4.1 Chronic inflammation

Liver pathology begins with lodging of schistosomal eggs in the liver sinusoids. These eggs, along with SEAs, trigger a significant T-helper 1 (Th1) type immune response. This involves an increased expression of Th1 type cytokines involving interleukin-1 (IL-1), IL-2, IL-6, interferon gamma, tumour necrosis factor alpha (Stadecker et al., Reference Stadecker, Asahi, Finger, Hernandez, Rutitzky and Sun2004; Butrous et al., Reference Butrous2019; Zheng et al., Reference Zheng, Zhang, Chen, Nie, Miller, Gong and Liu2020; Wang et al., Reference Wang, Tang, Bergquist, Zhou and Qin2023). Though Th-1 type immune response has a killing effect over schistosomes, it appears that humans lack the capacity to eliminate schistosomes, which might also be linked to escape mechanism used by these parasites (Trottein et al., Reference Trottein, Nutten, Angeli, Delerive, Teissier, Capron, Staels and Capron1999). Continuous stimulation by eggs and SEAs lead to an excessive Th1 inflammatory response.

After about four weeks of infection and egg production, the Th1 response shifts towards a Th2 response which exerts anti-inflammatory effects and regulates the immunopathology of Th1 response (Zheng et al., Reference Zheng, Zhang, Chen, Nie, Miller, Gong and Liu2020). This response is marked by heightened secretion of Th2 cytokines involving IL-4, IL-5, and IL-13. These promote polarization of macrophages to M2 type macrophages, creating a microenvironment having a high level of type 2 immune responses which further promotes the formation of granulomas (Oliphant et al., Reference Oliphant, Barlow and McKenzie2011; Yunna et al., Reference Yunna, Mengru, Lei and Weidong2020).

4.2 Granuloma formation

Granuloma formation is supposedly beneficial for the host because it blocks the toxic effects of egg antigens released from the eggs (Morais et al. Reference Morais, Souza, Melo, Aroucha, Miranda, Domingues, Abath and Montenegro2008; Jain & Rana Reference Jain and Rana2024). Once the type 2 immune responses set in, granuloma formation begins. Egg granulomas are generally composed of massive number of lymphocytes, monocytes, macrophages, eosinophils and neutrophils (Liu et al., Reference Liu, Zhang, Liang and Lu2022). Granulomas contain T-lymphocytes of both Th-1 and Th-2 response types (Bogen et al., Reference Bogen, Flores Villanueva, McCusker, Fogelman, Garifallou, el-Attar, Kwan and Stadecker1995).

The core of granuloma might have different immune cells depending upon the species of schistosome that has infected the host (Jain & Rana Reference Jain and Rana2024). Apart from Sj, Schistosoma mansoni (Sm) has also been implicated in LC induction but currently has been assigned to Group 3 of IARC classification (von Bülow et al., Reference von Bülow, Lichtenberger, Grevelding, Falcone, Roeb and Roderfeld2021). Regardless of LC induction, both parasites are capable of granuloma formation. Granulomas elicited by Sj and Sm show distinct cellular composition. Chemokine binding protein secreted by Sm eggs is capable of binding to a specific chemokine and inhibiting its action. It binds to neutrophil chemo-attractant CXCL8 but does not bind to eosinophil chemo-attractant CCL11. This blocks the infiltration of neutrophils but does not block the infiltration of eosinophils as shown in Figure 1, hence leading to a distinct granulomatous pathology where granulomas elicited by Sj will have core mainly of neutrophils, whereas that elicited by Sm will have a core mainly of eosinophils (Figure 1) (Chuah et al. Reference Chuah, Jones, Burke, McManus and Gobert2014; Jain & Rana Reference Jain and Rana2024). While granulomas rarely cause structural liver damage (Culver et al., Reference Culver, Watkins and Westbrook2016), they lead to fibrosis in the periportal space.

Figure 1. Granuloma’s core composition in case of infection with Schistosoma japonicum and Schistosoma mansoni. Binding of a specific chemo-attractant to chemokine-binding protein secreted by Schistosoma mansoni leads to recruitment of eosinophils in the core of granuloma elicited by it. This is distinct to the core of granuloma elicited by Schistosoma japonicum which is mainly composed of neutrophils.

A study conducted in mice showed that neutrophils can cause major hepatic necrosis and are progenitors of hepatic fibrosis in acute stage of schistosomal infection (Hirata et al., Reference Hirata, Hara, Kage, Fukuma and Sendo2002). The neutrophil core further produces pro-inflammatory cytokines, including IL-1α, IL-1β, IL-6, TNF, CCL3 and CXCL1, which contributes to overall local tissue damage (Chuah et al., Reference Chuah, Jones, Burke, Owen, Anthony, McManus, Ramm and Gobert2013; Llanwarne & Helmby, Reference Llanwarne and Helmby2021). These granulomas ultimately promote severe liver fibrosis, another step in LC induction (Wynn & Cheever, Reference Wynn and Cheever1995).

4.3 Liver Fibrosis

Liver fibrosis involves excessive deposition of extracellular matrix (ECM) in the liver which also involves an up-regulation of ECM production and down-regulation of ECM degradation. These accumulated ECM proteins start distorting the hepatic architecture (Bataller & Brenner, Reference Bataller and Brenne2005). Liver fibrosis occurs in the hepatic sinusoids and portal veins (Hoffmann et al., Reference Hoffmann, Cheever and Wynn2000; Wang et al., Reference Wang, Fan, Lei, He, Wang, Luo, Zhang and Pan2020).

These changes in hepatic architecture are followed by periportal fibrosis, which is also known as periportal hepatic fibrosis (PHF), the most serious manifestation of chronic schistosomal infection. PHF impedes the blood flow and hence impedes the nutrient reach to the liver, causing liver function impairment which can lead to an advanced stage of liver fibrosis leading to cirrhosis. It ultimately leads to portal hypertension and hepatomegaly, splenomegaly and predisposes the infected person to a potential failure of liver if not provided with appropriate treatment (Wiest et al., Reference Wiest, Wu, Zhong, McGarvey, Tan, Yuan, Peters, Olveda and Olds1993; Coutinho et al. Reference Coutinho, McGarvey, Acosta, Manalo, Langdon, Leenstra, Kanzaria, Solomon, Wu, Olveda, Kurtis and Friedman2005; Kamdem et al. Reference Kamdem, Moyou-Somo, Brombacher and Nono2018).

The initiation and progression of liver fibrosis to an advanced stage involves activation and proliferation of hepatic stellate cells (HSCs). Quiescent HSCs (qHSCs) are activated in response to fibrotic stimuli, including chronic inflammatory reactions, to become activated HSCs (aHSCs) (Wynn, Reference Wynn2008; Li et al., Reference Li, He, Guo, Chen and Shan2015). qHSCs are situated in the space of Disse, between hepatocytes and sinusoidal endothelial cells and constitute about 15% of the total liver resident cells. qHSCs play important roles in liver regeneration, vitamin A storage, immuno-regulation among others (Li et al., Reference Li, He, Guo, Chen and Shan2015; Zhang et al., Reference Zhang, Yuan, He, Lei and Wang2016). aHSCs are source of collagen and have the potential to abundantly secrete ECM proteins and tissue inhibitors of metalloproteinases, all of which are responsible for changing the liver architecture (Puche et al., Reference Puche, Saiman and Friedman2013).

A recent study found that schistosomal egg ribonuclease SjCP1412 promotes granuloma formation and accelerates liver fibrosis (Li et al., Reference Li, Li, Yang, Dong, Mei, Lu and Yang2023). Another study which used a mouse model, concluded that aHSCs play an important role in liver fibrosis in Sj-infected host (Huang et al., Reference Huang, Ma, Cao, Zhan, Zhang, Wang, Zhang, Xu and Xia2022a). All these changes lead to an advanced stage of liver fibrosis and cirrhosis, which are major risk factors for HCC (Dhar et al., Reference Dhar, Baglieri, Kisseleva and Brenner2020). Interestingly, an egg antigen Sjp40 supresses HSC activation; however, it appears that it does so to prevent egg death as collagen deposits near the eggs are detrimental for their survival. This observation is supported by the fact that HSCs tend to be located on the margins of granulomas (Zhu et al., Reference Zhu, Yang, Shen, Chen, Chen, Sun, Duan, Zhang, Zhu and Duan2018; Li et al., Reference Liu, Zhang, Liang and Lu2022). Figure 2 shows a proposed mechanism on how Sj infection alone is inducing LC through chronic inflammation and SEAs, leading to fibrosis and cirrhosis and ultimately paving way for carcinogenesis.

Figure 2. Proposed mechanism of liver cancer induction by Schistosoma japonicum. Deposited eggs lead to inflammation and release SEAs, which causes granuloma formation. Chronic infection and inflammation lead to liver fibrosis and cirrhosis and lead to DNA damage, creating a suitable microenvironment for induction of liver cancer.

4.4 Co-infection with HBV and HCV

Concurrent infections of Sj with HBV or/and HCV can lead to heightened liver deterioration and severe illness and is a risk factor for LC induction (Jain & Rana, Reference Jain and Rana2024). Chronic schistosomiasis coupled with HBV or/and HCV infection not only affects the architecture of hepatocyte arrangement but is detrimental for the viability of hepatocytes (Omar, Reference Omar2019).

A study reported that HBV/Schistosoma spp. concurrent infection from the years 1980–2014 ranged from as low as 9.6% to as high as 64% in Egypt among all reported schistosomiasis patients. For HCV, the figures for same period ranged between 1% in Ethiopia to 50% in Egypt. Same study also collated interesting data on the prevalence of co-infection HCV/HBV and hepatosplenic schistosomiasis (HSS). For HCV, of total patients with schistosomiasis, those with HCV and HSS ranged from as low as 0.06% in China to 40.2% in Egypt, whereas with all the same parameters, the cases for HBV and HSS ranged from 15.8% in Brazil to 58.4% in Egypt (Gasim et al., Reference Gasim, Bella and Adam2015). Concurrent infection of Sj and hepatitis viruses can lead to liver cirrhosis and a synergistic effect can be expected leading to exacerbated liver pathology (Gasim et al., Reference Gasim, Bella and Adam2015).

Studies have reported an increased susceptibility for HBV in patients with chronic schistosomiasis (Zhang et al., Reference Zhang, Xie, Chen, Chen, Dong and Tan2020). While few studies point out that this might be due to frequent blood transfusions which are necessary for chronic schistosomiasis patients (Gasim et al., Reference Gasim, Bella and Adam2015), other studies have rejected the hypothesis altogether for different schistosomal species (Larouzé et al., Reference Larouzé, Dazza, Gaudebout, Habib, Elamy and Cline1987; Ye at al., Reference Ye, Fu, Anderson and Nokes1998). Nonetheless, chronic schistosomiasis and HBV co-infection is expected to end in liver cirrhosis and HCC in advanced cases (Omar, Reference Omar2019).

Moreover, the co-infection prolongs the carriage stage which facilitates chronic hepatitis with significant cirrhosis (Abruzzi et al., Reference Abruzzi, Fried and Alikhan2016). Similar observations were made in patients with chronic schistosomiasis and HCV infection, where a decreased capability of resolving HCV infection was observed in schistosomiasis patients (Kamal et al., Reference Kamal, Rasenack, Bianchi, Al Tawil, El Sayed Khalifa, Peter, Mansour, Ezzat and Koziel2001) and higher liver fibrosis, cirrhosis and HCC were reported (Abruzzi et al., Reference Abruzzi, Fried and Alikhan2016; Omar, Reference Omar2019).

Conversely, an observational study has reported no increase in liver damage in schistosomiasis infected patients who were co-infected with HBV in non-endemic area (Marchese et al., Reference Marchese, Beltrame, Angheben, Marocco, Gaeta and Bisoffi2020).

4.5 Changes at genetic level

As discussed previously, chronic inflammation, granuloma formation, fibrosis, cirrhosis and co-infection with hepatitis viruses are probable factors leading to initiation of SJLC; however, it is important to note that all these factors work simultaneously to induce changes at genetic and molecular level, which ultimately leads to cancer induction.

An important aspect of chronic inflammation is the generation of reactive oxygen species (ROS) (Wang et al., Reference Wang, Li, Ye, Xie and Li2016). During early infection, the activated macrophages contribute to heightened generation of ROS. SEAs induce ROS generation by NADPH oxidase 2 and mitochondria in macrophages, which is essential for M2 macrophage differentiation, which further promotes granuloma formation and fibrosis (Yu et al., Reference Yu, Wang, Wang, Gu, Lei, Tang, Wei, Xu, Wang, Chen, Pu, Qi, Yu, Chen, Zhu, Li, Zhang, Zhou and Su2021). Oxidative stress, exerted by ROS, disrupts normal cell function, interferes with genetic elements and affects signal transduction pathways (Wang et al., Reference Wang, Li, Ye, Xie and Li2016). At genetic level, it induces oxidative damage to both nuclear and mitochondrial DNA, whereas it plays key roles in Wnt/β-catenin and Notch pathways (Li et al., Reference Li, Yu, Yang and Wang2023). All these changes predispose liver towards HCC.

Many studies have linked liver fibrosis and cirrhosis with shortening of telomere length in the hepatocytes (Donati & Valenti, Reference Donati and Valenti2016; Barnard et al., Reference Barnard, Moch and Saab2018; Shin et al., Reference Shin, Park, Yu, Jin, Suh, Lee and Kim2021). This shortening of telomere in hepatocytes has been previously correlated with chromosomal instability which is known to cause hepatoma in humans (Plentz et al., Reference Plentz, Caselitz, Bleck, Gebel, Flemming, Kubicka, Manns and Rudolph2004). Further, telomere length and its shortening has also been linked to HCC diversity (Ningarhari et al., Reference Ningarhari, Caruso, Hirsch, Bayard, Franconi, Védie, Noblet, Blanc, Amaddeo, Ganne, Ziol, Paradis, Guettier, Calderaro, Morcrette, Kim, MacLeod, Nault, Rebouissou and Zucman-Rossi2021). Development of hypoxic environment which is possibly related to cirrhosis has also been reported. The study has labelled this hypoxic liver parenchyma as highly angiogenic (Yu et al., Reference Yu, Chen and Ding2010), another factor probably linked with SJLC induction and progression.

A recent study confirmed the role of B-lymphoma Mo-MLV insertion region 1, a known proto-oncogene, which encodes a crucial histone modifying protein Polycomb group protein in SJLC. The group conducted bioinformatics study where multiple proteins were identified and role of B-lymphoma Mo-MLV insertion region 1 in SJLC was confirmed through molecular studies and studies in mouse models (Sheng et al., Reference Sheng, Chen, Xu, Han, Mao, Chen, Li, Su, Hu, Zhao, Lowe, Huang, Shao and Yao2024).

Further, recent studies have identified siRNA and miRNA, which are derived from Sj, and are capable of inhibiting hepatoma cell growth. In one study, miRNA-7-5p, which was detected in host hepatocytes during Sj infection, inhibited the growth and migration of hepatoma cells. Inhibition was achieved by cross-species down-regulation of the S-phase kinase-associated protein 2 gene in the human hepatoma cell lines in vitro (Hu et al., Reference Hu, Zhu, Wang, Lin, Ma, Zhu, Jiang, Li and Pan2019). In another study, the inhibition was achieved by miRNA-71a, which targeted the FZD4 host gene (Jiang et al., Reference Jiang, Wang, Zhu, Hu, Lin and Pan2022). Detection of these RNAs in human host cells probably hint towards the cancer inducing capabilities of Sj.

One interesting study found up-regulation of glycolysis linked genes during Sj infection. Ldha, Glut4, Pkm2, Pfkfb3, Aldoc and HK2 genes were found to be up-regulated (Xu et al., Reference Xu, Chang, Ni, Li, Chen, Hou and Ji2019). Interestingly, up-regulation of all these genes, except Aldoc, has been directly linked to liver fibrosis and LC as shown in Table 1. While Aldoc gene has not been directly linked with LC, it is known to promote non-small cell lung cancer by regulating Wnt signalling pathway (Shang et al., Reference Shang, Lu, Jiang, Xing, Mao, Yang and Zhang2023). It needs to be determined if Wnt signalling among hepatocytes is also altered by up-regulation of Aldoc gene.

Table 1. Up-regulated glycolysis genes due to Sj infection and their links with liver fibrosis and liver cancer

4.6 SJLC and LC not linked with Sj

Different mechanisms of cancer induction, both at cellular and genetic level, are observed in SJLC and LC not linked with Sj. As noted previously, LC can be induced by an array of factors not linked with Sj. HBV and/or HCV are independently capable of inducing LC. Hepatitis viruses are capable of insertional mutagenesis of both oncogenes as well as tumour suppressor genes. Hepatocyte transformation is further aided by the accumulation of preS1 large envelope proteins and preS2/S mutant proteins through unfolded protein response (Levrero & Zucman-Rossi, Reference Levrero and Zucman-Rossi2016).

Similarly, multiple factors are responsible for initiating LC in case of tobacco smoking. Tobacco constituents like tar and nicotine not only supress T-cell–related responses but p53 gene reduction is also reported. Chronic smoking further promotes pro-inflammatory cytokines and other toxins which are responsible for necro-inflammation and hepatic lesions and may also cause iron deposition leading to liver fibrogenesis (Jain et al., Reference Jain, Chaudhary, Varshney, Bin Razzak, Verma, Khan Zahra, Janmeda, Sharifi-Rad, Daştan, Mahmud, Docea and Calina2021). Excessive alcohol intake produces similar results, where the pathology begins with a fatty liver proceeding to hepatitis which leads to cirrhosis and ultimately causes HCC (Matsushita & Takaki, Reference Matsushita and Takaki2019).

NAFLD ranges from simple steatosis (fat build-up not linked with alcohol intake) to NASH (excessive amount of fat accumulation in hepatic tissue) (Dhamija et al., Reference Dhamija, Paul and Kedia2019; Peng et al., Reference Peng, Stewart, Woodman, Ritchie and Qin2020). While the exact cause of NASH is unknown, in case of certain obese people with NASH, fat deposition leads to inflammation, fibrosis and further to cirrhosis which transforms into HCC in significant number of patients (Dhamija et al., Reference Dhamija, Paul and Kedia2019; NIH, 2022).

From the previous discussion, although different factors might utilise overlapping mechanisms of inflammation, fibrosis and cirrhosis with a synergistic effect of HBC/HCV infection in HCC induction, the inducing factors are totally different. For example, deposited Sj eggs and SEAs seem to be playing a major carcinogenic through granuloma formation which is seldom reported in other cases discussed previously.