Infectious mononucleosis: antibodies to the Epstein-Barr virus in the blood

Infectious mononucleosis is a common systemic lymphoproliferative disease, most commonly caused by the Epstein-Barr virus. Toxoplasma gondii and other viruses (CMV, human immunodeficiency virus and human herpesvirus type 6, recognized as the cause of sudden exanthema) can cause clinically similar diseases. These same etiological agents are presumably capable of causing the development of chronic fatigue syndrome.

Epstein-Barr virus is a virus from the herpes group, has a tropism for B-lymphocytes, persistently persists in the host cells as a latent infection. It is widely distributed throughout the world. By structure and size, the Epstein-Barr virus is indistinguishable from other herpesviruses, but differs significantly from them in antigenic properties. The virus has a membrane antigen (MA-membrane antigen), a nuclear antigen (EBNA-Epstein-Barris nucleic antigen), and a viral capsid antigen (VCA).

Infection occurs when the virus is transmitted with saliva. The Epstein-Barr virus, when ingested, infects the pharyngeal epithelium, causing inflammation and fever-typical clinical signs of the onset of infectious mononucleosis. The virus is strictly lymphotropic, joining the C3α receptor of the cell membrane of B lymphocytes, it causes the proliferation of polyclonal B lymphocytes with a corresponding increase in tonsils, systemic lymphadenopathy and splenomegaly. B-lymphocytes are transformed (acquire the capacity for endless fission), and in the absence of an adequate cellular immune response, this process can evolve into a clearly malignant (eg, X-linked lymphoproliferative syndrome). If the cellular immunity factors control the replication of the Epstein-Barr virus in the body, then the clinical symptoms of infectious mononucleosis gradually disappear.

Like other herpesviruses, the Epstein-Barr virus can persist as a latent infection (its DNA is contained in the nucleus of a small number of B-lymphocytes). Episodic asymptomatic reactivation of infection is common, about 20% of healthy young people excrete the Epstein-Barr virus with saliva. In persons with impaired cellular immunity (eg, in AIDS, ataxia-telangiectasia, transplant recipients), a clear reactive infection with hairy leukoplakia, interstitial pneumonitis, or in the form of monoclonal B-cell lymphoma can develop. With the Epstein-Barr virus, the etiology of nasopharyngeal carcinoma and Burkitt's lymphoma is linked.

One of the manifestations of infectious mononucleosis is the appearance in the peripheral blood of atypical lymphocytes (up to 10% of the total number of lymphocytes). Atypical lymphocytes are found in the blood from the beginning of the period of clinical manifestations of infection. Their content in the blood reaches a peak at the end of the 2nd or the beginning of the 3rd week and can hold at this level up to 1.5-2 months, complete disappearance usually occurs by the beginning of the 4th month from the onset of the disease. The presence of atypical lymphocytes is a relatively insensitive sign of infection caused by the Epstein-Barr virus, but has a total specificity of about 95%.

The proliferation of polyclonal B lymphocytes in infection caused by the Epstein-Barr virus generates a large number of different autoantibodies in the patient's body, such as IgM-anti-i (cold agglutinin), rheumatoid factor, antinuclear antibodies. Most of the unusual Igs that appear in infectious mononucleosis are called the Paul-Bunnel heterophile antibodies. These antibodies are classified as IgM, they have an affinity for mutton and horse erythrocytes, not directed to any antigen of the Epstein-Barr virus. Heterophilic antibodies are a random product of B-lymphoid proliferation (caused by the Epstein-Barr virus), they appear in the first week of infectious mononucleosis and gradually disappear upon recovery, they are usually not detected after 3-6 months.

As the initial acute stage of infection becomes latent, the genomes of the Epstein-Barr virus (unique antigens) appear in large numbers in all cells, and the nuclear antigen is released into the environment. In response to the antigen, specific antibodies are synthesized - valuable markers of the disease stage. Shortly after infection of B lymphocytes, an early antigen (EA) is detected, a protein required for replication of the Epstein-Barr virus (and not a structural viral component). To the early antigen in the patient's body, antibodies of classes IgM and IgG are synthesized. Together with the full viral of the Epstein-Barr virus, antigens of the viral capsid (VCA) and membrane antigen (MA) appear. As the infectious process subsides, a small percentage of B-lymphocytes infected with the Epstein-Barr virus avoids immune destruction and retains the viral genome in a latent form. The nuclear antigen (EBNA) of the Epstein-Barr virus is responsible for its duplication and survival.

Laboratory tests can detect antibodies to various antigens.

From the serological methods of diagnosing infectious mononucleosis, the Paul-Bunnel (agglutination) reaction is most common, aimed at detecting heterophilic antibodies in serum. The titer of heterophilic antibodies 1: 224 and higher in the serum of the patient is recognized as diagnostically significant, confirming the diagnosis of infectious mononucleosis. Heterophilic agglutination is positive in 60% of young people after 2 weeks and in 90% after 4 weeks from the onset of clinical manifestations of the disease. Therefore, several studies are needed to diagnose infectious mononucleosis: during the first week of the disease (the reaction may be negative) and 1-2 weeks later (the reaction may become positive). The content of heterophilic antibodies decreases after the end of the acute period of the infectious process, but their titer can be determined within 9 months after the onset of clinical symptoms. The Paul-Bunnel reaction can turn from positive to negative, even against the background of residual hematological and clinical symptoms in the patient. Sensitivity of the method in adults is 98%, specificity is 99%. In children with infectious mononucleosis before the age of 2 years, heterophilic antibodies can be detected only in 30% of patients, at the age of 2-4 years - in 75%, over 4 years - in more than 90%. The sensitivity of the method in children is less than 70%, the specificity is 20%. Reduction, and then re-increase of the titer of heterophilic antibodies can occur in response to another infection (most often in viral infections of the upper respiratory tract). The Paul-Bunnel response is not specific for the Epstein-Barr virus. The titer of heterophilic antibodies does not give a cross reaction and does not correlate with specific antibodies to the Epstein-Barr virus, nor does it correlate with the severity of the disease course. The test is useless for diagnosing the chronic form of infectious mononucleosis (positive on average only in 10% of patients).

Titers 1:56 or less can be found in healthy people and in patients with other diseases (rheumatoid arthritis, rubella). False positive test results are met very rarely.

At present, the method of "single spot" (slide agglutination) is used to determine antibodies to erythrocytes of a ram, it is used initially as a screening test. By sensitivity, it is comparable to the Paul-Bunnel reaction. False-positive slide tests can be approximately 2% of studies (in leukemia, malignant lymphoma, malaria, rubella, viral hepatitis, pancreatic carcinoma), and false-negative in adults - in 5-7% of cases.

It should be noted that the spectrum of diagnostic test systems produced by firms based on the determination of antibody titer is very wide, so it is necessary to focus on the diagnostic titer of antibodies specified in the instructions for test systems.

If heterophilic antibodies are not detected, and the clinical picture of the disease corresponds to infectious mononucleosis, it is necessary to examine serum for specific antibodies of classes IgM and IgG. To detect specific antibodies to the Epstein-Barr virus, indirect immunofluorescence methods are used (detect antibodies to EA and VCA antigens), anti-complement-immunofluorescence (detect antibodies to EA, VCA and EBNA antigens) and ELISA.

Antibodies to the EA antigen D component (anti-EA-D) appear even in the latent period of the primary infection and quickly disappear with recovery.

Antibodies to the EA antigen R component (anti-EA-R) can be detected 3-4 weeks after the clinical manifestations of the disease. They persist in the blood serum for about a year, often detected with atypical or protracted currents of infectious mononucleosis. Usually, these antibodies are found with Burkitt's lymphoma.

Antibodies to VCA class IgM (anti-VCA IgM) appear very early, usually to clinical symptoms, they are detected at the onset of the disease in 100% of cases. High titers occur on the 1-6th week from the onset of infection, they begin to decrease from the 3rd week and usually disappear after 1-6 months. Anti-VCA IgM is almost always present in the serum with active infection, so the method of their detection is very sensitive and specific for an acute episode of infectious mononucleosis.

Antibodies to VCA class IgG (anti-VCA IgG) can appear early (in the 1-4 th week), their amount reaches a peak by the 2nd month of the disease. At the onset of the disease, they are found in 100% of cases. Only 20% of patients showed a 4-fold increase in the titer in the study of paired sera. The titre decreases on recovery, but is found within several years after the transferred infection, therefore it is useless for the diagnosis of infectious mononucleosis. The presence of anti-VCA IgG indicates a state after infection and immunity.

Antibodies to EBNA (anti-EBNA) appear later than all, are rarely present in the acute phase of the disease. Their content increases during the recovery period (within 3-12 months), they can persist in the blood for many years after the disease. The lack of anti-EBNA in the presence of anti-VCA IgM and anti-EA IgM indicates a current infection. The detection of anti-EBNA after a previously negative reaction indicates an existing infection. When using the ELISA method, it is possible to simultaneously detect the presence of anti-EBNA classes of IgM and IgG. If the amount of anti-EBNA IgM is greater than the anti-EBNA IgG, an acute infection should be considered, with the reverse relationship being the previous one.

In favor of an acute primary infection, one or more of the following symptoms indicate the presence of:

• anti-VCA IgG (detected early, and later the content is reduced);
• High titer (more than 1: 320) or 4-fold increase in the titre of anti-VCA IgG during the course of the disease;
• a transient rise in the titer of anti-EA-D (1:10 or more);
• early anti-VCA IgG without anti-EBNA, and later - the emergence of anti-EBNA.

Acute or primary infection caused by the Epstein-Barr virus is excluded if the anti-VCA IgG and anti-EBNA titers in the serum do not change during the study in dynamics (acute period and recovery).

The constant presence of early antigen and anti-VCA IgG in high titers indicates a chronic phase of infection.

The detection of antibodies to the Epstein-Barr virus is used to diagnose infectious mononucleosis and chronic infections caused by the Epstein-Barr virus.

Antibodies to the Epstein-Barr virus can be detected in the following diseases: secondary immunodeficiency states, including HIV infection, nasopharyngeal carcinoma, Burkitt's lymphoma, CMV infection, syphilis, Lyme disease, brucellosis, etc.

[1], [2], [3], [4], [5], [6], [7]