Tick-borne Rickettsia cause serious human disease in the United States and worldwide, including spotted fever. These bacteria obligately reside in the host cell cytosol where they parasitize over 50 nutrients from the host. We hypothesize that this obligate lifestyle has led Rickettsia to evolve distinctive strategies to manipulate innate immunity. We use bacterial and host genetic approaches paired with advanced microscopy techniques to decipher how virulence genes enable Rickettsia to avoid inflammasomes, ubiquitylation, and interferon-stimulated genes. We recently discovered a mouse model that recapitulates key manifestations of human disease, which we use to determine how Rickettsia evade innate immunity in vivo. This animal model facilitates our ability to ask new questions about Rickettsia pathogenesis in vivo, including studies on improved therapeutics and vaccines.
Mouse vs human: Why do humans develop disease while animal reservoirs do not?
Many arthropod-borne viral and bacterial pathogens cause severe disease in humans, yet no disease in certain animals. This includes SARS-CoV2 (which causes COVID-19), Dengue virus, Rickettsia, and many other mosquito- and tick-borne pathogens. Rickettsia infection in mice is a valuable model system to study this phenomenon, as Rickettsia use rodents for their propagation and horizontal transmission in the wild. We are pursuing the hypothesis that animals including mice evolved inhibitory innate immune responses that have been lost in humans. We found that mouse interferon signaling, via redundant type I and type II interferons, restricts Rickettsia parkeri in mice. Mice lacking both interferon pathways develop disease that recapitulates key hallmark manifestations of human disease, including eschar (skin lesion) formation and dissemination (see below figure) (Burke et al., eLife 2021).
These discoveries provide the first animal model to recapitulate key human disease manifestations and enable a variety of studies on bacterial pathogenesis, vaccines, therapeutics, and host innate and adaptive immune responses.
However, this animal model is not perfect, as humans encode functional IFNAR and IFNGR. Thus, the key question remains: what interferon-stimulated factors are different between mouse and humans that are responsible for human disease? We are answering this comparative high throughput sequencing approaches and CRISPR-based screens. Ultimately, uncovering human susceptibility versus mouse resistance phenomena will lead us to improved animal models, improved therapeutic strategies, and an understanding of the animals that serve as reservoirs for arthropod-borne pathogens in nature.
Obligate lifestyles: How does nutrient uptake promote Rickettsia intracellular growth and innate immune evasion?
The roles for metabolite acquisition in Rickettsia parkeri physiology, pathogenesis, and evading innate immunity are unclear. We recently observed that depletion of the abundant low molecular weight thiol glutathione (GSH) in host cells led to an impaired ability of R. parkeri to form plaques. Super-resolution microscopy revealed that GSH depletion in macrophages, endothelial or epithelial cells caused bacterial chaining, fewer actin-tails, and an impeded ability to spread (figure on right). These bacteria are more targeted by antibacterial autophagy and are completely eliminated in immune cells.
These observations make an unexpected connection between Rickettsia nutrient acquisition and cell division and pathogenesis. More broadly, we are interested in understanding how nutrient uptake promotes Rickettsia physiology. An important outcome of these studies will be towards generating an axenic media that sustains Rickettsia growth outside of cells, which would be a valuable resource for the research community.
Pathogenesis: What virulence factors do Rickettsia use for growth in the host cytosol?
The virulence factors used by Rickettsia remain largely unknown, as genetic tools in these bacteria are less well developed than their facultative counterparts. We recently performed a forward genetic screen to identify the Rickettsia factors that promote growth in immune cells (image to right). This screen identified <5 genes that are required for growth and survival in macrophages. We are now investigating how these factors enable innate immune evasion, including how they promote R. parkeri’s ability to evade inflammasomes, interferons, and autophagy.
Using innovative imaging techniques to answer: How does Rickettsia persist and disseminate in vivo?
Rickettsia parkeri causes an eschar-associated human spotted fever rickettsiosis in the Americas with an increasing incidence of infection. However, it is unclear whether R. parkeri persists in vivo for prolonged periods, if R. parkeri causes asymptotic infections, and what organs are targeted in vivo. Better understanding tissue tropism, dissemination, and persistence will improve our understand of human disease. We are filling these knowledge gaps with innovative in vivo imaging techniques. These studies will enable us to better understand persistence, dissemination, tissue tropism, and how interventions such as antibiotics and adaptive responses limit infection.