Project Summary
Microbial adaptation within different natural environments is a poorly understood area of emerging infectious
disease research and is relevant toward improving public health. For example, tick-borne relapsing fever (RF)
spirochetes are a significant yet understudied public health problem. The pathogens colonize insect and tick
vectors including Ornithodoros species. These ticks are long lived (10 – 20 years), possess multiple nymphal
stages, and maintain RF spirochetes transstadially and transovarially. Within the Ornithodoros species, RF
spirochetes persistently colonize the immunological and physiological disparate environments of the midgut and
salivary glands. An important aspect of vector competence for RF spirochetes is vector specificity. Here, a given
RF spirochete species has evolved to colonize a specific species of Ornithodoros tick. For example, nonnative
species (i.e. Borrelia hermsii) can infect Ornithodoros turicata, are detected post-molt in the midgut, are initially
detected in the salivary glands within 7 to 10 days post-acquisition, but cannot persistently colonize the salivary
gland acini post molt. This contrasts the native species (Borrelia turicatae) that persistently colonizes the salivary
glands for over five years. These findings and the vague understanding of argasid immunity indicates the need
of functional genomics approaches to delineate the mechanisms of vector colonization by RF spirochetes.
Recently, four putative antimicrobial peptides in the defensin family were identified. Designated OtdA-OtdD, they
have structural features common of defensins. Moreover, they are differentially expressed during the life cycle
of uninfected O. turicata, and preliminary data show that otdA and otdC remain up-regulated in infected tick
midguts. This further indicates a role of these peptides in immunity. Consequently, in this application a
comprehensive (Aim 1) and focused (Aim 2) approach will be implemented to understand argasid immunity.
The overall hypothesis is that B. turicatae has uniquely adapted to O. turicata immunity and are resistant to
defensin mediated killing. This hypothesis will be tested with two aims. The first aim will exploit the inability of B.
hermsii to persistently colonize the salivary glands of O. turicata. The hypothesis is that transcriptional responses
of immune related genes are consistent in O. turicata regardless of if the tick is infected with native (B. turicatae)
versus nonnative (B. hermsii) spirochetes. This comparative transcriptional analysis will indicate whether B.
turicatae has uniquely adapted to the natural immunological responses of the tick and/or if the species modulates
host gene expression to generate an environment suitable for colonization. The second aim will evaluate the role
of OtdA-OtdD in immunity. The hypothesis is that B. turicatae resists OtdA–OtdD killing while these small proteins
are borreliacidal to other species of RF spirochetes. The completion of this aim will determine whether O. turicata
OtdA–OtdD are novel tick antimicrobial peptides that are borreliacidal against nonnative species of RF
spirochete. Understanding the molecular events occurring in the tick will guide novel control strategies.
