Amblyomma maculatum Feeding Augments Rickettsia parkeri Infection in a Rhesus Macaque Model: A Pilot Study

Abstract

Rickettsia parkeri is an emerging eschar-causing human pathogen in the spotted fever group of Rickettsia and is transmitted by the Gulf coast tick, Amblyomma maculatum. Tick saliva has been shown to alter both the cellular and humoral components of the innate and adaptive immune systems. However, the effect of this immunomodulation on Rickettsia transmission and pathology in an immunocompetent vertebrate host has not been fully examined. We hypothesize that, by modifying the host immune response, tick feeding enhances infection and pathology of pathogenic spotted fever group Rickettsia sp. In order to assess this interaction in vivo, a pilot study was conducted using five rhesus macaques that were divided into three groups. One group was intradermally inoculated with low passage R. parkeri (Portsmouth strain) alone (n = 2) and another group was inoculated during infestation by adult, R. parkeri-free A. maculatum (n = 2). The final macaque was infested with ticks alone (tick feeding control group). Blood, lymph node and skin biopsies were collected at several time points post-inoculation/infestation to assess pathology and quantify rickettsial DNA. As opposed to the tick-only animal, all Rickettsia-inoculated macaques developed inflammatory leukograms, elevated C-reactive protein concentrations, and elevated TH1 (interferon-γ, interleukin-15) and acute phase inflammatory cytokines (interleukin-6) post-inoculation, with greater neutrophilia and interleukin-6 concentrations in the tick plus R. parkeri group. While eschars formed at all R. parkeri inoculation sites, larger and slower healing eschars were observed in the tick feeding plus R. parkeri group. Furthermore, dissemination of R. parkeri to draining lymph nodes early in infection and increased persistence at the inoculation site were observed in the tick plus R. parkeri group. This study indicates that rhesus macaques can be used to model R. parkeri rickettsiosis, and suggests that immunomodulatory factors introduced during tick feeding may enhance the pathogenicity of spotted fever group Rickettsia.

Fig 1. Experimental design for tick feeding, R. parkeri/Vero cell inoculation, and sample collection.

Adult Amblyomma maculatum ticks were placed on the hosts as indicated. Either a partially purified low passage human isolate of R. parkeri or an uninfected Vero cell inoculum was administered at the indicated time points. Blood collection, physical exams (PE), rectal temperatures, and skin and lymph node biopsies were taken from all animals at the indicated time points. Complete necropsies were performed at the end of the study as indicated.

Fig 2. Evidence of an acute phase inflammatory response in response to R. parkeri inoculation.

Comparisons of neutrophil (A), lymphocyte (B), and C-reactive protein (C) concentrations in peripheral blood of all animals at the various time points indicated. Neutrophilia, lymphopenia, and elevated C-reactive protein were noted in the acute phase of infection after R. parkeri inoculation with greater neutrophilia noted in the tick + R. parkeri group. For presentation purposes all of the final time points are plotted as 31 dpi as opposed to 31, 32, and 35 dpi for the tick-only, tick feeding + R. parkeri, and R. parkeri-only groups, respectively.

Fig 3. Concentrations of serum inflammatory cytokines are increased in response to R. parkeri inoculation.

Comparisons of interleukin-6 (D), interferon γ (E), and interleukin-15 (F) concentrations in serum of all animals at the various indicated time points as determined by a magnetic cytokine bead panel kit. Measurements were performed in duplicate with the bars indicating standard error. For presentation purposes all of the final time points are plotted as 31 dpi as opposed to 31, 32, and 35 dpi for the tick-only, tick feeding + R. parkeri, and R. parkeri-only groups, respectively.

Fig 4. Eschars form after intradermal R. parkeri inoculation and are exacerbated by tick feeding during inoculation.

Photographs of tick feeding/inoculation lesions of each group at 4 dpi (A-C), the same locations as pictured in B and C at 31–35 dpi (D and E), and another tick feeding/inoculation site at 0 dpi for comparison (F). (A) Tick feeding alone results in cutaneous erythema at 4 dpi. (B) Intradermal inoculation of R. parkeri results in eschar formation (well circumscribed ulcer surrounded by an erythematous halo) at 4 dpi. (C) Intradermal inoculation of R. parkeri during tick feeding results in a large area of necrosis surrounded by erythema at the inoculation site at 4 dpi. (D) R. parkeri inoculation alone results in the formation of a small scar at 35 dpi. (E) A large healing ulcer has replaced the eschar from the tick feeding + R. parkeri animal at 32 dpi. (F) No gross alterations are noted at the time of R. parkeri inoculation (3 days post female tick infestation) for comparison. Black marks were made adjacent to inoculation sites.

Fig 5. Intradermal inoculation of R. parkeri results in marked diffuse dermatitis characterized by infiltrates of neutrophils and macrophages, epidermal necrosis, and dermal vasculitis at 4 dpi.

Photomicrographs of an H&E-stained skin section from a primate from the R. parkeri-only group at 4 dpi. (A) The epidermis is diffusely necrotic and superficial dermis is effaced by inflammatory cells. (B) Magnified view showing a dermal vessel (arrow) effaced by neutrophils and macrophages (vasculitis) and another dermal vessel with intact endothelium (arrowhead) surrounded by neutrophils and macrophages.

Fig 6. Anti-Rickettsia immunohistochemistry demonstrating numerous organisms in the skin of animals inoculated with R. parkeri at 4 dpi as opposed to rare Rickettsia in the tick-only animal.

Photomicrographs of skin sections stained with a polyclonal anti-Rickettsia antibody at 4 dpi. (A) Rare cells contain positive, brown-staining, rickettsial organisms in the tick-only animal. (B) Abundant positive, brown-staining, organisms in a section from an animal in the R. parkeri-only group. (C) Similarly, many organisms are noted in an animal from the tick + R. parkeri group. The red-framed images at the bottom are higher magnification views of the red-boxed areas in the top images. Black-framed insets are higher magnification images of the black-boxed areas and highlight the coccobacilli morphology of the positively stained rickettsial organisms.

Fig 7. Rickettsial DNA was detected in the skin of R. parkeri-inoculated animals at 4 and 9 dpi.

Rickettsial load as detected by qPCR in skin samples from 4 and 9 dpi expressed as R. parkeri ompB copies per 10,000 M. mulatta OSM copies. No rickettsial DNA was isolated from the tick-only macaque at any time point.