Alternatively activated macrophages (AAMφ) are primarily associated with the chronic stages of parasitic infections and the development of a polarized Th2 response. We have shown thatFasciola hepaticainfection of BALB/c mice induces a polarized Th2 response during both the latent and chronic stage of disease. The activation status of macrophages was analyzed in this model of helminth infection by evaluating the expression of genetic markers of alternative activation, namely, Fizz1, Ym1, and Arg1. AAMφ were recruited to the peritoneum of mice within 24 h ofF. hepaticainfection and after intraperitoneal injection of parasite excretory-secretory (ES) products. Administration of a recombinant antioxidant thioredoxin peroxidase (TPx), which is contained within the ES products, also induced the recruitment of AAMφ to the peritoneum. In vitro studies showed that this recombinant TPx directly converts RAW 264.7 macrophages to an alternatively activated phenotype characterized by the production of high levels of interleukin-10 (IL-10), prostaglandin E₂, corresponding with low levels of IL-12. Our data suggest that the Th2 responses induced by the helminthF. hepaticaare mediated through the secretion of molecules, one of which is TPx, that induce the recruitment and alternative activation of macrophages.

To infect their mammalian hosts, Fasciola hepatica larvae must penetrate and traverse the intestinal wall of the duodenum, move through the peritoneum, and penetrate the liver. After migrating through and feeding on the liver, causing extensive tissue damage, the parasites move to their final niche in the bile ducts where they mature and produce eggs. Here we integrated a transcriptomics and proteomics approach to profile Fasciola secretory proteins that are involved in host-pathogen interactions and to correlate changes in their expression with the migration of the parasite. Prediction of F. hepatica secretory proteins from 14,031 expressed sequence tags (ESTs) available from the Wellcome Trust Sanger Centre using the semiautomated EST2Secretome pipeline showed that the major components of adult parasite secretions are proteolytic enzymes including cathepsin L, cathepsin B, and asparaginyl endopeptidase cysteine proteases as well as novel trypsin-like serine proteases and carboxypeptidases. Proteomics analysis of proteins secreted by infective larvae, immature flukes, and adult F. hepatica showed that these proteases are developmentally regulated and correlate with the passage of the parasite through host tissues and its encounters with different host macromolecules. Proteases such as FhCL3 and cathepsin B have specific functions in larvae activation and intestinal wall penetration, whereas FhCL1, FhCL2, and FhCL5 are required for liver penetration and tissue and blood feeding. Besides proteases, the parasites secrete an array of antioxidants that are also highly regulated according to their migration through host tissues. However, whereas the proteases of F. hepatica are secreted into the parasite gut via a classical endoplasmic reticulum/Golgi pathway, we speculate that the antioxidants, which all lack a signal sequence, are released via a non-classical trans-tegumental pathway.

During helminth infections, alternatively activated macrophages (AAMacs) are key to promoting Th2 responses and suppressing Thl‐driven inflammatory pathology. Th2 cytokines IL‐4 and/or IL‐13 are believed to be important in the induction and activation of AAMacs. Using murine models for the helminth infections caused by Fasciola hepatica (Fh) and Schistosoma mansoni (Sm), we show that a secreted antioxidant, peroxiredoxin (Prx), induces alternative activation of macrophages. These activated, Ym1‐expressing macrophages enhanced the secretion of IL‐4, IL‐5, and IL‐13 from naive CD4 ⁺ T cells. Administration of recombinant FhPrx and SmPrx to wild‐type and IL‐4 ^−/− and IL‐13 ^−/− mice induced the production of AAMacs. In addition, Prx stimulated the expression of markers of AAMacs (particularly, Ym1) in vitro , and therefore can act independently of IL‐4/IL‐13 signaling. The immunomodulatory property of Prx is not due to its antioxidant activity, as an inactive recombinant variant with active site Cys residues replaced by Gly could also induce AAMacs and Th2 responses. Immunization of mice with recombinant Prx or passive transfer of anti‐Prx antibodies prior to infection with Fh not only blocked the induction of AAMacs but also the development of parasite‐specific Th2 responses. We propose that Prx activates macrophages as an initial step in the induction of Th2 responses by helminth parasites and is thereby a novel pathogen‐associated molecular pattern.— Donnelly, S., Stack, C. M., O'Neill, S. M., Sayed, A. A., Williams, D. L., Dalton, J. P. Helminth 2‐Cys peroxiredoxin drives Th2 responses through a mechanism involving alternatively activated macrophages. FASEB J. 22, 4022–4032 (2008)

Summary: Fasciolosis, a food‐borne trematodiasis, results following infection with the parasites, Fasciola hepatica and Fasciola gigantica. These trematodes greatly affect the global agricultural community, infecting millions of ruminants worldwide and causing annual economic losses in excess of US $3 billion. Fasciolosis, an important zoonosis, is classified by WHO as a neglected tropical disease with an estimated 17 million people infected and a further 180 million people at risk of infection. The significant impact on agriculture and human health together with the increasing demand for animal‐derived food products to support global population growth demonstrate that fasciolosis is a major One Health problem. This review details the problematic issues surrounding fasciolosis control, including drug resistance, lack of diagnosis and the threat that hybridization of the Fasciola species poses to future animal and human health. We discuss how these parasites may mediate their long‐term survival through regulation and modulation of the host immune system, by altering the host immune homeostasis and/or by influencing the intestinal microbiome particularly in respect to concurrent infections with other pathogens. Large genome, transcriptome and proteomic data sets are now available to support an integrated One Health approach to develop novel diagnostic and control strategies for both animal and human disease.