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How B cells sense monovalent antigens

October 10, 2016

A new study by scientists of the Max Planck Institute of Immunobiology and Epigenetics in Freiburg reveals how monovalent antigens open and activated B cell antigen receptors to start the immune response.

Large amounts of antibodies are produced after an acute infection or a successful vaccination. This limits the duration of an infectious disease and protects us from re-infection. The production of antibodies during an immune response requires the activation of specific B lymphocytes. B cells detect antigens (specific molecules that trigger specific antibody formation) with their B cell antigen receptor (BCR), which lies on their surface. Upon the binding of an antigen to the BCR, B cells are activated and differentiate into antibody-producing plasma cells to support the immune response.

The lab of Michael Reth at the Max Planck Institute of Immunobiology and Epigenetics in Freiburg is studying the structural organization and signalling mechanisms of BCRs. In the course of this, the team developed a new model on how B cells sense and become activated upon contact with antigens. For a long time, it was thought that antigen-sensing by the BCR requires the cross-linking of at least two separate BCRs. After contact with matching antigens BCRs are cross-linked and send signals into the cell to activate the immune response.

However, with this cross-linking model (CLM) it is difficult to explain how monovalent antigens, that per se cannot »cross-link« two BCRs, are able to activate the BCR. The dissociation-activation model (DAM) of BCR activation, suggested by Jianying Yang and Michael Reth in 2010, offers a simple solution to this problem. “We developed methods to investigate the BCR organization on a nanometer scale. Upon these observations we suggest, that binding to antigen, be it mono- or polyvalent, disturbs a pre-ordered structure of the BCRs and leads to their opening (dissociation) and activation”, says Michael Reth, speaker of the cluster of excellence BIOSS Centre for Biological Signalling Studies at the University of Freiburg.

The two models of receptor activation in B cells at a glance

Left: The cross-linking model does not explain how cross-linking activates the B-cell receptors after the binding of monovalent antigens.

Right: The dissociation-activation model by Yang and Reth (2010) assumes that after binding of monovalent antigens, a specific arrangement of B-cell receptors is disrupted and leads to the opening and activation of the receptor. The study by Volkmann et al. (2016) also concluded that the opening of the receptor by monovalent antibody fragments requires the presence and activity of the protein Lyn.

To validate either model (CLM or DAM), it is important to directly monitor conformational alterations of the BCR upon binding to monovalent antigens. In the current study by the lab of Michael Reth the team employed a so-called Fab-based proximity ligation assay (Fab-PLA), which monitors alteration of the BCR conformation at 10-20 nanometer distances. With this assay, two PhD students of team, Christoph Volkmann and Naema Brings studied the organization of the BCR and their conformational alterations on splenic B cells derived from mice. “We found that exposure of these B cells to monovalent antigen results in the loss of the IgM:IgM Fab-PLA signal. This indicates a dissociation, rather than aggregation of the BCRs and further supports our new model”, says the team of the two (Fig. 1).

Previous experiments in this field showed that the exposure of B cells to full-length anti-BCR antibodies that have more than one binding site (divalent) could activate the BCR, whereas monovalent binding fragments of anti-BCR antibodies, so-called Fab fragements, failed to do so. These results were taken as evidence that “cross-linking” by divalent antibodies or polyvalent antigens is required for BCR activation. But the investigations by the lab of Michael Reth now showed that a monovalent Fab indeed can open the BCR, but only if it interacts directly with the antigen binding site. “Based on our dissociation-activation model we suggest that within the BCR islands the antigen binding sites of the receptors have specific alignments. This orientation allows them to touch each other and to cooperate in the antigen sensing”, says Jianying Yang.

Interestingly, the team also found that the sensing and opening process requires the presence and activity of the protein Lyn (Fig 1). “If B cells lack Lyn they are only able to sense polyvalent antigens, but no longer the monovalent antibody fragments”, says Christoph Volkmann.

This finding indicates that the BCR has evolved a special mechanism to sense monovalent antigens. “We are very satisfied that our new study cleary supports our suggested BCR activation model”, says Michael Reth. He and his team are convinced that in following this pathway they are able to get a more detailed view of the B cell activation process and may improve vaccination strategies within the next years.

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