Modulation of the host's immune response by schistosome larvae

Schistosomes appear to have evolved several strategies to down-regulate the host's immune response in order to promote their own survival. For the host, down-regulation is also beneficial as it can limit the extent of pathology. It is widely accepted that schistosomes modulate the immune response during the chronic phase of infection after egg deposition has started. However, there is increasing evidence that modulation of the immune response can occur much earlier at the time infective cercariae penetrate the host skin. In this review, we explore the various lines of evidence that excretory/secretory (ES) molecules from cercariae down-regulate the host's immune response. We highlight the immunological factors that are produced and may be involved in regulating the immune system (e.g. IL-10, and eicosanoids), as well as speculating on possible mechanisms of immune modulation (e.g. mast-cell activation, T-cell apoptosis, and/or the skewed activation of antigen-presenting cells [APCs]). Finally, we draw attention to several molecules of schistosome origin that have the potential to stimulate the regulatory response (e.g. glycans) and link these to potential host receptors (e.g. TLRs and C-type lectins).