Modulation of soluble CD40 ligand bioactivity with anti-CD40 antibodies

The B cell surface molecule CD40 may be activated either by its ligand CD40L or by anti-CD40 antibodies. In this study, five new anti-CD40 monoclonal antibodies (MAb) were characterized. Bioactivity of the MAb was assessed using a receptor hybrid consisting of the extracellular domain of CD40 and the intracellular domain of the p55 TNF receptor as a model for CD40 activation. Two agonistic MAb were able to enhance the activation of this CD40 hybrid CD40L. These MAb bound to an epitope that was not located within the CD40L-binding region indicating that activation of CD40 occurs epitope-independent. A second pair of ligand mimetic anti-CD40 MAb which appeared to bind to the CD40L binding site decreased CD40L bioactivity. With regard to ligand mimetic effects binding of the CD40L epitope was not of advantage. Combining anti-CD40 MAb with different epitope specificities or cross linking anti-CD40 MAB with secondary antibodies enhanced ligand mimetic effects. These data clearly show that ligand or antibody-mediated receptor aggregation is the major mechanism by which CD40 is activated. Furthermore, our data support that an aggregate of activated receptors is favorable in regard to CD40 activation.