A Model System for the Analysis of B‐Cell Activation and Effector T‐Cell Functions

We have attempted to develop an in vitro system where polyclonal B lymphocyte responses could be induced in ‘antigen‐like’ conditions, that is, where surface immunoglobulin‐dependant binding mediates interaction with a mitogen. Monoclonal anti‐μ and anti‐δ antibodies were covalently bound to lipopolysaccharide (LPS) and these complexes were shown to display mitogenic activity. Polyclonal plaque‐forming‐cell (PFC) responses, however, were diminished in cultures stimulated by anti‐μ‐LPS (but not by anti‐δ‐LPS) indicating that ‘anti‐μ inhibition’ of terminal B‐cell differentiation also applies to ‘specific’ antibody responses. Moreover, the analysis of the functional activity of monoclonal antibodies to major histocompatibility complex (MHC) class II molecules revealed a surprising synergy between low. non‐stimulatory concentrations of anti‐μ‐LPS (but not anti‐SdL‐LPS) with anti‐I‐A antibodies. These responses are T‐cell dependent and synergy with anti‐μ‐LPS conjugates can also be obtained with ‘naturally’ activated CD4+ cells isolated from normal donors. A model of molecular and cellular interactions was derived, which accounts for the present findings and is applicable in antigen‐dependent lymphocyte collaboration.