Phylogenomic diversity elucidates mechanistic insights into Lyme borreliae host association

Abstract

Host association– the selective adaptation of pathogens to specific host species – evolves through constant interactions between host and pathogens, leaving a lot yet to be discovered on immunological mechanisms and genomic determinants. The causative agents of Lyme disease (LD) are spirochete bacteria composed of multiple species of the Borrelia burgdorferi sensu lato complex, including B. burgdorferi (Bb), the main LD pathogen in North America – a useful model for the study of mechanisms underlying host-pathogen association. Host adaptation requires pathogens’ ability to evade host immune responses, such as complement, the first-line innate immune defense mechanism. We tested the hypothesis that different host adapted phenotypes among Bb strains are linked to polymorphic loci that confer complement evasion traits in a host-specific manner. We first examined the survivability of 20 Bb strains in sera in vitro and/or bloodstream and tissues in vivo from rodent and avian LD models. Three groups of complement-dependent host association phenotypes emerged. We analyzed complement-evasion genes, identified a priori among all strains and sequenced and compared genomes for individual strains representing each phenotype. The evolutionary history of ospC loci is correlated with host-specific complement-evasion phenotypes, while comparative genomics suggests several gene families and loci are potentially involved in host association. This multidisciplinary work provides novel insights into the functional evolution of host adapted phenotypes, building a foundation for further investigation of the immunological and genomic determinants of host association.IMPORTANCEHost association is the phenotype that is commonly found in many pathogens that preferential survive in particular hosts. The Lyme disease (LD) causing agent, B. burgdorferi (Bb), is an ideal model to study host association, as Bb is mainly maintained in nature through rodent and avian hosts. A widespread yet untested concept posits that host association in Bb strains is linked to Bb functional genetic variation conferring evasion to complement, an innate defense mechanism in vertebrate sera. Here, we tested this concept by grouping 20 Bb strains into three complement-dependent host association phenotypes based on their survivability in sera and/or bloodstream and distal tissues in rodent and avian LD models. Phylogenomic analysis of these strains further correlated several gene families and loci, including ospC, with host-specific complement-evasion phenotypes. Such multifaceted studies thus pave the road to further identify the determinants of host association, providing mechanistic insights into host-pathogen interaction.

Structural evolution of an immune evasion determinant shapes Lyme borreliae host tropism

“…Though it is possible that lack of selective pressure detection reflects the neutral selection experienced in cspZ, as a systemically redundant anti-complement protein (58), the possibility that cspZ is undergoing the same selective pressures as other functionally redundant proteins (e.g., ospC) cannot be excluded. In fact, balancing selection on one locus can increase diversity in another genetically-linked locus (59), and there is genetic linkage between CspZ types and ospC types (27,28,36). This may raise an intriguing possibility that the balancing selection on ospC may allow diversification through drift in other functionally redundant genes like cspZ without deleterious effect to the spirochetes, which warrants further investigations.…”

“…Some Lyme borreliae species or strains carry determinants that promoting host-specific phenotypes that differ from the known host range of these species or strains (36,(60)(61)(62). For example, the B. burgdorferi strain 297 is highly infectious in mice, but its CspZ loop-structures are identical to those in CspZB379 from the strain B379 that is not mouse adapted (36,62). This (which was not certified by peer review) is the author/funder.…”

“…This may raise an intriguing possibility that the balancing selection on ospC may allow diversification through drift in other functionally redundant genes like cspZ without deleterious effect to the spirochetes, which warrants further investigations. Some Lyme borreliae species or strains carry determinants that promoting host-specific phenotypes that differ from the known host range of these species or strains (36,(60)(61)(62). For example, the B. burgdorferi strain 297 is highly infectious in mice, but its CspZ loop-structures are identical to those in CspZB379 from the strain B379 that is not mouse adapted (36,62).…”

“…The ΔcspZ strain had significantly greater levels of mouse C5b-9 and quail C8 deposition than WT B31-A3 or pCspZB31 (Fig. 3C-D) (32,36). Compared to ΔcspZ, pCspZB379 had indistinguishable levels of mouse C5b-9 deposition but significantly lower levels of quail C8, whereas pCspZB408 harbored significantly lower levels of mouse C5b-9 but indistinguishable levels of quail C8 (Fig.…”

“…Mouse FH was purchased from MyBiosource (San Diego, CA). Quail FH and recombinant OmCI proteins were generated as described previously (24,32,36,39). The mouse and quail sera were obtained from Southern Biotech, Inc (Birmingham, AL) and Canola Live Poultry Market (Brooklyn, NY), respectively.…”