Application of microphysiological systems to unravel the mechanisms of schistosomiasis egg extravasation

Abstract

Despite decades of control efforts, the prevalence of schistosomiasis remains high in many endemic regions, posing significant challenges to global health. One of the key factors contributing to the persistence of the disease is the complex life cycle of the Schistosoma parasite, the causative agent, which involves multiple stages of development and intricate interactions with its mammalian hosts and snails. Among the various stages of the parasite lifecycle, the deposition of eggs and their migration through host tissues is significant, as they initiate the onset of the disease pathology by inducing inflammatory reactions and tissue damage. However, our understanding of the mechanisms underlying Schistosoma egg extravasation remains limited, hindering efforts to develop effective interventions. Microphysiological systems, particularly organ-on-a-chip systems, offer a promising approach to study this phenomenon in a controlled experimental setting because they allow the replication of physiological microenvironments in vitro. This review provides an overview of schistosomiasis, introduces the concept of organ-on-a-chip technology, and discusses its potential applications in the field of schistosomiasis research.

Keywords: 3D microphysiological systems (3D MPS); animal models; egg extravasation; granuloma; organ-on-a-chip (OOC); schistosomiasis.

Figure 1

Schistosoma life cycle. Infection begins when (Step 1) free-living cercariae penetrate human skin and (Step 2) cercariae lose tails and become schistosomula, followed by (Step 3) migration through the portal vein in the liver and maturation to adult schistosomes. Once in the liver, the female and male adults pair up (Step 4) and migrate to the intestines where the females release eggs. Interaction with the microbiome, epithelial cell death and remodeling lead to the active release of eggs, which are then released to the environment with host feces. (Step 5). Lastly, when in water eggs hatch into miracidia (step 6) which eventually penetrate the tissue of Biomphalaria spp snails to continue the cycle (step 7). Eggs that do not extravasate are encapsulated by vascular endothelia cells that trigger intravascular host-immune responses to induce VEC inflammation, proliferation, and migration. This then leads to the formation of granuloma in the liver that results in liver fibrosis after about 8 weeks. Created in BioRender.com.

Figure 2

Schistosome egg transition through gastrointestinal tissues. Modified from Schwartz and Fallon (2018) with further editing in Biorender.com. (A) Adult female schistosomes deposit eggs (about 300 eggs per day for S. mansoni) into the vasculature close to the lamina propria. Platelets and fibrinogen adhere to the eggs and activate the endothelium. Endothelial cells actively grow over the egg supporting its extravasation. Eggs that do not cross the endothelial border are disseminated by the blood flow and become trapped mostly in the liver portal system. (B, C) Immune cells, such as macrophages, T cells and eosinophils start to encapsulate the egg. Granuloma formation occurs around the egg and together with other processes, such as fibrinolysis, egg secretions-induced necrosis, leading to the passage of the egg toward the intestinal lumen. (D) Entrapped eggs become fibrotic and calcified in the liver during chronic schistosomiasis infection.

Figure 3

A conceptualized potential application of OOC to mimic schistosome egg migration through the gastrointestinal tissues. (A) A layout of the OOC showing the various components for tracking egg migration. (B) An enlarged diagram depicting the migration process on the OOC. The complex process involved in the egg migration, including the penetration of endothelial barrier wall, egg interaction with fibroblast and immune cells leading to the formation of granuloma in the stroma, and finally, egg transition through epithelial barrier into the intestinal lumen are depicted. (C) Fluorescence microscopy can be applied to track fluorescent beads embedded in the gel mimicking the stroma, and (D) force characterization using atomic force microscope (AFM) can yield quantitative information about the forces produced by host cells to propel Schistosome egg during the egg migration process. (A) is modified from Lee et al., 2019 (Lee et al., 2019).

Figure 4

Conceptualized representation of how fibroblast-dependent tissue mechanics may drive schistosome egg migration. (A) Recruitment phase where immune cells and fibroblasts respond to egg released immunogenically. Light blue arrows indicate migration direction. (B) Assembly phase involving ECM deposition and isotropic contraction without a resultant force due to tissue stress balance. Red arrows indicate hypothesized fibroblast-generated contractile forces within the granuloma (C) Propulsion phase where polarized ECM degradation or myofibroblast differentiation contributes to anisotropic contraction and egg propulsion. The red dashed arrows indicate diminishing forces due to matrix degradation. The solid black arrows in (B, C) represent the tissue stress balance or unbalance.

Figure 5

An illustration of a gel-based assay to decipher forces produced by host cells such as contractile fibroblasts to aid Schistosoma eggs migration. (A) A hydrogel is first characterized using (B) stiffness measurement equipment such as AFM or rheometer. (C) A stress-strain relationship obtained from the characterization is then used to derive quantitative stiffness information such as Young’s modulus. (D) Gel samples can be embedded with fluorescence particles such as beads for optical tracking and mapping of gel deformation during cell migration on or through the gel. Schistosoma eggs can be co-cultured on the gel surface in contact with host cells such as fibroblasts. (E) Tracking bead displacement using a fluorescent microscope would yield (F) a displacement field resulting from gel deformation. By applying particle image velocimetry (PIV) analysis, migration forces produced by host cells can be determined quantitatively.