Emergence of a surface immunoglobulin recycling process during B lymphocyte differentiation.

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Abstract
Surface Ig-mediated endocytosis was investigated in rat B lymphocytes and plasma cells, using horseradish peroxidase (HRP)-labeled sheep anti-rat Ig Fab'' fragment of antibody and HRP as monomeric ligands, respectively. Quantitative estimates of HRP activity associated either with plasma membrane or with endomembrane compartments were made in several experimental conditions. Binding of HRP-conjugate on B lymphocytes was followed by its endocytosis in combination with surface Ig, as shown by the progressive disappearance of plasma membrane-associated HRP activity. Between 1 and 6 h at 37.degree. C in presence of conjugate the total amount of cell-associated activity was constant. During this time no reappearance of surface Ig occurred by neosynthesis by the expression of an intracellular pool or by the recycling in a free form of the previously internalized molecules. At saturating doses, internalization of HRP by anti-HRP plasma cells increased linearly with time at 37.degree. C in the presence of antigen when during the same time, the plasma membrane HRP-binding capacity remained constant. Cycloheximide did not affect continuous HRP uptake. The existence of a large intracellular pool of receptors was ruled out by experiments of removal of binding sites with pronase. Monensin caused a progressive decrease in the number of surface receptors on plasma cells but not on B lymphocytes. Evidently, unlike B lymphocytes, plasma cells were able to recycle their surface Ig.