When injected subcutaneously into irradiated athymic (nude) mice, human B cells immortalized with Epstein-Barr virus (EBV) either fail to grow or give rise to small tumors that soon regress. In the same experimental system, EBV-negative human Burkitt's lymphoma cells generally develop into lethal malignant tumors. We examined whether inoculation of EBV-immortalized B cells might have an antitumor effect against experimental Burkitt's lymphoma in athymic mice. EBV-negative Burkitt's tumors regressed after intratumor inoculations with EBV-immortalized B cells. Other human malignant cell lines, representative of breast carcinoma,colon adenocarcinoma, sarcomas, myeloid leukemias, neuroblastoma, and other malignancies, regressed when tested in the same experimental nude mouse model. We have began to study the mechanisms mediating tumor regression in this experimental system, and found that the chemokines IP-10 and Mig contribute to tumor regression. When IP-10 was inoculated into established Burkitt's lymphoma tumors or expressed through transfection into Burkitt's lymphoma cells, the tumors consistently developed areas of tumor necrosis with vascular damage and intravascular coagulation. Mig caused similar effects. In the same system, IL-12 caused tumor necrosis similar to IP-10 and Mig, and tissue induction of IP-10 and Mig was present. In other studies, we wished to identify the signals that promote a host response leading to cytokine and chemokine production, and have focused on the role of the latent EBV gene products. Using Burkitt lymphoma cell lines expressing individual EBV latency genes, we have identified LMP-1 as being critical to the induction of a host response that includes IP-10 and Mig. Due to its previously recognized ability to promote NF kappa B expression and TRAF signaling, LMP1 may either promote the secretion of regulatory molecules or mimic receptor signaling in the absence of the ligand. Additional studies have focused on the mechanisms by which IL-12, IP-10, and Mig cause tumor tissue necrosis. First, we have demonstrated that in athymic mice the induction of IP-10 and Mig is critical to the antitumor effect of IL-12. Secondly, we have established that inhibition of angiogenesis is essential to the antitumor effect of IL-12, IP-10 and Mig. Thirdly, we have shown that IP-10 and Mig do not act directly on endothelial cells which do not express CXCR-3, the receptor for these chemokines. Rather, IL-12, IP-10 and Mig induce chemoattraction and activation of NK cells which can effectively kill proliferating endothelial cells. These studies have defined the antiangiogenic activities of IL-12 and its downstream mediators IP-10 and Mig.
