Abstract
Antibodies and B cell receptors often bind their antigen at cell-cell interface while both molecular species are surface-bound, which impacts bond kinetics and function. Despite the description of complex energy landscapes for dissociation kinetics which may also result in significantly different association kinetics, surface-bound molecule (2D) association kinetics usually remain described by an on-rate due to crossing of a single free energy barrier, and few experimental works have measured association kinetics under conditions implying force and two-dimensional relative ligand-receptor motion. We use a new laminar flow chamber to measure 2D bond formation with systematic variation of the distribution of encounter durations between antigen and antibody, in a range from 0.1 to 10 ms. Under physiologically relevant forces, 2D association is 100-fold slower than 3D association as studied by surface plasmon resonance assays. Supported by brownian dynamics simulations, our results show that a minimal encounter duration is required for 2D association; an energy landscape featuring a rough initial part might be a reasonable way of accounting for this. By systematically varying the temperature of our experiments, we evaluate roughness at 2kBT, in the range of previously proposed rough parts of landscapes models during dissociation.
Highlights
Ligand-receptor interactions have long been described with the formalism elaborated by chemists for reactions in solution, i.e., affinity at equilibrium, and kinetics by on-rate and off-rates
Probability of bond formation was not proportional to encounter duration: we proposed a bond formation model[23,24] based on a rough initial part in the energy landscape
We show that 2D association kinetics strongly differs from 3D association kinetics measured using surface plasmon resonance, and that a rough energy landscape resulting in a minimal encounter time may be more suitable than an on-rate to describe association kinetic
Summary
Ligand-receptor interactions have long been described with the formalism elaborated by chemists for reactions in solution, i.e., affinity at equilibrium, and kinetics by on-rate and off-rates. During somatic hypermutation (which may follow B lymphocyte activation and where several mutation-selection cycles in the lymph node lead to a strong increase in antibody affinity, from 104M up to 1010M13,14) B cells do probe their ligand by exerting a force on the BCR-antigen bond. This pulling phase is of considerable importance for ligand discrimination[12,15]. The molecular intrinsic association rate is not questioned in these studies
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