The relationship between immunization and circulating antigen-specific plaque-forming cells

The relationship between the appearance of cells producing antibody to tetanus toxoid (TT) in the circulation and the serum titers of anti-TT IgG following booster immunization has been studied. It was found that cells producing anti-TT antibody can be detected in the circulation in a hemolytic plaque assay using sheep red blood cells (SRBC) coated with TT by the chromic chloride method. In symmetric inhibition studies using cells from TT or keyhole limpet hemocyanin (KLH)-immune donors, the homologous antigen inhibited 100% of the PFC with no cross-inhibition. Thus, the plaque-forming cells (PFC) detected in this assay are specific for the immunizing antigen. No evidence of polyclonal B-cell activation in response to TT was found, as shown by a failure to detect any PFC against unmodified or KLH or human serum albumin-treated SRBC. In addition, the increase in total Ig-secreting cells observed in a staphylococcal protein A reverse hemolytic plaque assay was always accounted for by the number of anti-TT antibody-producing cells observed. The peak number of anti-TT antibody-producing cells varied between donors, but the kinetics of their appearance was highly reproducible--none before Day 5, peak numbers between Days 6 and 8, and a sharp decline with only rare anti-TT Ig-secreting cells in the circulation by Day 15 postimmunization. Anti-TT antibody-producing cells appeared in the circulation prior to any detectable increase in serum anti-TT antibody titers, and following the disappearance of PFC from the circulation, there was no further increase in serum IgG anti-TT levels. These observations demonstrate a marked specificity of B-cell activation on boosting with a recall antigen, and a parallelism between the appearance of activated B cells in the circulation and of IgG anti-TT synthesis by the subject as a whole.