Difference in Genesis between Antigen‐Induced IgM Antibody‐Forming Cells and B‐Cell Mitogen‐Induced IgM Antibody‐Forming Cells in Mouse Spleen Cell Cultures

Abstract

AbstractTo determine the mechanisms in the triggering of thymus‐independent lymphocytes (B cells) for development into antibody‐forming cells (AFC), genesis of IgM AFC elicited polyclonally by nonspecific stimulation with B‐cell mitogen, such as nystatin and bacterial lipopolysaccharide, was compared with that of IgM AFC specifically elicited by antigenic stimulation, using mouse spleen cell cultures as an experimental system and sheep erythrocytes (SRBC) as a test antigen. Considering that differentiation and proliferation are necessary cellular events for precursor B cells to develop into AFC, the effect of different antimetabolic agents on the generation of each type of AFC in spleen cell cultures was examined. The generation of anti‐SRBC IgM hemolysin plaque‐forming cells (PFC) in B‐cell mitogen‐stimulated spleen cell cultures was found to be less susceptible to X‐irradiation or mitomycin C than that in the SRBC‐stimulated cultures. These apparently paradoxical results were affiirmed using colcemid as an inhibitor of cell mitosis and hydroxyurea (HU) as an inhibitor of cellular DNA synthesis. Thus, when spleen cell cultures responding to either SRBC or B‐cell mitogen were exposed to colcemid or HU during a period from 2 days to 3 days after the stimulation, the exponential generation of anti‐SRBC IgM PFC in the cultures responding to SRBC was completely halted, whereas that in the cultures responding to B‐cell mitogen was not. Furthermore, N6, O2′ ‐dibutyryl adenosine 3′, 5′ ‐cyclic monophosphoric acid was found to halt the exponential generation of antigen‐induced anti‐SRBC IgM PFC but not that of the B‐cell mitogen‐induced anti‐SRBC IgM PFC. From these results it was suggested that B‐cell mitogen might stimulate precursor Bμ cells at a late stage in the differentiative pathway to develop into AFC without cell division, and that antigenic stimulation might stimulate relatively primitive precursor Bμ cells to proliferate and then differentiate into AFC. Based on this idea, mechanisms in the triggering of B‐cell activation are discussed.