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Following infection with Epstein–Barr virus (EBV), the virus is carried for life in the memory B-cell compartment in a silent state (latency I/0). These cells do not resemble the proliferating lymphoblastoid cells (LCLs) (latency III) that are generated after infection. It is of fundamental significance to identify how the different EBV expression patterns are established in the latently infected cell. In view of the prompt activatability of CD4 + T cells in primary EBV infection, and their role in B-cell differentiation, we studied the involvement of CD4 + T cells in the regulation of EBV latency. Lymphoblastoid cell lines (LCLs) were cocultured with autologous or allogeneic CD4 + T cells. Activated T cells influenced the expression of two key viral proteins that determine the fate of the infected B cell. EBNA2 was down-regulated, whereas LMP1 was unregulated and the cells proliferated less. This was paralleled by the down-regulation of the latency III promoter (Cp). Experiments performed in the transwell system showed that this change does not require cell contact, but it is mediated by soluble factors. Neutralizing experiments proved that the up-regulation of LMP1 is, to some extent, mediated by IL21, but this cytokine was not responsible for EBNA2 down-regulation. This effect was partly mediated by soluble CD40L. We detected similar regulatory functions of T cells in in vitro-infected lymphocyte populations. In conclusion, our results revealed an additional mechanism by which CD4 + T cells can control the EBV-induced B-cell proliferation.

Chronic lymphocytic leukemia (CLL) cells express the receptor for Epstein-Barr virus (EBV) and can be infected in vitro. Infected cells do not express the growth-promoting set of EBV-encoded genes and therefore they do not yield LCLs, in most experiments. With exceptional clones, lines were obtained however. We describe a new line, HG3, established by in vitro EBV-infection from an IGHV1–2 unmutated CLL patient clone. All cells expressed EBNA-2 and LMP-1, the EBV-encoded genes pivotal for transformation. The karyotype, FISH cytogenetics and SNP-array profile of the line and the patient's ex vivo clone showed biallelic 13q14 deletions with genomic loss of DLEU7, miR15a/miR16–1, the two micro-RNAs that are deleted in 50% of CLL cases. Further features of CLL cells were: expression of CD5/CD20/CD27/CD43 and release of IgM natural antibodies reacting with oxLDL-like epitopes on apoptotic cells (cf. stereotyped subset-1). Comparison with two LCLs established from normal B cells showed 32 genes expressed at higher levels (> 2-fold). Among these were LHX2 and LILRA. These genes may play a role in the development of the disease. LHX2 expression was shown in self-renewing multipotent hematopoietic stem cells, and LILRA4 codes for a receptor for bone marrow stromal cell antigen-2 that contributes to B cell development. Twenty-four genes were expressed at lower levels, among these PARD3 that is essential for asymmetric cell division. These genes may contribute to establish precursors of CLL clones by regulation of cellular phenotype in the hematopoietic compartment. Expression of CD5/CD20/CD27/CD43 and spontaneous production of natural antibodies may identify the CLL cell as a self-renewing B1 lymphocyte.

Epstein Barr virus (EBV) is carried by almost all adults, mostly without clinical manifestations. Latent virus infection of B lymphocytes induces activation and proliferation that can be demonstrated in vitro . In healthy individuals, generation of EBV induced malignant proliferation is avoided by continuous immunological surveillance. The proliferation inducing set of the virally encoded genes is expressed exclusively in B cells in a defined differentiation window. It comprises nine EBV encoded nuclear proteins, EBNA 1–6, and three cell membrane associated proteins, LMP-1, 2A and 2B, designated as latency Type III. Outside this window the expression of the viral genes is limited. Healthy carriers harbor a low number of B lymphocytes in which the viral genome is either silent or expresses one virally encoded protein, EBNA-1, latency Type I. In addition, EBV genome carrying B cells can lack either EBNA-2 or LMP-1, latency Type IIa or Type IIb respectively. These cells have no inherent proliferation capacity. Detection of both EBNA-2 and LMP-1 can identify B cells with growth potential. We devised therefore a method for their simultaneous detection in cytospin deposited cell populations. Simultaneous detection of EBNA-2 and LMP-1 was reported earlier in tissues derived from infectious mononucleosis (IM), postransplantation lymphoproliferative disorders (PTLD) and from “humanized” mice infected with EBV. We show for the first time the occurrence of Type IIa and Type IIb cells in cord blood lymphocyte populations infected with EBV in vitro . Further, we confirm the variation of EBNA-2 and LMP-1 expression in several Type III lines and that they vary independently in individual cells. We visualize that in Type III LCL, induced for plasmacytoid differentiation by IL-21 treatment, EBV protein expression changes to Type IIa (EBNA-2 negative LMP-1 positive). We also show that when the proliferation of EBV infected cord blood lymphocyte culture is inhibited by the immunomodulator, PSK, the majority of the cells express latency Type IIa pattern. These results show that by modifying the differentiation state, the proliferating EBV positive B cells can be “curbed”. Type IIa expression is a characteristic for EBV positive Reed–Sternberg (R/S) cells in EBV positive Hodgkin's lymphomas. For survival and proliferation, the R/S cells require the contribution of the in vivo microenvironment. Consequently, Type IIa lines could not be established from Hodgkin's lymphoma in vitro . We propose that these experimental cultures can be exploited for study of the Type IIa cells.

The potentially oncogenic Epstein-Barr virus (EBV) is carried by almost all humans in a well equilibrated coexistence. The phenotype of the cells that carry EBV genomes is determined by virally-encoded and cellular proteins. B lymphocyte is the main target of the virus and latent infection of this cell induces proliferation. Nine virus-encoded genes participate in the “growth program” that is expressed in a narrow differentiation window of the B cell. Such cells have the potential to develop malignant proliferations. However, several control mechanism eliminate this danger and the general chronic virus carrier state is most often asymptomatic. One mechanism exploits the normal regulation in the immune system, the T cell mediated modulation of the B cell differentiation state. Another is based on cognate recognition and elimination of the infected cells. The expression of EBV encoded genes in B lymphocytes can be also “restricted,” they do not express all components of the viral growth program. Here, we discuss a rare viral expression in B cells that has not been connected with malignant transformation yet.