Selective Effects of Bacterial Endotoxins on Various Subpopulations of Lymphoreticular Cells

Evidence has been presented that the in-vitro mitogenic activity of bacterial endotoxins on lymphoreticular cells of mice is a property of the lipid A fraction and that this mitogenic activity affects approximately one-half of the spleen cells of murine strains studied. This mitogenic effect was shown to be thymus-independent and a property of a low-density cell subpopulation. These transformed cells have unique characteristics (including a highly developed endoplasmic reticulum) that distinguish them from cells transformed by thymus-dependent mitogens such as phytohemagglutinin. Evidence was reviewed that the initial mitogen-cell interaction involves both membrane binding and internalization and that mitogenic activity depends more on the nature and rate of the internalization process that results from binding than on the actual degree of binding. It is proposed that bacterial lipopolysaccharide (LPS) has a bifunctional nature, acting as both antigen and mitogen, and that this bifunctional nature makes LPS thymus-independent as an antigen. No evidence presently exists showing that the antigen- and mitogen-responsive subpopulations are identical.