Theoretical and practical aspects of B-cell activation: murine and human systems.

We have reviewed observations which were made during studies of murine and human B-cell responses in vitro. One currently faces difficulties in drawing any clear schema as to which external signals elicit which responses (activation, proliferation, differentiation) in B cells. However, the most potent antigen-dependent or polyclonal B-cell responses in vitro occur when, in addition to various cytokines, accessory cells, serum etc., the cultures contain either a) intact T-helper cells which enter into cell-to-cell contact with B cells, or b) some B-cell "mitogen" (T-independent antigen). Murine B cells activated with LPS and anti-Ig antibodies represent a model for the study of IL2 receptor expression and function. LPS does not act on human B cells. Certain mutant EL-4 thymoma cells are potent activators of murine and human B cells via a direct cell-to-cell interaction. The majority of human B cells can be induced to proliferate and generate a few hundred antibody-secreting cells each in the presence of such thymoma cells and a mixture of cytokines. From a practical point of view, this observation should be useful in a variety of investigations such as the analysis of the human B-cell specificity repertoire.