As Rob tussled on the floor with his small cousins after Thanksgiving dinner, he was feeling on top of the world. He was doing very well in all his classes, had made good progress on his term paper, and had high hopes for a good GPA. The week after Thanksgiving he developed a low grade fever, lost his appetite, had a headache, and was very tired even in the morning. Thinking it was just the end of the semester, Rob tried to get some extra sleep over the weekend, but with a lab report due and an early final on Wednesday he didn't rest all that much. By the time finals officially began on Friday, he had a sore throat and the lymph nodes in his neck were swollen. "All I need to do is get through finals week!", he thought, as he dragged himself through his last in-class final and turned in his take-home immunology exam.
Fig 1. Swollen lymph nodes and tonsils in acute mononucleosis.
When the symptoms, especially the fatigue, persisted even after two weeks at home over the holidays, Rob decided to see his family physician before returning to campus. The physician noted swollen lymph nodes in the neck (above left) and enlarged tonsils (above right). Blood test results indicated a white blood cell count about twice normal, with increased numbers of lymphocytes and decreased percent of neutrophils. Thirty percent of the lymphocytes were atypical (probably activated rather than resting), with curled edges and vacuoles in the cytoplasm (below). These atypical lymphocytes were CD8 positive. IgM antibody to an EBV viral capsid antigen, VCA, was also detected. Based on these symptoms, Rob's physician diagnosed infectious mononucleosis (also called "glandular fever" or "mono"). Rob's sore throat and fever disappeared after about three weeks, but he was abnormally tired for several months following infection.
Fig 2. "Atypical" Lymphocytes in blood of mononucleosis patient.
Infectious mononucleosis is caused by Epstein Barr Virus (EBV), a member of the Herpes virus family. EBV is a relatively large virus with a DNA genome surrounded by a donut-shaped core protein, a protein capsid, a protein tegument, and a lipid bilayer envelope derived from host cell plasma membrane and bearing EBV surface glycoprotein gp350/220 (Membrane Antigen). Other EBV antigens include the capsid antigen VCA, proteins needed to establish EBV infection (Early Antigens, EA), several nuclear proteins called EBNA1 through EBNA6, and Latent Membrane Protein LMP1.
Fig 3. Infection cycle of EBV.
EBV is transmitted in saliva and initially infects pharyngeal (throat) epithelial cells. From there, gp350/220 on newly produced virus binds to a complement receptor (CR2) on B cells, and EBV replicates in the B cell nucleus. EBV can be shed in saliva as long as 18 months following symptomatic infection. Most of the virus is eliminated from the body, but some virus becomes latent in B cells and remains there without dividing. Occasionally the virus can be reactivated to produce new virions that can be transmitted to others.
Over 95% of people test positive for antibodies to EBV. Many people acquire EBV as children and have few or no symptoms. Rob remembered that among his small cousins who had been at Thanksgiving dinner with his family, 3-year-old Mandy had had a few sores on her face which could have been caused by EBV.
Fig 4. EBV Structure.
Wednesday February 21 B Cell Activation
1. REVIEW: Discuss how EBV is causing these symptoms of infectious mononucleosis:
2.REVIEW: Outline in a detailed bulleted list the steps a lymphoid progenitor cell takes to successfully become a mature naïve EBV-specific B cell.
Fig 5. Activation of B cells by Th.
3. Outline in a detailed bulleted list the events in B cell activation, including
Fig 6. Antibody-Dependent Cell Medicated Cytotoxicity (ADCC).
Friday February 23 Antibody Functions, T-Independent responses.
Fig 7. Antibody response to EBV. VCA = viral capsid antigen; EBNA = Epstein BArr Nuclear Antigen; EA-D = Early Antigen.
4. The common antibodies to EBV antigens and their order of appearance are shown above. Compare the VCA-IgM and VCA-IgG curves.
5. Ninety percent of people infected with EBV produce heterophile antibodies, a polyclonal IgM response that includes antibodies that agglutinate erythrocytes of many species (including human) and antibodies to T and B cells, antinuclear antibodies, and rheumatoid factor. Most of these antibodies are specific for carbohydrate antigens on the erythrocytes and lymphocytes. The heterophile antibody titer usually drops below 1:40 in about a year, but some of these antibodies can be detected for years.
Fig 8. Polyclonal B Cell Mitogens (T-Independent-1 antigens).
6. Discuss how each of these strategies of EBV might influence the ability of the immune system to eliminate the virus.
EBV can very rarely transform B cells to divide in an unregulated fashion, resulting in B cell lymphoma (a tumor or B lymphocytes). The photo at left above shows a child with Burkitt's Lymphoma, an EBV-induced lymphoma commonly seen in parts of Africa where malaria is endemic. In the U.S., EBV-lymphomas are rare; they occur most often in bone marrow transplant patients on immune suppressants. The EBV is latent in the transformed B cells (no virus is produced); the virus does not have to integrate its genome into that of the host cell to cause transformation. The B cell lymphoma cells express EBNA and secrete monoclonal IgM antibody. They also have a chromosome translocation, where part of the EBV genome that has homology with bcl-2 is moved into the Ig H chain locus. Promoters of antibody synthesis cause over-expression of the bcl-2 protein.
7. What is the antigen specificity of the monoclonal IgM antibody produced by the transformed B cells (think about this; the obvious answer may not be correct!) What does bcl-2 have to do with cancer? Do you think Rob will get B cell lymphoma from EBV? Explain your answer.
Supplementary Materials: Humoral Immunity, ToolBox
Cohen, J. I. Medical Progress Epstein-Barr Virus Infection, New England Journal of Medicine 343 (7): 481-492, August 17, 2000.