Turnover of radioiodinated and of leucine-labeled immunoglobulin M in murine splenic lymphocytes.

Lactoperoxidase-catalyzed radioiodination of cell surface proteins and biosynthetic incorporation of tritiated leucine were used to study the size and turnover of cell-associated immunoglobulins in splenic lymphocytes of different size from BALB/c, C3H, and nu/nu mice. Small spleen cells, separated from large cells by velocity sedimentation, showed a single slow rate of turnover (t1/2 = 10-30 h) of surface radioiodinated or leucine-labeled IgM released as the monomeric 7-8 S subunit of IgM. Mitogen-activated large B cells and nonstimulated large spleen cells, separated by velocity sedimentation, released leucine-labeled IgM in an initial rapid phase (t1/2 = 2-4 h) as 19 S IgM, and later released 7-8 S IgM slowly (t1/2 = 10-30 h). The iodination reaction changed neither the biphasic mode of turnover, nor, even at higher concentration of H2O2, the size of the actively secreted 19 S IgM. Radioiodination of mitogen-activated large cells or of unstimulated large cells labeled only the slowly released 7-8 S IgM, not the actively secreted 19 S IgM. Large cells of nonstimulated spleen, however, released radioiodinated as well as leucine-labeled IgM also rapidly (t1/2 = 1-3 h). These rapidly released IgM molecules were found to be 7-8 S IgM subunits. Our results indicate that these turnover rate and size differences of radioiodinated and of leucine-labeled IgM released from murine spleen cells are in part resulting from the differences in the methodology of labeling and are in other parts due to different small and large splenic lymphocytes releasing IgM of different size at different rates.