Contribution of parasite and host genotype to immunopathology of schistosome infections

Abstract

Background: The role of pathogen genotype in determining disease severity and immunopathology has been studied intensively in microbial pathogens including bacteria, fungi, protozoa and viruses but is poorly understood in parasitic helminths. The medically important blood fluke Schistosoma mansoni is an excellent model system to study the impact of helminth genetic variation on immunopathology. Our laboratory has demonstrated that laboratory schistosome populations differ in sporocyst growth and cercarial production in the intermediate snail host and worm establishment and fecundity in the vertebrate host. Here, we (i) investigate the hypothesis that schistosome genotype plays a significant role in immunopathology and related parasite life history traits in the vertebrate mouse host and (ii) quantify the relative impact of parasite and host genetics on infection outcomes.

Methods: We infected BALB/c and C57BL/6 mice with four different laboratory schistosome populations from Africa and the Americas. We quantified disease progression in the vertebrate host by measuring body weight and complete blood count (CBC) with differential over a 12-week infection period. On sacrifice, we assessed parasitological (egg and worm counts, fecundity), immunopathological (organ measurements and histopathology) and immunological (CBC with differential and cytokine profiles) characteristics to determine the impact of parasite and host genetics.

Results: We found significant variation between parasite populations in worm numbers, fecundity, liver and intestine egg counts, liver and spleen weight, and fibrotic area but not in granuloma size. Variation in organ weight was explained by egg burden and intrinsic parasite factors independent of egg burden. We found significant variation between infected mouse lines in cytokine levels (IFN-γ, TNF-α), eosinophils, lymphocytes and monocyte counts.

Conclusions: This study showed that both parasite and host genotype impact the outcome of infection. While host genotype explains most of the variation in immunological traits, parasite genotype explains most of the variation in parasitological traits, and both host and parasite genotypes impact immunopathology outcomes.

Keywords: Schistosoma mansoni; BALB/c mouse; C57BL/6 mouse; Host-parasite interaction; Immunopathogenesis.

Fig. 1

Experimental timeline. We investigated the influence of parasite and host genotype on disease progression during schistosome infection in BALB/c and C57BL/6 mice. Four laboratory schistosome populations from Africa (SmEG) and the Americas (SmOR, SmLE and SmBRE) were used to infect the mice. Over a 12-week infection period, we quantified disease progression in the vertebrate host by monitoring body weight and complete blood count (CBC). Upon sacrifice, we measured multiple parasitological, immunopathological and immunological traits (see box, top right and main text)

Fig. 2

Parasitological outcome is driven by parasite rather than host genotype. Box and whisker plots showing A cercarial penetration rate, B worm burden, C liver and intestine egg counts and D fecundity for infections with SmBRE, SmEG, SmLE and SmOR in BALB/c (left) and C57BL/6 mice (right). Parasite populations were compared separately for each host strain. Different letters indicate comparisons that are statistically different as analyzed using Kruskal-Wallis (K-W) followed by Dunn’s or ANOVA followed by Tukey HSD post hoc test. Use of ANOVA or Kruskal-Wallis is shown at the top of each graph. For (C), stats are shown for liver eggs only. Host differences were not significant in any of the figures

Fig. 3

Impact of parasite and host on mouse immunopathology. A Longitudinal plots of weight gain in mice infected with the four parasite populations (solid lines) and uninfected controls (dotted lines). Means shown with standard error. B Liver weight or C spleen weight from euthanized BALB/c (left) and C57BL/6 mice (right) infected with the four parasite populations or uninfected controls. Photos (right) show representative organ samples from C57BL/6 mice infected with the four parasite populations and the control group for reference. Differences between hosts were not significant. D Fibrotic area and E granuloma area measured from liver sections. For all box plots, statistical comparisons are between infected groups only. Comparisons between parasite populations were conducted for each host strain separately. Different letters mean groups are significantly different in comparisons using Kruskal-Wallis (K–W) followed by Dunn’s or ANOVA followed by Tukey HSD post hoc test. Differences between BALB/c (left) and C57BL/6 mice (right) in the pathology traits were calculated with Wilcoxon rank-sum or Student's t-test and are shown by: ^#P < 0.05, ^##P < 0.01, ^###P < 0.001

Fig. 4

White blood cells are influenced by host background. Longitudinal data of A eosinophil, B basophil, C lymphocyte and D monocyte levels in BALB/c (top) and C57BL/6 mice (bottom) infected with the four infected parasite populations (solid lines) or uninfected controls (dotted lines). For all plots, statistical comparisons are between infected groups only. Mice with missing data points were excluded from the analysis. Groups with different letters and separated by host strain are significantly different (Friedman’s test followed by Conover’s post hoc test). #P < 0.05, ##P < 0.01, ###P < 0.001: values are significantly different between infected (parasite populations combined) host strains using Wilcoxon rank-sum test

Fig. 5

Cytokine production in response to schistosome infection. Box and whisker plots showing cytokine production of A IFN-γ, B TNF-α, C IL-4, D IL-5 and E IL-6 in BALB/c (left) and C57BL/6 mice (right) infected with the four parasite populations or uninfected controls. For all plots, statistical comparisons are separated by host strain and between infected groups only using Kruskal-Wallis (K-W) followed by Dunn’s post hoc test. #P < 0.05, ##P < 0.01, ###P < 0.001: values are significantly different between host strains as calculated by Wilcoxon rank-sum test

Fig. 6

Effect size by parasite and host genotype on disease parameters. Heatmap showing effect size measures (eta squared) as calculated by ANOVA. Parasite includes the four parasite populations, host includes both mouse strains, and interactions applies to the interaction between the two. Traits are organized in rows and assigned to the parasitology, immunopathology, or immunology category. *P < 0.05, **P < 0.01, ***P < 0.001: values are significant per ANOVA result