Spirochetes of the Borrelia genus are the cause of several prevalent vector-borne diseases. The most
well-known pathogen from this group is Borrelia burgdorferi sensu stricto, which causes over 300,000 cases of
Lyme disease in the United States each year. B. garinii and B. afzelii, which belong to the B. burgdorferi sensu
lato complex, are the primary agent of Lyme disease in Europe and Asia. Borrelia spirochetes are also the
etiological agent of the ancient human disease relapsing fever, as well as a newly recognized infectious
condition called Borrelia miyamotoi disease. Lyme-associated, relapsing fever-associated, and B. miyamotoi
spirochetes have differing lifecycles and their infections are accompanied by distinct clinical presentations.
However, each of these pathogens are known to encode multifunctional surface-expressed lipoproteins that
interact with vertebrate host molecules. Among these proteins are a small arsenal of immunomodulators that
specifically target and inactivate a primary arm of innate immunity known as the complement system. We have
recently reported two independent lines of evidence that support the hypothesis that one of these pathways,
known as the classical pathway, is important in controlling B. burgdorferi infections. First, we have shown that
mice deficient in the pattern recognition molecule of the classical pathway, C1q, are significantly more
susceptible to B. burgdorferi infection. Secondly, we have shown that the lipoprotein B. burgdorferi BBK32 is a
high-affinity inhibitor of the initiating protease of the classical pathway, C1r.
In Aim 1 of this project we seek to understand the C1r inhibitory activity of BBK32 sensu lato proteins at
the molecular level. In Aim 2 we will determine the immunomodulatory roles and virulence contribution of three
BBK32 orthologues known as FbpA, FbpB, and FbpC which are found uniquely in relapsing fever and B.
miyamotoi spirochetes. In Aim 3 we will delineate the role of C1r inhibition in borrelial pathogenesis using in
vivo models of disease. To achieve this, we propose a multi-disciplinary strategy that employs x-ray
crystallography, biophysical approaches, and complement functional assays to pinpoint key ‘hot-spot’ residues
on BBK32 that give rise to its potent anti-C1r activity. These data will inform the design of bbk32 mutants which
will be used in mouse infectivity studies to connect structural features of BBK32, at the amino-acid level, to an
in vivo phenotype. Parallel studies will use genetic deletion mutants of fbp genes from the relapsing fever-
associated spirochetes B. turicatae and B. hermsii. These studies will be paired with experimental models of
Lyme and relapsing fever borrelioses using C1r-/- mice to better understand the role of the classical pathway
initiating protease in the control of borrelial infections. By addressing fundamental questions of how medically
important Borrelia spirochetes recognize and evade host immunity, the studies proposed here stand to have a
broad and significant impact on the field of bacterial pathogenesis.