Autocrine lymphotoxin production in Epstein-Barr virus-immortalized B cells: Induction via NF-κB activation mediated by EBV-derived latent membrane protein 1

Epstein-Barr virus (EBV)-immortalized lymphoblastoid cells express high levels of lymphotoxin and use this molecule as an autocrine growth factor. We hypothesized that the EBV-derived latent membrane protein 1 (LMP1) mediates lymphotoxin production by inducing NF-κB binding to the lymphotoxin promoter. We assessed lymphotoxin production, LMP1 expression, and NF-κB activation in Z-43 (EBV-positive lymphoblastoid cells), Daudi (EBV-positive Burkitt's cells), and 3A4 (EBV-negative Burkitt's cells containing a stably transfected tetracycline-inducible LMP1 construct). Z-43 cells expressed high levels of LMP1 (immunoblot) and lymphotoxin (ELISA); the EBV-positive Burkitt's lymphoma line Daudi expressed neither LMP1 nor lymphotoxin. Similarly, induction of LMP1 in the 3A4 cells (exposed to tetracycline) was accompanied by a 13-fold increase in lymphotoxin levels (ELISA) as compared to uninduced (LMP1-negative) cells. EMSAs demonstrated high levels of NF-κB activation in Z-43 and tetracycline-induced 3A4 cells, but much lower levels in the uninduced 3A4 cells. Exposure of these cells to Bay 11-7082 (an inhibitor of IκB phosphorylation and, therefore, NF-κB activation) abrogated NF-κB binding and lymphotoxin production in a dose-dependent manner in both Z-43 and 3A4 cells. Therefore, in our model system, autocrine lymphotoxin production is largely driven by NF-κB activation, which is in turn mediated by EBV-derived LMP1 signaling.