Human antibody responses to bacteriophage phi X 174: sequential induction of IgM and IgG subclass antibody.

We studied the appearance of antigen-specific immunoglobulin classes and IgG subclasses in normal adult human subjects in response to primary, secondary, and tertiary immunization with the T-cell-dependent neo-antigen bacteriophage phi X 174. To complete the study we developed a sensitive, specific, and reproducible ELISA assay which was closely comparable to the widely used neutralization assay for total antibody (r = +0.97) and for IgG antibody (r = +0.93), and reasonably comparable for IgM antibody (r = +0.76). We confirmed that the initial response to primary immunization was predominantly, but not exclusively, IgM antibody. The secondary and tertiary responses demonstrated memory, amplification, and switch from IgM to IgG antibody. There was an orderly appearance of phage-specific IgG subclasses. IgG3 and IgG1 antibodies appeared 2 to 6 weeks after primary immunization. In all subjects there was a marked increase in IgG1 and IgG3 antibody after secondary immunization, and IgG2 antibody followed closely; IgG4 antibody appeared in some subjects. IgM antibody persisted in significant amounts (approx 50%) throughout the secondary response period. Following tertiary immunization, IgG1, IgG2, and IgG3 antibody consistently increased, and IgG4 antibody appeared in all subjects; IgG1 antibody predominated. Low levels of IgM antibody (approx 1% of total) persisted during the tertiary response. The persisting antibody on long-term follow-up (median 4 years after immunization) was virtually all (greater than 90%) IgG1.