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Keywords:

  • Helminth;
  • Regulatory T cells;
  • Th17;
  • Th2

Abstract

Metazoan parasites of mammals (helminths) belong to highly divergent animal groups and yet induce a stereotypical host response: Th2-type immunity. It has long been debated whether this response benefits the host or the parasite. We review the current literature and suggest that Th2 immunity is an evolutionarily appropriate response to metazoan invaders both in terms of controlling parasites and repairing the damage they inflict. However, successful parasites induce regulatory responses, which become superimposed with, and control, Th2 responses. Beyond helminth infection, this superimposition of response profiles may be the norm: both Th1 and Th2 responses coexist with regulatory responses or, on the contrary, with the inflammatory Th17 responses. Thus, typical responses to helminth infections may differ from Th2-dominated allergic reactions in featuring not only a stronger regulatory component but also a weaker Th17 component. The similarity of immune response profiles to phylogenetically distinct helminths probably arises from mammalian evolution having hard-wired diverse worm molecules, plus tissue-damage signals, to the beneficial Th2 response, and from the convergent evolution of different helminths to elicit regulatory responses. We speculate that initiation of both Th2 and regulatory responses involves combinatorial signaling, whereby TLR-mediated signals are modulated by signals from other innate receptors, including lectins.