SUMMARY:Many nasopharyngeal carcinoma (NPC) biopsy specimens contain Epstein-Barr virus (EBV). However, the response of NPC cells to EBV infection in vitro and in vivo is not well characterized. In this experiment we infected NPC cells with EBV particles through endocytosis of a complex of EBV immunoglobulin A (IgA) secretory component (SC) protein to observe the response of host cells to the foreign viral infection in vitro. We found that EBV particles were endocytosed and stabilized in NPC nuclei 24 hours after infection; the EBV genomes were then gradually decreased after serial passages within 3 to 4 weeks by the following pathway: the EBV genomes first moved toward the nuclear envelope from the center of the nucleus; after crossing the nuclear envelope, they moved into the cytoplasm and toward the plasma membrane and were discharged by exocytosis. At the 10th day of EBV infection, EBV-latent membrane protein-1 and Epstein-Barr nuclear antigen (EBNA)-1 protein expressions could be detected, but not EBV-viral capsid antigen. Observation of EBNA-1 protein and host growth factor and cytokine gene expressions in the weeks after incubation revealed that the EBNA-1 protein expression was decreased proportionally with decrease of EBV genome. The mRNA expression of epithelial growth factor receptor, transforming growth factor (TGF)-␣, interleukin (IL)-1␤, IL-6, and granulocyte-macrophage colony-stimulating factor increased within 1 to 2 weeks after infection, and gradually recovered to the original level at 3 to 4 weeks, whereas the mRNAs of TGF␤1, TGF␤ receptor type I (TGF␤RI), TGF␤R type II, IL-8, and tumor necrosis factor-␣ remained unchanged. It is concluded that in vitro EBV infection in NPC cells results in increase of certain growth factor and cytokine gene expressions in host cells. The change in gene expression returns to the original level approximately 3 to 4 weeks after infection because of exocytosis of EBV DNA by the infected cells through an unidentified mechanism. (Lab Invest 2000, 80:1149-1160.N asopharyngeal carcinoma (NPC) is one of the most common cancers among Chinese living in Taiwan, Singapore, and South China (Chan, 1990;de The, 1982; Lin et al, 1971). The etiological factors for this cancer are not yet well defined. Epstein-Barr virus (EBV) has been suggested to be closely associated with NPC (Ablashi et al, 1991;Klein et al, 1974;Niedobitek et al, 1991;Zur Hausen et al, 1970). Hereditary and other environmental factors, such as eating salted fish and exposure to sulfuric acid vapor, have also been associated with NPC (Armstrong et al, 1983;Hirayama and Ito, 1981;Ho et al, 1999).For identification of the actual relationship between EBV and NPC, we have previously established nine NPC cell lines in our laboratory (Lin et al, 1990(Lin et al, , 1993.Results from the observation of EBV in these nine NPC cell lines and biopsy specimens revealed that EBV was present in only five of nine lines and was an episomal, but not an integrated form, in the infected cells. In each EBV-infected cell line, on...