Differential Morphology of Mouse Spleen Cells Stimulated In Vitro by Endotoxin, Phytohemagglutinin, Pokeweed Mitogen and Staphylococcal Enterotoxin B

The in vitro mitogenic effects of endotoxin (LPS), phytohemagglutinin (PHA), pokeweed mitogen (PWM) and staphylococcal enterotoxin B (SEB) on mouse spleen cells were studied by light and electron microscopy. By light microscopy, LPS was found to “transform” a culture largely composed of small lymphocytes into “large lymphocytes” which did not synthesize DNA and into blast cells which did synthesize DNA. The blast cells were morphologically similar to those transformed by PHA, PWM and SEB. By electron microscopy, a large percentage of the endotoxin-transformed cells developed extensive dilated rough ER. Their appearance was similar to that reported by others for cells stimulated by PWM, but in our hands the development of rough ER was more conspicuous in LPS-stimulated cells. In contrast, PHA and SEB produced few blast cells which contained significant rough ER. The morphologic observations are consistent with the hypothesis that the mitogenic effect of endotoxin is on bone-marrow-derived lymphocytes. This effect may relate to the known adjuvant effect of endotoxin in vivo.