Quantitative ultrastructural autoradiographic studies of iodinated plasma membranes of lymphocytes during segregation and internalization of surface immunoglobulins

Rat spleen lymphocytes were iodinated (125 I) with lactoperoxidase. Quantitative autoradiographic studies on cells fixed immediately after iodination showed 19-24% of intracytoplasmic grains at 3HD and over from the plasma membrane. Normalization of grain density distribution and comparison of resulting curves with the universal curve of grain scatter of 125 I showed that a significant percentage of intracytoplasmic grains (36%) originates from intracytoplasmic labeled sources rather than from scattering from the heavily labeled plasma membrane. Damaged cells had a threefold grain density than intact cells. Radioactivity counts in sliced polyacrylamide gels of iodinated cells revealed 65-72% of total radioactivity in five peaks of apparent mol wt of 44, 50, 57, 90 and 195 thousand daltons. Segregation and internalization of anti-immunoglobulin-Ig-horseradish peroxidase (HRP) complexes from the iodinated plasma membrane proteins of lymphocytes was studied with quantitative autoradiography (125 I) and peroxidase cytochemistry; 64% of grains at 1.5HD (1,500 A) from the plasma membrane were within the cap zone, and 36% of grains remained outside the capped immunoglobulins; 45-57% of grains internalized together with Fab-anti-Ig-Ig-HRP, and 68% of grains internalized together with anti-Ig-Ig-HRP. These studies indicate that (a) iodination of rat spleen lymphocytes results in a significant internal labeling and that (b) immunoglobulins segregate into caps and internalize together with other iodinated plasma membrane proteins while a significant percentage of iodinated proteins (36%) are excluded from the immunoglobulin caps or internalization sites (32-55%).