Exploring B cell receptor function using defined multivalent ligands

Antigen interactions with the B cell antigen receptor (BCR) mediate responses of both tolerance and immunity. A number of factors influence B cell activation: the developmental state of the B cell, antigen concentration, other cell surface receptors (coreceptors), and antigen valency. Here, we employ defined multivalent ligands as reagents to investigate the role of antigen valency and coreceptor engagement in B cell signaling.; To explore the issue of antigen valency, multivalent ligands of defined valencies were synthesized using ring-opening metathesis polymerization (ROMP). I found that the extent of signaling, degree of BCR clustering, and antibody production were all influenced profoundly by antigen valency. The data support models in which BCR clustering is required for antigen-dependent signaling. Moreover, they indicate that differences in the antigenicity of BCR ligands are related to the increases in intracellular Ca2+ concentration they elicit. Finally, I observed that unligated BCRs clustered with BCRs engaged by multivalent ligands, a result that indicates that BCR-mediated signals are amplified through receptor arrays.; To examine the influence of coreceptors on B cell activation, multivalent ligands were generated displaying recognition elements to the BCR and coreceptors implicated in modulating B cell signaling. I found that co-engagement of CD22 with the BCR inhibited antigen-induced increases in intracellular calcium levels. These observations demonstrate that antigen interaction with CD22 dampens BCR-mediated signals. In contrast, co-engagement of CD21 and the BCR enhances responses of multivalent ligands displaying T cell-independent antigens, and requires less antigen for signal generation. These data suggest that CD21 engagement lowers the threshold for signals initiated by T-independent antigens.; My results highlight the importance of antigen valency and coreceptor function in modulating B cell signaling. They demonstrate the utility of polymers generated by ROMP for investigating B cell signaling in vitro and modulating immune responses in vivo. Finally, my results set the stage for future studies using multivalent ligands to illuminate the mechanisms by which B cells respond to antigens with outcomes ranging from immunity to tolerance.