Regulation of innate responses during pre-patent schistosome infection provides an immune environment permissive for parasite development.

Blood flukes of the genus Schistosoma infect over 200 million people, causing granulomatous pathology with accompanying morbidity and mortality. As a consequence of extensive host-parasite co-evolution, schistosomes exhibit a complex relationship with their hosts, in which immunological factors are intimately linked with parasite development. Schistosomes fail to develop normally in immunodeficient mice, an outcome specifically dependent on the absence of CD4⁺ T cells. The role of CD4⁺ T cells in parasite development is indirect and mediated by interaction with innate cells, as repeated toll-like receptor 4 stimulation is sufficient to restore parasite development in immunodeficient mice in the absence of CD4⁺ T cells. Here we show that repeated stimulation of innate immunity by an endogenous danger signal can also restore parasite development and that both these stimuli, when administered repeatedly, lead to the regulation of innate responses. Supporting a role for regulation of innate responses in parasite development, we show that regulation of inflammation by neutralizing anti-TNF antibodies also restores parasite development in immunodeficient mice. Finally, we show that administration of IL-4 to immunodeficient mice to regulate inflammation by induction of type 2 responses also restores parasite development. These findings suggest that the type 2 response driven by CD4⁺ T cells during pre-patent infection of immunocompetent hosts is exploited by schistosomes to complete their development to reproductively mature adult parasites.