Immune responses to a Nippostrongylus brasiliensis infection in athymic and euthymic rats: surface expression of IgE receptors, IgE occupancy and secretory ability of mast cells.

The significance of T cells in an allergy-related immune response was investigated. Athymic, nude (Lewis rnu/rnu) rats lacking T lymphocytes and euthymic (Lewis+/+) rats were infected with the nematode Nippostrongylus brasiliensis. The density and occupancy of IgE receptors on peritoneal mast cells were quantified using a cytofluorometric method. The secretory ability of the mast cells as a function of anti-IgE challenge was evaluated in terms of histamine releasability in vitro. It was found that the IgE receptors were markedly up-regulated and the IgE occupancy increased on the mast cells of both athymic and euthymic rats during the parasite infection. However, at its peak, IgE occupancy was significantly higher in euthymic rats than in athymic ones. To our knowledge, this is the first proof of the possibility of a T-cell-independent IgE response in vivo. The IgE-mediated histamine releasability of mast cells was significantly enhanced 1 week after the infection in euthymic rats but not in athymic ones. These results thus indicate that the IgE immune response can occur in the absence of, but is augmented by, T cells. They also suggest that the concept of the IgE response should be widened to comprise not only increased IgE production but also an up-regulation of Ig receptors and an increase in IgE occupancy on mast cells, as well as an increase in the secretory ability of these cells. The latter is T cell dependent, although it is not directly related to the density of IgE-receptor complexes on the mast cells but is more likely due to a stimulation of the post-receptor signal transduction mechanism.