[Paper Review] The weakest link bridging germinal center B cells and follicular dendritic cells limits antibody affinity maturation

Rajat Desikan, Rustom Antia+1 more|arXiv (Cornell University)|Feb 7, 2020

T-cell and B-cell Immunology132 references4 citations

TL;DR

The paper proposes that the antibody affinity ceiling in vivo arises not from B cell receptor-Antigen (BCR-Ag) interactions alone, but from the weakest link in a multi-protein complex bridging germinal center B cells and follicular dendritic cells (FDCs) during antigen acquisition. By strengthening this weakest link—such as the FcγRIIB-antibody interaction—affinity maturation could be enhanced, raising the ceiling on antibody potency.

ABSTRACT

The affinity of antibodies (Abs) produced in vivo for their target antigens (Ags) is typically well below the maximum affinity possible. Nearly 25 years ago, Foote and Eisen explained how an 'affinity ceiling' could arise from constraints associated with the acquisition of soluble antigen by B cells. However, recent studies have shown that B cells in germinal centers (where Ab affinity maturation occurs) acquire Ag not in soluble form but presented as receptor-bound immune complexes on follicular dendritic cells (FDCs). How the affinity ceiling arises in such a scenario is unclear. Here, we argue that the ceiling arises from the weakest link of the chain of protein complexes that bridges B cells and FDCs and is broken during Ag acquisition. This hypothesis explains the affinity ceiling realized in vivo and suggests that strengthening the weakest link could raise the ceiling and improve Ab responses.

Motivation & Objective

• To resolve the longstanding paradox of why in vivo antibody affinities plateau far below theoretical maximums despite prolonged affinity maturation.
• To reconcile the classical Foote-Eisen model of affinity ceiling with modern immunological understanding that antigen is acquired not as soluble Ag, but as immune complexes on follicular dendritic cells (FDCs).
• To propose a new mechanistic explanation for the affinity ceiling based on the weakest link in a multi-protein complex bridging B cells and FDCs.
• To suggest that enhancing the weakest link in this complex could raise the ceiling on antibody affinity and improve vaccine and immunotherapy design.

Proposed method

• Modeling the antigen acquisition process as a chain of protein complexes connecting B cells and FDCs, with each complex contributing to the overall binding energy.
• Using thermodynamic free energy (ΔG) estimates to quantify the strength of each link in the chain, particularly focusing on the BCR-Ag, Fc-FCγRIIB, and complement-C3d-CR2 interactions.
• Analyzing in vitro experiments where Ag was presented via DNA or streptavidin tethers to stiff (PEG) or flexible (PMS) surfaces to simulate mechanical strength of links.
• Applying the concept of 'slip bonds' and 'catch bonds' to understand how mechanical forces during antigen extraction affect dissociation rates of the chain components.
• Proposing targeted interventions such as Fc engineering or use of high-affinity IgG subtypes to strengthen the weakest link and elevate the affinity ceiling.
• Drawing on passive immunization data to suggest that administered antibodies with enhanced FcγRIIB binding could improve endogenous B cell selection and raise the ceiling.

Experimental results

Research questions

• RQ1 Why do in vivo antibody affinities plateau at ~10^10–10^12 M⁻¹ despite the potential for much higher affinities in vitro?
• RQ2 How does the mechanism of antigen acquisition—via immune complexes on FDCs—alter the basis of the affinity ceiling compared to the classical model of soluble antigen uptake?
• RQ3 What determines the upper limit of antibody affinity maturation when antigen is presented as immune complexes on FDCs?
• RQ4 Can the affinity ceiling be experimentally raised by strengthening a specific molecular link in the B cell-FDC antigen transfer chain?
• RQ5 What role do mechanical forces and bond dynamics (e.g., slip vs. catch bonds) play in limiting antigen acquisition and thus affinity maturation?

Key findings

• The affinity ceiling in vivo arises not from the BCR-Ag interaction itself, but from the weakest link in a multi-protein complex bridging germinal center B cells and follicular dendritic cells (FDCs).
• In vitro experiments show that when the FcγRIIB-antibody interaction is the weakest link (as in flexible PMS surfaces), B cells internalize the entire complex, losing affinity discrimination and lowering the effective ceiling to the strength of the membrane patch.
• When the DNA-streptavidin bond is the weakest link (on stiff PEG surfaces), B cells rupture only that bond, preserving affinity discrimination and allowing higher-affinity B cells to be selected.
• Strengthening the weakest link—such as by engineering Fc regions for higher FcγRIIB affinity—could raise the overall affinity ceiling and improve antibody responses.
• Passive immunization with antibodies engineered to have higher FcγRIIB binding affinity may enhance endogenous B cell selection and raise the ceiling on affinity maturation.
• Clinically relevant polymorphisms in FcγRIIB or CR2 may correlate with differences in the observed affinity ceiling, suggesting a potential for genetic association studies.