The acquisition of b-cell competence and diversity

The in vitro splenic focus technique has been used to establish the characteristics of the repertoire of B cells in adult BALB/c mice, its stimulation and development. Evidence is presented that the technique yields an accurate estimate of the precursor cell frequency by providing monoclonal responses of a constant portion (4%) of transferred B cells. Since the monoclonally derived antibodies can be analyzed, it has also been possible to determine the isotype and clonotype of many of the resultant antibodies. The adult spleen can be shown to contain a vast repertoire (over 10(7) clonotypes) of primary B cells, each clonotype represented by from less than ten to as many as 10(4) responsive cells. Antigenic stimulation of these primary B cells is highly specific and apparently affinity dependent and leads to both antibody-forming cell clone formation and a population of secondary B cells. Secondary B cells, though derived from the same clonotypes, differ from primary B cell in several characteristics, including a lower degree of secificity of antigenic stimulation. In addition, secondary B cells can be stimulated in collaboration with allogeneic antigen-specific T cells to yield clones producing antibody of the IgG1 isotope, whereas primary B cells similarly stimulated produce only IgM antibodies. Analyses of neonatal B cells indicate that the acquisition of the adult repertoire is a highly patterned process. Thus, whereas 10(4) clonotypes are present at birth, more and more clonotypes are acquired in a highly ordered pattern during the first few weeks of life. It is concluded that this process is probably evolutionarily determined, since antigen-driven events appear to play no role. However, since developing B cells can be shown to be exquisitely sensitive to tolerance induction, negative antigenic selection may play an important role, particularly in eliminating clonotypes recognizing self-determinants for which the species is polymorphic.