Prior exposure to subimmunogenic amounts of some bacterial lipopolysaccharides induces specific immunological unresponsiveness.

Pretreatment (priming) of BALB/c mice with a low (subimmunogenic) dose of Escherichia coli O113 lipopolysaccharide (LPS) generates immunological memory 7 to 30 days later; the direct (immunoglobulin M) plaque-forming cell (PFC) responses produced after subsequent immunization with an optimal dose are 4 to 20 times greater than those of unprimed mice. By contrast, priming with a low dose of E. coli O55 LPS, followed by immunization with an optimally immunogenic dose 2 to 30 days later, resulted in a significantly reduced antibody response. Similar results were obtained with Serratia marcescens LPS. Dose-response studies indicated that such unresponsiveness is antigen specific and could be induced with subimmunogenic amounts of LPS. Priming reduced the magnitude of the PFC response to all immunizing doses of LPS tested. Unresponsiveness is not due to (i) an alteration in the time course of the PFC response or to (ii) a change in the isotype of the anti-LPS antibody produced after priming and immunization.