<Originals>Protective Immunity to Hymenolepis nana Infection in Mice

Several aspects of parasitological findings with the infection of Hymenolepis nana in mice are summarized in the present review and an attention is given on the acquired immunity to this parasite with a special reference to the immune effector mechanisms involved in this hostparasite interaction. H. nana is a small tapeworm parasitic in the small intestine of man and mice. The hosts are exposed to H. nana by oral ingestion of both eggs and cysticercoid larvae. The oral infection of eggs produces both the tissue phase with developing oncosphere larvae and cysticercoids in the intestinal villi of the host and the lumenal phase with adult tapeworms in the intestinal lumen, while the infection with cysticercoids produces only lumenal phase. The oral infection of a small number of eggs in mice induces a strong protective immunity that is established within 2 days after egg infection and is thought to be directed against the oncosphere larvae derived from challenged eggs. The oral cysticercoid infection does not induce such a drastic immunity. The protective immunity to H. nana is thymus-dependent, and Thy-2 positive cells in the mesenteric lymph nodes of immunized mice are probably relevant to this immunity. Many serological studies provide indirect evidences that specific antibody probably belonging to IgG1 or IgG2a, or both, classes of immunoglobulins may play a role in killing challenge larvae in immunized mice. There is no evidence for a role of complement in either induction of the response to H. nana nor its effect in the anamnestic response. The histopathological observations with the light and electron microscopy demonstrate that eosinophils and macrophages are closely applied to the epithelial surface of challenge oncospheres and are considered to exert some parasiticidal effect on the larvae. It is suggested that complement-independent antibody-mediated reactions with or without cooperation of some cellular components may contribute to the immunological killing and elimination of challenge oncospheres in the immunized mice. However, the exact effector mechanisms in this host-parasite model are still uncertain, and a sufficient number of questions remain to be investigated.