CD19: Lowering the Threshold for Antigen Receptor Stimulation of B Lymphocytes

TLDR

Lymphocytes must respond to low antigen concentrations, yet broad specificity and sensitivity conflict because low‑affinity receptors enable specificity but limit sensitivity. The study examines how the accessory protein CD19 enables B cells to activate when only a few antigen receptors are engaged. Co‑ligating CD19 with the B‑cell antigen receptor lowered the activation threshold by ~100‑fold, allowing proliferation with only ~100 receptors (0.03 %) per cell.

Abstract

Lymphocytes must proliferate and differentiate in response to low concentrations of a vast array of antigens. The requirements of broad specificity and sensitivity conflict because the former is met by low-affinity antigen receptors, which precludes achieving the latter with high-affinity receptors. Coligation of the membrane protein CD19 with the antigen receptor of B lymphocytes decreased the threshold for antigen receptor-dependent stimulation by two orders of magnitude. B lymphocytes proliferated when approximately 100 antigen receptors per cell, 0.03 percent of the total, were coligated with CD19. The B cell resolves its dilemma by having an accessory protein that enables activation when few antigen receptors are occupied.