Induction of immunity to Schistosoma mansoni: interaction of schistosomula with accessory leucocytes in murine skin and draining lymph nodes.

A single exposure to radiation-attenuated cercariae of Schistosoma mansoni induces a high level of protective immunity in C57BL/6 mice, which is mediated by Th1 responses. Events in the skin and/or draining lymph nodes early after exposure are crucial for the induction of protection, and we have investigated the interactions of vaccinating parasites with host leucocytes in these 2 locations. We observed extensive lateral spreading of cercarial secretions along layers of the stratum corneum but not between keratinocytes. There was little direct contact with host leucocytes during the first 1-2 days when the parasites lay at the base of the epidermis, but cells accumulated in the underlying dermis. In contrast to normal parasites, attenuated larvae persisted in the dermis for > 10 days, often surrounded by aggregates of macrophages/dendritic cells. Whilst cells bearing MHC II, CD11b or CD11c markers were present in the lymph nodes, particularly in the periphery and paracortical areas, no obvious redistribution was seen as a result of parasite residence there for 5-15 days. However, ultrastructural observations revealed numerous cells with macrophage/dendritic morphology in the vicinity of parasites, in some instances closely adherent to the tegument. The observations strongly suggest that the tegument is a potent source of the antigens which prime the immune system in the lymph nodes of vaccinated mice for a protective response.