VIROLOGY AND PATHOGENESIS
EBV is a DNA virus of the Gammaherpesvirinae subfamily. The virus is estimated to infect 90 percent of the world’s population. In the process of this infection, the EBV intercalates itself primarily into the long-lived memory B cell, not the naïve B cell,23 and thereafter establishes lifelong residence in its host. Early after infection, the virus is continuously shed into oral secretions. The virus then usually undergoes latency, but it may be reactivated periodically.24
Following primary infection, varying severity of disease ensues. Infection occurs via virus attachment to the cell surface CD21 glycoprotein, a 140-kDa complement receptor type 2. Infection induces polyclonal proliferation of infected B cells in the nodes in the pharynx. There is increasing evidence that host genetic factors predict the severity and duration of disease following primary EBV infection.25,26 Specifically, the interferon (IFN)-γ +874T/A or the interleukin (IL)-10 –592C/A polymorphisms are the important genetic factors. Using measures of severity of illness such as fatigue, myalgia, and height of fever, individuals who have the IFN-γ +874TT (high interferon production) allele have a striking increase in the risk of experiencing these severe manifestations compared to those with IFN-γ +874 A and IL-10 –592.24 When a subject’s blood monocytes are tested in vitro, the high-risk group has a higher production of IFN-γ in stimulated cells.27 Other factors in host response, which include the height of the virus load in the blood, the number of CD 8+ cells, and the T-cell granzyme expression in the CD8+ cell also have been described as contributing to disease severity.28 As a corollary to this, variability in the symptoms, signs, physical findings, and laboratory abnormalities occurs in primary infection.28,29 At the time serology confirms evidence of primary infection, some, but not all individuals with primary infection, have classic symptomatic disease.28,29 In those who develop the classical syndrome, T lymphocytes recognize viral replicative antigens on the infected B cell as foreign, and an exuberant polyclonal cytotoxic T-cell response ensues. The proliferative rate of CD8+ T cells is estimated to be approximately 50 percent of this population of cells proliferating per day. This translates into a population doubling time of 1.5 days so that 5 × 109 CD8+ T cells per day appear in the blood. The later rate of appearance is two orders of magnitude greater than normal.30 The surface activation marker SLAM (signaling lymphocyte activation molecule)-associated protein (SAP) on T lymphocytes stimulates cell activation in response to a signal from CD244 and CD150 (SLAM) on the T-cell surface.31 In the healthy individual, this process subsides over days to weeks. In parallel, the signs and symptoms of the infection subside, although fatigue may persist for a longer period of time.
There is an apparent seasonal pattern with its peak incidence in the summer. Close person-to-person contact is required for transmission to a susceptible individual.29 Subclinical primary infection28 or frequent asymptomatic reactivation of EBV in the previously infected individual provides an opportunity for transmission at all ages. Although, transmission from breast milk or from the cervix occurs, it is very uncommon.32 Nonetheless, nearly everyone in the developing world is infected by age 5 years and mononucleosis is rarely seen. Similar rates of infection occur in the lower socioeconomic class in the developed world. In the upper socioeconomic strata of the developed world, the majority of persons avoid infection when they are young. However, between the ages of 12 and 25, infection is very common. Characteristically, primary infection occurs in an individual a few months after the individual develops a relationship with someone who has latent infection.24,28 Transmission occurs from a virus-positive asymptomatic individual who transmits his or her virus to a previously uninfected person. Individuals who are raised in more protected environments may reach their 30s before they are infected. If both individuals in an initial relationship are seronegative, years may pass before they become infected. Individuals who are seropositive usually do not develop clinical disease upon reexposure, although a second infection with a different strain may occur.33,34,35
Clinical manifestations vary by age.15,18,36,37,38,39,40,41 When young children acquire illness they develop a typical childhood illness of upper respiratory infection (43 percent), otitis media (29 percent) pharyngitis (21 percent), gastroenteritis (7 percent), or typical mononucleosis (<10 percent). Rashes and/or periorbital swelling occur more frequently in younger children than in the teens. In the age group 12 to 25 years, many, but not all, present with the classical presentation of infectious mononucleosis. However, there are a substantial proportion of newly infected individuals who have minimal or very mild disease.28,29 Furthermore, evidence of virus in the blood may be present for several days before disease appears.28 For those who present with the classical disease of infectious mononucleosis, the earliest manifestations of disease develop 35 to 42 days after the individual develops infection (Table 82–2). Infection occurs via virus attachment to the cell-surface CD21 glycoprotein, a 140-kDa complement receptor type 2. Infection induces polyclonal proliferation of infected B cells in the nodes in the pharynx. Initial symptoms are lassitude and fever with no evidence of lymphocytosis or pharyngitis. Fever results from infection and proliferation of the B lymphocytes. From the nodes in the pharynx, the infected cells make their way into the circulating lymphocyte pool.42,43 Although the duration of virus in the blood is much shorter than it is in the secretions, it is the movement into the blood that leads to the manifestation of the disease. Subsequent, massive T-cell response to the neoantigens on the infected B lymphocyte is evident by the lymphocytosis with reactive blood lymphocytes and other disease manifestations. The pharyngitis that develops is a result of the T-cell response to the infected B cells that are found in Waldeyer ring in the tonsils. Sometimes enlargement of the tonsils occurs to the extent that they touch each other in the midline. Blood lymphocytosis occurs in response to the virus in the blood. Periorbital swelling, which occurs in mononucleosis, is an important clue to the diagnosis, even in young adults. Other manifestations include lymphadenopathy, hepatitis and splenic enlargement. (Table 82–2).8,28 Although the liver is not an organ rich in lymphocytes and hepatocytes are not damaged by the infection, CD4+ and CD8+ lymphocytes are trapped in that organ and their release of cytokines contributes to the inflammation in the liver and the changes in liver function that occur.44 However, hyperbilirubinemia is very uncommon. The frequency of each clinical finding of the typical syndrome in newly infected patients is variable (Table 82–3).28,29 The disease abates with the occurrence of a T-cell–mediated counterresponse to the virus-induced initial polyclonal B-cell proliferation. During this time, dramatic clinical improvement can occur in 24 to 48 hours. Subsequently, EBV remains in the patient’s B cells throughout life, but expresses only Epstein-Barr nuclear antigen-1 (EBNA-1), which does not elicit a T-cell response because of a glycine-alanine repeat that inhibits its processing.45
Table 82–2.Signs and Symptoms of Epstein-Barr Virus and Cytomegalovirus Mononucleosis: Effect of Age (Percent of Patients) ||Download (.pdf) Table 82–2. Signs and Symptoms of Epstein-Barr Virus and Cytomegalovirus Mononucleosis: Effect of Age (Percent of Patients)
|Signs and Symptoms ||EBV (Age 14–35 Years*) ||EBV (Age 40–72 Years†) ||CMV (Age 30–70 Years‡) |
|Fever ||95 ||94 ||85 |
|Pharyngitis ||95 ||46 ||15 |
|Lymphadenopathy ||98 ||49 ||24 |
|Splenomegaly ||65 ||33 ||3 |
|Hepatomegaly ||23 ||42 ||N/A |
|Jaundice ||8 ||27 ||24 |
Table 82–3.Complications of Epstein-Barr Virus and Cytomegalovirus Mononucleosis ||Download (.pdf) Table 82–3. Complications of Epstein-Barr Virus and Cytomegalovirus Mononucleosis
|Complication ||Epstein-Barr Virus ||Cytomegalovirus |
|Hemolytic anemia ||++ ||+ |
|Thrombocytopenia ||+ ||+ |
|Aplastic anemia ||+ ||– |
|Splenic rupture ||+ ||– |
|Jaundice (age >25 years) ||++ ||++ |
|Guillain-Barré syndrome ||+ ||++ |
|Encephalitis* ||++ ||+/– |
|Pneumonitis* ||+/– ||+ |
|Myocarditis* ||+ ||– |
|B-cell lymphoma ||+ ||– |
|Agammaglobulinemia ||+ ||– |
Group A streptococcus is found in the pharynx occasionally (3 to 4 percent of cases) in concert with an EBV primary infection. Although treatment of the streptococcus eradicates the organism, the severe pharyngitis changes little, and the disease follows its usual course. Thus, treatment should be administered only if the test result for β-streptococcus is positive. If a penicillin congener is used, quite often, but not always, a rash develops46,47 and the patient is labeled “penicillin allergic.” The patient should be reevaluated after the mononucleosis resolves to determine if the patient has a true allergy.
There are exceptions to the usual situation of most people becoming infected by age 25 years. The first is found in a woman who is protected by her family from intimate male contact. She avoids infection until she is married at which time she may become infected from her husband. The second situation occurs when a long-term relationship is established when a couple is young. If they are both seronegative, infection does not occur. When they reach parenting age or older, they become infected by a child or a grandchild. When that occurs, the presentation is less likely to include lymphadenopathy and pharyngitis (see Table 82–3).18,40,41 Fever almost always occurs and abdominal pain, hepatomegaly, and abnormal liver function develops in most. Older adults are less likely to develop lymphocytosis, they have fewer reactive lymphocytes, and splenomegaly is less evident. This leads to the clinical impression of infiltrative hepatic disease or cholecystitis. The illness at this age may be very protracted.
By week three of a mononucleosis caused by primary EBV infection, an heterophile antibody response will occur in approximately 85 percent of patients. The test is called the monospot test and may be falsely negative, especially in young children.36,48
The infection of B cells results in their production of a variety of antibodies against uninvolved infectious agents. The B-cell clones expanded, non-specifically, result in antibodies against Chlamydia, Borrelia burgdorferi, the yellow fever virus, and many other infectious agents. If the patient’s febrile illness is considered a fever of unknown origin, diagnostic conclusions may be misleading. A variety of other antibodies against antigens that are not those of infectious agents also are produced because of polyclonal B-cell activation. These include antiplatelet, anti–red cell (anti-i cold agglutinin), and antinuclear antibodies.
At the time clinical disease is evident, both immunoglobulin (Ig) G and IgM antibodies to Epstein-Barr virus capsid antigen (VCA) usually are detectable. Later, antibody to early antigen (EA) develops. A small proportion of individuals will also have developed antibody to EBNA-1 on presentation. However, usually EBNA-1 antibody does not appear until the recovery phase of the illness. For those patients suspected of having infectious mononucleosis but who do not develop heterophile antibody, detection of IgG and IgM antibody to VCA with the absence of EBNA-1 antibody leads to the diagnosis.49 A real-time positive polymerase chain reaction (PCR) is sometimes useful.50
Expansion of cytotoxic T lymphocytes produces lymphocytosis. Reactive lymphocytes are larger than the lymphocytes normally found in the blood (Fig. 82–1). They may have a vacuolated cytoplasm, lobulated and eccentrically placed nucleus, and a cell membrane that often is indented by neighboring erythrocytes. A more darkly staining peripheral cytoplasm, called “skirting,” occurs. Reactive lymphocytes are a hematologic hallmark of infectious mononucleosis, but they are not always found29 and are not pathognomonic. They also are found in CMV infection, roseola (caused by human herpes virus-6), viral hepatitis, toxoplasmosis, rubella, mumps, and drug reactions.
A-D. Blood films from patients with Epstein-Barr virus–induced mononucleosis. These reactive lymphocytes exhibit the characteristic changes seen in patients with infectious mononucleosis: large lymphocytes with abundant cytoplasm. The cytoplasmic margin often spreads around (is indented by) neighboring red cells and the margin may take on a densely basophilic coloration (skirting). This type of reactive T lymphocyte may be seen in a variety of diseases and is not a specific change but is characteristic and helpful in pointing to the diagnosis in concert with other characteristic clinical findings. (Reproduced with permission from Lichtman’s Atlas of Hematology, www.accessmedicine.com.)
Sheets of lymphocytes are noted on a stained slide preparation of tonsillar exudate. The immunophenotype of lymphocytes in mononucleosis syndromes assessed by multiparametric flow cytometry has confirmed that lymphocytosis results from CD8+ T cells. CD4+ T cells and B cells are not increased. In EBV mononucleosis, the notable populations increased are CD8+CD57– and CD3+γδ+ T cells.51 If β-streptococcal infection accompanies the EBV infection segmented neutrophils may be present in the tonsillar exudate.
Other Blood Test Abnormalities
Liver function abnormalities are common, especially elevated serum alkaline phosphatase and γ-aminotransferase. There is no or only slight elevation of bilirubin. Studies in Israel have found a higher frequency of hyperbilirubinemia (15 percent), a lower incidence of leukocytosis (46 percent), and elevated liver enzymes (58 percent) than previously reported. The differences may be geographical or genetic.29
Complications of Epstein-Barr Virus Mononucleosis
Virtually all subjects with acute mononucleosis develop a mildly decreased platelet count (see Table 82–2). More-severe hematologic complications occur infrequently, but include severe immune thrombocytopenia with petechiae, immune hemolytic anemia, immune-mediated granulocytopenia, and aplastic anemia.52,53,54,55,56,57,58,59,60 Uncommonly, the splenomegaly that accompanies the lymphoid proliferation accentuates an underlying, previously undiagnosed, hereditary spherocytosis.60 Splenic rupture is estimated to occur in 1 to 5 per 1000 cases. It is the leading cause of death from EBV mononucleosis.61,62 Avoidance of athletic activities is prudent until the signs of the disease have disappeared and the spleen has returned to normal size.62
Neurologic complication include Guillain-Barré syndrome, acute encephalitis, acute disseminated encephalomyelitis (Alice-in-Wonderland syndrome), acute cerebellar ataxia, viral meningitis, transverse myelitis, and cranial nerve palsies.57,58,63,64,65 There is evidence that antibody to gangliosides plays a role in the pathogenesis of Guillain-Barré syndrome Neurologic complications may occur in the absence of clinical mononucleosis. Diagnosis of EBV-induced neurologic disease requires obtaining specific antibodies to EBV (see “Antibody Responses” above) and a positive PCR for EBV in the cerebrospinal fluid.63 Neurologic disease can be associated with primary infection, reactivated infection or chronic EBV infection.63 Table 82–2 lists other complications.
Fatigue is a very prominent feature of acute infectious mononucleosis. Most recover from this fatigue fairly quickly but a few remain fatigued for a very long period of time. The source of this fatigue is not certain but there is evidence that dysfunction of the midbrain plays a role.68,69 Furthermore, there are genetic factors that are present in those who remain fatigued. Limited information suggests improvement in some with antiviral treatment.69
There are reports indicating that EBV infection is linked to the development of multiple sclerosis.70,71 Further studies to clarify which molecular mechanisms link the immune response to a natural infection of humans with EBV to the subsequent development of chronic inflammatory damage to the CNS are required to explain this relationship
Systemic Lupus Erythematosus and Rheumatoid Arthritis
There are epidemiologic links between previous infection with EBV and development of systemic lupus erythematosus (SLE).72 Not unexpectedly, virtually everyone with SLE has had previous infection with EBV. Therefore, the link may be fortuitous. Alternatively, a history of infection with EBV might lead to induction of autoimmunity.73,74 There has also been a suggested relationship between increased viral load in rheumatoid arthritis leading to expansion of CD8+ cells and its consequences.75
Chronic Progressive Epstein-Barr Virus Infections, T-Cell or Natural Killer–Cell Lymphoproliferation, Lymphoma, and Hemophagocytic Syndrome
Chronic EBV infection is a rare outcome of primary EBV infection, notable in persons with an immunodeficiency state.76,77 In chronic EBV infection, fever, marrow hypoplasia, interstitial pneumonia, hepatosplenomegaly, persistent hepatitis often to the point of hepatic failure, lymphadenitis, and uveitis are frequent clinical manifestations. These findings may persist for months or years and eventuate in a high fatality rate.75 EBV antibodies may be very elevated (VCA in excess of 1:5120, anti-EA greater than 1:640) but with no detectable EBNA-1 antibody. A persistently elevated blood PCR for EBV is a feature. The more-severe form of this manifestation may evolve into a natural killer (NK)- or T-cell lymphoproliferative disease that ranges from chronic to fulminant.78 Alternatively, EBV hemophagocytic syndrome may develop. The latter is a severe multiorgan, inflammatory disease provoked by massive cytokine elaboration. In some case, clonal proliferation induced by EBV develops (Chap. 71).78,79,80,81
OTHER EPSTEIN-BARR VIRUS–ASSOCIATED DISEASE PROCESSES
Neoplastic Potential of Epstein-Barr Virus
EBV was the first human tumor virus identified82 from the cultured cells of a patient with African Burkitt lymphoma. EBV can confer unlimited proliferative potential of infected B lymphocytes in culture.83 EBV has since been associated with tumors other than Burkitt lymphoma, including some patients with Hodgkin lymphoma (Chap. 97). Although proof of causality still eludes investigators, there is an intriguing relationship between EBV and Hodgkin lymphoma.84,85,86,87 EBV is detectable in the neoplastic B cells (Reed-Sternberg cells) of a significant percent of patients with Hodgkin lymphoma. The etiologic role of EBV in this setting is unknown.84,85,86,87
There is also a relationship between EBV and lymphoma in immune-deficient individuals including the posttransplantation lymphoproliferative disease (PTLD).88 Recipient PTLD is a more serious problem than donor PTLD89 and investigators avoid certain immunosuppressive programs if there is a risk of PTLD.90 Use of positron emission tomography helps determine who of those at risk for PTLD have evidence for disease.91 X-chromosome–linked lymphoproliferative disease,92 T-cell and NK-cell lymphomas that follow chronic EBV infection,93,94,95 nasopharyngeal carcinoma in patients in the Far East,96 the latter disease generating efforts to develop a vaccine,97 leiomyoma and leiomyosarcoma in patients with HIV infection or immunodeficiency posttransplantation,98 and a small fraction of cases of gastric carcinoma99 are each associated with EBV infection (Table 82–4). In three types of lymphomas, Burkitt, Hodgkin, and PTLD, the cell that mutates to produce the clonal disease is a germinal center B cell with a circular viral genome in the tumor cells that expresses the EBV-encoded latent genes.100
Table 82–4.Special Problems with Epstein-Barr Virus or Cytomegalovirus Infection ||Download (.pdf) Table 82–4. Special Problems with Epstein-Barr Virus or Cytomegalovirus Infection
|Epstein-Barr Virus ||Cytomegalovirus |
|Rare congenital infection129,130 ||Congenital infection110 |
|Chronic progressive mononucleosis65,66,67,72,73,74 ||Posttransplant primary infection114,115 |
|Hemophagocytic syndrome77,78 ||Graft-versus-host disease association143 |
|XX-linked B cell lymphoma92 ||Transfusion-related infection144 |
|Posttransplant lymphoproliferative disease88,89,90,91 ||Aspergillus and/or Pneumocystis infection122,123 |
|T or NK lymphoproliferative disease94,95 || |
|African Burkitt lymphoma5 || |
|Approximately 20% of Burkitt lymphoma in the United States5 || |
|Approximately 35% of Hodgkin lymphoma84,85,86,87 || |
|Nasopharyngeal carcinoma96,97 || |
|Approximately 5% of gastric carcinoma99 || |
|Leiomyoma and leiomyosarcoma in HIV or immunosuppressed patients98 || |
|Oral hairy leukoplakia102 || |
In the case of PTLD, the most characteristic clinical setting involves an EBV seronegative person receiving an organ from an EBV seropositive donor.89 EBV is latent in the B lymphocytes of the transplanted marrow or solid organ. Immunosuppression allows reactivation of the latent virus. Because basiliximab, calcineurin inhibitor, sirolimus, and glucocorticoids seem to increase the frequency of PTLD, that regimen is avoided where the potential for PTLD is present.90 Because the recipient is not immune, there is no T-cell response and the B cells may proliferate unchecked, sometimes eventuating in PTLD. In the EBV-seronegative recipient who develops PTLD posttransplantation, this disease usually develops within the first posttransplantation year and often within the first few months. The abnormally proliferating cell almost always is a B cell and the process at an early stage may be monoclonal. In the initial states, the disease may respond to lowering of immunosuppressive medications. Although administration of antiviral prophylaxis seems to lower the frequency of development of the disease, once the disease develops, antiviral agents are ineffective.101 When disease develops, in addition to reduction in immunosuppression, administration of anti-CD20 therapy with rituximab is used. Occasionally, PTLD occurs in a person who is EBV antibody-positive pretransplantation. When this occurs, the onset is usually greater than 1 year posttransplantation.
In young males with an X-linked lymphoproliferative syndrome, primary EBV infection leads to unabated B-cell proliferation and evolution into a frank B-cell lymphoma, the so-called Duncan syndrome.92 These young males do not develop a T-cell response and hence do not develop mononucleosis. Although control of this effect of EBV infection by treatment with antiviral agents and/or chemotherapy has been attempted, the Duncan syndrome usually is fatal.
Oral hairy leukoplakia, a characteristic white lingual lesion with hairy projections, occurs in patients with HIV infection. It is caused by EBV infection of the lingual epithelium.102
FUTURE THERAPEUTIC APPROACHES TO EPSTEIN-BARR VIRUS INFECTION AND NEOPLASIA
Because of the severe consequences of EBV infection, several approaches to preventing or treating these disorders are underway, such as an EBV vaccine,103 adoptive transfer of activated cytotoxic T cells,104 and the development of peptides that inhibit viral replication.105