Schistosomiasis affects more than 200 million people worldwide (23). The majority of cases affect the urogenital tract, caused mostly by S. haematobium. As a blood fluke, S. haematobium adult worms live primarily in the pelvic veins, where worm pairs lay eggs that lodge in the pelvic organs. Eggs laid in the bladder induce granuloma formation, which is thought to facilitate passage of eggs into the urinary stream (24, 25). Eggs voided into fresh bodies of water hatch into miracidia, which infect intermediate snail hosts. Infected snails release cercariae, the larval stage which infects humans.
The complex life cycle of S. haematobium depends upon the parasite successfully negotiating host tissue and immune responses that may threaten its survival. For instance, hematuria is a cardinal sign of urogenital schistosomiasis, and represents a compromised urothelial and endothelial barrier which allows S. haematobium to continue reproducing. However, unchecked damage to the urothelium and bladder blood vessels results in hemorrhage and even host death, which is counterproductive to any parasite (26). Hence, it is possible that S. haematobium and humans have co-evolved survival strategies. The exuberant proliferative response of the urothelium and endothelium to S. haematobium eggs may be one such strategy, since this likely promotes bladder tissue repair.
Schistosomes may induce host tissue repair responses, in particular angiogenesis, to promote egg expulsion. Turner et al. reported that deposition of S. mansoni eggs in Peyers’ patches of the mouse small intestine is associated with vascular remodeling and an expanded venule network (27). In mice deficient in Peyers’ patches, egg excretion is lessened, leading to more eggs entrapped in tissues, and consequently worsened host morbidity. We postulate that S. haematobium eggs similarly initiate angiogenesis in the bladder to facilitate their expulsion into the urinary stream.
Angiogenesis and urothelial proliferation benefit the S. haematobium-infected host in the short term but have been postulated to facilitate bladder carcinogenesis. S. haematobium is one of a handful of helminths that are known to be carcinogenic (28). Angiogenesis, such as that seen in bladders containing S. haematobium eggs, is a crucially important process for tumors, since their rapid growth puts them at risk of outstripping their blood supply. Expansion of local vascular beds may also enhance hematogenous spread of metastases. Urogenital schistosomiasis is associated with urothelial hyperplasia, a potentially pre-cancerous feature in the bladder.
Despite the prominence of urothelial and endothelial aberrations in urogenital schistosomiasis, our comprehension of the underpinnings of these pathological processes is limited. One mystery is whether S. haematobium eggs, without adult worms, are sufficient to activate urothelial hyperplasia and angiogenesis in the bladder. This is a significant matter because S. haematobium worms can live for years within their human hosts, whereas parasite eggs can remain in the bladder wall for decades and continue to drive chronic inflammation, even after successful treatment of infection. Understanding the contributions of S. haematobium worms to bladder pathogenesis is important to improving our knowledge base of chronic urogenital schistosomiasis. To that end, we observed that when S. haematobium eggs are injected into the bladder walls of mice, this elicits significant urothelial hyperplasia, angiogenesis, and vascular leakiness. These bladder changes are not seen when mice are injected with control vehicle. These findings intimate that S. haematobium worms play no or minimal role in the pathological changes of the bladder associated with urogenital schistosomiasis. Finally, these findings point to one or more S. haematobium egg-associated factors which mediate urothelial and endothelial changes.
We hypothesized that one of these egg-associated factors was the Interleukin-4 inducing Principle of Schistosoma mansoni Eggs (IPSE) (29). IPSE, also known as α-1 (8), features many host immunomodulatory functions. First, IPSE ligates Fcε receptor-bound IgE on the surface of mast cells and basophils to trigger IL-4 secretion (30–32). It also binds to immunoglobulins on the surface of B regulatory cells (Bregs) and thereby activates these cells (33). IPSE can also sequester chemokines; it was previously known as S. mansoni chemokine-binding protein (smCKBP) (34). Lastly, IPSE contains a nuclear localization sequence which directs the protein to host cell nuclei (9, 11), where it modulates transcription (10, 35).
Although IPSE has multiple immunomodulatory properties, our in vitro work indicates that H03-H-IPSE mediates its urothelial and endothelial effects via non-immune mechanisms, including through its nuclear localization sequence. The similar uptake of H03-H-IPSE by endothelial and urothelial cells hints that endocytosis occurs in either a non-specific fashion, or that these cell types share a receptor for H03-H-IPSE. We speculate that once IPSE is internalized by host cells, it may trigger pro-carcinogenic programs. Indeed, Roderfeld et al. showed that IPSE activated hepatocellular carcinoma-associated proto-oncogenes, namely c-Jun and its associated signaling molecule, STAT3 (35). This supports a potential role for H03-H-IPSE in promoting bladder oncogenesis.
Another interesting issue regarding the potential causal link between IPSE and schistosomal bladder carcinogenesis is whether this molecule promotes the high rates of squamous cell carcinoma seen in association with S. haematobium infection (36–39). This remains an open question, since we did not have an immortalized schistosomiasis-associated squamous cell bladder carcinoma cell line available for testing. However, schistosomal bladder cancer can still be associated with carcinomas arising from the urothelium, the tissue of origin for the HCV-29 cells tested herein.
Superficially, targeting IPSE to prevent schistosomal bladder cancer appears to be a worthy prophylactic approach. However, it has been reported that immunization of S. mansoni mice with IPSE leads to larger granulomas with enhanced macrophage activity and a mixed type 1 and type 2 immune response (40). Since granulomas contribute to host tissue fibrosis, neutralization of IPSE during schistosomiasis may actually be undesirable.
This investigation has noteworthy limitations. Although we hypothesize that H03-H-IPSE is sufficient to drive urothelial and endothelial proliferation and related effects, we have not demonstrated that it is necessary. There may be additional S. haematobium egg-derived factors which contribute to urothelial hyperplasia and angiogenesis during urogenital schistosomiasis. Ideally, transgenic approaches would be used to show that H03-H-IPSE is critical in angiogenesis and urothelial hyperplasia in vivo. This approach could be in reach for future studies given that both germline transgenesis and genome editing of egg-expressed genes have now been reported in (41, 42)
In conclusion, we have provided data that support the hypothesis that S. haematobium eggs are sufficient to initiate bladder urothelial hyperplasia, angiogenesis, and vascular permeability associated with urogenital schistosomiasis. These changes may be orchestrated by IPSE. Our observations are particularly striking considering that IPSE can increase proliferation of transformed urothelial and endothelial cell lines that, at baseline, already exhibit significant proliferation. IPSE may indeed be a pro-oncogenic factor of S. haematobium.