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.