A Nonhuman Primate Scrub Typhus Model: Protective Immune Responses Induced by pKarp47 DNA Vaccination in Cynomolgus Macaques

Abstract

We developed an intradermal (ID) challenge cynomolgus macaque (Macaca fascicularis) model of scrub typhus, the leading cause of treatable undifferentiated febrile illness in tropical Asia, caused by the obligate intracellular bacterium, Orientia tsutsugamushi. A well-characterized animal model is required for the development of clinically relevant diagnostic assays and evaluation of therapeutic agents and candidate vaccines. We investigated scrub typhus disease pathophysiology and evaluated two O. tsutsugamushi 47-kDa, Ag-based candidate vaccines, a DNA plasmid vaccine (pKarp47), and a virus-vectored vaccine (Kp47/47-Venezuelan equine encephalitis virus replicon particle) for safety, immunogenicity, and efficacy against homologous ID challenge with O. tsutsugamushi Karp. Control cynomolgus macaques developed fever, classic eschars, lymphadenopathy, bacteremia, altered liver function, increased WBC counts, pathogen-specific Ab (IgM and IgG), and cell-mediated immune responses. Vaccinated macaques receiving the DNA plasmid pKarp47 vaccine had significantly increased O. tsutsugamushi–specific, IFN-γ–producing PBMCs (p = 0.04), reduced eschar frequency and bacteremia duration (p ≤ 0.01), delayed bacteremia onset (p < 0.05), reduced circulating bacterial biomass (p = 0.01), and greater reduction of liver transaminase levels (p < 0.03) than controls. This study demonstrates a vaccine-induced immune response capable of conferring sterile immunity against high-dose homologous ID challenge of O. tsutsugamushi in a nonhuman primate model, and it provides insight into cell-mediated immune control of O. tsutsugamushi and dissemination dynamics, highlights the importance of bacteremia indices for evaluation of both natural and vaccine-induced immune responses, and importantly, to our knowledge, has determined the first phenotypic correlates of immune protection in scrub typhus. We conclude that this model is suitable for detailed investigations into vaccine-induced immune responses and correlates of immunity for scrub typhus.

Comparative Accuracy of the InBios Scrub Typhus Detect IgM Rapid Test for the Detection of IgM Antibodies by Using Conventional Serology

“…Two important questions remain unresolved, (i) the longevity of IgM and IgG in human scrub typhus and (ii) which isotype appears earlier in naive and exposed populations. Nonhuman primate time course studies have shown that IgM and IgG can appear almost simultaneously in cynomolgus macaques ( 6 , 7 ).…”

The estimated burden of scrub typhus in Thailand from national surveillance data (2003-2018)

“…The incubation period ranges from 6 to 21 days and clinically, scrub typhus is characterised by fever, eschar, maculo-papular rash, headache, cough, nausea, vomiting, myalgia and lymphadenopathy [18]. The eschar at the site of the mite bite is the most characteristic feature but is not always present, being dependent on the degree of pre-existing immunity to the infecting Orientia tsutsugamushi strain [19,20]. Severe disease may present with complications affecting multiple organ systems [18].…”

Case Report: Concurrent Sympatric Scrub Typhus and Japanese Spotted Fever in Japan

“…The formation of an eschar, according to a nonhuman primate model, is reduced by preexisting cellular immunity in homologous infections. 16 On day 3, significant IgG titers were already present against all O. tsutsugamushi serotypes tested-especially against the Irie/Kawasaki and Gilliam types. The subsequent dynamic increase of IgG titers, rising rapidly before any IgM response, strengthens the suspicion of previous exposure, with subsequent reactivation of a preexisting B-cell immune memory response.…”

“…Had this patient been isolation positive and/or PCR positive for both pathogens, the case would have been unequivocally resolved on the basis of the proposed grading system. 7 However, previous findings in ST have shown that protective immunity results in a reduction of the bacterial load and a shorter bacteremic dissemination phase, 16 potentially limiting the likelihood of providing isolation-based and/or PCR-based evidence. The current case revealed the antibody dynamics to be even more complex in concurrent infections with JSF and emphasized the importance of more active screening for concurrent infection.…”