Lysosomal enzyme transfer from different types of lymphoid cell

The direct transfer of certain lysosomal enzymes during cell-to-cell contact between normal lymphocytes and enzyme-deficient recipient cells has previously been reported in vitro and may play an important role in the correction of lysosomal storage diseases by bone marrow transplantation in vivo. In the present study we have used a number of different T, B, and plasma cell lines to examine the expression and immunological specificity of the transfer of the lysosomal enzyme, beta-glucuronidase (Gus). Each of these groups of cell had differing intracellular and secreted levels of Gus activity, which were nevertheless similar within each group. Dermal fibroblasts deficient in the Gus enzyme acquired substantial amounts of additional activity when they were cultured together with the T cells, the B cells, or the plasma cells. This occurred by the direct transfer of Gus from all three types of cell. In addition, with plasma cells, which had very high intracellular enzyme activity and also secreted high levels of Gus into their culture medium, the secreted enzyme was readily internalized by the fibroblasts via the mannose 6-phosphate receptor (MPR). It was notable that the purified endogenous enzymes from plasma cells as well as from B cells, but not from T cells, were also endocytosed by the fibroblasts utilizing this receptor-mediated process. Although the Gus activity from all the cell lines examined had the same molecular size, polyacrylamide electrophoresis and isoelectric focusing patterns showed that the immunologically distinct types of lymphoid cell have characteristic, unique pathways of post-translational lysosomal enzyme processing. These results show that the transfer of lysosomal enzymes from lymphoid cells can occur by two distinct mechanisms, both likely to have important roles in enzyme replacement therapy.