B cell receptors and free antibodies have different antigen-binding kinetics

M. García-Sánchez, M. Castro, J. Faro

Proceedings of the National Academy of Sciences of the United States of America Vol. 120, nº. 35, pp. e2220669120-1 - e2220669120-8

Summary:

Since the pioneering works of Berg and Purcell, discriminating between diffusion followed by binding has played a central role in understanding cell signaling. B cell receptors (BCR) and antibodies (Ab) challenge that simplified view as binding to the antigen follows after a chain of diffusion and rotations, including whole molecule rotation and independent tilts and twists of their Fab arms due to their Y-shaped structure and flexibility. In this paper, we combine analytical calculations with Brownian simulations to derive the first-passage times due to these three rotations positioning the Fab paratopes at a proper distance and orientation required for antigen binding. Our results indicate that when measuring Ab–Ag effective kinetic binding rates, using experimental methods in which the analyte is in solution only gives values proportional to the intrinsic binding rates, k⁺, and k^-, for values of k⁺ up to 10⁹ s^-1. Beyond that, a plateau of the effective 3D on rate between 10⁸ M^-1 s^-1 and 10⁹ M^-1 s^-1 is attained. Additionally, for BCR–Ag interactions, the effective 2D on and off binding rates can only be inferred from the corresponding effective 3D on and off rates for values of effective 3D on rates lower than 10⁶ M^-1 s^-1. This is highly relevant when trying to relate BCR–antigen-binding strength and B cell response, especially during germinal center reactions. Therefore, there is a pressing need to reexamine our current understanding of the BCR–antigen kinetic rates in germinal centers using the latest experimental assays for BCR–Ag interactions.

Spanish layman's summary:

Las células B se unen a los antígenos a través de las membranas celulares. Sin embargo, muchos experimentos se realizan en solución con anticuerpos análogos en entornos de laboratorio controlados. Nuestro modelo matemático explica y cuantifica por qué hay una diferencia entre la función inmune esperada y real.

English layman's summary:

B cells link up with antigens across cell membranes. However, many experiments are done in solution with analogous antibodies in controlled lab settings. Our math model explains and quantifies why there's a difference between expected and actual immune function.

Keywords: B cell receptor; humoral response; immunoglobulins; first-passage times; kinetic rates

JCR Impact Factor and WoS quartile: 11,100 - Q1 (2022)

DOI reference: https://doi.org/10.1073/pnas.2220669120

Published on paper: August 2023.

Published on-line: August 2023.

Citation:
M. García-Sánchez, M. Castro, J. Faro, B cell receptors and free antibodies have different antigen-binding kinetics. Proceedings of the National Academy of Sciences of the United States of America. Vol. 120, nº. 35, pp. e2220669120-1 - e2220669120-8, August 2023. [Online: August 2023]