A system for studying in vitro the antibody response against a single determinant and to all the determinants of a macromolecule (β-D-Galactosidase of Escherichia coli) is described. It consists of culturing fragments of rabbit lymph nodes (either preimmunized in vivo or not) and exposing them to antigen in vitro. Antibodies secreted into the culture during several days, and up to 3 months in the secondary response, were titrated for: (a) one-hit activation AMEF, the cross-reacting material produced by a point mutant Lac- E. coli; and (b) precipitation of wild type enzyme. Titrations of activating and binding antibodies are very sensitive owing to the amplification potential inherent in the enzymatic assays, which allows several antibody measurements on minute samples. In addition antigen decay in vitro was followed and correlated with the antibody response, showing faster disappearance when the latter took place. Time-course studies of the in vitro antibody response demonstrated that precipitating titres are higher and last longer than activating antibody titres. Repeated in vitro challenges showed decay of the memory potential of in vivo primed lymph nodes, as well as the possibility of inducing an immune response in vitro using non-primed lymph nodes. The results underline the amenability of the present system to the study of in vitro primary and secondary immune responses toward restricted portions of a macromolecule.
