Induction of lymphocyte blast transformation by purified fibronectin in vitro

Purified rat plasma fibronectin (Fn) induces a dose-dependent, nonspecific proliferation of lymphoid cells isolated from spleen and lymph nodes, but has no effect on thymocytes. Proliferation of cells occurred after 4 to 5 days of incubation and was generally 5- to 10-fold greater than control cells cultured without Fn or in the presence of the same concentrations of rat serum albumin. The responding cell appears to be the T cell and requires the presence of adherent accessory cells (macrophages). Although purified T and B cells failed to demonstrate a significant increase in blastogenesis in the presence of Fn, reconstitution of T, but not B, cells with irradiated peritoneal exudate macrophages restored the stimulatory effect of Fn. Furthermore, comparison of the effect of various concentrations of macrophages on the restored T cell response shows that proliferative activity in the presence of Fn is dependent on a critical number of macrophages. Adding higher numbers of macrophages beyond the optimal concentration results in a sharp decline in lymphocyte responsiveness to Fn, although the proliferation of control cells continues to increase at these macrophage concentrations. Macrophages pulsed with Fn for 1 hr failed to evoke an increase in T cell responsiveness after 3 or 5 days of incubation. In addition, incubation of Fn-pulsed T cells with macrophages that had been precultured with or without Fn also failed to result in T cell activation. The effect of Fn on T lymphocyte transformation appears to be mediated indirectly through its interaction with the macrophage, because 125I-Fn, which shows significant binding to peritoneal exudate macrophages, fails to interact with purified T cells. Radiolabeled Fn also demonstrated little or no binding activity for unseparated lymph node cells.