Organelle Dynamics in B Lymphocytes following Activation with Antigen-Coated Beads

B cell receptors, BCRs, on B lymphocytes, bind antigens, causing cell activation. This leads to antigen internalization for processing into peptide fragments, which are loaded on class II MHC molecules for presentation to helper T lymphocytes to generate an effective immune response. 

To study B lymphocyte activation in vitro using antigen-coated beads, first, obtain antigen-coated beads. These beads are covalently coated with antigens, ligands specific for BCRs; a non-reactive protein blocks the unbound bead sites, preventing non-specific B lymphocyte binding.

Add the beads to the B lymphocyte suspension. Seed the mixture onto a poly-L-lysine-coated coverslip.

The negatively-charged B lymphocytes bind to the positively-charged coverslip, immobilizing the cells. Further, the beads' antigens bind specifically to BCRs. The interaction of the BCR with the antigen activates B lymphocytes, forming a tight junction — an immune synapse.

The interaction initiates intracellular signaling pathways, resulting in actin cytoskeleton reorganization. This causes cell membrane spreading followed by contraction, during which the BCR-antigen microcluster concentrates at the immune synapse center.

With actin remodeling, the centrosome — a microtubule organizing center — repositions near the synapse. The repositioning recruits organelles like lysosomes and the Golgi apparatus near the immune synapse to extract and present antigens.