Changes in immunoferritin labeling of Rickettsia tsutsugamushi after serial cultivation in 60Co-irradiated BHK cells

Novel polysaccharide antigen of Orientia tsutsugamushi revealed by a monoclonal antibody

Orientia tsutsugamushi, the causative agent of scrub typhus, is an obligate intracellular bacterium that replicates in the cytosol of host cells. Although several protein antigens have been characterized and cloned, little information exists regarding the polysaccharide antigen of this bacterium. In this study, we identified and characterized a novel antigen defined by a monoclonal antibody (MAb), NT19, against O. tsutsugamushi. Immunofluorescence microscopic studies showed that the NT19 antigen is released from the bacteria in the cytosol of host cells forming aggregates with bacteria. Immunoblot analysis showed that MAb NT19 recognized a strong band with a molecular mass of 20 kDa that was resistant to proteinase K digestion and sensitive to periodate oxidation, suggesting that the NT19 antigen is a polysaccharide. The function of this polysaccharide is not known, but considering its distribution within a bacterial microcolony, it is suspected to be involved in forming a biofilm-like structure within host cells.

Surface-exposed proteins of Ehrlichia chaffeensis

The surface proteins of Ehrlichia chaffeensis provide an important interface for pathogen-host interactions. To investigate the surface proteins of E. chaffeensis, membrane-impermeable, cleavable Sulfo-NHS-SS-Biotin was used to label intact bacteria. The biotinylated bacterial surface proteins were isolated by streptavidin-agarose affinity purification. The affinity-captured proteins were separated by electrophoresis, and five relatively abundant protein bands containing immunoreactive proteins were subjected to capillary-liquid chromatography-nanospray tandem mass spectrometry analysis. Nineteen out of 22 OMP-1/P28 family proteins, including P28 (which previously was shown to be surface exposed), were detected in E. chaffeensis cultured in human monocytic leukemia THP-1 cells. For the first time, with the exception of P28 and P28-1, 17 OMP-1/P28 family proteins were demonstrated to be expressed at the protein level. The surface exposure of OMP-1A and OMP-1N was verified by immunofluorescence microscopy. OMP-1B was undetectable either by surface biotinylation or by Western blotting of the whole bacterial lysate, suggesting that it is not expressed by E. chaffeensis cultured in THP-1 cells. Additional E. chaffeensis surface proteins detected were OMP85, hypothetical protein ECH_0525 (here named Esp73), immunodominant surface protein gp47, and 11 other proteins. The identification of E. chaffeensis surface-exposed proteins provides novel insights into the E. chaffeensis surface and lays the foundation for rational studies on pathogen-host interactions and vaccine development.

Orientia tsutsugamushi infection: overview and immune responses

Orientia tsutsugamushi, an obligate intracellular bacterium, was isolated for the first time in 1930. Infections by virulent strains are characterized by fever, rash, eschar, pneumonia, myocarditis, and disseminated intravascular coagulation. Here we review the general aspects of O. tsutsugamushi and immune responses in terms of inflammation, protective immune mechanisms, and immunogenic antigens.

Clinical, histopathological, and immunological responses of ponies to Ehrlichia sennetsu and subsequent Ehrlichia risticii challenge

Ehrlichia risticii has a close antigenic relationship to E. sennetsu. Sera of ponies experimentally infected with E. risticii, the etiologic agent of Potomac horse fever, consistently reacted with E. sennetsu, a human pathogen, in indirect fluorescent-antibody (IFA) testing, while human E. sennetsu convalescent serum reacted with E. risticii by IFA testing and immunoferritin labeling of cells infected in vitro. Two ponies injected intravenously with live E. sennetsu did no develop clinical illness. Subsequent injection with live E. sennetsu did not develop clinical illness. Subsequent injection with live E. risticii also did not induce any disease, in contrast to two control ponies given E. risticii without prior exposure to E. sennetsu. Both controls developed fever, anorexia, depression, dehydration, and diarrhea, which are typical clinical signs of Potomac horse fever, and had characteristic lesions of enteritis and lymph node histiocytosis at postmortem examination. E. sennetsu-exposed ponies had normal gastrointestinal morphologies and lymph node hyperplasia. Ponies primed with E. sennetsu before E. risticii challenge developed high titers of immunoglobulin G antibody which reacted against both E. sennetsu and E. risticii antigens by IFA testing. The most prominent antigenic polypeptide in Western (immuno-) blot analysis of sera collected from ponies primed with E. sennetsu before subsequent challenge with E. risticii was present in lysates of both Ehrlichia species and had an apparent molecular mass of 44 kilodaltons. This band was not prominent in Western blots performed with sera of ponies injected with E. risticii alone. Thus, injection of E. sennetsu protects ponies from clinical and pathological manifestations of the disease induced by injection with E. risticii. Immunologic cross-reactivity of the two organisms with IFA testing and strong immunologic recognition by ponies of the 44-kilodalton antigen common to the two organisms may be related to the development of protective immunity against E. risticii.

Glycogen autophagosomes in polymorphonuclear leukocytes induced by rickettsiae

Guinea pig polymorphonuclear leukocytes (PMNs), rich in glycogen granules, were collected from sodium-caseinate-induced peritoneal exudate. When these cells were incubated with rickettsiae, many microorganisms were phagocytized within 30 minutes at 35 degrees C and vacuoles up to 5 microns in diameter containing glycogen granules were present. Contained within these vacuoles were phagocytized extracellular material and a dense, lysosomelike substance that was acid phosphatase positive. These vacuoles, which were interpreted to be autophagosomes, were absent from PMNs that had not been stimulated with microorganisms. The number of rickettsiae in the PMN did not appear to be related to the number of autophagosomes. About 8% and 80% of thin-sectioned profiles of PMNs contained these vacuoles after 30 minutes and 4 hours incubation, respectively. After 4 hours, the PMNs contained multiple autophagosomes. Almost all of the glycogen granules were in autophagosomes in some of the cells. In some PMNs, discontinuous membranes encircled some glycogen. When PMNs were initially incubated with thorium dioxide and ferritin, and extensively washed prior to incubation with rickettsiae, glycogen was found surrounded by flattened secondary lysosomes containing the dense tracers. Some autophagosomes also contained the electron-dense tracers. These results suggest that rickettsiae induce the rapid formation of glycogen-containing autophagosomes in guinea pig peritoneal PMNs in vitro.

Effect of antibody on entry of Rickettsia tsutsugamushi into polymorphonuclear leukocyte cytoplasm

The effects of rickettsial antibodies on the entry of Rickettsia tsutsugamushi, Gilliam strain, into guinea pig polymorphonuclear leukocyte cytoplasm were studied by electron microscopy. Immunoglobulin G fractions were obtained from four rabbit antisera against: yolk sac-propagated rickettsiae; baby hamster kidney cell (BHK-21)-propagated rickettsiae; formaldehyde-fixed, BHK-21 cell-propagated rickettsiae; and glutaraldehyde-fixed, BHK-21 cell-propagated rickettsiae. A fuzzy coating was observed on the rickettsiae after reaction with each of the antibodies. All of the antibodies increased the uptake of rickettsiae by polymorphonuclear leukocytes. The opsonization effect was higher with an antibody against BHK-21 cell-propagated rickettsiae than with an antibody against yolk sac-propagated rickettsiae, and an antibody to live rickettsiae had a higher opsonization effect than did antibodies to chemically fixed rickettsiae. Rickettsiae were released from phagosomes into the cytoplasm with the four antibodies. The highest number of rickettsiae were released into the cytoplasm with antibody against live rickettsiae propagated in BHK-21 cells. The four antibodies inhibited the translocation of the cytoplasmic rickettsiae from the filamentous area to glycogen-rich zones. Almost 100% inhibition of translocation was observed with antibodies against live rickettsiae. These results indicate that rabbit antibodies against rickettsiae, when used alone, were unable to completely prevent rickettsial entry into polymorphonuclear leukocyte cytoplasm in vitro.

Entry of Rickettsia tsutsugamushi into polymorphonuclear leukocytes

Factors involved in the phagocytosis and entry into polymorphonuclear leukocytes (PMNs) of Rickettsia tsutsugamushi were studied by electron microscopy. R. tsutsugamushi propagated in baby hamster kidney cell cultures was incubated with guinea pig peritoneal PMNs in vitro at 35 degrees C. Structurally intact and degenerating rickettsiae were found in phagosomes, but only intact rickettsiae escaped phagosomes and specifically entered the glycogen-rich cytoplasm. The extraphagosomal cytoplasmic rickettsiae were found within 30 min after incubation; continued incubation for 4 h increased the rickettsial entry about fourfold as seen in ultrathin sections. Most rickettsiae in phagosomes were degenerating after 4 h of incubation. When incubated at 25 degrees C, no entry and very few phagocytized rickettsiae were observed. At 40 degrees C, rickettsial entry was greatly reduced, but more rickettsiae were found in phagosomes than at 35 degrees C. Preincubation of rickettsiae at 56 degrees C for 20 min with trypsin or with 2,4-dinitrophenol inhibited entry, but many rickettsiae were in phagosomes. Glutaraldehyde or formaldehyde fixation of rickettsiae and addition of 2-deoxyglucose, iodoacetamide, cytochalasin B, colchicine, or vinblastine inhibited all rickettsial uptake by PMNs. Acid phosphatase cytochemistry of infected PMNs revealed the enzyme activity only in phagosomes with degenerated rickettsiae and not in those with intact rickettsiae. These observations indicated that rickettsiae are passively phagocytized by PMNs, and only those that are intact actively escape from phagosomes, which selectively inhibits lysosomal fusion.