Characterization of the cellular immune response following in vivo immunization with pneumococcal polysaccharides in man.

The in vivo and in vitro immune responses following in vivo immunization with pneumococcal polysaccharides (PPS) has been analyzed in man. Within 6 days following immunization, specific PPS antigen-binding cells (ABC), specific plaque-forming cells (PFC) and cells capable of spontaneously synthesizing in vitro substantial amounts of specific anti-PPS IgG and IgA and lesser amounts of specific IgM appeared in the peripheral blood. The ABC, PFC and total amounts of specific spontaneous antibody production in vitro followed nearly identical kinetics after immunization. The ABC were isolated and shown to be large B cells, predominantly surface IgG positive and surface IgD negative, and to express an activation antigen called 4F2. Low doses of in vitro irradiation markedly inhibited spontaneous anti-PPS antibody production by lymphocytes obtained 7 or 8 days after immunization, suggesting a requirement for in vitro proliferation for full expression in vitro of antibody secretion following in vivo activation. 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). These findings strongly suggest that the in vivo immunization with PPS delivered to these spontaneously secreting PPS-specific B cells the signals for proliferation as well as differentiation, both of which were expressed in vitro. By 2 weeks after immunization, spontaneous anti-PPS antibody production in vitro was no longer detected. Subsequent stimulation of lymphocytes 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, they were capable of specific anti-PPS antibody production following stimulation with PWM. This finding demonstrates that PPS-specific B cells are indeed present in the circulation at this time. The lack of ability of PPS antigen itself to trigger these B cells at this point in time strongly suggests that PPS-specific T cells are either not present or are untriggerable, and PWM can bypass the need for specific T cells by polyclonally stimulating the T cell pool. This system should prove useful in delineating certain aspects of B cell physiology not readily approachable with standard soluble protein antigens.