Lipopolysaccharide and lipid A‐induced human blood lymphocyte activation as detected by a protein A plaque assay

Various purified cell wall lipopolysaccharides (LPS) from gram‐negative bacteria and derivatives of these LPS were tested for their stimulatory capacity for human peripheral blood cells. Immunoglobulin (Ig) production was tested by an indirect plaque‐forming cell assay using Staphylococcus aureus protein A‐coupled erythrocytes and specific anti‐Ig as developing serum. This method allows the detection of the majority of cells secreting Ig of a single class, and the number of plaque‐forming cells detected are approximately 100–1000 times the amount obtained using normal sheep red cells as targets. LPS containing the O antigen‐specific chain, as well as mutant products only containing lipid A and ketodeoxyoctonate trisaccharide, could induce cell division and antibody synthesis. The polypeptide antibiotic polymyxin B was found to inhibit LPS‐induced activation. Furthermore, purified lipid A, complexed with bovine serum albumin, was also found to activate human peripheral blood B cells. These findings demonstrate that human peripheral blood lymphocytes can be activated by LPS and also indicate that lipid A is the active part of these molecules.