Inhibition of MHC class I and class II cell surface expression on bovine endothelial cells upon infection with Cowdria ruminantium

Infection of the endothelium by members of the order Rickettsiales

The vascular endothelium is the main target of a limited number of infectious agents, Rickettsia, Ehrlichia ruminantium, and Orientia tsutsugamushi are among them. These arthropod-transmitted obligately-intracellular bacteria cause serious systemic diseases that are not infrequently lethal. In this review, we discuss the bacterial biology, vector biology, and clinical aspects of these conditions with particular emphasis on the interactions of these bacteria with the vascular endothelium and how it responds to intracellular infection. The study of these bacteria in relevant in vivo models is likely to offer new insights into the physiology of the endothelium that have not been revealed by other models.

Ixodes ovatus Ehrlichia exhibits unique ultrastructural characteristics in mammalian endothelial and tick-derived cells

Tick-borne pathogens in the genus Ehrlichia cause emerging zoonoses. Although laboratory mice are susceptible to Ehrlichia infections, many isolates do not cause clinical illness. In contrast, the Ixodes ovatus Ehrlichia-like agent (IOE) causes disease and immune responses in mice comparable to the human illness caused by Ehrlichia chaffeensis. No culture system had been developed for IOE, however, which limited studies of this pathogen. We reasoned that endothelial and tick cell lines could potentially serve as host cells, since the IOE is found in ticks and in endothelial cells in mice. Infected spleen cells from RAG-deficient mice were overlaid onto ISE6 and RF/6A cultures, and colonies typical of Ehrlichia were noted in RF/6A cells within 2 weeks. Infection of ISE6 cells was established after transfer of IOE from RF/6A cells. Electron microscopy revealed densely packed inclusions in infected RF/6A and ISE6 cells; these inclusions contained copious amounts of filamentous structures, apparently originating from Ehrlichial cells. In particular, within RF/6A cells the structures assumed an ordered morphology of finely combed hair. IOE from RF/6A cells, when inoculated into C57BL/6 and RAG-deficient mice, induced fatal disease. These data reveal unique structural features of IOE that may contribute to the pathogen's high virulence.

Prostaglandin E(2) modulation of gene expression in an Atlantic salmon (Salmo salar) macrophage-like cell line (SHK-1)

Following lipopolysaccharide (LPS)-stimulation of Atlantic salmon (Salmo salar) macrophage-like SHK-1 cells, prostaglandin E(2) (PGE(2)) exhibited dose-dependent inhibition of the antigen presenting molecules major histocompatability class I and II and the pro-inflammatory cytokine interleukin-1 beta gene expression. Prostaglandin E(2) was found to be stimulatory towards cyclooxygenase-2 (COX-2) expression at higher concentrations (1 x 10(-6) and 1 x 10(-8)M) and inhibitory at lower concentrations (1 x 10(-10) and 1 x 10(-12)M) after 4h exposure. After 24h exposure, however, LPS-induced COX-2 expression decreased and was completely inhibited by all PGE(2) concentrations (1 x 10(-6)-1 x 10(-10)M). Incubation of SHK-1 cells with LPS alone had no effect on tumour necrosis factor alpha (TNFalpha)-like gene or transforming growth factor beta-like gene expression after 4h, however, LPS and PGE(2) showed a synergistic effect on TNFalpha-like gene expression after 24h. This study provides evidence for the existence of a PGE(2)-mediated negative feedback mechanism in the control of PGs through down-regulation of COX-2, as well as for inflammatory responses by the down-regulation of both COX-2 and IL-1 beta. The differential regulation of immune-related genes under these conditions further demonstrates the usefulness of the SHK-1 cell line for studying aspects of salmonid immunology.

Mechanisms of immunity to Ehrlichia muris: a model of monocytotropic ehrlichiosis

Ehrlichia species can cause life-threatening infections or chronic persistent infections. Mechanisms of protective immunity were examined in an Ehrlichia muris mouse model of monocytotropic ehrlichiosis. C57BL/6 mice possessed strong genetic resistance to E. muris of an undetermined mechanism. CD8 T lymphocytes were particularly important, as revealed by 81% fatalities for E. muris-infected, major histocompatibility complex class I gene knockout mice compared with no deaths for wild-type C3H mice. Moreover, 80% of C3H mice depleted of CD8 and CD4 cells died of E. muris infection compared with only 44% of CD4 cell-depleted mice. CD8 T lymphocytes were demonstrated for the first time in an Ehrlichia infection to exhibit cytotoxic T-lymphocyte activity against Ehrlichia-infected target cells. Both gamma interferon and tumor necrosis factor were shown to play synergistic roles in protective immunity in vivo for the first time, as demonstrated by 75% fatalities when both cytokines were neutralized compared with minimal mortality when they were depleted separately. Passive transfer of antibodies, but not Fab fragments, to E. muris protected C3H/SCID mice against lethal infection. The mechanism of increased susceptibility (22% lethality) of C57BL/6 major histocompatibility complex class II gene knockout mice and CD4 cell-depleted C3H mice (i.e., through a gamma interferon or antibody mechanism), as well as the more important role of CD8 T lymphocytes (in the form of cytotoxic T-lymphocyte activity and/or gamma interferon production), remains to be elucidated. Protective immunity against monocytotropic E. muris is mediated by a combination of CD8 and CD4 T lymphocytes, gamma interferon, tumor necrosis factor alpha, and antibodies.

Down-regulation of MHC class II receptors of DH82 cells, following infection with Ehrlichia canis

In order to evaluate whether infection with E. canis alters the expression of major histocompatibility complex (MHC) class I and/or MHC class II receptors, and by doing so alters the immune response to the organism, flow cytometry was performed on DH82 cells infected with Ehrlichia canis (90% infection) and on uninfected DH82 cells of the same passage, using anti-canine MHC class I and II antibodies. MHC class II expression was evident in 47.6 and 46.2% (mean 46.9%) of uninfected DH82 cells using two different anti-MHC class II antibodies, while no MHC class II expression was evident in DH82 cells infected with E. canis. The present results indicate that infection of DH82 macrophages with E. canis down-regulates their MHC class II receptors. These results suggest a possible mechanism by which E. canis evades the immune system.

Immune responses to Cowdria ruminantium infections

Understanding the basis of protective immunity to Cowdria ruminantium will facilitate the development of an effective subunit vaccine against heartwater in ruminants and contribute to a better definition of protective immune mechanisms to obligate intracellular pathogens in general. Until recently, immunological studies of heartwater in ruminants concentrated solely on antibody responses. Since 1995, the mechanisms underlying cell-mediated immunity of heartwater have been analysed. Progress achieved in these areas is discussed here by Philippe Totté and colleagues, with special emphasis on ruminants, the natural hosts of C. ruminantium.

Chlamydia inhibits interferon gamma-inducible major histocompatibility complex class II expression by degradation of upstream stimulatory factor 1

We report that chlamydiae, which are obligate intracellular bacterial pathogens, can inhibit interferon (IFN)-gamma-inducible major histocompatibility complex (MHC) class II expression. However, the IFN-gamma-induced IFN regulatory factor-1 (IRF-1) and intercellular adhesion molecule 1 (ICAM-1) expression is not affected, suggesting that chlamydia may selectively target the IFN-gamma signaling pathways required for MHC class II expression. Chlamydial inhibition of MHC class II expression is correlated with degradation of upstream stimulatory factor (USF)-1, a constitutively and ubiquitously expressed transcription factor required for IFN-gamma induction of class II transactivator (CIITA) but not of IRF-1 and ICAM-1. CIITA is an obligate mediator of IFN-gamma-inducible MHC class II expression. Thus, diminished CIITA expression as a result of USF-1 degradation may account for the suppression of the IFN-gamma-inducible MHC class II in chlamydia-infected cells. These results reveal a novel immune evasion strategy used by the intracellular bacterial pathogen chlamydia that improves our understanding of the molecular basis of pathogenesis.