glycoprotein orthologs. We previously demonstrated that recombinant E. canis p28 and the 140- and 200-kDa glycoproteins gp140 and gp200, respectively, react strongly with serum antibodies from suspect canine ehrlichiosis cases that were positive for E. canis by immunofluorescent antibody test and in various phases of acute or chronic infection (J. Clin. Microbiol. 39:315-322, 2001). The kinetics of the antibody response to these potentially important vaccine and immunodiagnostic candidates is not known. Acute-phase serum antibody responses to whole-cell E. canis lysates and recombinant p28, gp140, and gp200 were monitored for 6 weeks in dogs experimentally infected with E. canis. Irrespective of the inoculation route, a T-helper 1-type response was elicited to E. canis antigens consisting of immunoglobulin G2 antibodies exclusively in both acute and convalescent phases in most dogs. Analysis of immuoreactive antigens for peak intensity and relative quantity identified major immunoreactive E. canis antigens recognized early in the infection as the 19-, 37-, 75-, and 140-kDa proteins. Later in infection, additional major immunoreactive E. canis proteins were identified, including the 28-, 47-, and 95-kDa proteins and the recently identified 200-kDa glycoprotein. All dogs had developed antibody against the recombinant gp140, gp200, and p28 in the convalescent phase. Immunoreactivity and antibody response kinetics suggest that major immunoreactive proteins identified are immunodominant, but early recognition suggests increased dominance by some antigens.

Animals↗
PubMed2003

Identification of CD8 T-lymphocyte epitopes in OmpB of Rickettsia conorii.

The 1.2-kb DNA fragment of the Rickettsia conorii outer membrane protein B gene (OmpB(451-846)) was subcloned using site-specific PCR primers and expressed as six smaller fragments: OmpB(458-652), OmpB(595-744), OmpB(595-654), OmpB(645-692), OmpB(689-744), and OmpB(739-848). NCTC cells transfected with a mammalian expression vector expressing the fragments OmpB(689-744) and OmpB(739-848) stimulated immune anti-R. conorii CD8 T lymphocytes, suggesting the presence of CD8 T-lymphocyte-stimulating epitopes on these fragments. In order to further characterize the CD8 T-lymphocyte-stimulatory elements, CD8 T-lymphocyte epitopes on OmpB(689-744) and OmpB(739-848) were mapped by overlapping synthetic peptides. The ability of these synthetic peptides to stimulate immune CD8 T lymphocytes was determined by gamma interferon (IFN-gamma) production and cell proliferation after incubation with simian virus 40-transformed murine vascular endothelial cells in the presence of a 20 micro M solution of each synthetic peptide. Five synthetic peptides, SKGVNVDTV (OmpB(708-716)), ANVGSFVFN (OmpB(735-743)), IVSGTVGGQ (OmpB(749-757)), ANSTLQIGG (OmpB(789-797)), and IVEFVNTGP (OmpB(812-820)), induced secretion of IFN-gamma at significantly higher levels than the controls. Three of these five peptides, SKGVNVDTV (OmpB(708-716)), ANSTLQIGG (OmpB(789-797)), and IVEFVNTGP (OmpB(812-820)), also stimulated the proliferation of immune CD8 T lymphocytes. Significantly higher levels of specific cytotoxic T-lymphocyte killing were observed with the same three synthetic peptides, SKGVNVDTV (OmpB(708-716)), ANSTLQIGG (OmpB(789-797)), and IVEFVNTGP (OmpB(812-820)).

Animals↗
PubMed2002

Current status of immune mechanisms of killing of intracellular microorganisms.

The interaction between intracellular pathogens and the mammalian host follows different pathways that reflect evolved survival mechanisms of both the pathogen and the host to assure each one's own survival. From the host's perspective, different immune mechanisms predominate at different stages of infection. Both phagocytic and non-phagocytic target cells participate in microbial uptake and, in some cases, intracellular destruction. In addition, the development of specific immunity ensures sustained activation of intracellular microbicidal mechanisms in the target cells, and induction of apoptotic or lytic target cell death by cytotoxic T lymphocytes. From the pathogen's perspective, different evasion strategies are employed to counteract host defenses. Understanding microbial survival strategies and the immune mechanisms that result in killing of intracellular pathogens will deepen our insight into the pathogenesis of infection that could be applied towards the development of effective vaccination and immunotherapy.

Animals↗
PubMed2002

Human ehrlichioses.

Human ehrlichioses represent one of the best examples of newly emergent infectious diseases in which the classic triad of host, infectious agent, and environment are intertwined closely. These pathogens have existed for eons on the planet, and some were described as veterinary pathogens decades ago. Because of dramatic increases of deer and small mammal populations in certain areas and the subsequent increased populations of particular blood-feeding ticks, the risk of developing these diseases is higher than before. Increasing human populations in suburban areas and increased immunosuppressed populations (transplant patients, human immunodeficiency virus patients, and cancer survivors) also have increased risk of developing severe forms of these diseases.

Anaplasma↗
PubMed2002

Mechanisms of immunity against rickettsiae. New perspectives and opportunities offered by unusual intracellular parasites.

Investigation of the biology, pathology and immunology of rickettsial diseases offers new insights useful not only for the field of rickettsiology, but more importantly for the understanding of general principles of host-intracellular parasite relationships and, in particular, the immune interaction between endothelial cells and immune cells in the context of infection.

Animals↗
PubMed2002

Antigenic variation of Ehrlichia chaffeensis resulting from differential expression of the 28-kilodalton protein gene family.

The transcriptional activity and allele variation of the 28-kDa outer membrane protein gene (p28) of Ehrlichia chaffeensis were analyzed to determine the mechanism of the antigenic variation of the 28-kDa outer membrane proteins. Reverse transcriptase PCR amplification of mRNA indicated that 16 of the 22 members of the p28 multigene family were transcribed. Amino acid sequence analysis indicated that the p28-19 protein was produced in vitro in the Arkansas strain. The p28-19 gene and its promoter region were sequenced and compared in 12 clinical isolates of E. chaffeensis to determine allele variation. The variation of the p28-19 gene among the isolates is limited to three types represented by strains Arkansas, 91HE17, and Sapulpa, respectively. These results indicate that the majority of the p28 genes are active genes and that antigenic variation of the E. chaffeensis 28-kDa proteins may result from differential expression of the p28 gene family members rather than gene conversion.

Alleles↗
PubMed2002

Identification and functional analysis of an immunoreactive DsbA-like thio-disulfide oxidoreductase of Ehrlichia spp.

Novel homologous DsbA-like disulfide bond formation (Dsb) proteins of Ehrlichia chaffeensis and Ehrlichia canis were identified which restored DsbA activity in complemented Escherichia coli dsbA mutants. Recombinant Ehrlichia Dsb (eDsb) proteins were recognized by sera from E. canis-infected dogs but not from E. chaffeensis-infected patients. The eDsb proteins were observed primarily in the periplasm of E. chaffeensis and E. canis.

Animals↗
PubMed2002

Rickettsia felis in Ctenocephalides spp. fleas, Brazil.

In June 2000, suspected cases of Brazilian spotted fever (BSF) occurred in Coronel Fabriciano Municipality, Minas Gerais State, Brazil. Pooled fleas collected near two fatal cases contained rickettsial DNA. The nucleotide sequence alignment of the 391-bp segment of the 17-kDa protein gene showed that the products were identical to each other and to the R. felis 17-kDa gene, confirming circulation of R. felis in Brazil.

Animals↗
PubMedDate not supplied

[Emerging and reemerging rickettsiosis in an endemic area of Minas Gerais State, Brazil].

This article describes a serological survey for rickettsiosis in the county of Novo Cruzeiro, Minas Gerais State, Brazil, in 1998, testing schoolchildren and dogs. Sera included 331 samples from schoolchildren from an endemic area and 142 samples from schoolchildren from a non-endemic area in the county. All children examined were healthy and had not reported clinical symptoms of Brazilian spotted fever prior to the serological survey. Some 35 children in the endemic area were reactive to Rickettsia rickettsiiby indirect fluorescent antibody (IFA) with a titer of 1:64, corresponding to 10.6%. Sera from 73 dogs were tested, showing seroreactivity (IFA 1:64) to Rickettsia rickettsi, Ehrlichia chaffeensis, and Ehrlichia canisin 3 (4.11%), 11 (15.07%), and 13 (17.81%), respectively. The results in schoolchildren and the presence of canine seroreactivity to Ehrlichiaspecies that are potentially pathogenic to humans suggests the risk of transmission of other Rickettsiaein the study area.

Animals↗