Antibody-secreting cell precursor frequencies among the sheep-erythrocyte-binding cells after immunization

Controversy concerning the immunologic role of antigen-binding cells (ABC) has prompted us to attempt to quantitate the proportion of stimulable ABC, in immunized animals, which are precursors for cells producing antibody specific for the antigen bound. Using a lipopolysaccharide (LPS)-driven limiting dilution analysis system, the precursor frequency (PF) of cells secreting IgM and IgG and sheep erythrocyte (SRBC)-specific IgM and IgG was established for highly purified SRBC antigen-binding cell (SRBC-ABC) and unfractionated populations taken from CBA/J mouse spleens on days 5, 12 and 180 of the in vivo primary immune response to SRBC. At all these times, almost all SRBC-ABC spontaneously secreting immunoglobulin (Ig) secreted SRBC-specific Ig, and almost all precursors of Ig-secreting cells in the ABC populations were precursors of cells secreting specific anti-SRBC antibody. In SRBC-ABC populations, the PF for total and SRBC-specific Ig secretion was seen to decrease on days 5 and 12 after immunization and to increase to 3.5 to 7 times nonimmune levels 180 days after immunization. The absolute number of precursors, within the SRBC-ABC population, for the secretion of SRBC-specific Ig decreased on day 12 after immunization. In the unfractionated population, the PF for SRBC-specific Ig secretion temporarily increased after immunization, reaching peak levels 5 days (IgM) and 12 days (IgG) after immunization. These two changes may be related, representing the progress of stimulated cells out of the ABC pool as they lose receptors en route to full maturation. The small clone sizes on days 5 and 12 indicate that ABC divide less in response to LPS when already engaged in a response to antigen. In contrast, the PF for total IgM and IgG secretion in the unfractionated population was not greatly affected by immunization.