Schistosome Infection in the Humanized Mouse Model of Sickle Cell Disease

Introduction

Sickle cell disease (SCD) is a hemoglobinopathy due to a single amino acid substitution in the β-chain of hemoglobin (Hb), in which glutamic acid is substituted by valine. The individuals carrying the substitution on both β-chains (HbSS) develop the disease, characterized by chronic hemolytic anemia, systemic inflammatory state, and development of acute painful vaso-occlusive crises. Globally distributed, the disease has its highest incidence in sub-Saharan Africa (SSA), where its superimposes with the distribution of infections by parasitic worms, including schistosomes. However, there is scarcity of the literature on the possible interactions between parasitic worm infections and SCD.

Schistosomes are parasitic worms responsible for the disease named schistosomiasis which affects over 240 million people in the world, of which over 90 % are found in SSA. The worms live within the blood vessels of infected individuals where females lay eggs which are strongly immunogenic, causing a systemic type 2 inflammation. The disease is characterized by anemia and the formation of granulomas in the liver and intestines (for Schistosoma mansoni & S. japonicum), leading to hepatic fibrosis, pulmonary hypertension, and hepatosplenomegaly. The overlapping of the clinical features of schistosomiasis and SCD highlights the existence of possible interactions between them in areas where they are both present. Leveraging the humanized mouse model of SCD, the aim of this study is to demonstrate how schistosome infection could affect the pathophysiology of SCD or vice versa.

Methods

Twenty-five - 28 weeks old male and female Townes (HbAA, HbAS, HbSS) were infected with S. mansoni . Complete blood count was performed before and 8 weeks after infection. Liver and small intestines (ileum) were chemically digested with 4% KOH and eggs counted in the suspension under a microscope. Fecal pellets were also collected and dispersed in 2x PBS and eggs were counted in the suspension as for liver and intestines. Data were analyzed with GraphPad Prism 9.5.1 software by using paired t-test or Wilcoxon test. Genotypes were compared by one-way ANOVA or Kruskall-Wallis tests followed by two-stage linear step-up procedure of Benjamini-Krieger-Yekutieli for the control of false discovery rate.

Results

Compared to HbAA and HbAS, which did not differ between them, naïve HbSS mice had bigger livers and spleens. This pattern remained even after infection, with infected HbSS having bigger livers and spleens than infected HbAA and HbAS. After the infection, the spleen size increased only in HbAA and HbAS but not in HbSS while liver size increased in all 3 genotypes.

Schistosome infection caused a significant increase in the numbers of neutrophils, monocytes, eosinophils, and basophils. However, lymphocytes number significantly decreased in HbSS only. Red blood cells (RBCs) number decreased in HbAA and HbAS but not in HbSS. Accordingly, the mean corpuscular volume (MCV) and the mean corpuscular hemoglobin (MCH) increased in HbAA and HbAS, but not in HbSS. The Hb rate was not affected by the infection while the hematocrit slightly decreased only in HbAS. However, while the platelets number decreased only in HbSS, the mean platelet volume (MPV) increased in all 3 genotypes.

HbSS mice had fewer total S. mansoni egg number than HbAA and HbAS. This trend was corroborated by the number of eggs in intestinal tissues in which HbSS had fewer eggs than HbAA and HbAS. Although the number of eggs in the liver did not differ significantly between them, HbSS tended to have fewer liver eggs than HbAA and HbAS. All 3 mouse groups excreted similar number of eggs in the feces.

Conclusions

All 3 genotypes of Townes mice supported the development of S. mansoni worms as illustrated by the presence of eggs in their liver, intestines, and feces, making this model a good model for the study of parasitic worm infections in SCD. Schistosome infection has affected less the RBC components in HbSS as shown by no changes in RBC, Hb, MCV, and MCH values, and by no change in the spleen size. Further investigations are underway.