Abstract
Human B cell lines expressing membrane immunoglobulin specific for tetanus toxoid/toxin were used to study the receptor-mediated endocytosis of antigen. Monovalent antigen, initially bound to cell surface immunoglobulin at 0 degree C, was rapidly endocytosed upon warming the cells to 37 degrees C. The kinetics of endocytosis of antigen were independent of the number of occupied binding sites and indicated a half-life for antigen on the cell surface of 8.5 min. Endocytosis of antigen apparently ceased after approximately 15 min at 37 degrees C, although some 40-50% remained on the cell surface at this time. We show, using biotinylated antigen and an avidin detection assay, that this is due to recycling of antigen to the cell surface. By labelling the antigen on the cell surface with Fabs against different epitopes we show that antigen continues to be endocytosed for at least 1 h after the initial rapid phase of endocytosis, again indicating that there must be recycling of immunoglobulin/antigen complexes. As a consequence of the stable interaction between antigen and membrane immunoglobulin, the capacity of the cells to accumulate antigen was limited when the synthesis of membrane immunoglobulin was blocked; under these conditions only 2-3 times as much antigen was endocytosed and degraded when antigen was supplied continuously over a 4-h period at 37 degrees C as could be bound to the cells at 0 degree C. These results reveal a rapid and efficient pathway for the endocytosis and recycling of monovalent antigen in B cells.