Epstein-Barr Virus - Host Interaction
Epstein-Barr virus (EBV) exhibits two very different behaviors: growth promotion and true latency. EBV is capable of infecting virtually any human B lymphocyte and driveing the cell to become an activated lymphoblast which can proliferate indefinitely. EBV does this through the expression of nine latent genes. Several of these genes promote cellular growth and behave as oncogenes in experimental systems. We refer to this as the "growth transcription program". It is this ability to drive cellular growth that makes EBV a candidate tumor virus for several common human cancers. EBV is also able to establish an ongoing infection with no clinical manifestations. My laboratory has shown that, in the peripheral blood, the virus persists latently in B lymphocytes but these cells are not proliferating blasts, they are resting cells. Furthermore, the virus is found only in memory B cells, and none of the growth promoting viral genes are expressed. We have proposed that EBV is able to persist for the life of the host by gaining access to the long lived memory B cell. Because these cells have little gene expression, no viral proteins are made, so the virus can not be "seen" by the immune response.
Figure 1. T his figure shows how EBV infection interacts with normal B cell differentiation pathways and can perturb normal cell growth and function.
So why does the virus have the ability to make B cells grow? We believe the answer lies in the normal biology of a B cell. In order for a B cell to enter the memory compartment it must first be activated by antigen and then rescued into the memory compartment by additional signals from antigen and helper T cells in a specific location in the lymph node referred to as the germinal center. We believe that EBV infects and activates B cells so that they are now able to differentiate into memory cells. This concept is supported by the fact that EBV infected cells in germinal centers express only two critical genes for two proteins (called LMP1 and LMP2a) which mimic the signals provided by antigen and T cell help. We refer to this as the default program. Thus the virus can provide, through its own gene products, constitutive signaling molecules that provide surrogate signals to "trick" the B cell into entering the memory compartment. Interestingly the default program is expressed in several EBV associated cancers including Hodgkin's lymphoma and nasopharyngeal carcinoma, suggesting that the survival/rescue signals provided by this program can be oncogenic if not properly regulated.
Currently we are interested in:
- finding out how EBV gains access to B lymphocytes in vivo
- how the transition from growth program to default program to resting memory cell is made
- how the latently infected memory cells are maintained
- how the virus gets back our again i.e. how does the latent virus get turned on and replicate so it can be shed into saliva to spread to other hosts.
To accomplish these goals we are using a multi-disciplinary approach that includes:
- genomic screening to identify novel cellular genes deregulated by EBV in EBV associated tumors.
- sophisticated cell fractionation techniques including magnetic bead and flow cytometry based isolation of cell population and laser capture microdissection to isolate single infected cells.
- highly sensitive DNA-PCR, RT-PCR and immunohistochemistry that can detect single virus infected cells and the viral gene products expressed therein.
- analysis of the disregulation of EBV infection in particular disease states including infectious mononucleosis, immunosuppressed individuals, individuals with genetic predisposition to fatal EBV infection and patients with autoimmune diseases such as systemic lupus erythematosus.
- using high speed super computers and advanced, agents based techniques to develop sophisticated computer models of EBV infection.
An example of how we can use high speed computing to simulate virus infection is shown in the figure below.
Figure 2. The levels of virus infected cells in the blood are shown for multiple runs of the computer simulation (colored lines) versus actual levels seen in 15 patients (green diamonds).