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					                                                                        The   new england journal          of   medicine



                                                                                   review article


                                                                                mechanisms of disease


                                                         Persistence of the Epstein–Barr Virus
                                                       and the Origins of Associated Lymphomas
                                                               David A. Thorley-Lawson, Ph.D., and Andrew Gross, M.D.

From the Department of Pathology, Tufts
University School of Medicine (D.A.T.-L.),
and the Department of Rheumatology,
Tufts–New England Medical Center (A.G.)
— both in Boston. Address reprint re-
quests to Dr. Thorley-Lawson at the De-
                                                  e        pstein–barr virus (ebv) is perhaps best known for its ability to
                                                           immortalize human B lymphocytes in culture.1 This property makes it a candi-
                                                           date for causing human disease, particularly cancer and autoimmune dis-
                                                  ease.2,3 Recent work, however, has shown that EBV has evolved strategies that reduce
                                                  its potential to become pathogenic.4-6 These new findings have encouraged a reassess-
partment of Pathology, Jaharis Bldg., Tufts
University School of Medicine, 150 Harri-         ment of how and when EBV may cause human disease. In this article, we review current
son Ave., Boston, MA 02111, or at david.          knowledge of the ways in which EBV establishes and maintains a persistent infection
thorley-lawson@tufts.edu.                         at the same time that it minimizes its pathogenicity; we also discuss how these charac-
N Engl J Med 2004;350:1328-37.                    teristics influence the understanding of the role of EBV in lymphomagenesis.
Copyright © 2004 Massachusetts Medical Society.

                                                                                     ebv infection
                                                  in vitro and in vivo infection
                                                  In vitro, EBV promiscuously infects resting B cells and almost always transforms them
                                                  into proliferating blasts. The result is unregulated polyclonal expansion of latently in-
                                                  fected lymphoblasts.7,8 The mechanism of this remarkable effect depends on the ex-
                                                  pression of nine viral latent proteins that are under the control of a master transcrip-
                                                  tion factor, EBV nuclear antigen 2 (EBNA-2).1 The pattern of viral gene expression that
                                                  drives this process is called the growth program5 (Table 1). In vitro events are very dif-
                                                  ferent from what occurs in the blood of healthy carriers of the virus.3,5 In healthy carri-
                                                  ers, the B cells are also latently infected with EBV, but because these cells are all mem-
                                                  ory cells9 that are in a resting state,10 they express no viral proteins.11 Cells that express
                                                  the growth program are found only in the lymph nodes.12,13 This restriction of the virus
                                                  in the blood to resting memory B cells is maintained even in immunosuppressed pa-
                                                  tients, in whom the number of virus-infected cells, on average, is 50 times as high as in
                                                  immunocompetent patients.14
                                                      Primary EBV infection in vivo generally occurs at an early age15 and is usually asymp-
                                                  tomatic.16 However, if the infection is acquired during adolescence or later, it can result
                                                  in infectious mononucleosis.17 During the early stages of infectious mononucleosis,
                                                  extremely large numbers of EBV-infected B cells circulate in the blood, but they are all
                                                  resting memory cells (up to 50 percent of such cells may be infected).18 They are not
                                                  proliferating blasts and do not enter the growth program.11
                                                      Therefore, even under the extreme conditions of infectious mononucleosis or sup-
                                                  pression of the immune response, the proliferating-lymphoblastoid stage of viral in-
                                                  fection does not occur in the blood, and the infected cells remain in a nonpathogenic
                                                  resting state. Why, then, has EBV developed the capacity to transform B cells by means
                                                  of the growth program when such cells pose a potential risk to the host? What is the re-
                                                  lation of these transformed B cells to the latently infected, resting memory cells in the
                                                  peripheral blood? Central to the discussion in this article is the idea that EBV uses a


1328                                                                  n engl j med 350;13   www.nejm.org        march 25, 2004



                                                         The New England Journal of Medicine
                 Downloaded from www.nejm.org by Julio Girón on November 7, 2010. For personal use only. No other uses without permission.
                                         Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                              mechanisms of disease



  Table 1. Five Transcription Programs Used by EBV to Establish and Maintain Infection.

  Type of Infected B Cell*                 Program                  Genes Expressed              Function of the Program
  Naive cell                       Growth                    EBNA-1 through EBNA-6,           Activates B cell
                                                             LMP-1, LMP-2A, and LMP-2B
  Germinal-center cell             Default                   EBNA-1, LMP-1, and LMP-2A        Differentiates activated
                                                                                                  B cell into memory cell
  Peripheral-blood memory cell     Latency                   None                             Allows lifetime persistence
  Dividing peripheral-blood        EBNA-1 only               EBNA-1                           Allows viral DNA in latency-
     memory cell                                                                                  program cell to divide
  Plasma cell                      Lytic                     All lytic genes                  Replicates virus in plasma
                                                                                                 cell

* Except where indicated, the types of cell are primarily restricted to the lymphoid tissue of Waldeyer’s ring. EBNA denotes
  EBV nuclear antigen, and LMP latent membrane protein.



strategy of transforming latently infected B cells              to produce the signals of the germinal center27,28
into proliferating blasts because only in this way              that cause latently infected B-cell blasts to form ger-
can the virus convert these cells into long-lived               minal centers29 and make the transition into the
memory cells and thereby make the cells nonpatho-               memory compartment (Fig. 1).
genic.                                                              The latently infected memory cells that have
                                                                been produced shut down the expression of viral
the persistence of ebv                                          proteins, enter the latency program11 (Table 1), and
The current model of persistent EBV infection holds             circulate primarily between the peripheral blood
that the growth program of the virus activates B cells          and Waldeyer’s ring.30 An exception occurs when
to become proliferating blasts so that they can then            the latently infected memory cells divide, in which
differentiate into resting memory B cells through the           case they express the EBNA-1 protein (Table 1),11
process of the germinal-center reaction (Fig. 1).20,21          thereby allowing viral DNA to replicate.31 In this
It is in the germinal center that an activated naive            case, cell division is not driven by the virus, because
B-cell blast that is responding to a foreign antigen            none of the growth-promoting latent proteins are
during an immune response undergoes the transi-                 present, but is instead regulated by the cell as part
tion into a long-lived memory B cell.20,22 The mod-             of the normal mechanism of memory B-cell ho-
el stipulates that the difference between the im-               meostasis.
mune-activated B-cell blast and the virus-infected                  Ultimately, the latent virus in memory B cells is
blast that expresses the growth program is that vi-             reactivated and replicates at a site that allows it to
ral genes, not antigens, provide all or some of the             spread to new hosts. Recent studies have shown
signals required to effect the transition to a memo-            that infectious virus is produced when memory cells
ry B cell.                                                      in Waldeyer’s ring differentiate into plasma cells
    Consistent with this model are the findings that            (Fig. 1 and unpublished data). This event closes the
lymphoblasts produced by the growth program re-                 cycle of viral infection and persistence (Fig. 2) and
semble antigen-activated B cells both in their cell-            underscores how extensively EBV makes use of the
surface phenotype23,24 and in their morphologic                 biology of normal B cells.
features25 and that the only type of EBV-infected                   In summary, EBV uses its growth program to ac-
B cell that expresses the growth program in the                 tivate newly infected B cells so that they can differen-
Waldeyer’s ring in healthy carriers is the naive                tiate into resting memory B cells. In memory cells
B cell.12,13 The virus is thought to push infected              the virus finds a perfect niche. It can persist in them
naive B cells into the memory state by switching the            for long periods, because memory B cells rarely die,
cell from the growth program to another pattern of              and it is safe in these cells because they express no vi-
transcription called the default program (Table                 ral proteins that can be detected by the immune sys-
1).5,26 The default program involves the expression             tem. Moreover, in cells that are in the resting state,
of only three latent proteins, two of which, latent             the virus poses no threat to its host, because the
membrane protein 1 (LMP-1) and LMP-2, are able                  growth-promoting genes are no longer expressed.


                                 n engl j med 350;13     www.nejm.org      march 25, 2004                                       1329


                                            The New England Journal of Medicine
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                            Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                                         The   new england journal              of   medicine



 Normal B-cell differentiation

   Antigen                                                        T-cell help           Survival                               Cell division
                                                                  + antigen             signals
               T-cell help




            Naive            B-cell blast             GC B cell           Post-GC B cell        Memory B cell
            B cell




                Growth
                program                                            Default              Latency
    Virus                                                                                                                      EBNA-1 only
                                                                  program               program
 EBV infection

 Figure 1. How EBV May Establish and Maintain Persistent Infection in Memory B Cells.
 EBV infects naive B cells that are in the resting state in the lymphoid tissue of Waldeyer’s ring and uses the growth program to activate these
 cells to become proliferating blasts. This process parallels the activation of a naive B cell on exposure to an antigen. The antigen-activated
 B-cell blast is rescued through entry into the pool of memory B cells when it receives signals from antigen and antigen-specific helper T cells.
 The virus switches from the growth program to the default program in order to deliver these rescue signals to the latently infected blast. Then
 the memory cells exit the cell cycle and enter the peripheral circulation. For infected cells, entry into the peripheral circulation results in the
 shutdown of all protein-encoding genes — known as the latency program. Memory B cells occasionally divide, as part of the homeostatic
 mechanism for maintaining stable numbers of cells. When a cell that is carrying the virus undergoes division, it expresses EBV nuclear antigen
 1 (EBNA-1) alone to allow the viral genome to divide along with the cell. In response to unknown signals (perhaps bystander T-cell help 19),
 memory cells may differentiate into plasma cells and secrete antibody. This differentiation may be related to the mechanisms that sustain life-
 time production of antibody. If such a cell contains the virus, it will reactivate viral replication and infectious virus will be produced. GC de-
 notes germinal center.



                     resolution of the infection                                       minimizing the pathogenic effect of ebv
                     Infection by EBV is controlled by both cellular and               A central tenet of the ideas discussed above is that
                     humoral immune mechanisms (Fig. 2). Antibody                      EBV uses the strategy of activating latently infected
                     limits the spread of infectious virus,32 and cytotoxic            B cells to become proliferating blasts because this
                     T cells destroy infected cells that express viral pro-            is the only way the virus can convert these cells into
                     teins.33 The cellular response can be extremely                   long-lived memory cells. This strategy has impor-
                     vigorous; in infectious mononucleosis, up to 50                   tant implications for the pathogenesis of EBV-
                     percent of all CD8-positive cells in the blood are                associated diseases. On the one hand, activation of
                     cytotoxic T cells that are directed against cells in              the newly infected cell is dangerous to both the
                     which EBV is replicating.34 The cytotoxic T cells are             host and the virus, because it risks the develop-
                     the main component of the classic lymphocytosis                   ment of a potentially fatal neoplastic disease that
                     (atypical lymphocytes) in infectious mononucleo-                  could limit the period of time in which the virus can
                     sis.35 It is likely that the major targets for control of         spread to new hosts. On the other hand, the virus
                     EBV by the immune system are memory cells that                    has the means to ensure that the proliferating lym-
                     have initiated viral replication. By killing these cells          phoblasts are short-lived. In the case of newly in-
                     and preventing the spread of infectious virus by an-              fected naive B cells in Waldeyer’s ring, EBV rapidly
                     tibody, the immune response reduces the level of                  pushes the cells out of the cell cycle and into a rest-
                     infection (Fig. 2). However, the immune system is                 ing memory state.
                     unable to eliminate the virus completely, and as a                    A risk to the host arises if EBV infects a B cell un-
                     consequence, viral shedding and virus-infected cells              der conditions in which the infected cell cannot dif-
                     persist at a low level, approximately 1 in 10,000 to              ferentiate out of the cell cycle or if memory cells are
                     100,000 memory B cells.30                                         accidentally triggered into expressing the growth


1330                                                   n engl j med 350;13      www.nejm.org         march 25 , 2004



                                                      The New England Journal of Medicine
              Downloaded from www.nejm.org by Julio Girón on November 7, 2010. For personal use only. No other uses without permission.
                                      Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                              mechanisms of disease



   Saliva                Tonsil                                                               Periphery
                                                    Activated blast
                                                                                                  Memory cell



                            Infection                                  Differentiation



       Virus




                                                                                                             Cell
                                                Immune response
                                                                                                           division


                                                                  Plasma cell



                                                                                Differentiation


                           Replication



 Figure 2. The Cycle of Persistent EBV Infection.
 Every element of the cycle of infection is susceptible to attack by the host’s immune system, with the exception of resting memory B cells, in
 which the virus is quiescent. Cytotoxic T cells recognize all other types of infected cells, and antibody neutralizes the virus. Purple lines with
 bars denote blocking.




program (as is the case with bystander B cells, Fig.            gram in germinal-center cells, and the latency pro-
3). Both cases could lead to uncontrolled growth                gram in memory B cells). The relation between
and the development of a tumor. Ordinarily, these               such cells has been inferred from parallels with the
possibilities are prevented, because EBV has the                biology of normal B cells but has yet to be demon-
apparently paradoxical property of conserving the               strated experimentally. Furthermore, the model may
viral targets that the cytotoxic T cells recognize on           be oversimplified in several respects. For example,
infected lymphoblasts.6 The conservation of these               it implies that viral genes completely replace anti-
antigens by EBV is very different from the continu-             gen signaling during the germinal-center reaction.
ous mutations that allow influenza virus and the                However, the relative contributions of antigen and
human immunodeficiency virus to avoid recogni-                  viral genes have not been established. Similarly,
tion by the immune system.36,37 Consequently,                   cells with a germinal-center phenotype that express
EBV in a proliferating lymphoblast is a sure target             the default program have been described, but it is
for the immune response, and conservation of the                not known where they reside and whether they ex-
targets guarantees that lymphoblastoid cells that               pand to form a true germinal center.
express the growth program but have not differen-                   An alternative explanation for the persistence of
tiated or cannot differentiate out of the cell cycle            EBV infection has been suggested by Kurth et
will be destroyed.                                              al.,38,39 who have proposed that EBV-infected cells
                                                                do not participate in the germinal-center reaction
other interpretations                                           but, rather, that EBV directly infects memory B cells.
The ideas discussed here are based on a large body              This idea is based on studies in which EBV-infected
of evidence that, in vivo, EBV uses different tran-             cells undergoing clonal expansion were identified
scription programs in different types of B cells (the           in the germinal centers of tonsils from patients with
growth program in lymphoblasts, the default pro-                infectious mononucleosis. The cells expressed


                                  n engl j med 350;13    www.nejm.org       march 25, 2004                                                     1331


                                           The New England Journal of Medicine
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                           Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                                          The   new england journal               of   medicine




 Figure 3. Putative Checkpoints in the EBV Life Cycle That Might Give Rise to Lymphoma.
 EBV normally infects naive B cells in Waldeyer’s ring, which differentiate into memory B cells, exit the cell cycle (thick red arrows), and are there-
 fore not pathogenic. Hodgkin’s disease arises from a virus-infected cell that is blocked at the germinal-center stage, which results in constitu-
 tive expression of the default program. Burkitt’s lymphoma arises from a germinal-center cell that is entering the memory compartment but is
 stuck at the point of proliferation owing to the activated c-myc oncogene. Consequently, the cell expresses EBNA-1 only. In both Hodgkin’s dis-
 ease and Burkitt’s lymphoma, the critical event may be a cellular mutation during the immunologic disturbance associated with acute EBV in-
 fection. Because the number of infected cells is so high at this point, there is a reasonable possibility that the cell undergoing mutation will have
 the virus in it by chance. Any cell other than the naive B cell in Waldeyer’s ring that becomes infected (thin red arrows) and expresses the growth
 program will continue to proliferate, because it cannot differentiate out of the cell cycle (thin dashed purple arrows). The rarity of such an event
 highlights the extent to which EBV infection is carefully controlled. Normally, bystander B-cell blasts would be destroyed by cytotoxic T cells
 (CTL, blue arrow), but if the CTL response is suppressed, the blasts can lead to post-transplantation lymphoproliferative disease (PTLD).




                     EBNA-2, which is characteristic of the growth pro-                 naive B cells rapidly differentiate out of the cell cy-
                     gram. Although this idea is attractive because of its              cle by means of the default program to become
                     simplicity, it provides no mechanism to explain                    resting memory cells. Owing to the high level of
                     how the proliferating memory cells exit the cell cy-               viremia and the disruption of lymphoid tissue in in-
                     cle, what the role of the default program may be,                  fectious mononucleosis, germinal-center or memo-
                     and why latently infected germinal-center and mem-                 ry B cells may incidentally become directly infected.
                     ory cells from the tonsils of healthy carriers do not              These bystander infected cells (Fig. 3) will express
                     express EBNA-2.12 They always express the default                  the growth program (that is, they will be EBNA-2–
                     program.                                                           positive), not the default program.12 They will then
                         In fact, the observations of Kurth et al.38,39 are             expand rapidly because they cannot differentiate
                     consistent with and predicted by the model shown                   out of the cell cycle and will therefore be the domi-
                     in Figures 1 and 2, which holds that latently infected             nant population of infected cells in the tonsils of



1332                                                    n engl j med 350;13     www.nejm.org           march 25 , 2004



                                                     The New England Journal of Medicine
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                                     Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                            mechanisms of disease


patients with infectious mononucleosis. Expanding             mucosal epithelium (nasopharyngeal and gastric
populations of such infected cells are exactly what           carcinoma2,3).
Kurth et al. observed.38,39 Eventually, the cytotoxic
T-cell response destroys these cells, leaving behind                         ebv and disease
only the small number of infected germinal-center
cells that express the default program — as is seen           EBV has been associated with a number of diseases
in healthy carriers of the virus.                             (Table 2), particularly autoimmune disease and
    It has also been suggested that EBV-driven differ-        cancer.2,3,45,46 Demonstration of a causative role of
entiation occurs in the extrafollicular regions of the        EBV in autoimmune disease has been difficult, how-
tonsil, rather than in the germinal centers. This idea        ever, because worldwide, more than 90 percent of
is not inconsistent with the model and was based              people are infected with EBV by the time they are
on studies of transgenic mice in which expression             adults,15 most of them in early childhood, and EBV
of LMP-1 from a constitutive promoter resulted in             persists in them for the rest of their lives. In order
lymphoma and blocked germinal-center develop-                 to associate EBV with a particular disease, it must be
ment.40 However, LMP-1 is not expressed from a                explained why the virus causes disease in only a few
constitutive promoter in the virus,41,42 it is not on-        persons, when almost everyone is infected with it.
cogenic in healthy carriers, and in germinal-center           Furthermore, because the virus is carried in the
cells it is not expressed alone but, rather, expressed        blood by infected memory B cells,9,47 sensitive tests
in the presence of LMP-2,12 which, from experi-               will detect it in any inflamed tissue, regardless of the
ments in transgenic mice, is known to cause gut               role of the virus in causing the inflammation. The
mucosal B cells to undergo germinal-center devel-             model of EBV infection discussed here adds a fur-
opment.29 For these reasons, the phenotype of these           ther complication in that EBV is exquisitely sensitive
mice is difficult to interpret, and it can be concluded       to changes in the immune system. Changes in the vi-
only that constitutive expression of LMP-1 by itself          ral burden or atypical behavior of the virus may be an
blocks germinal-center development.                           indirect effect of a compromised immune system
                                                              that results from the autoimmune disease, rather
ebv and epithelium                                            than evidence that the virus has a role in the disease.
Although the focus of this review is on the infec-                The evidence of an association between EBV and
tion of B lymphocytes, it is worth noting that there          cancer is stronger than is the case for autoimmune
is increasing evidence to suggest that the epithelium         disease, and the ability of the virus to drive cellular
of Waldeyer’s ring also has a role in both primary            proliferation identifies it as a potential carcinogen.
infection and viral shedding.43,44 Although the par-
ticipation of epithelium remains to be clearly estab- lymphoma in immunosuppressed patients
lished, it might have major implications for under- Immunosuppressed patients are at risk for B-cell
standing the role of EBV in carcinoma of the lymphoproliferative diseases, such as post-trans-



  Table 2. Diseases in Which EBV May Have a Causative Role.

                                                                                                Controversial
  Definite Role                                     Strong Association                    or Discredited Association
  Infectious mononucleosis               Burkitt’s lymphoma                        Rheumatoid arthritis
  X-linked lymphoproliferative disease   Hodgkin’s lymphoma                        Systemic lupus erythematosus
                                         B-cell lymphoma in patients with          Multiple sclerosis
                                            immunosuppressive disease              Breast cancer
                                         Nasal NK–T-cell lymphoma*                 Chronic fatigue syndrome
                                         Nasopharyngeal carcinoma
                                         Leiomyosarcoma in immunosuppressed
                                             patients

* NK denotes natural killer.




                                n engl j med 350;13   www.nejm.org       march 25, 2004                                         1333


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                            Copyright © 2004 Massachusetts Medical Society. All rights reserved.
                                                         The   new england journal              of   medicine


                    plantation lymphoproliferative disease. These dis-                clonal EBV (up to 80 percent in developing coun-
                    eases are a heterogeneous collection of disorders48               tries).52,53 Although there are no obvious differ-
                    that usually carry the virus and express the growth               ences to indicate that EBV-positive and EBV-nega-
                    program.49 The obvious explanation for post-trans-                tive Hodgkin’s disease are distinct entities, there is
                    plantation lymphoproliferative disease is that an                 evidence that infectious mononucleosis is a risk fac-
                    impaired cytotoxic T-cell response permits unin-                  tor only for EBV-positive Hodgkin’s disease.54
                    hibited growth of EBV-infected cells, but the situa-                  EBV-positive tumors express the viral genes
                    tion is not so simple. Two events must occur for                  EBNA-1, LMP-1, and LMP-255-58 of the default tran-
                    lymphoblastoid cells that express the growth pro-                 scription program, which is the program that is
                    gram to survive and evolve into a lymphoma. First,                used by latently infected germinal-center B cells.
                    the EBV-infected cell must be unable to exit the cell             The immunoglobulin genes of Reed–Sternberg cells
                    cycle and become a resting memory B cell. Second,                 are hypermutated to the same extent as germinal-
                    the cytotoxic T-cell response must be impeded so                  center B cells.59 Thus, the immunoglobulin muta-
                    that the lymphoblasts are not killed.                             tions and the data on viral gene expression inde-
                        Post-transplantation lymphoproliferative dis-                 pendently support the idea that Hodgkin’s disease
                    ease may be initiated when a type of B cell other                 arises from an EBV-infected germinal-center B cell.
                    than a naive B cell in Waldeyer’s ring becomes in-                    The presence of EBV in approximately 40 per-
                    fected and expresses the growth program (as in the                cent of tumors would seem to rule out a chance as-
                    case of bystander cells, Fig. 3). These cells cannot              sociation of the virus with Hodgkin’s disease, but
                    exit the growth program, and they continue to pro-                this view does not take into account the fact that
                    liferate owing to the absence of effective T-cell im-             the number of EBV-infected cells is extremely high
                    munity. This mechanism may explain the heteroge-                  in infectious mononucleosis (up to 50 percent of
                    neity of the disease.50 Even in immunosuppressed                  all memory B cells are infected18). Therefore, if the
                    patients, this event must be rare, because only one or            immunologic disruption of infectious mononucle-
                    two of the millions of EBV-infected cells in the hu-              osis, rather than EBV itself, is the risk factor for
                    man body develop into tumors. That every infected                 Hodgkin’s disease, there is a high probability that
                    B cell does not simply proliferate out of control fur-            the premalignant germinal-center cell will have
                    ther attests to the tight regulation of EBV-driven                EBV in it by chance.
                    proliferation in vivo, even in the presence of a crip-                A reasonable scenario for the association of
                    pled immune response.                                             EBV with Hodgkin’s disease is that a germinal-cen-
                                                                                      ter B cell acquires a mutation during infectious
                    hodgkin’s disease                                                 mononucleosis that blocks its differentiation.60 If
                    The first recognition of an association between                   that cell happens also to contain EBV, it will be-
                    EBV and Hodgkin’s disease came from the observa-                  come a germinal-center cell that constitutively ex-
                    tion that infectious mononucleosis is a risk factor               presses LMP-1 and LMP-2 (Fig. 3 and Table 3). The
                    for this form of lymphoma.15 Subsequently, Reed–                  virus may then be carried as a chance passenger, or,
                    Sternberg cells were found in some cases of infec-                more likely, the constitutive expression of LMP-1
                    tious mononucleosis,51 and approximately 40 per-                  and LMP-2 will provide growth and survival signals
                    cent of Hodgkin’s tumors were shown to contain                    that enhance tumor growth.

                                                                                      burkitt’s lymphoma
 Table 3. Putative Infected Cell of Origin and Viral Role for the Three               EBV was discovered 40 years ago in tumor cells
 EBV-Associated Lymphomas.*                                                           from patients who had Burkitt’s lymphoma,61 yet
                                                                                      its contribution to the development of this tumor
                                                      Transcription       Causative
 Type of Lymphoma         Putative Cell of Origin       Program             Role      remains enigmatic. The consistent genetic lesion
                                                                                      in Burkitt’s lymphoma is deregulated activation of
 Immunoblastic         Bystander B cell of any type Growth            Likely
                                                                                      the c-myc oncogene owing to reciprocal transloca-
 Hodgkin’s             Germinal-center cell           Default         Unsure
                                                                                      tion with an immunoglobulin gene.62-64 Burkitt’s
 Burkitt’s             Latency-program memory         EBNA-1 only     Unsure          lymphoma has the same pattern of immunoglobu-
                       B cell stuck in cycle
                                                                                      lin gene hypermutation as germinal-center and
* EBNA denotes EBV nuclear antigen.                                                   memory B cells,65 but it has the cellular phenotype
                                                                                      of a germinal-center cell.66


1334                                                   n engl j med 350;13       www.nejm.org        march 25 , 2004



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                                                      mechanisms of disease


    The most compelling evidence of the involve-                          push an EBV-infected cell toward the germinal-
ment of EBV in Burkitt’s lymphoma is the high fre-                        center phenotype if the genes that promote viral
quency of tumors that carry the virus67 in endemic                        growth are not expressed.73 This explanation im-
areas (98 percent) and the presence of clonal EBV                         plies that the virus is present in the tumor cells
in all the tumor cells.68 There is no satisfactory ex-                    solely by chance, as a passenger. It is difficult, how-
planation of how EBV participates in the patho-                           ever, to decipher the origin of tumors on the basis
genesis of Burkitt’s lymphoma.69-71 None of the                           of the phenotype of the end-stage tumor. Given
growth-promoting latent genes are expressed, and                          that tumorigenesis is a multistep process that oc-
the only latent protein of the virus present is EBNA-                     curs over long periods of time, it is virtually impos-
1.72 According to current knowledge, an EBNA-1–                           sible to know how directly the final cellular or viral
only phenotype is present in nontumor cells only                          phenotype of Burkitt’s lymphoma relates to the
when a latently infected memory cell that expresses                       original infected precursor.
the latency program divides.11 This mechanism
raises the possibility that Burkitt’s lymphoma aris-                                           conclusions
es if a translocation in the c-myc gene occurs in an
EBV-infected germinal-center cell that is on its way                      We are beginning to develop a comprehensive un-
to becoming a memory cell. This cell would nor-                           derstanding of how EBV persists in vivo, and this
mally express the latency program, but owing to                           knowledge may provide insights into the origin of
the activated c-myc, it is instead stuck in the prolif-                   EBV-associated diseases. However, the virus has
erating mode and therefore constitutively express-                        evolved strategies to minimize or eliminate its patho-
es only EBNA-1 (Fig. 3 and Table 3). The mainte-                          genic potential, in the interest of maintaining in-
nance of the germinal-center phenotype in this                            fection and the survival of the host in which it per-
presumptive memory-cell tumor can be explained                            sists. Therefore, causal relationships between the
by the observation that an activated c-myc gene will                      virus and disease should be interpreted with care.

references
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