Activation of human B lymphocytes: frequency of antigen-specific B cells triggered by alloreactive or by antigen-specific T cell clones.

Human T cell clones specific for tetanus toxoid (TT) or for alloantigens were isolated and expanded in culture using antigen stimulation and soluble growth factors. When stimulated by the specific antigen, the clones were able to proliferate in the absence of exogenous growth factors and to provide help to B cells. The alloreactive and TT-specific clones were compared for their capacity to help an anti-TT as well as a polyclonal antibody response. The frequency of the B cells activated to the production of specific antibody was determined by limiting dilution analysis in cultures containing limiting numbers of responding B cells and optimal numbers of cloned T helper cells. In these conditions a single activated b cell was able to produce about 10 ng of antibody. TT-specific clones, in the presence of low TT concentrations, selectively induced a number of TT-specific B cells to produce IgG antibody in the absence of a detectable polyclonal B cell activation. On the other hand the alloreactive clones activated higher numbers of TT-specific B cells as a part of a strong polyspecific B cell activation. In this case the antibody production did not require the presence of TT in culture; furthermore cells producing antibodies of unrelated specificities were also activated in allostimulated cultures. IgG anti-TT were produced only by immune donors, while IgM anti-TT were produced by immune and nonimmune donors, providing the B cells were activated by alloreactive clones. These data demonstrate that human memory B cells can be triggered by both antigen-specific and alloreactive T cells to antibody production. The alloreactive clones can therefore be used to analyze at the clonal level the repertoire of a pool of B cells that contain most, if not all, the memory antibody specificities.