Association of an inflammatory I region-associated antigen-positive macrophage influx and genetic resistance of inbred mice to Rickettsia tsutsugamushi

Dissemination of Orientia tsutsugamushi and inflammatory responses in a murine model of scrub typhus

Central aspects in the pathogenesis of scrub typhus, an infection caused by Orientia (O.) tsutsugamushi, have remained obscure. Its organ and cellular tropism are poorly understood.

The purpose of this study was to analyze the kinetics of bacterial dissemination and associated inflammatory responses in infected tissues in an experimental scrub typhus mouse model, following infection with the human pathogenic strain Karp. We provide a thorough analysis of O. tsutsugamushi infection in inbred Balb/c mice using footpad inoculation, which is close to the natural way of infection. By a novel, highly sensitive qPCR targeting the multi copy traD genes, we quantitatively monitored the spread of O. tsutsugamushi Karp from the skin inoculation site via the regional lymph node to the internal target organs. The highest bacterial loads were measured in the lung. Using confocal imaging, we also detected O. tsutsugamushi at the single cell level in the lung and found a predominant macrophage rather than endothelial localization. Immunohistochemical analysis of infiltrates in lung and brain revealed differently composed lesions with specific localizations: iNOS-expressing macrophages were frequent in infiltrative parenchymal noduli, but uncommon in perivascular lesions within these organs. Quantitative analysis of the macrophage response by immunohistochemistry in liver, heart, lung and brain demonstrated an early onset of macrophage activation in the liver. Serum levels of interferon (IFN)-γ were increased during the acute infection, and we showed that IFN-γ contributed to iNOS-dependent bacterial growth control.

Our data show that upon inoculation to the skin, O. tsutsugamushi spreads systemically to a large number of organs and gives rise to organ-specific inflammation patterns. The findings suggest an essential role for the lung in the pathogenesis of scrub typhus. The model will allow detailed studies on host-pathogen interaction and provide further insight into the pathogenesis of O. tsutsugamushi infection.

Many details of the pathogenesis of scrub typhus, an infection caused by the intracellular bacterium Orientia tsutsugamushi that is endemic in Southeast Asia, have remained unclear until today. In this study, we present an experimental self-healing mouse model of scrub typhus based on footpad skin inoculation of the human pathogenic Karp strain of O. tsutsugamushi that shares many features with human infection. We established a novel quantitative PCR with increased sensitivity for the measurement of bacterial organ loads of infected mice. It was thereby shown that O. tsutsugamushi initially accumulated in the regional lymph node and subsequently spread to many organs with the highest bacterial loads found in the lung. The predominant host cells in the lung were macrophages located in the parenchymal interstitium, rather than endothelial cells. Our data also show unexpected organ-specific differences in the dynamics of macrophage activation. This mouse model will help to advance our understanding of scrub typhus pathogenesis.

Effect of genetic modificationsby selection for immunological toleranceon fungus infection in mice

Two strains of mice genetically selected for extreme phenotypes of immunological tolerance to ovalbumin, susceptible (TS) and resistant (TR), were experimentally infected with Sporothrix schenckii. The objective was to observe whether the genetic modifications produced by the selection might be associated with interstrain differences in adaptive immune and innate responses to infection. Therefore, we evaluated the LD(50), CFU, phagocytic index, fungicidal activity, pro-inflammatory cytokines, specific antibody titres, and the delayed-type hypersensitivity reactivity. TR mice were tenfold more susceptible to infection than TS mice, as shown by LD(50) (5 x 10(6) conidia i.v.). In TS mice, the resistance was a consequence of the tissue fungal load reduction, consistent specific T-cell-mediated immunity, and tumour necrosis factor (TNF)-alpha activity at onset of infection. In TR mice, these responses were not precociously detected. Therefore, the absence of CD4(+) T-cell response in the first week of infection might explain the non-clearance of pathogen in TR mice. However, TR mice did show an increase in TNF level and delayed-type hypersensitivity response after the first week post-infection; there was also expansion and increase in granulomatous foci and CFU in the spleen. The expansion of granulomatous foci and the increase in TNF-alpha and tissue fungal load to damaging levels induced severe tissue destruction, general failure of the organs, cachexy and death in TR mice. The results show that genetic selection for extreme phenotypes of immunological tolerance also modified the responses to S. schenckii infection.

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.

Expression of chemokine genes in murine macrophages infected with Orientia tsutsugamushi

Scrub typhus, caused by Orientia tsutsugamushi infection, is characterized by local as well as systemic inflammatory manifestations. Inflammation is initiated by O. tsutsugamushi-infected macrophages and endothelial cells in the dermis. We investigated the regulation of chemokine induction in macrophage cell line J774A.1 in response to O. tsutsugamushi infection. The mRNAs for macrophage inflammatory proteins 1alpha/beta (MIP-1alpha/beta), MIP-2, and macrophage chemoattractant protein 1 were induced within 30 min, and their levels showed a transitory peak for 3 to 12 h. However, the lymphotactin, eotaxin, gamma interferon-inducible protein 10, and T-cell activation gene 3 mRNAs were not detected by RNase protection assays. Heat-killed O. tsutsugamushi induced a similar extent of chemokine responses. Induction of the chemokine genes was not blocked by the eukaryotic protein synthesis inhibitor cycloheximide, suggesting that de novo synthesis of host cell protein is not required for these transcriptional responses. The induction of chemokine mRNAs by O. tsutsugamushi was blocked by the inhibitors of NF-kappaB activation. Furthermore, O. tsutsugamushi induced the nuclear translocation and activation of NF-kappaB. These results demonstrate that heat-stable molecules of O. tsutsugamushi induce a subset of chemokine genes and that induction involves activation of the transcription factor NF-kappaB.

Activation of mouse peritoneal macrophages in vitro or in vivo by recombinant murine gamma interferon inhibits the growth of Chlamydia trachomatis serovar L1

Peritoneal mouse macrophages activated in vitro with recombinant murine gamma interferon (10 ng/ml) or in vivo (10 micrograms per mouse) showed a significant decrease in the growth and yield of Chlamydia trachomatis. The restriction of the growth of C. trachomatis paralleled the expression of Iad on the macrophages. Mice that received macrophages activated in vitro with recombinant murine gamma interferon showed a significant decrease in the yield of chlamydial infection-forming units from their spleens and peritoneal fluids.

Antigen presentation and the response to parasitic infection

Antigen presentation to T-cells is central to the induction and maintenance of the cell-mediated immune response. Many approaches have been used to define a complex accessory cell compartment, which performs the variety of functional roles encompassed by this term. Resulting from this, antigen presentation has become the cornerstone behind many theories of disease susceptibility and is an important consideration for vaccine design. In this article, Paul Kaye reviews aspects of antigen presentation by accessory cells with particular emphasis on how parasites and their antigens interact with this heterogeneous group of cells.

Gamma interferon-mediated increase in the number of Ia-bearing macrophages during infection with Listeria monocytogenes

The role of gamma interferon (IFN-gamma) in an increase in Ia-bearing macrophages during Listeria monocytogenes infection was studied. The peritoneal macrophages from L. monocytogenes-infected mice contained a high proportion of Ia. Intraperitoneal injection of the supernatant from a culture of spleen cells from L. monocytogenes-infected mice induced Ia-rich exudates in normal mice. The Ia-inducing activity in the culture supernatant was abrogated by the pretreatment of spleen cells with anti-Thy-1.2 antibody plus complement. Immunoadsorption of the culture supernatant with anti-recombinant IFN-gamma antibody and protein A-Sepharose CL-4B completely abrogated its Ia-inducing activity. These results suggested that an increase in Ia-bearing macrophages during L. monocytogenes infection was attributable to T-cell-derived IFN-gamma.

Regulation of macrophage accessory cell activity by mycobacteria. I. Ia expression in normal and irradiated mice infected with Mycobacterium microti

CBA/Ca mice were infected by either the intravenous or intraperitoneal route with Mycobacterium microti and the subsequent changes in local macrophage populations examined. Following infection, the number of macrophages increased and they showed greater expression of both MHC Class II molecules. This response was not dependent on viability of the mycobacteria, in contrast to reports with other microorganisms such as Listeria. Studies in sublethally irradiated mice indicated that persistent antigen could give rise to a response after a period of host recovery which was radiation dose dependent. This procedure also highlighted differences in the regulation of different murine class II antigens in vivo, as seen by delayed re-expression of I-E antigens. Macrophage accessory cell function, as assessed by an in vitro T cell proliferation assay, correlated with Ia expression after fixation, but not after indomethacin treatment; this highlights the diverse nature of regulatory molecules produced by these cells.

Immunosuppression associated with the development of chronic infections with Rickettsia tsutsugamushi: adherent suppressor cell activity and macrophage activation

Measures of general immunocompetency such as lymphocyte responses to mitogens and alloantigens and the ability to produce antibody to T-dependent and T-independent antigens were evaluated during the development of chronic infections with Rickettsia tsutsugamushi resulting from subcutaneous infection of BALB/c mice. It was found that a transient immunosuppression was demonstrable regardless of the infecting strain of rickettsiae; however, the immunosuppression produced by the Karp and Kato strains was more pronounced and longer lived. As a marked splenomegaly resulting from inflammatory macrophage influx accompanied this immunosuppression, mitogen- and antigen-induced lymphocyte proliferation was also evaluated after adherent cell depletion or in the presence of indomethacin, and both treatments significantly improved the responses. Isolated splenic macrophages were shown to suppress the responses of lymphocytes from naive mice as well as to exhibit parameters of activation including tumor cell cytolysis and cytostasis and the ability to inhibit the replication of R. tsutsugamushi in vitro. These data suggest an association between macrophage activation involved in rickettsial clearance and a transient immunosuppression.

Immunoregulatory pathways in murine leishmaniasis: different regulatory control during Leishmania mexicana mexicana and Leishmania major infections

The effect of whole body sublethal gamma irradiation on the subsequent growth of Leishmania mexicana mexicana and Leishmania major was studied in CBA/Ca and BALB/c mice. Whereas BALB/c mice are highly susceptible to both parasites developing non healing progressively growing lesions at the site of cutaneous infection, CBA/Ca mice develop small healing cutaneous ulcers following subcutaneous infection with L. major but non healing lesions following subcutaneous infection with L.m. mexicana. Prior whole body sublethal irradiation of CBA/Ca mice, but not BALB/c mice, resulted in strong resistance against infection with L.m. mexicana: no lesions developed at the site of cutaneous infection. Irradiated BALB/c mice did, however, develop small lesions which healed when infected with L. major. The protective effects of irradiation coincided with the development of delayed type hypersensitivity. Both naive and sensitized nylon wool purified lymphocytes could restore susceptibility to L. major in irradiated BALB/c mice but only lymphocytes from long term infected donor mice adoptively transferred a non healing response to irradiated CBA/Ca mice infected with L.m. mexicana. Non-irradiated, L. major infected, CBA/Ca mice, but not similarly treated BALB/c mice, were found to be resistant to subsequent infection with L.m. mexicana. On the other hand, irradiated BALB/c mice infected with L. major were resistant to subsequent infectious challenge with L.m. mexicana. We suggest that the susceptibility of CBA/Ca mice to L.m. mexicana is under the control of an as yet unidentified gene which is not dependent on the generation of T suppressor cells and is bypassed by previous infection with L. major. Therefore, BALB/c mice immunized against L. major by prior sublethal irradiation are also resistant to L.m. mexicana.

Gamma interferon production in response to homologous and heterologous strain antigens in mice chronically infected with Rickettsia tsutsugamushi

The ability of antigen-responsive, thymus-derived lymphocytes to produce immune (gamma) interferon was investigated during the development and expression of cellular immunity to Rickettsia tsutsugamushi. C3H/HeDub mice infected subcutaneously with the Gilliam strain developed the ability to produce serum interferon in response to intravenously inoculated antigen which correlated with the development of resistance to intraperitoneal rechallenge. Antigen-responsive lymphocytes, measured by interferon production and proliferation, were first apparent in draining lymph node cells, but spleen cell responses were detectable relatively soon after the appearance of reactive lymph node cells. The peak spleen cell response was of a greater magnitude and was found to be relatively long-lived. Reactivity to heterologous strains of R. tsutsugamushi also developed after immunization and paralleled the homologous responses, although reactivity was greatest to homologous antigens. Responses to heterologous strains differed in magnitude and time of appearances; however, immune mice resisted challenge with all strains of R. tsutsugamushi tested.