The effect of lipopolysaccharide on the primary immune response to the hapten NNP

We have studied the effects of lipopolysaccharide (LPS) on the primary in vivo immune response to the hapten (4-hydroxy-3,5-dinitrophenyl)acetyl (NNP), with special reference to the avidity and affinity of the early appearing 19S and 7S antibodies. Comparisons were made of the immune response to NNP in groups of mice given either antigen alone, LPS alone, or antigen plus LPS. The avidity of antibodies induced by LPS plus antigen were similar to that found after injection of antigen alone, in spite of the fact that the antibodies were more numerous. However, when comparing the avidity of antibodies produced in animals given only LPS with those given LPS plus antigen, the latter group was often found to have fewer low-avidity 19S-antibody-producing cells. The affinity of 7S antibodies was also similar in the two groups given antigen or antigen plus LPS. Kinetic studies of the effect of LPS on the primary immune response to NNP showed that synergy was observed only before or after the peak response in groups given antigen alone. It is concluded that LPS under synergy conditions acts preferentially on specific antigen-sensitive cells, which are distinct from those that are activated to polyclonal antibody synthesis by LPS alone. Possible mechanisms for the adjuvant effect of LPS are discussed.