Macrophages in resistance to rickettsial infection: strains of mice susceptible to the lethal effects of Rickettsia akari show defective macrophage Rickettsicidal activity in vitro

Atg5 Supports Rickettsia australis Infection in Macrophages In Vitro and In Vivo

Rickettsiae can cause life-threatening infections in humans. Macrophages are one of the initial targets for rickettsiae after inoculation by ticks. However, it remains poorly understood how rickettsiae remain free in macrophages prior to establishing their infection in microvascular endothelial cells. Here, we demonstrated that the concentration of Rickettsia australis was significantly greater in infected tissues of Atg5^flox/flox mice than in the counterparts of Atg5^flox/flox Lyz-Cre mice, in association with a reduced level of interleukin-1β (IL-1β) in serum. The greater concentration of R. australis in Atg5^flox/flox bone marrow-derived macrophages (BMMs) than in Atg5^flox/flox Lyz-Cre BMMs in vitro was abolished by exogenous treatment with recombinant IL-1β. Rickettsia australis induced significantly increased levels of light chain 3 (LC3) form II (LC3-II) and LC3 puncta in Atg5-competent BMMs but not in Atg5-deficient BMMs, while no p62 turnover was observed. Further analysis found the colocalization of LC3 with a small portion of R. australis and Rickettsia-containing double-membrane-bound vacuoles in the BMMs of B6 mice. Moreover, treatment with rapamycin significantly increased the concentrations of R. australis in B6 BMMs compared to those in the untreated B6 BMM controls. Taken together, our results demonstrate that Atg5 favors R. australis infection in mouse macrophages in association with a suppressed level of IL-1β production but not active autophagy flux. These data highlight the contribution of Atg5 in macrophages to the pathogenesis of rickettsial diseases.

Liver Necrosis and Lethal Systemic Inflammation in a Murine Model of Rickettsia typhi Infection: Role of Neutrophils, Macrophages and NK Cells

Rickettsia (R.) typhi is the causative agent of endemic typhus, an emerging febrile disease that is associated with complications such as pneumonia, encephalitis and liver dysfunction. To elucidate how innate immune mechanisms contribute to defense and pathology we here analyzed R. typhi infection of CB17 SCID mice that are congenic to BALB/c mice but lack adaptive immunity. CB17 SCID mice succumbed to R. typhi infection within 21 days and showed high bacterial load in spleen, brain, lung, and liver. Most evident pathological changes in R. typhi-infected CB17 SCID mice were massive liver necrosis and splenomegaly due to the disproportionate accumulation of neutrophils and macrophages (MΦ). Both neutrophils and MΦ infiltrated the liver and harbored R. typhi. Both cell populations expressed iNOS and produced reactive oxygen species (ROS) and, thus, exhibited an inflammatory and bactericidal phenotype. Surprisingly, depletion of neutrophils completely prevented liver necrosis but neither altered bacterial load nor protected CB17 SCID mice from death. Furthermore, the absence of neutrophils had no impact on the overwhelming systemic inflammatory response in these mice. This response was predominantly driven by activated MΦ and NK cells both of which expressed IFNγ and is considered as the reason of death. Finally, we observed that iNOS expression by MΦ and neutrophils did not correlate with R. typhi uptake in vivo. Moreover, we demonstrate that MΦ hardly respond to R. typhi in vitro. These findings indicate that R. typhi enters MΦ and also neutrophils unrecognized and that activation of these cells is mediated by other mechanisms in the context of tissue damage in vivo.

Rickettsialpox in a patient with HIV infection

We describe the first case of rickettsialpox in a patient infected with HIV. Immunohistochemical staining of biopsied lesions showed a relatively large number of rickettsiae within the papulovesicular rash. Rickettsialpox is easily treated and may resemble more serious cutaneous eruptions in patients infected with HIV. This diagnosis should be considered in immunocompromised city-dwellers, with fever and a papulovesicular rash.

The role of tumor necrosis factor in host defense against scrub typhus rickettsiae. I. Inhibition of growth of Rickettsia tsutsugamushi, Karp strain, in cultured murine embryonic cells and macrophages by recombinant tumor necrosis factor-alpha

Recombinant murine tumor necrosis factor-alpha (TNF-alpha) inhibited intracellular growth of Rickettsia tsutsugamushi, Karp strain, in the mouse embryo cell line C3H/10T1/2 clone 8 at doses of 100 to 10 U/ml. The growth inhibitory effect of TNF-alpha was also evident when peritoneal exudate macrophages or bone marrow-derived macrophages were used as the host cell for rickettsial growth. Interferon-gamma (IFN-gamma), at doses up to 1,000 U/ml, did not affect the growth of this strain of rickettsiae in the mouse embryo cell line but, as expected, profoundly inhibited rickettsial growth in peritoneal exudate macrophages and bone marrow-derived macrophages. The effect of TNF-alpha on rickettsial growth in the mouse embryo cell line was not reproducibly enhanced by IFN-gamma. Treatment of the cell line with TNF-alpha delayed rickettsial cytopathic effects, but the rickettsiae ultimately grew to high numbers in the cells and caused cell death. These findings show that, at least in our system, R. tsutsugamushi is resistant to IFN-gamma-mediated antirickettsial effects in cells other than macrophages. The results of this study support the suggestion that TNF-alpha may inhibit rickettsial growth in cells other than macrophages.

Variation in susceptibility of ten mouse strains to infection with a strain of Ehrlichia risticii

Eight inbred strains of mice (A/J, AKR/NCrlBR, BALB/cAnNHsd, C3H/HeJ, C3H/HeNHsd, C57BL/6J, C57BL/10SnJ and CBA/J) and two outbred strains (CF1 and ICR) were inoculated with a strain of Ehrlichia risticii and their relative susceptibility to disease was determined. The strains varied widely in susceptibility, some showing profound illness, with illness being barely detectable in others. Severity of illness was uniform within strains. This study confirmed that the A/J and BALB/cAnNHsd strains were very susceptible to illness, but the C3H/HeJ, CBA/J and the CF1 strains were slightly less susceptible. The C57BL/6J, C57BL/10SnJ, C3H/HeNHsd, AKR/NCrlBR and ICR strains were resistant.

Susceptibility of Rickettsia tsutsugamushi Gilliam to gamma interferon in cultured mouse cells

Recombinant rodent gamma interferon (IFN-gamma) inhibited the infection of cultured BALB/3T3 mouse fibroblasts by Rickettsia tsutsugamushi Gilliam, apparently mainly by clearance of intracellular rickettsiae. No significant effect on rickettsial entry into the cells was noted; IFN-gamma was toxic to infected cells, as measured by the capacity of treated, infected cells to attach to the surfaces of culture vessels. In a small proportion of IFN-gamma-treated cells, rickettsial replication appeared to persist at normal levels. A fraction (28%) of rickettsiae clonally isolated from cultures treated with IFN-gamma was resistant to IFN-gamma-mediated inhibition, but four serial passages of these resistant clones in the absence of additional IFN-gamma resulted in the loss of resistance. In several respects, therefore, the IFN-gamma-mediated inhibition of scrub typhus rickettsiae in cultured fibroblasts was similar to that reported for Rickettsia prowazekii.

Interleukin-2, anti-interleukin-2 receptor antibody, and activation of macrophages

Macrophages treated with IFN-gamma and IL-2 before exposure to parasites develop the ability to resist infection with amastigotes of Leishmania major. In this cooperative interaction of cytokines, IL-2 can be replaced with any of several mAb directed against the beta chain of the IL-2 receptor, but not by antibodies to a number of other cell receptors or antigens. Thus, antibodies to the IL-2 receptor act as agonists of IL-2 in the induction of a biologic activity in macrophages, and macrophages, a nonproliferating cell type, respond to signals transmitted through the beta chain of the IL-2 receptor.

Lymphokine-induced macrophage resistance to infection with Leishmania major

Resistance to infection is an effector activity of macrophages that is induced by the cooperation of several molecularly distinct factors in LK: IFN and another nonIFN macrophage activation factor. Unlike many other effector activities of activated macrophages, signal sequence is not critical for induction of resistance to infection. Nor is the activation of macrophages for resistance to infection dependent upon the presence of T lymphocytes in the culture vessel: T cell-depleted peritoneal cell cultures and bone marrow-derived macrophages both develop resistance to infection with L. major in the presence of LK that contains IFN. Further characterization of this activity of activated macrophages will include the identification of LK that cooperate with IFN for induction of resistance to infection, and characterization of the LK-induced changes in macrophage function that mediate this resistant state.

Establishment and characterization of a T-cell line specific for Rickettsia tsutsugamushi

To study the immunological protective system against rickettsial infection, a T-cell line specific for Rickettsia tsutsugamushi antigen was established by long-term culture of splenocytes from mice immunized with live Gilliam strain R. tsutsugamushi and then propagated in the presence of homologous rickettsial antigen and syngeneic filler cells. The characteristics of the T-cell line and its capacity to induce antirickettsial protection in vivo were studied. Flow cytometric analysis demonstrated that the T-cell line showed the phenotype Thy-1.2+ L3T4+ Lyt-2-, suggestive of helper T cells. In a lymphocyte proliferation assay, this cell line showed a specific response to Gilliam antigen, partial cross-reactivity to Karp antigen, but no response to Kato antigen. The proliferative response of this T-cell line was filler cell dependent, and genetic restriction was observed between the T-cell line and filler cells. The T-cell line produced gamma interferon, one of the macrophage-activating factors, in cultures with specific antigen and was able to adoptively mediate antirickettsial protection in vivo. The data presented here suggest that antigen-specific helper T cells play an important role in protection against rickettsial infection.

Activation of macrophages to kill rickettsiae and Leishmania: dissociation of intracellular microbicidal activities and extracellular destruction of neoplastic and helminth targets

Mononuclear phagocytes undergo dramatic changes during differentiation from bone marrow stem cells to resident tissue macrophages. Throughout differentiation, cells lose or acquire numerous morphologic, metabolic and functional capacities such that mature, resident macrophages of one tissue often bear little resemblance to resident cells of another. Superimposed on the intrinsic continuum of mononuclear phagocyte differentiation are the reactive changes in macrophages induced by endogenous and exogenous stimuli: the ability of mononuclear phagocytes to respond to a particular stimulus may also change with cell differentiation. This dynamic interaction of cell differentiation and response to a micro-environment, and the resulting heterogeneity among mononuclear phagocytes for many functional characteristics, is clearly illustrated by the effector activities of activated macrophages that we describe in this report. Despite the common regulatory events for induction and expression of transient nonspecific cytotoxic reactions effective against such diverse targets as rickettsiae, leishmania, schistosomula, and neoplastic cells, these effector functions can be dissociated by the cells that perform the effector activity, and the signals that regulate these activities. The differential susceptibility of the various targets to particular killing mechanisms induced by LK in responsive populations only adds to the complexity of these in vitro analyses. The details of effector functions of activated macrophages are unique for each target.

Antileishmanial activities of macrophages from C3H/HeN and C3H/HeJ mice treated with Mycobacterium bovis strain BCG

C3H/HeN and C3H/HeJ mice were infected ip with viable BCG, a macrophage-activating agent, and their peritoneal exudate macrophages exposed to Leishmania tropica amastigotes. Macrophages from BCG-infected C3H/HeN mice had both leishmanicidal activities described for lymphokine activation of C3H/HeN macrophages in vitro: increased resistance to L. tropica infection, followed by intracellular killing of the parasite. Macrophages from BCG-infected C3H/HeN mice were also activated to kill tumor cells in vitro. In contrast, macrophages from BCG-treated C3H/HeJ mice were not resistant to L. tropica infection, did not kill intracellular amastigotes over 72 hr in culture, and were not cytotoxic to tumor cells.

Susceptibility of inbred mice to Leishmania tropica infection: correlation of susceptibility with in vitro defective macrophage microbicidal activities

Eleven mouse strains were inoculated in footpads with amastigotes of Leishmania tropica and observed for 12 weeks. Liver and spleen impression smears from infected mice were examined for the presence of intracellular parasites. Four strains (BALB/cJ, C57L/J, NZW/N, and P/J) failed to heal the subcutaneous lesion and showed evidence of systemic infection; the remaining seven strains (A/J, C3H/HeJ, C3H/HeN, C3HeB/FeJ, C57BL/6J, C57BL/10J, and C57BL/10ScN) were each resistant to infection and resolved their lesions by Week 10. Macrophages from the four susceptible strains could not be activated to kill L. tropica amastigotes by treatment with soluble lymphocyte products in vitro. In contrast, macrophages from all seven resistant strains responded to lymphokine treatment and eliminated 80-90% of intracellular parasites. These results suggest that in vitro macrophage microbicidal activities predict the course of systemic leishmanial disease.