Surface-associated antigen induces permeabilization of primary mouse B-cells and lysosome exocytosis facilitating antigen uptake and presentation to T-cells

Abstract

B-cell receptor (BCR)-mediated antigen internalization and presentation are essential for humoral memory immune responses. Antigen encountered by B-cells is often tightly associated with the surface of pathogens and/or antigen-presenting cells. Internalization of such antigens requires myosin-mediated traction forces and extracellular release of lysosomal enzymes, but the mechanism triggering lysosomal exocytosis is unknown. Here we show that BCR-mediated recognition of antigen tethered to beads, to planar lipid-bilayers or expressed on cell surfaces causes localized plasma membrane (PM) permeabilization, a process that requires BCR signaling and non-muscle myosin II activity. B-cell permeabilization triggers PM repair responses involving lysosomal exocytosis, and B-cells permeabilized by surface-associated antigen internalize more antigen than cells that remain intact. Higher affinity antigens cause more B-cell permeabilization and lysosomal exocytosis and are more efficiently presented to T-cells. Thus, PM permeabilization by surface-associated antigen triggers a lysosome-mediated B-cell resealing response, providing the extracellular hydrolases that facilitate antigen internalization and presentation.

Non-Muscle Myosin II Is Essential for the Negative Regulation of B-Cell Receptor Signaling and B-Cell Activation

“…This study shows that BCR activation by Fab’-PLB induces rapid recruitment of NMII to BCR activation sites, consistent with previous reports ( 40 , 43 ). Phosphorylated MLC is also recruited to the B-cell contact zone, suggesting that the recruited NMII is activated by BCR signaling.…”

“…NMII may negatively regulate BCR signaling by facilitating BCR endocytosis. NMII is critical for B-cells to pull Ag from Ag-presenting surfaces for endocytosis ( 40 , 42 , 43 , 55 ). BCR-mediated Ag endocytosis initiates Ag processing and presentation and also removes BCRs from surface signalosomes, potentially attenuating BCR proximal signaling.…”

“…However, NMII is not required for BCR endocytosis of soluble Ag ( 40 , 42 ). Additionally, B-cell endocytosis of surface-associated Ag occurs much later than BCR proximal signaling ( 40 , 42 , 43 ). These findings argue against NMII-dependent BCR endocytosis as a possible mechanism for BCR signaling attenuation.…”

“…NMII-driven traction forces pull Ag from presenting surfaces, enabling B-cells to acquire Ag based on BCR binding affinity ( 40 – 42 ). We have recently shown that NMII mediates antigen capture by causing B-cell membrane permeabilization, which induces a membrane repair response involving lysosome exocytosis ( 43 ). NMIIA-deficiency ultimately causes impaired antibody responses against antigenic challenges ( 39 ).…”

Lysosomes and lysosome‐related organelles in immune responses

“…Interestingly, B cell engagement with surface‐bound antigen seems also to trigger a lysosome‐mediated membrane resealing response. Maeda et al [ 81 ] have recently shown that BCR signalling following contact with surface‐bound antigen induced transient membrane permeabilization and lysosome exocytosis whose extent was proportional to BCR/antigen affinity. It seems that an ancient membrane repair response has been co‐opted by the immune system to aid antigen processing and presentation by B cells with high affinity for surface bound antigens.…”