Cross-Immunity and Community Structure of a Multiple-Strain Pathogen in the Tick Vector

Abstract

bMany vector-borne pathogens consist of multiple strains that circulate in both the vertebrate host and the arthropod vector. Characterization of the community of pathogen strains in the arthropod vector is therefore important for understanding the epidemiology of mixed vector-borne infections. Borrelia afzelii and B. garinii are two species of tick-borne bacteria that cause Lyme disease in humans. These two sympatric pathogens use the same tick, Ixodes ricinus, but are adapted to different classes of vertebrate hosts. Both Borrelia species consist of multiple strains that are classified using the highly polymorphic ospC gene. Vertebrate cross-immunity against the OspC antigen is predicted to structure the community of multiple-strain Borrelia pathogens. Borrelia isolates were cultured from field-collected I. ricinus ticks over a period spanning 11 years. The Borrelia species of each isolate was identified using a reverse line blot (RLB) assay. Deep sequencing was used to characterize the ospC communities of 190 B. afzelii isolates and 193 B. garinii isolates. Infections with multiple ospC strains were common in ticks, but vertebrate cross-immunity did not influence the strain structure in the tick vector. The pattern of genetic variation at the ospC locus suggested that vertebrate cross-immunity exerts strong selection against intermediately divergent ospC alleles. Deep sequencing found that more than 50% of our isolates contained exotic ospC alleles derived from other Borrelia species. Two alternative explanations for these exotic ospC alleles are cryptic coinfections that were not detected by the RLB assay or horizontal transfer of the ospC gene between Borrelia species. Many vector-borne pathogens consist of multiple genetically distinct strains (1-4). The adaptive arm of the vertebrate immune system plays a key role in generating and maintaining this diversity of pathogen strains (5-7). Genetic diversity is often the highest at loci coding for surface-exposed pathogen molecules that function during the invasion and infection of host tissues (8, 9). The study of these highly polymorphic pathogen molecules is important for understanding how cross-reactive acquired immunity can mediate indirect competition and superinfection in the vertebrate host (10, 11). In addition, these pathogen outer surface proteins are often used to characterize pathogen strains because they provide an upper estimate of pathogen strain richness.In vector-borne diseases, the community of pathogen strains can be studied in both the vertebrate host and the arthropod vector. The vertebrate immune system creates nonrandom associations between pathogen strains (1, 12) that are subsequently transmitted to the arthropod vector. Conversely, the study of mixed infections in the arthropod vector can provide information on the processes that structure the community of multiple pathogen strains in the vertebrate host (13,14). In addition, estimates of strain richness in the arthropod vector are important for understanding the frequency with which ...

Multistrain Infections with Lyme Borreliosis Pathogens in the Tick Vector

“…This gene codes for outer surface protein C (OspC), which is a critical virulence factor for the infection of the vertebrate host (45,46). The ospC alleles can be clustered into ospC major groups (oMGs), which are more than 8% divergent in DNA sequence from other such clusters (5,24,(42)(43)(44). We have previously shown that genetic divergence among sequences belonging to the same oMG is Ͻ2% and that there are no intermediately divergent (2 to 8%) ospC sequences (24).…”

“…However, field studies that characterize the patterns of cooccurrence and abundance of multiplestrain vector-borne pathogens in the arthropod vector can indicate which pathogen strains are likely to have negative (or positive) interactions with each other in nature. Such field studies have made significant contributions to our understanding of these important infectious diseases (23)(24)(25)(26)(27)(28)(29)(30)(31).…”

“…The ospC alleles can be clustered into ospC major groups (oMGs), which are more than 8% divergent in DNA sequence from other such clusters (5,24,(42)(43)(44). We have previously shown that genetic divergence among sequences belonging to the same oMG is Ͻ2% and that there are no intermediately divergent (2 to 8%) ospC sequences (24). Thus, one advantage of this strain-typing system is that minor sequencing errors do not create new oMGs or result in the misclassification of the ospC sequences to the wrong oMG.…”

“…Strains are often defined by the highly polymorphic, single-locus ospC gene (5,24,30,31,40,(42)(43)(44). This gene codes for outer surface protein C (OspC), which is a critical virulence factor for the infection of the vertebrate host (45,46).…”

“…LB pathogens establish mixed-strain infections in both the vertebrate host (23,30,31,39) and the tick vector (23,24,(40)(41)(42). Strains are often defined by the highly polymorphic, single-locus ospC gene (5,24,30,31,40,(42)(43)(44).…”