Evidence for specific genotype-dependent immune priming in the lophotrochozoan Biomphalaria glabrata snail

Abstract

Historically, the prevailing view in the field of invertebrate immunity was that invertebrates that do not possess acquired adaptive immunity rely on innate mechanisms with low specificity and no memory. Several recent studies have shaken this paradigm and suggested that the immune defenses of invertebrates are more complex and specific than previously thought. Mounting evidence has shown that at least some invertebrates (mainly Ecdysozoa) show high levels of specificity in their immune responses to different pathogens, and that subsequent reexposure may result in enhanced protection (recently called 'immune priming'). Here, we investigated immune priming in the Lophotrochozoan snail species Biomphalaria glabrata, following infection by the trematode pathogen Schistosoma mansoni. We confirmed that snails were protected against a secondary homologous infection whatever the host strain. We then investigated how immune priming occurs and the level of specificity of B. glabrata immune priming. In this report we confirmed that immune priming exists and we identified a genotype-dependent immune priming in the fresh-water snail B. glabrata.

Fig. 1

Reinfection rates of BgGUA and BgBRE primary infected with 10 miracidia SmBRE (a) and reexposed to 10 miracidia of the homologous strain SmBRE (b). ‘Unprimed’ corresponds to snails that were not primary infected and exposed solely to the secondary infection.

Fig. 2

S. mansoni intramolluskal-stage development in the intermediate snail host, B. glabrata. A SpIs at 3 DPI in the snail foot. B SpIs full of SpIIs at 14 DPI in the foot. C SpIs degenerating at 25 DPI in the foot. D One SpII migrating in the snail kidney at 10 DPI. E The kidney full of migrating SpIIs at 20 DPI. F The first SpII (black arrowhead) observed at the digestive/genital gland interface at 14 DPI. G Higher magnification of the adjacent image. H The digestive and genital gland full of SpIIs at 25 DPI, showing some developing cercariae. I Higher magnification of the adjacent image. J SpIIs full of cercariae in the digestive and genital glands at 35 DPI. K Higher magnification of the adjacent image. L Cercariae in the snail mantle at 35 DPI. All scale bars are indicated.f = Foot; m = mantle; k = kidney; h = heart; i = intestine; st = stomach; ag = albumen gland; dg = digestive gland; gg = genital gland.

Fig. 3

a Infection rates of BgBRE snails subjected to primary infection with 10 irradiated or nonirradiated SmBRE miracidia, and then secondarily infected with 10 SmBRE miracidia. ‘Unprimed’ corresponds to snails that were not primarily infected and exposed solely to the secondary infection. b Detection of irradiate SmBRE miracidia in infected snails using diagnostic PCR amplification of the S. mansoni-specific SmAlphaFem gene (GenBank accession No. U12442.1) using primer pairs SmAlphaFem1 (270 bp) and SmAlphaFem2 (120 bp). * p < 0.05.

Fig. 4

Infection rates of BgBRE snails infected with 10 miracidia of SmBRE at 5 or 10 days DPTI. ‘Uninjured’ corresponds to healthy snails that did not receive tissue injuries. a Results from snails subjected to needle-induced tissue injuries. b Results from snails subjected to biolistic particle-induced tissue injuries.

Fig. 5

Experimental vaccination of B. glabrata with SmBRE miracidium extracts. Prevalence of naïve snails (control), snails injected with 20 µl of TBS-T and snails vaccinated with 1 µg of SmBRE miracidium extracts in 20 µl of TBS-T. Snails were treated 15 days before the exposure to 10 miracidia of SmBRE. n = Number of snails used in each group. * p < 0.05.

Fig. 6

Specific genotype-dependent immune priming in B. glabrata snails. Effect of a primary infection with SmBRE on prevalence (a) and intensity (b) after secondary infections with different Schistosoma strains. ND = Not determined - no intensity rate could be calculated because there was no snail infected. c Nei's genetic distances between the strain used for the primary infection and the strain used for the secondary infections (table 2b); Nei's distance could not be calculated because microsatellite markers for S. rodhaini were not available. Effect of a primary infection with SmBRE-LE on prevalence (d) and intensity (e) after secondary infections with different Schistosoma strains. f Nei's genetic distances between the strain used for the primary infection and the strain used for the secondary infections (table 2b); Nei's distance could not be calculated because microsatellite markers for S. rodhaini were not available.

Fig. 6

Specific genotype-dependent immune priming in B. glabrata snails. Effect of a primary infection with SmBRE on prevalence (a) and intensity (b) after secondary infections with different Schistosoma strains. ND = Not determined - no intensity rate could be calculated because there was no snail infected. c Nei's genetic distances between the strain used for the primary infection and the strain used for the secondary infections (table 2b); Nei's distance could not be calculated because microsatellite markers for S. rodhaini were not available. Effect of a primary infection with SmBRE-LE on prevalence (d) and intensity (e) after secondary infections with different Schistosoma strains. f Nei's genetic distances between the strain used for the primary infection and the strain used for the secondary infections (table 2b); Nei's distance could not be calculated because microsatellite markers for S. rodhaini were not available.