Résultat de la recherche
Cross-immunity and community structure of a multiple-strain pathogen in the tick vector
Many 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.
Serological signature of tick-borne pathogens in Scandinavian brown bears over two decades
Background: Anthropogenic disturbances are changing the geographic distribution of ticks and tick-borne diseases. Over the last few decades, the tick Ixodes ricinus has expanded its range and abundance considerably in northern Europe. Concurrently, the incidence of tick-borne diseases, such as Lyme borreliosis and tick-borne encephalitis, has increased in the human populations of the Scandinavian countries. Methods: Wildlife populations can serve as sentinels for changes in the distribution of tick-borne diseases. We used serum samples from a long-term study on the Scandinavian brown bear, Ursus arctos, and standard immunological methods to test whether exposure to Borrelia burgdorferi sensu lato, the causative agent of Lyme borreliosis, and tick-borne encephalitis virus (TBEV) had increased over time. Bears had been sampled over a period of 18 years (1995-2012) from a southern area, where Ixodes ricinus ticks are present, and a northern area where ticks are uncommon or absent. Results: Bears had high levels of IgG antibodies against B. burgdorferi sensu lato but not TBEV. Bears at the southern area had higher values of anti-Borrelia IgG antibodies than bears at the northern area. Over the duration of the study, the value of anti-Borrelia IgG antibodies increased in the southern area but not the northern area. Anti-Borrelia IgG antibodies increased with the age of the bear but declined in the oldest age classes. Conclusions: Our study is consistent with the view that ticks and tick-borne pathogens are expanding their abundance and prevalence in Scandinavia. Long-term serological monitoring of large mammals can provide insight into how anthropogenic disturbances are changing the distribution of ticks and tick-borne diseases.
Effects of acquired immunity on co-feeding and systemic transmission of the Lyme disease bacterium, "Borrelia afzelii"
La borréliose de Lyme est une zoonose transmise par les tiques. Les bactéries qui en sont la cause, des spirochètes, se trouvent principalement chez des hôtes réservoirs sauvages tels que des rongeurs ou des petits oiseaux terrricoles. Borrelia afzelii est l’un des pathogènes responsables de la borréliose de Lyme les plus importants en Europe et est transmis par la tique du mouton Ixodes ricinus. Pendant le repas sanguin de la tique, le pathogène peut infecter l’hôte. La bactérie change l’expression des protéines de surfaces externes (Osp) pendant l’infection pour pouvoir s’adapter efficacement à l’hôte ou à la tique. L’une de ces protéines, OspC, joue un rôle important dans le développement de l’infection chez l’hôte.
Pendant le développement de l’infection systémique dans l’hôte vertébré, ce dernier va développer une immunité acquise (ex : anticorps) contre diverses protéines des spirochètes incluant OspC. Une des stratégies développée par le pathogène pour contourner les anticorps anti-OspC d’un hôte déjà infecté par une autre souche de Borrelia est de porter un allèle différent pour ospC. Nous pouvons ainsi distinguer des souches de B. afzelii en fonction de l’ospC dont elles disposent.
Borrelia afzelii dispose de deux modes de transmission: la transmission systémique (de l’hôte à la tique) et la transmission par co-feeding (de la tique à la tique). La transmission systémique requiert le développement d’une infection très répandue, multi-systémique, dans l’hôte vertébré dans le but de faciliter la transmission à de nouvelles tiques. Pendant la transmission par co-feeding, l’hôte crée simplement un pont qui va amener la tique infectée et la tique non-infectée à proximité l’une de l’autre. La transmission par co-feeding peut ainsi être une stratégie pour les spirochètes d’éviter le système immunitaire acquis et d’infecter de nouvelles tiques naïves.
Pour tester cette hypothèse, nous avons immunisé des souris de laboratoire avec l’une des deux protéines recombinantes OspC (rOspC) qui correspondent à deux souches différentes de B. afzelii : A3 et A10. Les anticorps contre un antigène OspC particulier bloquent la souche ciblée (homologue) mais pas la souche non-ciblée (hétérologue). L’immunisation réduit aussi drastiquement l’efficacité de la transmission par co-feeding. Chez les souris non-immunisées (contrôles) et les homologues, la transmission par co-feeding a atteint une prévalence de 51,6 % tandis que pour les souris homologues, ce taux n’a atteint que 3.3 %, correspondant à une baisse de 15,6 fois.
Nous avons recherché les effets de l’immunité acquise croisée en comparant les phénotypes d’infection entre les hétérologues et les souris contrôles. Les souris hétérologues ont une première expérience avec le ‘mauvais’ antigène OspC, et ces souris sont donc suspectées de répondre plus efficacement à l’infection avec B. afzelii que les souris contrôles naïves. Nous avons trouvé que l’immunité croisée avait un effet sur la charge en spirochètes des nymphes qui se sont nourries à l’état de larve sur les souris 1 mois après l’infection. La charge en spirochète moyenne dans les nymphes était 2,1 fois plus haute dans le groupe contrôle que dans le groupe hétérologue.
Il y avait aussi un grand effet du mode de transmission sur la charge en spirochètes des nymphes. La charge en spirochètes était en moyenne 4 fois plus basse dans les tiques de co-feeding que dans les tiques infectées par transmission systémique. Finalement, nous avons vu un effet important du vieillissement de la tique sur la charge en spirochètes des nymphes. Les nymphes qui ont été tuées 4 mois après la mue de la larve à la nymphe avaient une charge en spirochètes qui étaient 6,3 à 15,3 fois plus basse que les nymphes qui ont été tuées 3 mois plus tôt. Cette baisse de la charge en spirochètes à mesure que l’infection vieillit dans la nymphe pourrait avoir un effet sur la fitness de B. afzelii.
Cette thèse de doctorat a démontré que la transmission par co-feeding ne permet pas à B. afzelii d’échapper aux anticorps spécifiques anti-OspC et que la réponse immunitaire de l’hôte avait un effet négatif sur la fitness de Borrelia. Ce travail fournit de nouvelles informations sur les mécanismes de la transmission par co-feeding, sur les interactions hôte-parasite d’un pathogène responsable de la maladie de Lyme, et sur les effets de l’immunité acquise d’un hôte vertébré sur la transmission du pathogène., Lyme borreliosis is a tick-borne zoonotic disease and the causative spirochete bacteria are predominantly found in wildlife reservoirs such as rodents and ground-dwelling birds. Borrelia afzelii is one of the most common Lyme borreliosis pathogens in Europe, and is transmitted by the sheep tick Ixodes ricinus. During the tick blood meal, the pathogen can infect the host. The bacteria change the expression of their outer surface proteins (Osp) during the infection to adapt efficiently to the vertebrate host or the tick vector. One of these proteins, OspC, plays an important role in the development of host infection.
The systemically infected vertebrate host develops an acquired immune response (e.g. antibodies) against various spirochete proteins including OspC. One of the strategies developed by the pathogen to avoid the OspC-specific antibodies of a host already infected by a given Borrelia strain is to carry a different ospC allele. The ospC gene is therefore a useful genetic marker for classifying B. afzelii into different strains.
Borrelia afzelii has two modes of transmission: systemic (host-to-tick) transmission and co-feeding (tick-to-tick) transmission. Systemic transmission requires the development of a widespread, multi-system infection in the vertebrate host to facilitate transmission to new vector ticks. During co-feeding transmission, the host merely forms the bridge that brings the infected and uninfected ticks in close proximity to each other. Co-feeding transmission could therefore be a strategy for the spirochete to avoid the host immune system and to infect new naïve ticks.
To test this hypothesis, we immunized lab mice with one of two recombinant OspC (rOspC) proteins that belonged to two different strains of B. afzelii: A3 and A10. Antibodies against a particular OspC antigen blocked infection of the targeted (homologous) strain but not the non-targeted (heterologous) strain. Immunization also drastically reduced the efficacy of co-feeding transmission. In non-immunized or heterologous mice, the co-feeding transmission rate was 51.6 % whereas in homologous mice, this rate was 3.3%, corresponding to a 15.6-fold decrease.
We investigated the effects of cross-reactive acquired immunity by comparing the infection phenotypes between heterologous and control mice. The heterologous mice had previous experience with the heterologous OspC antigen, and we therefore predicted that these mice would respond more efficiently to infection with B. afzelii than the completely naive control mice. We found that cross-immunity had an effect on the spirochete load in the nymphal ticks that had fed on the mice one month post-infection. The mean nymphal spirochete load was 2.1 times higher in the control group than in the heterologous group.
There was also a large effect of the mode of transmission on the nymphal spirochete load. The spirochete load was on average 6 times lower in co-feeding ticks than in ticks infected via systemic transmission. Finally, there was an important effect of nymphal ageing on the spirochete load inside the nymph. Nymphs that were examined four months after the larva-to-nymph molt had a spirochete load that was 6.3 to 15.3 times lower than nymphs that were examined 3 months earlier. This decrease in spirochete load as the infection ages inside the nymphal tick could have an effect on the fitness of B. afzelii.
This PhD thesis demonstrated that co-feeding transmission does not allow B. afzelii to escape the OspC-specific antibodies and that host acquired immunity had a negative effect on the fitness of Borrelia spirochetes. This work provides new information on the mechanism of co-feeding transmission, on the host-parasite interactions of an important Lyme disease pathogen, and on the effect of acquired immunity in the vertebrate host on pathogen transmission.