Cellular and lipopolysaccharide subunit requirements for the caprine lymphoblastogenic response to endotoxin

Caprine B lymphocytes were established as the cell type that divided when cultured with lipopolysaccharide (LPS), and lipid A was defined as the in vitro mitogenic component of LPS. The conclusion that the caprine B lymphocyte was stimulated by LPS was based on 3 observations: (i) Numbers of B lymphocytes increased in cultures containing LPS, but not in unstimulated or concanavalin A-stimulated cultures, (ii) mixtures of T lymphocytes and monocytes did not incorporate tritiated thymidine when LPS was added, and (iii) removal of monocytes from mixtures of T and B lymphocytes did not reduce the LPS-stimulated reaction. Stimulation of B lymphocytes by LPS occurred when less than 1% monocytes were present and was augmented by greater than 5% monocytes. The lipid A subunit of LPS was most likely responsible for mitogenesis, since purified lipid A stimulated lymphocytes and the addition of polymyxin B, a specific inhibitor of lipid A activity, blocked the lymphocyte reaction.