In addition to nasopharyngeal carcinoma and gastric carcinoma, many examples of EBV infection of epithelial cells in vivo are known, including AIDS related hairy leukoplakia, which is a proliferative EBV lesion in the lingual epithelium,43
and EBV infection of the salivary gland epithelium.44
However, in vitro, in comparison with the infection of B cells, infection of epithelial cells is more difficult. EBV adheres to B cells through CD21 molecules expressed on these cells, and this determines the B cell tropism.45
In contrast, epithelial cells are CD21 negative and this is believed to be why infection is not established, especially because they become susceptible to EBV infection when the barrier is overcome by CD21 expression via gene transfer46
or membrane implantation.47
We have shown clear evidence for direct infection of various human epithelial cells by EBV in vitro.48,49
The infection was achieved by the use of recombinant EBV (Akata strain) carrying a selectable marker gene,50,51
but without any other artificial operations, such as introduction of the CD21 gene. The cells used were 21 human carcinoma cell lines including those of the stomach, lung, and colon, normal human fibroblasts, and five non-human epithelial and fibroblast cell lines. Infection was accomplished by two methods: direct contact with high titre virus supernatant; and mixed culture with recombinant EBV producing Akata cells. After infection, cells were cultured in G418 containing medium and EBV infected cells were selected. Continuous EBV infected cell clones were obtained from three human carcinoma lines by the virus supernatant, and from 15 human carcinoma lines by the mixed culture method (table 1). In all of the continuously infected cell clones, only EBER, EBNA1, LMP2A, and BARF0 were expressed constitutionally .
Efficiency of Epstein-Barr virus (EBV) infection of epithelial cell lines
Sixbey and Yao have reported that in the pathway whereby IgA in the blood is processed into the secreted form by epithelial cells, EBV is incorporated into the epithelial cells in a bound state with IgA antibody against envelope protein.52
This phenomenon could explain the involvement of EBV infection in the development of nasopharyngeal carcinoma and possibly of gastric carcinoma, which are typically preceded and accompanied by the appearance of virus specific IgA in serum. Our data, however, point to another conceivable method of infection in vivo, in which cell to cell contact with virus producers is another efficient mode of EBV infection. The virus donors are most likely EBV infected B cells migrating into the epithelial stroma or intraepithelial space.
In our studies, most of the epithelial cell lines that were successfully infected with EBV were CD21 negative, and the infection was not inhibited by anti-CD21 monoclonal antibodies (OKB7). The results indicate that EBV infects epithelial cells through a receptor other than CD21. On the other hand, Fingeroth et al
have reported that EBV can infect a human epithelial cell line, 293, in a CD21 dependent manner.53
These results suggest that EBV might be able to infect epithelial cells by more than one mechanism.
As in nasopharyngeal carcinoma, attempts to establish EBV positive gastric carcinoma cell lines have been unsuccessful. However, an EBV positive gastric carcinoma cell line that is transplantable in severe combined immunodeficient mice (SCID) mice (KT) has been established.54
The cell line retains the same clonal EBV genome and pattern of EBV gene expression as the original tumour biopsy. Thus, only the EBERs and Qp driven EBNA1 are expressed. This KT cell line could be an excellent in vivo model for studying the role of EBV in the development of gastric carcinoma. Two EBV positive epithelial cell lines have been established from non-cancerous portions of the stomachs of two patients with gastric carcinoma.55,56
Both cell lines express EBNA2 and LMP1, and spontaneously produce infectious viruses. These viruses are unique in that they have the ability to transform primary B cells and induce EA in latently EBV infected Raji cells. Apart from the P3HR-1 cell line, EA inducing virus has been reported in the nasopharyngeal carcinoma–KT cell line57
and a nasopharyngeal carcinoma cell line.58
The former is a nasopharyngeal carcinoma hybrid cell line obtained by fusing primary nasopharyngeal carcinoma cells with an epithelial cell line derived from a human adenoid tissue. EA inducing virus might therefore be more prevalent in epithelial tissues.
With regard to the virus strain dependent difference in infection efficiency suggested previously,46,59
our preliminary results indicate that the B95-8 strain of EBV is also infectious to epithelial cells. However, we have not determined the relative infection efficiencies of Akata and B95-8 viruses.