Immunoelectron microscopic investigations of patching, capping, endocytotic and shedding processes in T and B lymphocytes

Lymphocytes were isolated from the blood of healthy juvenile test persons by the FICOLL method. Subsequently, CD4-, CD8-, and CD19-positive cells were obtained by the use of magnetic beads. The sandwich technique and gold labelling method (preembedding) served for the demonstration of receptors in the electron microscope. The gold-labelled receptors were primarily endocytosed via smooth-walled micropinocytotic processes, less frequently by coated pits/vesicles. The endocytosis cycle lasted only as far as the level of multivesiculated bodies. Lysosomes and structures of the Golgi apparatus were free from gold particles. It was surprising that after activation the capping phenomena were not associated with increased endocytotic activity. The inner surface of the membrane of endocytotic vesicles underneath the cap does not explain modulation or turnover of the receptor under these conditions, not even in view of a fast endocytotic cycle. Another possibility of a membrane turnover is the "shedding" process. We were indeed able to demonstrate gold labelling and surface coat-like material in the extracellular matrix.