Comparison of Models for Bubonic Plague Reveals Unique Pathogen Adaptations to the Dermis

Abstract

H ost-pathogen interactions begin with transmission and continue as the pathogen disseminates into deeper tissues. These tissues are in themselves microenvironments with defined immunological characteristics and can serve as barriers to prevent spread. For arthropod-borne pathogens, the skin epithelium is the first barrier that must be surpassed to penetrate into deeper organs (1, 2). The complexity of the immune responses of each layer of the skin reflects the specificity at which this tissue can act upon invaders.Based on its histological architecture, the skin can be divided into three main layers, namely, epidermis, dermis, and subcutaneous space (also known as hypodermis) (3). Each layer encompasses not only specific architectural qualities but also unique immunological characteristics that define them (2). How these characteristics influence the progression of infection is unknown. The epidermis is the outermost layer of the skin and constitutes a strong physical barrier that pathogens must cross to enter the body. Pathogens that survive in bloodfeeding arthropods can cross this layer through the mechanical action of the arthropod's probing mouth parts (4). Consequently, the dermis, located immediately adjacent to the epidermis, is highly exposed to these pathogens. Because most arthropod vectors do not penetrate beyond the dermis to the subcutaneous (s.c.) space, the dermis is, most likely, the first tissue where initial host-pathogen interactions occur during vector-borne infections (1,(5)(6)(7).Yersinia pestis is a Gram-negative bacterium and the causative agent of plague. This highly virulent pathogen was responsible for major pandemics in history and continues to survive in animal reservoirs in many parts of the world (8, 9). Bubonic plague is the most prevalent form of the disease and occurs after bacterial inoculation into the skin, typically by a flea vector (10). From the skin, Y. pestis disseminates to draining lymph nodes (LNs) via lymphatic vessels (11) and then to deeper organs through the bloodstream (12). Systemic dissemination results in septic shock, severe organ failure, and death.Models of infection that target the s.c. space are broadly used in plague research (13-16) as this is a relatively easy tissue to target. However, the dermis is strongly implicated as the layer of the skin probed by fleas during transmission (5, 17). In comparison with s.c. models, intradermal (i.d.) inoculations require more dexterity and can be more challenging to implement in biosafety level three facilities, which are mandatory when handling fully virulent strains of Y. pestis. In this study, we further characterize the i.d. model we used in a previous study (11) and compare i.d. and s.c. inoculations to investigate whether infection model impacts progression of bubonic plague. We found substantial differences in disease progression after inoculation by the two different routes. In addition, we found that dissemination of bacteria from the LNs is restricted. These findings are relevant for the understanding ...

Intramuscular Immunization of Mice with a Live-Attenuated Triple Mutant of Yersinia pestis CO92 Induces Robust Humoral and Cell-Mediated Immunity To Completely Protect Animals against Pneumonic Plague

“…Recently, it was reported that intradermal inoculation of Y. pestis in C57BL/6J mice resulted in faster kinetics of infection compared to that of the subcutaneous route of inoculation due to the organisms' greater ability to access the vascular and lymphatic vessels in the dermis (69). Studies have shown that the dermis of the skin is enriched in terminal lymphatic vessels, which facilitate antigen uptake and the infiltration of immune cells to mount a stronger immune response compared to that of the subcutaneous layer (70)(71)(72).…”

A Deadly Path: Bacterial Spread During Bubonic Plague

“…Infected fleas deposit bacteria in the dermal layer of the skin while obtaining a blood meal [3]. The bacterial population inoculated in the dermis can be divided into two: a large population that remains in the skin, and a very small population that leaves this tissue [4].…”

“…The bacteria that remain in the skin trigger the recruitment of neutrophils [4–6], which can be seen in close proximity to the bacteria or engulfing them. Neutrophils arrest bacterial proliferation in the skin but are unable to clear infection in this tissue [3, 4]. Accordingly, Y. pestis must interfere, at least to some extent, with the normal function of neutrophils.…”

“…Approximately 10 4 –10 5 colony forming units (CFU)/LN can be detected 24 hours after ID inoculation of ~200 CFU [3]. This number increases to ~10 8 CFU/LN around 48 hours after inoculation.…”

“…However, two possible mechanisms are (i) escape through efferent lymphatic vessels, to eventually drain into the subclavian vein, and (ii) escape through the LN vasculature, ruptured as a result of LN colonization. The use of genetically tagged bacteria revealed a reduction in the number of tags in the spleen (used to assess systemic dissemination) in comparison with tags in the LN [3]. This second bottleneck is a barrier the bacteria pass to reach the bloodstream.…”