Secreted Molecules from the Helminth Parasite Fasciola Hepatica Prevent Pro-Inflammatory Immune Responses to Prevent Autoimmune Diabetes

Abstract

Introduction:Parasitic worms (helminths) have evolved mechanisms to regulate host protective immune responses, to ensure their own survival, and minimise immune-mediated tissue pathology for the host. Helminths achieve this by inducing potent anti-inflammatory (Th2) and regulatory immune responses, while simultaneously inhibiting the development of pro-inflammatory Th1 and Th17 immune responses. Understanding how helminths modulate the immune response will provide novel mechanisms to induce a regulatory immune environment, with significant clinical applications for the treatment of autoimmune/inflammatory diseases, and in transplantation. Methods:To investigate the therapeutic potential of helminths, we have isolated the excretory/secretory (ES) products of the liver fluke Fasciola hepatica, and have delivered them intraperitoneally to female nonobese diabetic (NOD) mice at 4 weeks of age (co-incident with the initiation of autoimmunity directed against the insulin-secreting pancreatic beta cells). Development of Type 1 diabetes (T1D) and the effects of ES-treatment on the developing (auto)immune response were both assessed. Blood glucose was monitored and mice were deemed diabetic when levels exceeded 14mmol/L on two consecutive occasions. Flow cytometry was used to phenotype immune cell populations within the pancreas, pancreatic lymph nodes and peritoneum after treatment with ES. The activation status of macrophages was determined using genetic markers for M1 and M2 phenotypes by RT-PCR. Cytokines secreted by T cells and the isotype of serum autoantibodies were determined by ELISA. Results: Treatment of NOD mice with parasite ES prevented the development of autoimmune diabetes in 80-86% of animals. By comparison, only 10-15% of NOD mice treated with PBS (vehicle control) remained normoglycaemic. Protection from autoimmune disease in ES-treated NOD mice was associated with the suppression of the normal secretion of IFN-γ from autoreactive T cells in response to stimulation with autoantigen, and a corresponding switch towards the production of a regulatory isotype (IgG1) of autoantibody. In addition, treatment with ES resulted in the expansion of a population of regulatory (IL-10 secreting) B cells within the pancreatic lymph nodes and the activation of regulatory (M2 phenotype) macrophages. Importantly, NOD mice treated with ES were capable of mounting an effective immune response to a third party antigen, suggesting that ES treatment did not cause immune suppression per se. Conclusion: Soluble products secreted by Fasciola hepatica prevent the development of antigen-specific inflammatory responses, thereby preventing the development of autoimmunity in a murine model of Type 1 diabetes. Our data indicates that protection is achieved via the induction of a regulatory immune environment in the pancreas (the site of beta cell destruction) and pancreatic lymph nodes (the site of T cell priming) at a time point co-incident with T cell priming events. Fasciola hepatica ES modulates the function of B cells and macrophages, which, via IL-10 secretion, suppress the polarisation of autoreactive T cells. These results suggest that ES contains novel immune-therapeutic agents which hold great potential for the treatment of autoimmune disease and transplant rejection, where a regulatory immune environment is required.