Hereditary Hemochromatosis Restores the Virulence of Plague Vaccine Strains

Yersinia pestis is the causative agent of plague, an ancient scourge that precipitates intermittent pandemics [1, 2]. Plague is a disease of mammals that is transmitted by fleabite or aerosol droplets, causing either bubonic or pneumonic plague [3]. Inoculation of humans with live-attenuated Y. pestis strains, identified as nonpigmented (pgm) colonies on hemin agar, affords protection against bubonic and pneumonic plague [4]. Nonpigmented strains, most notably Y. pestis EV76, have been used extensively as live-attenuated vaccines to successfully stem the spread of the last plague pandemic in Asia [5]. The pathogenesis of Y. pestis infections relies on the pathogen's type III secretion pathway, which delivers Yop effectors into immune cells via type III needle complexes that are capped by the protective antigen LcrV [6–8]. The protective immunity raised by live-attenuated plague vaccines is based on antibodies that are directed against the fraction 1 capsular antigen (Caf1, or F1 pilus) of Y. pestis and that interfere with the delivery of effectors by LcrV-capped type III needle complexes [9, 10]. Y. pestis variants defective for the expression of F1 capsular antigen caused plague in animals that had been immunized with the live-attenuated vaccine, indicating that protection is not universal for all Y. pestis isolates [10]. Spontaneous excision and loss of the 102-kb high-pathogenicity island and pigmentation (pgm) locus occurs on the Y. pestis chromosome at flanking IS100 elements [11, 12]. Y. pestis pgm strains are attenuated in animal models of bubonic and pneumonic plague because of their inability to synthesize yersiniabactin, a siderophore that scavenges iron from transferrin during host infection [13, 14]. Live-attenuated pgm plague vaccine–related illnesses were not reported during the immunization campaign involving Asian populations [5]. However, in a small clinical trial in the United States, some recipients of the live-attenuated plague vaccine experienced malaise, fever, and severe reactogenicity requiring hospitalization [4]. Recently, the nonpigmented Y. pestis strain UC91309 was isolated from a researcher with fatal, septicemic plague [15]. Genome sequencing suggested that the isolated strain had been acquired in the laboratory [15]. Autopsy findings included abnormally high levels of iron deposits in the liver and markedly elevated levels of serum ferritin, iron, total iron-binding capacity, and iron saturation [15]. The presence of a HFE C282Y mutation supported the postmortem diagnosis of hereditary hemochromatosis [15]. These clinical and pathological findings suggested that iron overload in tissues of an individual with hereditary hemochromatosis may complement the iron-scavenging defect in mutants that are unable to synthesize yersiniabactin, thereby restoring the virulence of pgm variants and their ability to cause plague. Formal proof for this hypothesis is not yet available. To address this question, we analyzed the virulence properties of Y. pestis UC91309 and used a mouse model of hereditary hemochromatosis to examine whether increased iron load in host tissues may restore the virulence of nonpigmented Y. pestis strains. Further, we asked whether the rV10-2 plague subunit vaccine [16], which elicits LcrV-specific antibodies that block Y. pestis type III injection of immune cells [17], can protect mice with hereditary hemochromatosis against plague disease.