The biosynthesis of Ly-5 in T and B cells.

Studies on the molecular structure of the murine alloantigen, Ly-5, have demonstrated the existence of an unusual m.w. variation, with only particular Ly-5 m.w. forms, or combinations of forms, being expressed on certain lymphoid cell populations. In this report we compared the biosynthesis of the 187,000, 200,000, and 210,000 m.w. forms of Ly-5 (designated 187ly5, 200ly5, and 210ly5), detected on splenic T cells, with the biosynthesis of the 220,000 m.w. Ly-5 form (220ly5), which is usually detected only on B cells. EL-4 cells and density gradient-purified B cell blasts induced by the B cell mitogen lipopolysaccharide (LPS blasts) were used as experimental cell populations since they expressed Ly-5 phenotypes closely characteristic to splenic T and B cells, respectively. Biosynthetic labeling of EL-4 cells and LPS blasts with 14C-amino acids followed by Ly-5 immunoprecipitation and SDS PAGE analysis demonstrated that cell surface Ly-5 was synthesized, in EL-4 cells from a precursor of approximately 170,000 daltons (designated pr170ly5) whereas the precursor in LPS blasts, (pr190ly5), was approximately 20,000 daltons larger. Neither precursor was readily detected on the cell surface. Although Ly-5 synthesis in both cell types proceeded by increasing the m.w. of the precursors, synthesis of 220ly5 in the LPS blasts, significantly, did not involve the intermediate m.w. Ly-5 forms such as 187ly5 or 200ly5 detected in EL-4 cells. All forms of Ly-5 in EL-4 cells and LPS blasts incorporated 3H-mannose and 3H-glucosamine, however, the precursors incorporated a relatively higher 3H-mannose to 3H-glucosamine ratio compared to their immediate products. Restricted phosphorylation of Ly-5 was observed in LPS blasts and EL-4 cells with 32phosphate incorporation into 187ly5, 200ly5, 220ly5 but not into pr170ly5 or pr190ly5, being detected. These results demonstrate that specialization of Ly-5 biosynthesis has occurred in these T and B cell populations, resulting in the selective expression of different Ly-5 m.w. forms on the cell surface.