Molecular Pathogenesis of Coxiella burnetii, the agent of Q fever

Our laboratory works with the obligate intracellular bacterial pathogen,Coxiella burnetii, the etiologic agent of Q fever and a category B biothreat agent. Because of their obligate intracellular growth restriction, they have become exquisitely adapted to their specific niche, which is similar to a typical terminal phagolysosome that evolves into a large, replicative vacuole. C. burnetii depends on various strategies to down-regulate the normal innate host response to bacterial infection. The organism is extremely sensitive to oxidative stress, lacking several repair genes essential to mitigate oxidative DNA damage, has a reduce requirement for and uptake systems for acquisition of iron, and actively inhibits activation of an oxidative burst by phagocytic cells through the secretion of an acid phosphatase. Isolates that originate from acute Q fever patients are able to induce acute, atypical pneumonia in rodent challenge models while isolates from chronic Q fevers patients (most commonly endocarditis and hepatitis) do not cause acute disease in animal models, confirming distinct pathotype virulence potentials between isolate groups.
The long-term goal of this research is to understand the molecular pathogenic mechanisms involved in the host-pathogen interaction. To accomplish this broad goal, project in the lab are designed to test the molecular mechanisms employed by both the host and pathogen. Current pathogen studies include 1) broad survey of proteins secreted via a type 4 secretion system (T4SS) followed by determination of essentiality of each substrate for virulence and detailed analysis of mechanism of host modulation 2) survey of essential virulence loci identified by specific mutant screens, and 3) definition of the relative virulence of phylogenetically distinct isolate groups.
Our studies and those of other key research groups in this field have allowed the development of a more refined model of host-pathogen interactions and mechanisms used for survival and pathogenesis (Figure 1). These studies have been enabled by recent paradigm-shifting advances in experimental approach allowing for growth in extracellular media and generation of defined mutants and resulting in a renaissance of discovery for this important, but seldom-studied pathogen.

Figure 1. Roles of T4SS effectors during C. burnetii intracellular infection. At least three T4SS effectors localize to the nucleus (CBU1314, CBU1524/CaeA, and CBU1976) during ectopic expression and may be involved in the transcriptional modulation that occurs during C. burnetii infection.  Located next to the golgi apparatus, CBU0635 interferes with the host secretory pathway during ectopic expression.  Several T4SS effectors have F-box domains and are potentially involved in promoting proteasome-mediated degradation of proteins including CpeC (CBUA0006), which localizes to ubiquitin rich compartments in host cells. A new family of effectors has been identified that localize to the CCV and may have diverse functions, such as CCV stability and fusogenicity. Exogenously stimulated apoptosis is modulated byC. burnetii and an ankyrin repeat-containing protein, (AnkG, CBU0781) has anti-apoptotic activity through interaction with p32, a host protein that is involved in pathogen-induced apoptosis.  A second T4SS effector, (CaeB, CBU1532) alters mitochondrial membrane permeability during exogenously stimulated apoptosis.  While other effectors may be involved in modulation of apoptosis, C. burnetii infection increases the synthesis of anti-apoptotic proteins A1/Bf-1 and c-IAP2 and causes the activation of pro-survival kinases Akt and Erk1/2.

Vaccine and Diagnostic Development

These current studies are focused on 1) the response to infection by the host, especially by cells of the immune system, 2) understanding the components of protective immunity elicited by a killed whole cellular antigen (WCV-1) and, 3) identify appropriate vaccine strategies composed of recombinant and native antigens to elicit protective immunity. Protection against Q fever can be induced by vaccination with (WCV-1), yet due to the adverse reaction to vaccination of previously sensitized individuals, wide use of the vaccine has not been employed and no Q fever vaccine is available in the US. Our recent studies demonstrated that antibody against the O side chain polysaccharide of C. burnetii LPS is an important component of vaccine-induced immunity. Additionally, control of infection by vaccinates involves the ability to stimulate recall responses to antigen by memory T cells that activate interferon gamma-mediated killing by host macrophages. These vaccine strategies are being evaluated in rodent models of human Q fever with future studies planned for non-human primate testing.