Epidemic typhus infection in cynomolgus monkeys (Macaca fascicularis)

Recent molecular insights into rickettsial pathogenesis and immunity

Human infections with arthropod-borne Rickettsia species remain a major global health issue, causing significant morbidity and mortality. Epidemic typhus due to Rickettsia prowazekii has an established reputation as the 'scourge of armies', and as a major determinant of significant 'historical turning points'. No suitable vaccines for human use are currently available to prevent rickettsial diseases. The unique lifestyle features of rickettsiae include obligate intracellular parasitism, intracytoplasmic niche within the host cell, predilection for infection of microvascular endothelium in mammalian hosts, association with arthropods and the tendency for genomic reduction. The fundamental research in the field of Rickettsiology has witnessed significant recent progress in the areas of pathogen adhesion/invasion and host immune responses, as well as the genomics, proteomics, metabolomics, phylogenetics, motility and molecular manipulation of important rickettsial pathogens. The focus of this review article is to capture a snapshot of the latest developments pertaining to the mechanisms of rickettsial pathogenesis and immunity.

Progress in the functional analysis of rickettsial genes through directed mutagenesis of Rickettsia prowazekii phospholipase D

Evaluation of: Driskell LO, Yu X-J, Zhang L et al.: Directed mutagenesis of the Rickettsia prowazekii pld gene encoding phospholipase D. Infect. Immun. 77(8), 3244–3248 (2009). Rickettsioses have afflicted humans worldwide throughout the course of history. Rickettsia prowazekii is the etiological agent of epidemic typhus, a disease transmitted by body lice and capable of massive outbreaks under conditions of compromised hygiene, such as famine, mass migration and war. Fastidious growth requirements and an obligately intracellular lifestyle, preferably within the cytoplasm of the host cell, pose a number of challenges in genetic manipulation of rickettsiae. Driskell et al. describe the successful application of a combination of molecular approaches to generate, isolate and characterize a R. prowazekii mutant lacking a 93-bp sequence of the pld gene responsible for phospholipase D activity. In initial studies, this Δpld mutant of R. prowazekii is shown to be capable of infecting macrophage-like RAW 264.7 cells in culture and quickly escaping from the phagosome into the cytosol. However, in guinea pigs infected by intraperitoneal inoculation, the pld deletion mutant exhibits attenuation of virulence and the ability to induce protective immune responses against virulent R. prowazekii. The fundamental importance of this study lies in the generation of a site-directed gene mutant for subsequent evaluation of the target gene’s function(s) in rickettsial pathogenesis and immune defense mechanisms. The results also lend support to the possibility that rickettsiae may either possess multiple phosholipases with different substrate specificities or as yet unknown alternative mechanisms for quick phagosomal escape into the host cytoplasm. Studies aimed at detailed characterization of the combinatorial mutant generated in this study and possibly other complete knockouts for genes with putative functions using relevant in vitro and in vivo models of infection are necessary to further elucidate their roles in the biology of rickettsiae.

Host-cell interactions with pathogenic Rickettsia species

Pathogenic Rickettsia species are Gram-negative, obligate intracellular bacteria responsible for the spotted fever and typhus groups of diseases around the world. It is now well established that a majority of sequelae associated with human rickettsioses are the outcome of the pathogen's affinity for endothelium lining the blood vessels, the consequences of which are vascular inflammation, insult to vascular integrity and compromised vascular permeability, collectively termed 'Rickettsial vasculitis'. Signaling mechanisms leading to transcriptional activation of target cells in response to Rickettsial adhesion and/or invasion, differential activation of host-cell signaling due to infection with spotted fever versus typhus subgroups of Rickettsiae, and their contributions to the host's immune responses and determination of cell fate are the major subtopics of this review. Also included is a succinct analysis of established in vivo models and their use for understanding Rickettsial interactions with host cells and pathogenesis of vasculotropic rickettsioses. Continued progress in these important but relatively under-explored areas of bacterial pathogenesis research should further highlight unique aspects of Rickettsial interactions with host cells, elucidate the biological basis of endothelial tropism and reveal novel chemotherapeutic and vaccination strategies for debilitating Rickettsial diseases.

Rickettsial diseases: from Rickettsia-arthropod relationships to pathophysiology and animal models

Rickettsiae cause spotted fevers and typhus-related diseases in humans. Some of these diseases occur worldwide and are life-threatening, for example, epidemic typhus is still a major health problem despite the apparent efficiency of antibiotic treatment. In addition, Rickettsia prowazekii, the agent of epidemic typhus, and R. rickettsii, the agent of Rocky Mountain spotted fever, are microorganisms that could potentially be used as bioweapons to induce panic in the population. Rickettsiae are obligate intracellular bacteria in both vertebrate and invertebrate hosts, but rickettsial species differ in terms of association with arthropods, behavior of the vector to infection, pathophysiology and outcome of the disease. Understanding the pathogenic steps of rickettsioses is essential to develop protective strategies against these bacteriological threats. Unfortunately, the mechanisms involved in the pathogenesis of many rickettsioses are poorly characterized, and protective immunity is incompletely understood, in part because accurate animal models that mimic human diseases are lacking. In the past, murine models have been of limited value because infection of mice was without effect or resulted in erratic mortality. Recent studies have reported that rickettsial infection can be established in mice, depending on the genetic background of mice, the type of rickettsial species and the route of inoculation. These models may be useful for analyzing the pathogenesis of rickettsioses, especially epidemic typhus, evaluating new therapeutic molecules and vaccine candidates, and preventing future outbreaks.

Macaque models of human infectious disease

Macaques have served as models for more than 70 human infectious diseases of diverse etiologies, including a multitude of agents—bacteria, viruses, fungi, parasites, prions. The remarkable diversity of human infectious diseases that have been modeled in the macaque includes global, childhood, and tropical diseases as well as newly emergent, sexually transmitted, oncogenic, degenerative neurologic, potential bioterrorism, and miscellaneous other diseases. Historically, macaques played a major role in establishing the etiology of yellow fever, polio, and prion diseases. With rare exceptions (Chagas disease, bartonellosis), all of the infectious diseases in this review are of Old World origin. Perhaps most surprising is the large number of tropical (16), newly emergent (7), and bioterrorism diseases (9) that have been modeled in macaques. Many of these human diseases (e.g., AIDS, hepatitis E, bartonellosis) are a consequence of zoonotic infection. However, infectious agents of certain diseases, including measles and tuberculosis, can sometimes go both ways, and thus several human pathogens are threats to nonhuman primates including macaques. Through experimental studies in macaques, researchers have gained insight into pathogenic mechanisms and novel treatment and vaccine approaches for many human infectious diseases, most notably acquired immunodeficiency syndrome (AIDS), which is caused by infection with human immunodeficiency virus (HIV). Other infectious agents for which macaques have been a uniquely valuable resource for biomedical research, and particularly vaccinology, include influenza virus, paramyxoviruses, flaviviruses, arenaviruses, hepatitis E virus, papillomavirus, smallpox virus, Mycobacteria, Bacillus anthracis, Helicobacter pylori, Yersinia pestis, and Plasmodium species. This review summarizes the extensive past and present research on macaque models of human infectious disease.

A mutation inactivating the methyltransferase gene in avirulent Madrid E strain of Rickettsia prowazekii reverted to wild type in the virulent revertant strain Evir

Rickettsia prowazekii Madrid E (E) strain is an effective vaccine, but can revert to virulent status when passaged in animals. The aim of this study is to identify the reverse mutation that may determine the virulence of R. prowazekii by comparing the genetic structures of E strain and its virulent revertant Evir strain. We determined that the gene (Rp028/Rp027) encoding the methyltransferase was mutated by frameshift in avirulent E strain but not in virulent revertant Evir strain and wild type virulent Breinl strain. We conclude that the mutation in the E strain gene reverts to wild type in the virulent revertant Evir strain. Whether the mutation plays an essential role in the attenuation of E strain needs to be further investigated.

Comparison of properties of virulent, avirulent, and interferon-resistant Rickettsia prowazekii strains

Several properties of virulent, avirulent, and interferon-resistant Rickettsia prowazekii strains were compared. All of the interferon-resistant rickettsial strains (which were derived from the avirulent Madrid E strain) resembled the virulent Breinl strain in that they grew well in untreated mouse macrophagelike RAW264.7 cells. In contrast, the avirulent Madrid E strain grew poorly in untreated RAW264.7 cells. Pretreatment of interferon-resistant rickettsiae or R. prowazekii Breinl with antirickettsial serum or immunoglobulin G suppressed the ability of the rickettsiae to grow in untreated RAW264.7 cells. Interferon-resistant R. prowazekii strains, like the Madrid E and Breinl strains, rapidly killed a substantial proportion of RAW264.7 cells that had been treated with gamma interferon or very high concentrations of alpha/beta interferon. Untreated infected RAW264.7 cells and interferon-treated mock-infected RAW264.7 cells were not killed during the same period. In cultures of RAW264.7 cells treated with either alpha/beta interferon (120 to 1,200 U/ml) or a subsaturating concentration of gamma interferon (0.5 U/ml), R. prowazekii Breinl organisms killed a higher percentage of the cells than did comparable numbers of R. prowazekii Madrid E organisms or interferon-resistant rickettsiae. Although R. prowazekii Breinl (like R. prowazekii Madrid E) was quite sensitive to gamma interferon in mouse L929 cells, the Breinl strain was resistant to murine alpha/beta interferon compared with the Madrid E strain and the two strains selected for resistance to murine gamma interferon. One of the interferon-resistant strains (strain 60P, which was selected for resistance to murine alpha/beta interferon) differed from the other R. prowazekii strains in that it induced little or no detectable interferon in L929 cell cultures.