MMCP-8, the first lineage-specific differentiation marker for mouse basophils. Elevated numbers of potent IL-4-producing and MMCP-8-positive cells inspleens of malaria-infected mice

in mice infected with the non-lethal malaria parasite Plasmodium chabaudi c habaudi AS, a prominent switch from a Th1 to a Th2 type of response occurs in CD4(+) T cells at the time of peak parasitemia or shortly thereafter (9- 15 days after infection). This is accompanied by a major increase in IL-4, and a similar decrease in IFN-gamma-producing cells. Non-B-non-T cells have been shown to be the main source of the IL-4 in these mice. The IL-4-produ cing cells are hyperresponsive to IL-3, indicating mast cell or basophil or igin. To further characterize this cell population we have studied various organs at different time points of malarial infection by Northern blot anal ysis. No significant increase in the expression of any of the classical mou se mast cell serine proteases (MMCP)-1 to 7 or carboxypeptidase A was detec ted in the spleen during the entire infection. However, a marked increase i n the expression of MMCP-8 was observed in the spleen at around day 15 post infection. Isolation of IgE receptor-positive cells from spleen shortly af ter peak parasitemia led to a prominent enrichment of MMCP-8-expressing cel ls. Fifty thousand of these cells were, after IL-3 stimulation, found to pr oduce IL-4 to levels comparable with more than one million fully activated T cells. Our results show that basophil-like cells are very potent producer s of IL-4 and that IL-4 produced by these cells may be of major importance for the initiation of a Th2 response. in addition, the detection of MMCP-8 in these cells has led to the identification of the first basophil-specific differentiation marker in the mouse.