Biology subjects

Burke, T. P.

Publications and source records attributed to Burke, T. P..

9 recordsLinked to original sources

bioRxiv2026

A genetic screen identifies O-antigen as essential for Rickettsia parkeri survival in macrophages by protecting from inflammasomes and interferon-stimulated genes

Lipopolysaccharide (LPS) is highly immunostimulatory, yet it is evolutionarily conserved among many obligate intracellular bacteria for unknown reasons. We report a forward genetic screen to identify factors required for survival of the tick-borne obligate cytosolic pathogen Rickettsia parkeri in primary macrophages. The most critical factors were WecA and RmlD, which synthesize O-antigen, the outermost layer of LPS. wecA and rmlD mutants grew at similar rates to wild type bacteria in epithelial cells, yet in macrophages they were targeted by guanylate binding proteins (GBPs) and they hyperactivated inflammasomes. Survival of O-antigen-deficient mutants was restored >1,000-fold in macrophages lacking Caspases-1 and -11, interferon signaling, and nitric oxide production, suggesting a multifaceted role for O-antigen in protecting against innate immunity. O-antigen was essential for causing disease in mice and protected R. parkeri against complement in vitro. Despite O-antigen being known as a major target of antibodies, mice immunized with O-antigen-deficient mutants were protected from a lethal rechallenge, suggesting that protection can be elicited independently of O-antigen-targeting antibodies. Together, these findings help resolve a paradox as to why obligate cytosolic bacteria evolutionarily maintain LPS despite it being immunostimulatory, which is that it serves as a multifunctional shield against innate immunity. SignificanceEukaryotic innate immune systems evolved to detect conserved microbial structures as danger signals of infection. Intracellular pathogens, in turn, evolved to hide from innate immunity, yet these mechanisms remain incompletely understood. Here, we performed an unbiased forward genetic screen in macrophages that identified lipopolysaccharide O-antigen as a critical virulence determinant in the tick-borne obligate cytosolic pathogen Rickettsia parkeri. We found that O-antigen shields the bacteria from multiple innate immune defenses, including guanylate-binding proteins, inflammasomes, nitric oxide, and complement. These findings reveal why a highly immunostimulatory molecule such as lipopolysaccharide is maintained by an obligate intracellular pathogen and establish O-antigen as a central determinant of Rickettsia cytosolic survival with implications for vaccine development.

microbiology↗

bioRxiv2025

Differences between human and rodent nitric oxide production dictate susceptibility to tick-borne Rickettsia.

Arthropod-borne pathogens cause serious human infections, yet they only cause limited disease in rodent reservoirs. Wild type mice resist infection by tick-borne Rickettsia parkeri, which causes spotted fever in humans, and it remains unclear why humans are vulnerable. Here, we report that whereas mouse type I interferon (IFN-I) or interferon-{gamma} (IFN-{gamma}) dramatically restrict R. parkeri in macrophages, human interferons do not. Differential RNA-seq revealed a significant induction of nitric oxide synthase 2 (Nos2, encoding inducible nitric oxide synthase, iNOS) in infected mouse but not human macrophages upon interferon treatment. Chemical iNOS inhibition or Nos2 deletion restored IFN-{gamma}-mediated restriction in mouse cells. Human cells treated with cytokine cocktails or with iNOS cofactors and substrates were still unable to restrict R. parkeri. In vivo, whereas wild type mice restricted R. parkeri, infected Nos2-/-mice developed mild skin eschars, recapitulating a key human disease manifestation. Together, our findings suggest that there is a threshold of NO production required to restrict R. parkeri, which mouse cells reach but human cells do not, and this is a key explanation for why humans develop tick-borne rickettsial diseases while rodents can be tolerant, asymptomatic reservoirs. Differences in NO abundance may provide an evolutionary explanation for human susceptibility to pathogens that propagate themselves in rodent reservoirs.

microbiology↗

bioRxiv2025

Comparative virulence analysis of seven diverse strains of Orientia tsutsugamushi reveals a multifaceted and complex interplay of virulence factors responsible for disease.

Orientia tsutsugamushi is an obligate intracellular bacterium found in Leptotrombidium mites that causes the human disease scrub typhus. A distinguishing feature of O. tsutsugamushi is its extensive strain diversity, yet differences in virulence between strains are not well defined nor well understood. We sought to determine the bacterial drivers of pathogenicity by comparing murine infections using seven strains combined with epidemiological human data to rank each strain in terms of relative virulence. Murine cytokine expression data revealed that the two most virulent strains, Ikeda and Kato, induced higher levels of IL-6, IL-10, IFN-{gamma} and MCP-1 than other strains, consistent with increased levels of these cytokines in severe human scrub typhus patients. We sought to identify the mechanistic basis of the observed differential virulence between strains by comparing their genomes, in vitro growth properties and cytokine/chemokine induction in host cells. We found that there was no single gene or gene group that correlated with virulence, and no clear pattern of in vitro growth rate that predicted disease. However, microscopy-based analysis of the intracellular infection cycle revealed that the only fully avirulent strain in our study, TA686, differed from all the virulent strains in its subcellular localisation and expression of its surface protein ScaC. We conclude that drivers of pathogenicity in Orientia tsutsugamushi are distributed throughout the genome, likely in the large and varying arsenal of effector proteins encoded by different strains, and that these interact in complex ways to induce differing immune responses and thus differing disease outcomes in mammalian hosts. Author SummaryScrub typhus is a vector-borne human disease caused by the bacterium Orientia tsutsugmushi and spread by mites. There are numerous different strains of this bacterium with some causing more severe disease in humans than others, and some that do not cause any illness at all. The factors driving these differences are not yet understood, and gaining insight into them could aid in vaccine development and help predict the severity of disease caused by new isolates. To better determine the mechanistic basis of pathogenicity in scrub typhus, we carried out experiments in which we compared seven diverse strains for virulence in animals. We measured their ability to cause disease in mice, so that we could reliably classify them as virulent or avirulent in this model. We then analysed various genomic and biological aspects to identify disease markers in both mice and humans. We report here that there is no single factor that predicts whether a strain will be pathogenic or not, but that disease in scrub typhus is a complex process resulting from the activity of multiple bacterial genes working together to drive different immune responses in the host, resulting in either clearance of the bacteria from the host, or escalating disease. Future work exploring the relationship of bacterial effector proteins will help to disentangle this complex relationship in mechanistic detail.

microbiology↗

bioRxiv2024

Cytosolic bacterial pathogens activate TLR pathways in tumors that synergistically enhance STING agonist cancer therapies

Bacterial pathogens that invade the eukaryotic cytosol are distinctive tools for fighting cancer, as they preferentially target tumors and can deliver cancer antigens to MHC-I. Cytosolic bacterial pathogens have undergone extensive preclinical development and human clinical trials, yet the molecular mechanisms by which they are detected by innate immunity in tumors is unclear. We report that intratumoral delivery of phylogenetically distinct cytosolic pathogens, including Listeria, Rickettsia, and Burkholderia species, elicited anti-tumor responses in established, poorly immunogenic melanoma and lymphoma in mice. We were surprised to observe that although the bacteria required entry to the cytosol, the anti-tumor responses were largely independent of the cytosolic sensors cGAS/STING and instead required TLR signaling. Combining pathogens with TLR agonists did not enhance anti-tumor efficacy, while combinations with STING agonists elicited profound, synergistic anti-tumor effects with complete responses in >80% of mice after a single dose. Small molecule TLR agonists also synergistically enhanced the anti-tumor activity of STING agonists. The anti-tumor effects were diminished in Rag2-deficient mice and upon CD8 T cell depletion. Mice cured from combination therapy developed immunity to cancer rechallenge that was superior to STING agonist monotherapy. Together, these data provide a framework for enhancing the efficacy of microbial cancer therapies and small molecule innate immune agonists, via the co-activation of STING and TLRs.

cancer biology↗

bioRxiv2023

Host glutathione is required for Rickettsia parkeri to properly septate, avoid ubiquitylation, and survive in macrophages.

Spotted fever group Rickettsia obligately reside in the cytosol where they parasitize over fifty metabolites from their hosts. However, the role for metabolite acquisition in pathogenesis remains unclear. Here, we find that depletion of the abundant low molecular weight thiol glutathione led to an impaired ability of Rickettsia parkeri to form plaques. Super-resolution microscopy revealed that glutathione depletion with buthionine sulfoximine (BSO) in endothelial or epithelial cells led to the formation of bacterial chains that increased in length over time. Chained bacteria had fewer actin-tails and were impeded for their ability to spread from cell to cell. Glutathione depletion also caused an increased frequency of colocalization between the bacterial surface and polyubiquitin. R. parkeri was significantly more restricted upon glutathione depletion in primary macrophages than in epithelial cells in a mechanism that avoided activating the inflammasome and the production of type I interferon. Together, these data suggest that host glutathione is critical for rickettsial septation, actin-based motility, avoiding ubiquitylation, and survival in immune cells.

microbiology↗

bioRxiv2021

A patatin-like phospholipase mediates Rickettsia parkeri escape from host membranes

Spotted fever group Rickettsia species are arthropod-borne obligate intracellular bacteria that can cause mild to severe human disease. These bacteria invade host cells, replicate in the cell cytosol, and then spread from cell to cell. To access the host cytosol and avoid detection by immune surveillance mechanisms, these pathogens must have evolved efficient ways to escape membrane-bound vacuoles. Although Rickettsia are predicted to express factors that disrupt host membranes, little is known about how and when these proteins function during infection. Here, we investigated the role of a Rickettsia patatin-like phospholipase A2 enzyme (Pat1) during host cell infection by characterizing a Rickettsia parkeri mutant with a transposon insertion in the pat1 gene. We show that Pat1 is important for infection in a mouse model and in host cells. We further show that Pat1 is critical for efficiently escaping from the single and double membrane-bound vacuoles into the host cytosol, and for avoiding host galectins that mark damaged membranes. In the host cytosol, Pat1 is important for avoiding host polyubiquitin, preventing recruitment of autophagy receptor p62, and promoting actin-based motility and cell-cell spread. Our results show that Pat1 plays critical roles in escaping host membranes and promoting cell-cell spread during R. parkeri infection and suggest diverse roles for patatin-like phospholipases in facilitating microbial infection. ImportanceSpotted fever group Rickettsia are bacteria that reside in ticks and can be transmitted to mammalian hosts, including humans. Severe disease is characterized by high fever, headache, and rash, and results in occasional mortality despite available treatment. Rickettsia interact with host cell membranes while invading cells, escaping into the cytosol, and evading cellular defenses. Bacterial phospholipase enzymes have been proposed as critical factors for targeting host cell membranes, however the specific roles of rickettsial phospholipases are not well defined. We investigated the contribution of one conserved patatin-like phospholipase, Pat1, in Rickettsia parkeri. We observed that Pat1 is important for virulence in an animal model. Moreover, Pat1 plays critical roles in host cells by facilitating access to the cell cytosol, inhibiting detection by host defense pathways, and promoting cell-cell spread. Our study indicates that Pat1 performs several critical functions, suggesting a broad role for phospholipases throughout the Rickettsia lifecycle.

microbiology↗

bioRxiv2020

Lysine methylation shields an intracellular pathogen from ubiquitylation

Many intracellular pathogens avoid detection by their host cells. However, it remains unknown how they avoid being tagged by ubiquitin, an initial step leading to anti-microbial autophagy. Here, we show that the intracellular bacterial pathogen Rickettsia parkeri uses two protein-lysine methyltransferases (PKMTs) to modify outer membrane proteins (OMPs) and prevent their ubiquitylation. Mutants deficient in the PKMTs were avirulent in mice and failed to grow in macrophages due to ubiquitylation and autophagy. Analysis of the lysine-methylome revealed that PKMTs modify a subset of OMPs by methylation at the same sites that are recognized by host ubiquitin. These findings show that lysine methylation is an essential determinant of rickettsial pathogenesis that shields bacterial proteins from ubiquitylation to evade autophagic targeting.

microbiology↗

bioRxiv2020

Rickettsia parkeri Sca2 promotes dissemination in an intradermal infection mouse model

Rickettsia are arthropod-borne pathogens that cause severe human disease worldwide. The spotted fever group (SFG) pathogen Rickettsia parkeri elicits skin lesion (eschar) formation in humans after tick bite. However, intradermal inoculation of inbred mice with millions of bacteria fails to elicit eschar formation or disseminated disease, hindering investigations into understanding eschar-associated rickettsiosis. Here, we report that intradermal infection of mice deficient for both interferon receptors (Ifnar-/-Ifngr-/-) with R. parkeri causes eschar formation, recapitulating the hallmark clinical feature of human disease. Intradermal infection with doses that recapitulate tick infestation caused eschar formation and lethality, including with as few as 10 bacteria. Using this model, we found that the actin-based motility protein Sca2 is required for R. parkeri dissemination from the skin to internal organs and for causing lethal disease, and that the abundant R. parkeri outer membrane protein OmpB contributes to eschar formation. We also found that immunizing mice with sca2 and ompB mutant R. parkeri protects against subsequent rechallenge with wild-type bacteria, revealing live-attenuated vaccine candidates. Thus, interferon receptor-deficient mice are a tractable model to investigate rickettsiosis, bacterial virulence factors, and immunity. Our results suggest that differences in interferon signaling in the skin between mice and humans may explain the discrepancy in susceptibility to SFG Rickettsia.

microbiology↗

bioRxiv2019

Inflammasome-mediated antagonism of type I interferon enhances Rickettsia pathogenesis

Inflammasomes and interferons constitute two critical arms of innate immunity. Most facultative bacterial pathogens that inhabit the host cell cytosol avoid activating inflammasomes and are often resistant to killing by type I interferon (IFN-I). We report that the human pathogen Rickettsia parkeri, an obligate intracellular pathogen that resides in the cytosol, is sensitive to IFN-I. The mechanism of IFN-I-dependent restriction requires the transcription factor IRF5, which upregulates anti-rickettsial factors including guanylate-binding proteins and iNOS. However, R. parkeri curtails cGAS-dependent IFN-I production by causing caspase-11-dependent pyroptosis. In vivo, inflammasome activation antagonizes IFN-I production, enhancing R. parkeri abundance in the spleen. Mice lacking either IFN-I or IFN-{gamma} signaling are resistant to infection, but mice lacking both rapidly succumb, revealing that both interferons are required to control R. parkeri. This study illuminates how an obligate cytosolic pathogen exploits the intrinsic trade-off between cell death and cytokine production to escape killing by innate immunity.\n\nHighlightsO_LIRickettsia killed by GBPs activates caspase-11 and GSDMD, promoting pyroptosis\nC_LIO_LIRickettsia exploits pyroptosis to avoid cGAS-dependent type I interferon\nC_LIO_LIIRF5, GBPs, and iNOS contribute to controlling R. parkeri infection\nC_LIO_LIIfnar-/-Ifngr-/- mice succumb to infection, uncovering a mouse model to study R. parkeri\nC_LI

immunology↗