Activation of human B lymphocytes after immunization with pneumococcal polysaccharides

The in vivo and in vitro immune response after in vivo immunization with pneumococcal polysaccharides (PPS) has been analyzed in man. Substantial differences were noted in this system when compared with human responses to soluble protein antigens. Within 6 d after immunization, specific PPS antigen-binding cells (ABC), specific plaque-forming cells (PFC), and cells capable of spontaneously synthesizing in vitro large amounts of specific anti-PPS immunoglobulin (Ig) G. IgA, and lesser amounts of specific IgM appeared in the peripheral blood. The ABC, PFC, and the total amount of specific spontaneous antibody production followed nearly identical kinetics after immunization. Low doses of irradiation markedly inhibited spontaneous anti-PPS antibody production by lymphocytes obtained 7 or 8 d after immunization, suggesting a requirement for in vitro proliferation for full expression of antibody-secreting capability of these cells that are activated in vivo and are capable of spontaneous antibody production in vitro. Spontaneous secretion by B lymphocytes in vitro was independent of T cells, unmodified by the addition of T cell factors, and readily suppressible by pokeweed mitogen (PWM). By 2 wk after immunization, spontaneous anti-PPS antibody production in vitro was no longer detected. Subsequent stimulation of lymphocytes in culture with a wide range of concentrations of specific antigen did not trigger either proliferation or specific antibody synthesis. Despite the unresponsiveness of these cells to antigenic stimulation at this time, they were capable of specific antiPPS antibody production after stimulation with PWM. In vivo booster immunization 4 mo after an initial immunization did not reproduce the increased numbers of ABC, PFC, or in vitro specific antibody production that had been found 4 mo earlier. The dichotomy in capacity for activation of PPS-specific B cells by PWM vs. specific antigen, and the in vivo and in vitro unresponsiveness to in vivo booster immunization with PPS, contrast sharply with previous studies in man with soluble protein antigens such as keyhole limpet hemocyanin and tetanus toxoid. Furthermore, the lack of T cell activation by PPS also contrasts with previous results with tetanus toxoid and other protein antigens. This system should prove useful in delineating certain aspects of human B cell physiology not readily approachable with standard soluble protein antigens.