Endotoxin-stimulated spleen cells: characterization of the responding cells

The percentage of endotoxin-stimulated lymphoblasts possessing C3 receptors (CR) decreases during a 72-hr period of in vitro cultivation. In studies reported here we attempted to determine whether the drop in the percentage of CR+ blast cells resulted from receptor loss by transformed cells or whether it resulted from the proliferation of additional blast cell types which lacked the CR. Using methods which permitted simultaneous detection of cytoplasmic IgM, the presence of the CR, and 3H-thymidine uptake we found that: a) LPS stimulation resulted in a progressive increase in the number of IgM+ blasts, b) these constituted approximately 30% of the total cell population at 72 hr after LPS stimulation, c) of the IgM+ blasts 72 to 84% were CR+ and 16 to 28% were CR- at 72 hr, d) from 48 to 72 hr there was no decrease in the percentage of IgM+ blasts possessing the CR, and e) an additional blast cell type was observed which incorporated 3H-thymidine but which was IgM- and lacked the CR. These cells constituted from 11 to 27% of total 3H-thymidine+ cells at 72 hr after LPS stimulation. We conclude that the decrease in the percentage of CR+ blasts in LPS-stimulated spleen cell cultures is not due to receptor loss from IgM+ blasts cells but apparently is partly due to the stimulation and proliferation of other cell type(s) which lack(s) the CR. This cell(s), although its functional identity is unknown, does not appear to be of thymic origin. The heterogeneity of the markers on LPS-stimulated blast cells suggests that a heterogeneous population of cells responds to LPS.