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Epstein-Barr virus strategy in normal and neoplastic B cells
Cell, Vol. 77, 791-793,
June 17, 1994, Copyright
0 1994 by Cell Press
Epstein-Barr Virus Strategy in Normal and Neoplastic B Cells
Minireview
The Paradoxical Coexistence of Epstein-Barr Virus and the Human Species The mutually successful and essentially harmonious relationship of the Epstein-Barr virus (EBV) with our species stems from the resolution of two apparent contradictions. First, EBV is the most highly transforming virus known in any species, but it causes no disease in the vast majority of all infected individuals. In a relatively small minority, it causes a self-limiting disease, mononucleosis. Second, EBV-infected B lymphocytes are highly immunogenic for autologous T cells. They elicit vigorous T cell proliferation responses and generate highly cytotoxic killer cells, called cytotoxic T lymphocytes (CTLs). These interactions between autologous T and B cells are as intense as the allogeneic interactions across major histocompatibility barriers. Nevertheless, the virus is not eliminated from the hemopoetic system. Actually, it persists in the B lymphocyte fraction itself. The resolution of these two paradoxes is closely related. The high immunogenicity of the EBV-transformed immunoblasts invites immunological surveillance against their unlimited growth. The persistence of the virus in the B lymphocyte fraction is dependent on the down-regulation of all growth transformation-associated viral proteins, except Epstein-Barr nuclear antigen 1 (EBNAl). This protein may be indispensable, owing to its role in maintaining the viral episomes (Yates et al., 1985). Current evidence is consistent with the possibility that EBNAl may not be able to provoke a CTL response. This minireview will mainly focus on the behavior of the virus in two different B cell subtypes, the immunoblast and the resting B cell.
representative (group I) lines, the WC promoters are inactive. An alternative promoter, tentatively designated as Fp, is active (Sample et al., 1992). It initiates an mRNA that encodes only EBNAl, a protein that binds to the origin of latent viral replication (orip) and is expressed in all known latently infected cells, irrespective of the cellular phenotype. The different programs of viral expression in LCLs and BLs must have evolved as part of the viral strategy in a corresponding normal cell in vivo. Activated immunoblasts of the LCL type proliferate in the blood and lymphoid tissues during acute mononucleosis (Klein et al., 1976). They disappear during convalescence. Group I BL cells that have not yet changed phenotypitally during in vitro culturing resemble resting B cells more than activated blasts (reviewed by Nilsson and Klein, 1982). The BL cell is therefore often referred to as a ‘resting cell that is not resting.” Their phenotypically closest normal counterpart was found in the germinal centers of lymphoid follicles (Gregory et al., 1987). Phenotypic features of the BL cell may stem from the juxtaposition of c-myc to an immunoglobulin locus by chromosomal translocation that is the universal cytogenetic feature of all BLs (reviewed by Klein, 1989). The normal c-myc gene is expressed in all proliferating, but not in resting, cells. After the translocation has brought it under the control of an immunoglobulin locus, it is no longer down-regulated when proliferating B cells become resting virgin or memory cells that await an appropriate antigenic stimulus to act on their constitutively expressed surface immunoglobulin receptors. The continued expression of the translocated myc gene prevents the translocation-carrying cell from leaving the cycling compartment. It appears likely that the exclusive usage of the so-called EBNAl-only program by the group I BL cell reflects the viral strategy in a normal counterpart. Using reverse transcriptase-polymerase chain reaction, we have recently found that a subfraction of normal and chronic lymphocytic leukemia-derived B cells express EBNAl but not EBNA2 and LMPl.
Alternative Programs of Latent Virus Expression Depend on the Host Cell Phenotype In EBV-immortalized immunoblastic lines of nonneoplastic origin, also designated as lymphoblastoid cell lines (LCLs), the virus expresses nine proteins (Rowe et al., 1992). Six nuclear antigens are designated EBNAlEBNAG. Two membrane-spanning proteins are called LMPl and LMP2. The latter exists in two forms, 2A and 28. The virus uses two alternative programs for EBNA expression. Evidence today indicates that a giant 85 kb transcript is generated from two alternative promoters, designated as Wp and Cp (Woisetschlaeger et al., 1989). The choice between W and C is outside the scope of this text. I shall refer to this alternative as the WC program. All six EBNAs are spliced from the WC-initiated transcript. LCLs regularly express all six EBNAs. In Burkitt lymphoma (BL) biopsies and phenotypically
Cell Phenotype-Dependent Switches of EBNA Expression Cell lines that express EBNAl only or all six EBNAs can switch on the opposite program under certain conditions. Group I BL cells can be induced to express the full program by exposure to 5azacytidine, indicating that DNA methylation may be involved in the down-regulation of EBNA2EBNAG (Masucci et al., 1989). This is borne out by the close relationship between the methylated or unmethylated state of various regulatory sequences and the expression of the corresponding protein (Hu et al., 1991). The spontaneous switch of most EBVcarrying BL lines to the LCL-like (group Ill) phenotype and the concurrent expression of all six EBNAs is another case in point. The opposite switch to exclusive EBNAl expression can be induced by fusing LCLs with non-B cells that impose theirfibroblastic, epithelial, or myelocytic phenotype on the hybrids (Con-
George Klein Microbiology and Tumor Biology Center Karolinska Institute S-171 77 Stockholm Sweden
Cell 792
As nearly half of the Hodgkin’s lymphomas and a certain proportion of T cell-derived lymphomas harbor EBV (reviewed by Pallesen et al., 1993) it cannot be excluded that the latent virus may also reside in other cells of the hemopoetic system.
Figure 1. CTL Responses
to EBV-Transformed
lmmunoblasts
Figure adapted from Masucci and Ernberg (1994). These results have bean compiled from Burrows et al. (MJO), Gavioli et al. (1993), Khanna et al. (1992), and Murray et al. (1992).
trerasBrodin et al., 1991). These findings suggest that the virus exploits host cell phenotype-dependent regulatory factors to modulate its own expression. Cytotoxic T Cell Responses to EBV-Transformed lmmunoblssts EBV-specific CTLs are readily generated in mixed cultures of T cells and autologous EBV-transformed LCLs derived from EBV-seropositive donors. They are targeted against human leukocyte antigen (HLA) class l-associated peptides derived from the EBNAs and the LMPs, with the notable exception of EBNAl . The choice of the target depends on the HLA phenotype of the responder (Murray et al., 1992; Khanna et al., 1992; Gavioli et al., 1993). Figure 1 summarizes the findings of the three main laboratories involved. They indicate that a sufficient variety of immunogenic peptides can be presented by the HLA spectrum to provide us with a nearly watertight surveillance against the progressive growth of virally transformed immunoblasts. The same is true for other Old World primates that carry EBV-related viruses, but not for New World primates that develop massive lymphoproliferative disease on primary infection. Numerous attempts have been made to generate EBNAl-specific CTLs, without success. This raises the question whether this uniformly expressed viral protein, which is required for the maintenance of the viral episomes, may have been modified selectively during viral evolution to avoid CTL recognition. In contrast with LMPl , EBNAl is not recognized bythe mouseT cell system either (Trivedi et al., 1994), suggesting a possible defect in processing or transport. The Strategy of Viral Latency Which tissue compartment harbors the persistent latent virus? Successful bone marrow transplantation can replace the resident with the donors virus or, if the donor is seronegative, convert the recipient to seronegativity and render him or her susceptible to new infection, suggesting that the hemopoetic compartment may be the principal site of the latent virus (Gratama et al., 1988). This is also indicated by the finding that small heavy B cells of the blood can give rise to EBVcarrying LCLs in the presence of a late viral inhibitor and neutralizing antibodies that prevent indirect, two-step outgrowth (Lewin et al., 1987). The small B cell is thus a probable reservoir of latent virus.
The EBV-B Cell Scenario The following speculative scenario may be considered to describe the latent interaction between EBV and B cells. Following its adsorption to CD21, the B cell-specific EBVIC3d receptor, the internalized viral genome induces an only partially known cascade of events, including the activation of the infected cell, appearance of the virally encoded transformation-associated proteins in a tightly controlled order, induction of blast transformation and immunoglobulin secretion, and transition from G O to cell cycling. The virally transformed blasts express CD23, a B cell growth factor receptor, and they also secrete B cell growth factors. This has led to the suggestion that the initial immunoblast proliferation in the blood and lymphoid tissues may involve an autocrine component. After a week or two, the specific CTLs appear and the EBV blasts vanish. The virus is postulated to stay in the small minority of infected B blasts that have switched to resting B cells, following their normal program. Since they express EBNAl only, they are not recognized by the immune system. Owing to the absence of the immunoblastassociated EBNA2-EBNAG proteins, they are not driven to proliferate, unless c-myc has been juxtaposed to immunoglobulin sequences by the rare translocation accident. Following physiological activation, the latently EBVinfected resting normal B cells may later turn into immunoblasts. Owing to the up-regulation of the immunogenic EBNA2-EBNAG and LMP proteins, such cells will be eliminated by CTL-mediated lysis. In the absence of such activation, the infected resting B cells would be subject to the same homeostatically controlled renewal as their uninfected counterparts, without any tendency to expand. The postulated lifelong presence of a small, relatively constant infected cell population fits well with the maintenance of steady antiviral antibody titers in healthy individuals over several decades (Henle and Henle, 1979) and with the persistence of small EBV-carrying, nonexpanding leukemic subpopulations in chronic lymphocytic leukemia (Lewin et al., 1988). EBV and Neoplsstic Disease The association of EBV with the human host is nonpathogenie in the vast majority of all infected individuals. The dynamics of primary infection, virally transformed immunoblast expansion, rejection, and latent persistence occur without any identifiable symptoms in young children. This is the prevalent mode of infection in developing countries and in the low socioeconomic groups of industrialized countries. In the high socioeconomic groups, primary infection is often postponed to the teens, when approximately half of the infected individuals develop mononucleosis (reviewed by Henle and Henle, 1979). Similar to the more serious EBV-associated diseases, mononucleosis is thus also a departure from the normal and undoubtedly very ancient nonpathogenic equilibrium between virus and host.
Minireview 793
Whether symptomatic or asymptomatic, the early expansion of the infected immunoblasts may be important for viral survival, as indicated by the mutation of an immunodominant EBNA4 epitope, the main target of HLA-Al lrestricted CTLs, in virus strains isolated from Papua-New Guinea and from southern China, where All is a frequently occurring allotype, but not in virus strains from Europe and Africa, where Al 1 is rare (de Campos-Lima et al., 1994). Since EBNA4 is only expressed in activated B blasts, this finding suggests that viral survival may depend on the ceiling level to which the total number of EBV-infected cells isallowed to rise priorto the rejection response. Eventually, the EBNM mutant-carrying immunoblasts are rejected as well, owing to the presence of alternative targets, but even a minor postponement of rejection may raise the level of persistently infected B cells. A high virus load may increase the probability of lytic incidents in the oral epithelium and subsequent shedding to the outside world. Lymphoproliferslive Diseases in hnmunodefectives EBVcarrying immunoblastomas represent a frequent complication in organ transplant recipients. They also appear in some congenital immunodeficiences, XLP (X-linked lymphoproliferative syndrome) in particular. The lymphoproliferative disease can take the form of fatal mononucleosis or appear more lymphoma-like. Part of the various lymphoproliferative diseases seen in AIDS patients also belong to the category of immunoblastomas. Phenotypically, the immunoblastomas resemble the LCLs in vitro. They express EBNAl-EBNAG and LMP. Obviously, these cells proliferate because the surveillance of the host has broken down, rather than owing to any cellular escape. This is also consistent with the clinical features of the disease. The immunoblastomas of kidney transplant recipients may regress without any treatment if the immunosuppressive regimen is lifted and the patient is allowed to reject his or her kidney. No second lymphoma appears after a second kidney transplantation, as a rule. BL BL develops in immunocompetent patients. The EBVcarrying form of the tumor (97% of the high endemic cases, approximately 20% of the sporadic cases) illustrates the pathways of cellular escape. Three features of the BL cell phenotype contribute to this: the downregulation of the immunogenic EBNA2-EBNAG and LMP proteins; the low expression of adhesion molecules (Gregoryet al., 1967), known to be requiredforefficient effectortarget cell interactions; and allele-specific down-regulation of certain major histocompatibility complex class I antigens (Andersson et al., 1991). All three features may be due to the freezing of the BL cell in a certain phenotypic window by the immunoglobulin-myc translocation. The virtually 100% occurrence of the translocations in BL may be related to the double effect of constitutive myc activation: blocking the normal exit of the cell from the cycling compartment and fixing its phenotype in an “immune escape-prone” window. Nasopharyngeal Carcinoma Nasophatyngeal carcinoma (NPC) provides another example of EBVcarrying tumor cell escape in immunocompetent hosts. All NPCs express EBNAl . They do not ex-
press the immunogenic EBNA2-EBNAG proteins, but 60% of them express LMP (Fghraeus et al., 1966). We have recently introduced a Chinese NPC-derived LMFV gene into a nonimmunogenic mouse mammary carcinoma cell. In contrast with a B cell-derived LMP7 gene that rendered the target cells highly immunogenic in syngeneic murine hosts, the NPC-derived LMP7 failed to do so (Trivedi et al., 1994). If confirmed on additional LMP7-positive NPCs, it would indicate that NPC cells may escape rejection by mutation in the potentially immunogenic LMPl protein. References Andersson, M. L., Stam, N. J., Klein, G., Ploegh, H. L.. and Masucci, M. G. (1991). Int. J. Cancer 47, 544-550. Burrows, S. R., Sculley, T. B., Misko, I. S., Schmidt, D. J. (1990). J. Exp. Med. 777. 345-350.
C., and Moss,
Contreras-Brodin, 8. A., Anvret, M., Imreh, S., Altiok, E., Klein, G., and Masucci, M. G. (1991). J. Gen. Virol. 72, 30253033. de Campos-Lima, P. 0.. Levitsky, V., Brooks, J., Lee, S. P., Hu, L.-F., Rickinson, A. B., and Masucci, M. G. (1994). J. Exp. Med., in press. Fihraeus, R., Hu, L.-F., Ernberg, I., Finke, J., Rowe, M., Klein, G., Falk. K., Nilsson, E., Yadaw, M., Busson, P., Tursz, T., and Kallin. 8. (1966). Int. J. Cancer 42, 329-336. Gavioli. R., Kurilla, M. G., de Campos-Lima, P. O., Wallace, L. E., Dolcetti, R., Murray, R. J., Rickinson, A. B., and Masucci, M. G. (1993). J. Virol. 67, 1572-1576. Gratama, J. W., Costerveer, M. A. P., Zwaan, F. E., Lepoutre, J., Klein, G., and Ernberg, I. (1966). Proc. Natl. Acad. Sci. USA 856693-6596. Gregory, C. D., Tursz, T., Edwards, G. F., Tetaud, C., Talbot, M., Caillou, B., Rickinson, A. B., and Lipinski, M. (1987). J. Immunol. 739, 313-316. Henle, G.. and Henle, W. (1979). In The Epstein-Barr Virus, M. A. Epstein and 8. G. Achong, eds. (New York: Springer-Verlag), pp. 296320. Hu, L.-F., Minarovits. J., Cao, S.-L., Contreras-Salazar, B., Rymo, L., Falk, K., Klein, G., and Ernberg, I. (1991). J. Virol. 65, 1556-1567. Khanna, R., Burrows, S. R., Kurilla, M. G., Jacob, C. A.. Misko, I. S., Sculley, T. B., Kietf, E., and Moss, D. J. (1992). J. Exp. Med. 176, 169-176. Klein, G. (1989). Genes Chromosom. Klein, G., Svedmyr, Cancer 17, 21-26.
Cancer
1, 3-6.
E., Jondal, M., and Persson, P. 0. (1976). Int. J.
Lewin, N., Aman. P., Masucci, M. G., Klein, E., Klein, G.. Cberg. B., Strander, H., Henle, W., and Henle, G. (1987). Int. J. Cancer 39,472476. Lewin, N., Aman, P.. Mellstedt, H., Zech, L., and Klein, G. (1966). Int. J. Cancer 41, 692-695. Masucci,
M. G., and Ernberg,
I. (1994). Trends Microbial.,
in press.
Masucci, M. G., Contreras-Salazar, B., Ragnar, E., Falk. K., Minarovits, J., Ernberg, I., and Klein, G. (1969). J. Virol. 63, 3135-3141. Murray, R., Kurilla, M. G., Brooks, J. M., Thomas, W. A., Rowe, M., Kieff, E., and Rickinson, A. B. (1992). J. Exp. Med. 776, 157-166. Nilsson, K., and Klein, G. (1962). Adv. Cancer Res. 37, 319-380. Pallesen, G., Hamilton-Dutoit, Res. 62, 179-239.
S. J., andZhou,
Rowe, M., Lear, A. L., Croom-Carter, A. B. (1992). J. Virol. 66, 122-131. Sample, J., Henson, 4654-4661.
X. (1993). Adv. Cancer
D., Davies, A. H., and Rickinson,
E. B. D., and Sample, C. (1992). J. Virol. 66,
Trivedi, P., Hu, L.-F., Chen. F., Christensson, B., Masucci, M. G., Klein, G., and Winberg, G. (1994). Eur. J. Cancer 3OA. 64-66. Woisetschiaeger, M., Strominger, J. L., and Speck, S. H. (1969). Proc. Natl. Acad. Sci. USA 86, 6496-6502. Yates, J. L., Warren, 615.
N., and Sugden,
8. (1965). Nature 373, 612-
June 17, 1994, Copyright
0 1994 by Cell Press
Epstein-Barr Virus Strategy in Normal and Neoplastic B Cells
Minireview
The Paradoxical Coexistence of Epstein-Barr Virus and the Human Species The mutually successful and essentially harmonious relationship of the Epstein-Barr virus (EBV) with our species stems from the resolution of two apparent contradictions. First, EBV is the most highly transforming virus known in any species, but it causes no disease in the vast majority of all infected individuals. In a relatively small minority, it causes a self-limiting disease, mononucleosis. Second, EBV-infected B lymphocytes are highly immunogenic for autologous T cells. They elicit vigorous T cell proliferation responses and generate highly cytotoxic killer cells, called cytotoxic T lymphocytes (CTLs). These interactions between autologous T and B cells are as intense as the allogeneic interactions across major histocompatibility barriers. Nevertheless, the virus is not eliminated from the hemopoetic system. Actually, it persists in the B lymphocyte fraction itself. The resolution of these two paradoxes is closely related. The high immunogenicity of the EBV-transformed immunoblasts invites immunological surveillance against their unlimited growth. The persistence of the virus in the B lymphocyte fraction is dependent on the down-regulation of all growth transformation-associated viral proteins, except Epstein-Barr nuclear antigen 1 (EBNAl). This protein may be indispensable, owing to its role in maintaining the viral episomes (Yates et al., 1985). Current evidence is consistent with the possibility that EBNAl may not be able to provoke a CTL response. This minireview will mainly focus on the behavior of the virus in two different B cell subtypes, the immunoblast and the resting B cell.
representative (group I) lines, the WC promoters are inactive. An alternative promoter, tentatively designated as Fp, is active (Sample et al., 1992). It initiates an mRNA that encodes only EBNAl, a protein that binds to the origin of latent viral replication (orip) and is expressed in all known latently infected cells, irrespective of the cellular phenotype. The different programs of viral expression in LCLs and BLs must have evolved as part of the viral strategy in a corresponding normal cell in vivo. Activated immunoblasts of the LCL type proliferate in the blood and lymphoid tissues during acute mononucleosis (Klein et al., 1976). They disappear during convalescence. Group I BL cells that have not yet changed phenotypitally during in vitro culturing resemble resting B cells more than activated blasts (reviewed by Nilsson and Klein, 1982). The BL cell is therefore often referred to as a ‘resting cell that is not resting.” Their phenotypically closest normal counterpart was found in the germinal centers of lymphoid follicles (Gregory et al., 1987). Phenotypic features of the BL cell may stem from the juxtaposition of c-myc to an immunoglobulin locus by chromosomal translocation that is the universal cytogenetic feature of all BLs (reviewed by Klein, 1989). The normal c-myc gene is expressed in all proliferating, but not in resting, cells. After the translocation has brought it under the control of an immunoglobulin locus, it is no longer down-regulated when proliferating B cells become resting virgin or memory cells that await an appropriate antigenic stimulus to act on their constitutively expressed surface immunoglobulin receptors. The continued expression of the translocated myc gene prevents the translocation-carrying cell from leaving the cycling compartment. It appears likely that the exclusive usage of the so-called EBNAl-only program by the group I BL cell reflects the viral strategy in a normal counterpart. Using reverse transcriptase-polymerase chain reaction, we have recently found that a subfraction of normal and chronic lymphocytic leukemia-derived B cells express EBNAl but not EBNA2 and LMPl.
Alternative Programs of Latent Virus Expression Depend on the Host Cell Phenotype In EBV-immortalized immunoblastic lines of nonneoplastic origin, also designated as lymphoblastoid cell lines (LCLs), the virus expresses nine proteins (Rowe et al., 1992). Six nuclear antigens are designated EBNAlEBNAG. Two membrane-spanning proteins are called LMPl and LMP2. The latter exists in two forms, 2A and 28. The virus uses two alternative programs for EBNA expression. Evidence today indicates that a giant 85 kb transcript is generated from two alternative promoters, designated as Wp and Cp (Woisetschlaeger et al., 1989). The choice between W and C is outside the scope of this text. I shall refer to this alternative as the WC program. All six EBNAs are spliced from the WC-initiated transcript. LCLs regularly express all six EBNAs. In Burkitt lymphoma (BL) biopsies and phenotypically
Cell Phenotype-Dependent Switches of EBNA Expression Cell lines that express EBNAl only or all six EBNAs can switch on the opposite program under certain conditions. Group I BL cells can be induced to express the full program by exposure to 5azacytidine, indicating that DNA methylation may be involved in the down-regulation of EBNA2EBNAG (Masucci et al., 1989). This is borne out by the close relationship between the methylated or unmethylated state of various regulatory sequences and the expression of the corresponding protein (Hu et al., 1991). The spontaneous switch of most EBVcarrying BL lines to the LCL-like (group Ill) phenotype and the concurrent expression of all six EBNAs is another case in point. The opposite switch to exclusive EBNAl expression can be induced by fusing LCLs with non-B cells that impose theirfibroblastic, epithelial, or myelocytic phenotype on the hybrids (Con-
George Klein Microbiology and Tumor Biology Center Karolinska Institute S-171 77 Stockholm Sweden
Cell 792
As nearly half of the Hodgkin’s lymphomas and a certain proportion of T cell-derived lymphomas harbor EBV (reviewed by Pallesen et al., 1993) it cannot be excluded that the latent virus may also reside in other cells of the hemopoetic system.
Figure 1. CTL Responses
to EBV-Transformed
lmmunoblasts
Figure adapted from Masucci and Ernberg (1994). These results have bean compiled from Burrows et al. (MJO), Gavioli et al. (1993), Khanna et al. (1992), and Murray et al. (1992).
trerasBrodin et al., 1991). These findings suggest that the virus exploits host cell phenotype-dependent regulatory factors to modulate its own expression. Cytotoxic T Cell Responses to EBV-Transformed lmmunoblssts EBV-specific CTLs are readily generated in mixed cultures of T cells and autologous EBV-transformed LCLs derived from EBV-seropositive donors. They are targeted against human leukocyte antigen (HLA) class l-associated peptides derived from the EBNAs and the LMPs, with the notable exception of EBNAl . The choice of the target depends on the HLA phenotype of the responder (Murray et al., 1992; Khanna et al., 1992; Gavioli et al., 1993). Figure 1 summarizes the findings of the three main laboratories involved. They indicate that a sufficient variety of immunogenic peptides can be presented by the HLA spectrum to provide us with a nearly watertight surveillance against the progressive growth of virally transformed immunoblasts. The same is true for other Old World primates that carry EBV-related viruses, but not for New World primates that develop massive lymphoproliferative disease on primary infection. Numerous attempts have been made to generate EBNAl-specific CTLs, without success. This raises the question whether this uniformly expressed viral protein, which is required for the maintenance of the viral episomes, may have been modified selectively during viral evolution to avoid CTL recognition. In contrast with LMPl , EBNAl is not recognized bythe mouseT cell system either (Trivedi et al., 1994), suggesting a possible defect in processing or transport. The Strategy of Viral Latency Which tissue compartment harbors the persistent latent virus? Successful bone marrow transplantation can replace the resident with the donors virus or, if the donor is seronegative, convert the recipient to seronegativity and render him or her susceptible to new infection, suggesting that the hemopoetic compartment may be the principal site of the latent virus (Gratama et al., 1988). This is also indicated by the finding that small heavy B cells of the blood can give rise to EBVcarrying LCLs in the presence of a late viral inhibitor and neutralizing antibodies that prevent indirect, two-step outgrowth (Lewin et al., 1987). The small B cell is thus a probable reservoir of latent virus.
The EBV-B Cell Scenario The following speculative scenario may be considered to describe the latent interaction between EBV and B cells. Following its adsorption to CD21, the B cell-specific EBVIC3d receptor, the internalized viral genome induces an only partially known cascade of events, including the activation of the infected cell, appearance of the virally encoded transformation-associated proteins in a tightly controlled order, induction of blast transformation and immunoglobulin secretion, and transition from G O to cell cycling. The virally transformed blasts express CD23, a B cell growth factor receptor, and they also secrete B cell growth factors. This has led to the suggestion that the initial immunoblast proliferation in the blood and lymphoid tissues may involve an autocrine component. After a week or two, the specific CTLs appear and the EBV blasts vanish. The virus is postulated to stay in the small minority of infected B blasts that have switched to resting B cells, following their normal program. Since they express EBNAl only, they are not recognized by the immune system. Owing to the absence of the immunoblastassociated EBNA2-EBNAG proteins, they are not driven to proliferate, unless c-myc has been juxtaposed to immunoglobulin sequences by the rare translocation accident. Following physiological activation, the latently EBVinfected resting normal B cells may later turn into immunoblasts. Owing to the up-regulation of the immunogenic EBNA2-EBNAG and LMP proteins, such cells will be eliminated by CTL-mediated lysis. In the absence of such activation, the infected resting B cells would be subject to the same homeostatically controlled renewal as their uninfected counterparts, without any tendency to expand. The postulated lifelong presence of a small, relatively constant infected cell population fits well with the maintenance of steady antiviral antibody titers in healthy individuals over several decades (Henle and Henle, 1979) and with the persistence of small EBV-carrying, nonexpanding leukemic subpopulations in chronic lymphocytic leukemia (Lewin et al., 1988). EBV and Neoplsstic Disease The association of EBV with the human host is nonpathogenie in the vast majority of all infected individuals. The dynamics of primary infection, virally transformed immunoblast expansion, rejection, and latent persistence occur without any identifiable symptoms in young children. This is the prevalent mode of infection in developing countries and in the low socioeconomic groups of industrialized countries. In the high socioeconomic groups, primary infection is often postponed to the teens, when approximately half of the infected individuals develop mononucleosis (reviewed by Henle and Henle, 1979). Similar to the more serious EBV-associated diseases, mononucleosis is thus also a departure from the normal and undoubtedly very ancient nonpathogenic equilibrium between virus and host.
Minireview 793
Whether symptomatic or asymptomatic, the early expansion of the infected immunoblasts may be important for viral survival, as indicated by the mutation of an immunodominant EBNA4 epitope, the main target of HLA-Al lrestricted CTLs, in virus strains isolated from Papua-New Guinea and from southern China, where All is a frequently occurring allotype, but not in virus strains from Europe and Africa, where Al 1 is rare (de Campos-Lima et al., 1994). Since EBNA4 is only expressed in activated B blasts, this finding suggests that viral survival may depend on the ceiling level to which the total number of EBV-infected cells isallowed to rise priorto the rejection response. Eventually, the EBNM mutant-carrying immunoblasts are rejected as well, owing to the presence of alternative targets, but even a minor postponement of rejection may raise the level of persistently infected B cells. A high virus load may increase the probability of lytic incidents in the oral epithelium and subsequent shedding to the outside world. Lymphoproliferslive Diseases in hnmunodefectives EBVcarrying immunoblastomas represent a frequent complication in organ transplant recipients. They also appear in some congenital immunodeficiences, XLP (X-linked lymphoproliferative syndrome) in particular. The lymphoproliferative disease can take the form of fatal mononucleosis or appear more lymphoma-like. Part of the various lymphoproliferative diseases seen in AIDS patients also belong to the category of immunoblastomas. Phenotypically, the immunoblastomas resemble the LCLs in vitro. They express EBNAl-EBNAG and LMP. Obviously, these cells proliferate because the surveillance of the host has broken down, rather than owing to any cellular escape. This is also consistent with the clinical features of the disease. The immunoblastomas of kidney transplant recipients may regress without any treatment if the immunosuppressive regimen is lifted and the patient is allowed to reject his or her kidney. No second lymphoma appears after a second kidney transplantation, as a rule. BL BL develops in immunocompetent patients. The EBVcarrying form of the tumor (97% of the high endemic cases, approximately 20% of the sporadic cases) illustrates the pathways of cellular escape. Three features of the BL cell phenotype contribute to this: the downregulation of the immunogenic EBNA2-EBNAG and LMP proteins; the low expression of adhesion molecules (Gregoryet al., 1967), known to be requiredforefficient effectortarget cell interactions; and allele-specific down-regulation of certain major histocompatibility complex class I antigens (Andersson et al., 1991). All three features may be due to the freezing of the BL cell in a certain phenotypic window by the immunoglobulin-myc translocation. The virtually 100% occurrence of the translocations in BL may be related to the double effect of constitutive myc activation: blocking the normal exit of the cell from the cycling compartment and fixing its phenotype in an “immune escape-prone” window. Nasopharyngeal Carcinoma Nasophatyngeal carcinoma (NPC) provides another example of EBVcarrying tumor cell escape in immunocompetent hosts. All NPCs express EBNAl . They do not ex-
press the immunogenic EBNA2-EBNAG proteins, but 60% of them express LMP (Fghraeus et al., 1966). We have recently introduced a Chinese NPC-derived LMFV gene into a nonimmunogenic mouse mammary carcinoma cell. In contrast with a B cell-derived LMP7 gene that rendered the target cells highly immunogenic in syngeneic murine hosts, the NPC-derived LMP7 failed to do so (Trivedi et al., 1994). If confirmed on additional LMP7-positive NPCs, it would indicate that NPC cells may escape rejection by mutation in the potentially immunogenic LMPl protein. References Andersson, M. L., Stam, N. J., Klein, G., Ploegh, H. L.. and Masucci, M. G. (1991). Int. J. Cancer 47, 544-550. Burrows, S. R., Sculley, T. B., Misko, I. S., Schmidt, D. J. (1990). J. Exp. Med. 777. 345-350.
C., and Moss,
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