The divergent roles of alternatively activated macrophages in helminthic infections

Macrophages play crucial roles in the immune response, as they can initiate, modulate and also be final effector cells during immune responses to infections. Macrophages are derived from myeloid precursor cells in bone marrow and are widely distributed in every tissue of the body. Over the past 10 years, the concepts about macrophage activation have clearly changed; macrophages are not called activated or inactivated as they used to be. These changes in the concept of macrophage response is the result of many in vitro and in vivo studies, but the major support for the current concept of alternatively activated macrophages (AAMphi) comes from parasitic helminth infections. Parasitic helminths have developed complex mechanisms to evade and modulate host immunity. Infections with these parasites induce strong polarized Th2-type immune responses frequently associated with impaired T-cell proliferative responses to parasitic or unrelated antigens. Given the recent advances in understanding the immunoregulatory capabilities of helminthic infections, it has been suggested that macrophages can be a target for immunomodulation. Furthermore, they become altered when a host experiences chronic exposure to helminth parasites or their by-products, which favour the induction of AAMphi. How AAMphi participate in modulating host immunity during helminth infections and what their roles are in clearing or favouring parasite survival remains elusive. Here we review the most recent advances in the literature on AAMphi at the host-parasite interface, including three classes of helminths: nematodes (Brugia, Nippostrongylus, Litomosoides, Heligmosomoides), trematodes (Schistosoma, Fasciola) and cestodes (Taenia, Echinococcus, Hymenolepis).