AIDS of Epstein-Barr Virus Infections
AIDS
EBV-associated tumours occur frequently in patients with AIDS.
Lymphomas are the second commonest malignancy in AIDS patients.
However, not all lymphomas in AIDS patients are associated with
EBV. The EBV associated tumours are (i) primary lymphoma of the
brain and, (ii) Burkitt's lymphoma (50% of these tumours are
similar to the African type and associated with EBV, whilst the
other 50% are EBV genome negative). Recently, oral hairy
leukoplakia has been described in HIV- seropositive individuals,
forming multiple characteristic lesions on the lateral side of
the tongue. DNA hybridization studies have shown active EBV
replication in these lesions. In infants with AIDS, a lymphocytic
infiltration of the lungs termed chronic interstitial pneumonitis
in which the lymphocytes carry the EBV genome. However, it is
thought that the syndrome is probably due to the direct action of
HIV rather than EBV.
Laboratory_Diagnosis of EBV Infections
1. Infectious Mononucleosis - the classical finding in IM is the presence of atypical mononuclear cells in the blood. Lymphocytes would account for more than 50% of leucocytes present and of these, 20% are atypical. Atypical lymphocytes may also be seen in infections by CMV, hepatitis, influenza B, and rubella, but they are most prominent in IM. Abnormal LFTs are also present in the majority of patients. The diagnosis of IM may be suspected on clinical grounds and the findings of atypical lymphocytes, However, a firm diagnosis can only be made on serological testing. 2 types of serological tests are generally used for the diagnosis of IM.
The Heterophil antibody test is commonly called the Paul-Bunnel test, or the Monospot test (if done on slides). This test detects an antibody which causes agglutination of red blood cells from another species. False negative heterophil Ab test results occur and are more common in children under 14 years, especially under the age of 5 years. A possibility is that priming exposure to the unknown "heterophil antigen" has not taken place in very young children and thus no secondary response will arise on polyclonal stimulation. False positive heterophil Ab results are fewer in number than false negative results. Positive heterophil Ab results may last more than 6 months after the onset of IM and can occur in asymptomatic primary infection. This may be responsible for some of the "false" positive results.
IgM to VCA by indirect immunofluorescence is the best serological test available for the diagnosis of IM. However, this test is time consuming and results may vary between different laboratories. Also false positive results in the IgM may result from the cross linking between EBV specific IgG and anti-IgM conjugate by rheumatoid factor. If rheumatoid factor is present in the serum, it should be absorbed with staphylococcal protein A before testing. For these reasons, most laboratories rely mainly on the heterophil antibody test. A pre- illness specimen is rarely available to demonstrate a rise in IgG antibodies to VCA. High levels of VCA IgG are not diagnostic of acute infection. VCA IgM is generally used in the diagnosis of acute infection and IgG as an immune status screen.
Testing for EBNA antibodies may be of use in the window period. Anti-EBNA-1 antibodies do not usually arise until convalescence. Anti-EBNA-2 antibodies arise earlier in the illness and fall to low or undetectable levels during convalescence. The absence of EBNA- antibodies should not be regarded as diagnostic for IM as they are often undetectable in chronic IM, and conversely, they may be present soon after the onset of IM. Another possible confirmatory test is EBV-IgG avidity. The elution principle (avidity-index) is generally used for VCA-IgG.
2. Chronic IM - signs and symptoms of chronic IM range from fever, Pharyngitis, malaise, myalgia, and lymphadenopathy to potentially life-threatening problems such as anemia, thrombocytopenia, hupoglobulinaemia, and pneumonitis. Onset of chronic IM follows acute IM and may be due to impaired cell-mediated response to the virus. To meet the criteria for a diagnosis of chronic IM, three conditions need to be satisfied; (1) the symptoms should have persisted for at least 12 months (2) onset of persistent symptoms should have been preceded by a proven case of IM; (3) there should be evidence of active EBV infection . Highly elevated VCA and EA antibodies are often observed (>1024), EBNA-2 antibodies are frequently higher than those of EBNA-1 (the reverse of the situation is found in normal seropositive individuals), a positive heterophil antibody result may be seen, and more rarely, anti-VCA IgM may be detected. Similar antibody profiles have been observed in patients suffering from post-viral fatigue syndrome. It may be that some PVFS patients were, in fact suffering from chronic IM.
3. Burkitt’s Lymphoma - histology of biopsy specimens should reveal a poorly differentiated lymphocytic lymphoma. The tumour can be stained with antibodies to lambda light chains which should reveal a monoclonal tumour of B-cell origin. In over 90% of cases, the cells express IgM at the cell surface. The presence of EBV in tumour cells can be demonstrated by hybridization or the detection of EBNA-1. However, both these methods are technically demanding and therefore a diagnosis of BL is usually made on clinical and histological grounds. Children with BL have highly elevated titre of antibodies to EBV which may decrease following treatment and remission. Therefore, the determination of antibody levels may have a role in the monitoring of treatment. Although EBV serology might be of value in the early diagnosis of BL, such monitoring is not feasible on financial and practical terms.
4. Nasopharyngeal Carcinoma - the diagnosis of NPC is usually made on histological examination of biopsy material. However, the presence of EBV DNA and EBNA-1 can be readily demonstrated. Serum antibodies to EBV antigens can be used to confirm the diagnosis and monitor the progress of the disease, especially serum IgA to EA, VCA and ENA-1.. Recent studies have demonstrated the value of testing for persistent high levels of serum IgA to VCA in screening for early lesions of the disease and also for monitoring treatment.
5. X-linked Lymphoproliferative syndrome - the diagnosis of XLPS is much the same as for IM. In addition, EBNA-positive infiltrating lymphocytes can often be detected in post mortem or biopsy material. Serological studies of XLPS families commonly reveal a carrier state in healthy female relatives as evidenced by elevated EA and/or VCA titres. In such cases, genetic counseling may be given.
6. Post transplant lymphomproliferative disease - it is
possible to detect EBV-DNA and EBNA in most of these lesions. The
demonstration of EBNA positive cells is probably the most
suitable method. Unlike BL and NPC, other latent antigens, ie.
all the EBNAs and LMPA are expressed. The majority of
EBV-associated post transplant lymphoproliferative lesions appear
to occur following primary EBV infection. Serological diagnosis
of primary infection is usually made retrospectively on sera
taken for other purposes, as these patients rarely exhibit
symptoms of IM. It may be possible to demonstrate a
seroconversion. Patients who were seropositive prior to the
transplant may have an antibody profile suggestive of
reactivation.
VACCINE_DEVELOPMENT
A vaccine against EBV which prevents primary EBV
infection should be able to control both BL and NPC. Such a
vaccine must be given early in life. Such a vaccine would also be
useful in seronegative organ transplant recipients and those
developing severe IM, such as the male offspring of X-linked
proliferative syndrome carriers. The antigen chosen for vaccine
development is the MA antigen gp 340/220 as antibodies against
this antigen are virus neutralizing. Inoculation of cotton top
tamarins by purified gp 340/220 was able to protect the animals
by subsequent virus challenge.
