Modulation of the host's immune response by schistosome larvae

Abstract

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).

Figure 1

A schematic diagram showing the generalized pattern of schistosome migration through the skin, and the possible mechanisms of immunoregulation that might operate in this tissue. Full details of the regulatory mediators released by both the parasite and the host are given in the main text of the review, as are descriptions of the possible mechanisms of immune regulation and identities of probable ligands. The diagram encompasses data obtained from studies of different Schistosoma and Trichobilarzia species with specific references given in the main text.