Journal
FRONTIERS IN IMMUNOLOGY
Volume 4, Issue -, Pages -Publisher
FRONTIERS RESEARCH FOUNDATION
DOI: 10.3389/fimmu.2013.00427
Keywords
BCR; oligomerization; electron microscopy; immuno-gold-labeling; Monte Carlo simulation; maximum-likelihood method
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Funding
- Excellence Initiative of the German Research Foundation (BIOSS Centre for Biological Signalling Studies) [EXC294]
- Excellence Initiative of the German Research Foundation (Spemann Graduate School of Biology and Medicine) [GSC-4]
- open access publication fund of the Albert Ludwigs University Freiburg
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The B cell antigen receptor (BCR) plays a crucial role in adaptive immunity, since antigen-induced signaling by the BCR leads to the activation of the B cell and production of antibodies during an immune response. However, the spatial nano-scale organization of the BCR on the cell surface prior to antigen encounter is still controversial. Here, we fixed murine B cells, stained the BCRs on the cell surface with immuno-gold and visualized the distribution of the gold particles by transmission electron microscopy. Approximately 30% of the gold particles were clustered. However the low staining efficiency of 15% precluded a quantitative conclusion concerning the oligomerization state of the BCRs. To overcome this limitation, we used Monte-Carlo simulations to include or to exclude possible distributions of the BCRs. Our combined experimental-modeling approach assuming the lowest number of different BCR sizes to explain the observed gold distribution suggests that 40% of the surface IgD-BCR was present in dimers and 60% formed large laminar clusters of about 18 receptors. In contrast, a transmembrane mutant of the mIgD molecule only formed IgD-BCR dimers. Our approach complements high resolution fluorescence imaging and clearly demonstrates the existence of pre-formed BCR clusters on resting B cells, questioning the classical cross-linking model of BCR activation.
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