Epstein-Barr virus (EBV) is a cancer-causing human herpesvirus that persists in B lymphocytes for the life of the host in greater than 95% of the world’s population. EBV is associated with endemic Burkitt lymphoma in equatorial Africa, nasopharyngeal cell carcinoma in South-east Asia, Hodgkin’s disease, AIDS lymphomas, and B cell lymphoproliferative diseases (LPD)/lymphomas during T cell immunocompromise worldwide. EBV is also the causative agent of infectious mononucleosis, a form of EBV-driven lymphoproliferation during primary infection that is eventually controlled by the immune system. EBV-LPD/lymphomas during immunocompromise are aggressive tumors that can affect as many as 20% of solid-organ transplant recipients. Treatment of EBV-mediated diseases is challenging and no specific therapy or vaccine directed against the virus exists.
We find EBV exciting not just for the multitude of diseases it can cause, but also for the unique and long history of peaceful coexistence of humans with this virus. This evolutionary coexistence has likely shaped the human host’s anti-pathogen defenses such as the immune system, while at the same time providing the virus with the ability to establish and maintain persistence in B lymphocytes. Our goal is to better understand such EBV-host interactions, aberrations in which often lead to EBV-mediated disease, by adopting a three-pronged approach. The first focuses on dissecting the early cellular events that are critical for susceptibility of B cells to EBV-driven proliferation. The second is to reveal the determinants of susceptibility of latently-infected B cells to lytic cycle-inducing triggers. The third focuses on delineating EBV-directed immune responses during health and disease.
EBV-driven B cell proliferation
Following infection of B cells, EBV-oncogenes must drive cell proliferation so that viral latency, and therefore persistence, can be established. Using this infection-transformation model of primary B cells, we discovered a novel function for cellular STAT3 (Signal Transducer and Activator of Transcription 3) in suppression of the DNA damage response; suppression of the cellular DNA damage response is essential for any tumor to develop. Thus, EBV exploits host STAT3 to interrupt intra-S phase ATR-to-Chk1 signaling to facilitate EBV-oncogene-driven transformation. How DNA damage response is suppressed during development of sporadic cancers is largely unknown and our findings now provide a medically-relevant mechanism. Therapeutic implications are substantial since most human cancers including EBV-cancers have overactive STAT3.
Susceptibility to EBV lytic cycle induction
Establishment of viral latency is essential for persistence of EBV in the host; on the other hand, latently infected B cells must periodically undergo lytic replication to produce infectious virus that can infect new cells in the same host and spread to other hosts. However, a balance between maintenance of latency and exit from latency to lytic replication is necessary to ensure that all cells do not support lytic replication simultaneously – this balance is critical to ensure long-term persistence by EBV and other herpesviruses. By exploiting novel strategies that we developed to separate lytic from non-lytic (or refractory) EBV-infected B cells, we have learned that EBV uses cellular STAT3 to regulate susceptibility to lytic activation. STAT3 regulates this process by transcriptionally activating members of the KRAB-ZFP repressor proteins which are then recruited to EBV lytic genes to suppress lytic activation. Our studies are among the few to reveal biologic targets and functions of members of this major family of transcriptional repressors, while simultaneously unraveling mechanisms underlying herpesvirus persistence.
EBV-directed immune response
Both primary infection with EBV and lytic (re)activation in persistently infected individuals are characterized by salivary shedding of infectious EBV particles. In addressing the relevance of serum IgA to EBV-related diseases, we discovered unique IgA responses to EBV replication proteins in patients with Hodgkin lymphoma and during infectious mononucleosis. These findings highlight the importance of mucosal immune responses to components of the viral DNA replication machinery during primary EBV infection, and provide tools for biomarker development for Hodgkin lymphomas.
Cell mediated immune responses to EBV are critical for prevention of outgrowth of EBV-infected B cells into EBV-lymphomas. We developed innovative tools that led us to discover the importance of CD4+ and γ-δ T cells in controlling EBV-transformed B cells. Of translational significance, our findings indicate that CD4+ T cells and γ-δ T cells are likely to be important components of reactivated T cell lines used in adoptive immunotherapy against EBV-lymphomas.