Evidence that pokeweed-mitogen-reactive B cells are pre-committed in vivo to the high-rate secretion of a single immunoglobulin isotype in vitro.

Normal human peripheral blood B cells that respond to pokeweek mitogen (PWM)-activated irradiated T cells with high-rate immunoglobulin secretion in vitro were analysed with respect to the frequency of the cells stimulated to high-rate immunoglobulin secretion in vitro and whether the progeny of each cell had the potential to secrete one or multiple immunoglobulin isotypes. In vitro cultures containing limiting numbers of human B cells were initiated in the presence of PWM and excess irradiated T cells, and the quantity of IgM, IgG and IgA secreted was determined after 9 days. The level of immunoglobulin secretion per cell in limiting-dilution microcultures was shown to be equivalent to that seen in the routinely used macrocultures, indicating that major loss of B-cell function was not occurring in the microcultures. At limiting B-cell numbers, individual microcultures were of ten shown to produce immunoglobulin of a single isotype, either IgM, IgG or IgA. Cultures that did produce multiple immunoglobulin isotypes occurred with a frequency predicted by the random distribution of B cells committed to production of a single isotype. A similar independent distribution of IgM anti-tetanus toxoid and IgG anti-tetanus toxoid antibody-producing precursors was observed when B cells selected on the basis of surface IgM were used in the microcultures. These results suggest that PWM-reactive B cells are at a stage of maturation in vivo such that they have the potential to secrete a single immunoglobulin isotype when activated in vitro.