In vitro antigen-specific antibody response to the species-specific surface protein antigens of typhus group rickettsiae by human peripheral blood mononuclear cells: generation of an antigen-dependent suppressor T cell.

We defined conditions suitable for in vitro synthesis of rickettsia-specific antibody by human PBMC cultured with the SPA of Rickettsia typhi or R. prowazekii and without addition of mitogens or polyclonal stimulators. Antibody synthesis, as measured by an enzyme-linked immunosorbent assay, was cycloheximide-inhibitable and antigen-specific. PBMC from individuals with prior rickettsial infection made antibody, whereas PBMC from those receiving vaccine or with undetectable levels of serum anti-SPA antibody did not. Antibody production was T helper cell-dependent because isolated B cells did not generate antigen-specific antibody in the absence of autologous T cells. Furthermore, prior exposure of T cells to high concentrations of SPA led to the generation of an antigen-dependent population of cells capable of suppressing the anti-SPA response when co-cultured with autologous PBMC and optimal SPA concentrations. This system should serve as an effective tool for analyzing the cellular interactions involved in the in vitro regulation of antigen-specific antibody synthesis by human PBMC.