Tuning of CD40–CD154 Interactions in Human B-Lymphocyte Activation: A Broad Array of In Vitro Models for a Complex In Vivo Situation

Naive and memory B-lymphocyte populations can be activated through the binding of CD154 to CD40, a receptor that is constitutively expressed on the surface of these cells. Models based on the in vitro stimulation of human B lymphocytes through CD40 have greatly contributed to our understanding of the human immune response in healthy individuals and patients suffering from immune disorders. The nature of the engineered CD40 ligands is as diverse as the in vitro models used in studies of CD40-activated B lymphocytes. Monoclonal anti-CD40 antibodies, recombinant CD154 proteins, soluble CD154+ membranes as well as CD154+ cell lines have turned out to be very useful tools, and are still in use today. As for any receptor–ligand interaction, parameters such as duration and strength of contact, timing, affinity, and receptor density are major determinants of CD40 binding by CD154 or anti-CD40. Furthermore, variation in the intensity of CD40 stimulation has been shown to influence proliferation, differentiation and immunoglobulin secretion of human hybridomas, B-cell lines, tonsil and blood B lymphocytes. The objective of this review is to present an overview of the great diversity of CD40 agonists used in in vitro models of B-lymphocyte activation, with a particular emphasis on variations in the resulting strength of CD40 signaling generated by these models. A better understanding of these models could open up new avenues for the rational use of human B lymphocytes as antigen-presenting cells in cellular therapies.