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.
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) and spleen. Blood test results indicated a slightly lowered eerythrocyte count and a white blood cell count about twice normal, with increased numbers of lymphocytes and decreased numbers of neutrophils. Thirty percent of the lymphocytes were atypical, 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.
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. Other EBV antigens include the capsid antigen VCA, several nuclear proteins called EBNA 1 through 6, and Latent Membrane Protein LMP1.
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.
1. Discuss how EBV is causing these signs and symptoms of infectious mononucleosis: fever; sore throat; swollen lymph nodes, tonsils, and spleen; elevated CD8+ lymphocyte count.
2. Do dendritic cells (DC) have the CR2 (CD21) membrane molecule required for EBV infection? [Note: antigen-presenting DC are not the same as follicular DC] If they cannot be infected by EBV, how can DC present EBV to CD8 T cells to activate them?
3. The common antibodies to EBV antigens and their order of appearance are shown above. Compare the VCA-IgM and VCA-IgG curves. a) What is wrong with the way these curves are drawn? b) Explain why the IgG levels are higher than the IgM levels and why they persist longer. c) Would you expect to find high levels of IgA to EBV? Why or why not?
4. Describe the activation of an EBV gp350-specific B cell to become an IgG-secreting plasma cell. a) What key molecular interactions are required for B cell activation? b) What is the role of follicular dendritic cells and how are they different from antigen-presenting DC? c) How does somatic hypermutation lead to affinity maturation?
5. Most of the effector functions of antibodies require accessory cells or molecules. a) Which function does not? Why is this important and what site(s) in the body require this for protection? b) What are the two main categories of effector functions that do require accessory cells and molecules? How are they similar and how are they distinct?
6. Discuss how each of these strategies of EBV might influence the ability of the immune system to eliminate the virus.
7. Ninety percent of people infected with EBV produce heterophile antibodies (literally: loving others); antibodies that bind non-EBV antigens. This is 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 (IgM anti-IgG). The heterophile antibody titer usually drops below 1:40 in about a year, but some of these antibodies can be detected for years. a) How might EBV stimulate this polyclonal IgM response to a variety of self antigens? b) What symptoms might result from Rob's having these heterophile antibodies in his circulation? c) Why is Rob able to make anti-self antibodies, and what is the significance of their isotype? Would these be high or low affinity antibodies?
8. EBV can very rarely transform B cells to divide in an unregulated fashion, resulting in B cell lymphoma (a tumor of B lymphocytes). 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 EBNAs 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.
a) What is the antigen specificity of the monoclonal IgM antibody produced by the transformed B cells? b) What does bcl-2 have to do with cancer? c) Do you think Rob will get B cell lymphoma from EBV? Explain your answer.