The parasitic worm Fasciola hepatica induces strong Th2 and T-regulatory immune responses while simultaneously suppressing Th1-driven immune responses to bystander microbial infections. It also prevents the initiation of Th1-mediated autoimmune disorders in mice through the suppression of Th17 and Th1 immune responses, and this can be mimicked by parasite-derived molecules. We have isolated F. hepatica tegumental coat Ag (FhTeg) and demonstrated its suppressive effect in vivo by directly targeting dendritic cells, impairing their ability to drive Th1 responses. Mast cells are critical in promoting Th1 protective immunity during bacterial infection and in driving Th1-mediated pathological conditions in autoimmune diseases. In this article, we show that FhTeg inhibits the ability of mast cells to drive the Th1 immune response by suppressing cytokine secretion (TNF-α, IL-6, IFN-γ, and IL-10) and ICAM1 expression in mast cells stimulated with LPS or heat-inactivated Bordetella pertussis Ag. These heat-inactivated B. pertussis Ag/LPS-stimulated mast cells fail to promote Th1 immune responses in CD4(+) T cells when pretreated with FhTeg, and a role for ICAM1 in this process was demonstrated. FhTeg suppresses the activation of transcription factors in the TLR signaling pathway, which explains the decrease in cytokine production and cell surface marker expression. We demonstrated that FhTeg suppresses MAPK and NF-κB activation and enhances SOCS3 expression, which could explain its negative effect on the TLR pathways. We conclude that FhTeg targets innate immune cells, inhibiting their ability to drive Th1 immune responses.
