Characterization of Lymphocyte Fibronectin

In vitrocultured “activated” peripheral blood lymphocytes and T-cell lines synthesized a high-molecular-weight gelatin binding molecule (MW 500 kDa), whereas resting lymphocytes showed poor or negligible synthesis of the same component. Concanavalin A-mediated anchorage of the lymphocytes to a substratum potentiated synthesis of the high-molecular-weight molecule. Western blotting of the gelatin-binding lymphocyte molecule demonstrated reactivity with antibodies specific for human fibronectin. Furthermore, immunocytochemistry showed reactivity of anti-fibronectin antibodies with T-lymphocytes at the single-cell level. The lymphocyte-derived fibronectin was preferentially cell associated and relatively small amounts were present in the culture medium. RT-PCR of total RNA from CD4⁺T-cells and the lymphoid T-cell line MOLT-4 showed that the most abundant species of fibronectin mRNA lacked the entire III CS exon encoding the α₄β₁binding region LDV. Amplification of the III CS region from other T-cell lines revealed that these cells expressed several fibronectin mRNA isoforms most of which were lacking the LDV coding sequence. In conclusion, synthesis of fibronectin is demonstrated to occur in T-lymphocytes and to be regulated by signals which activate the cells.