Immunological responses during the incubation period and acute phase of naturally acquired primary Epstein-Barr virus infection

Abstract

Epstein-Barr virus (EBV) in a human herpesvirus. It infects about 90% of the human population, and is the main causative agent of infectious mononucleosis. The incubation period, the time between viral acquisition and onset of symptoms, is unusually long in patients presenting with infectious mononucleosis, lasting about six weeks. In addition to causing acute illness, there can also be long-term consequences as the result of acquisition of the virus, including nasopharyngeal carcinoma and lymphoma. Nevertheless, there remains a surprising dearth of knowledge regarding the establishment of and immune response to persistent EBV infection in its natural hosts, especially during the incubation period. We sought to address many of these gaps by studying the incubation period, acute phase, and convalescence of undergraduates experiencing infectious mononucleosis during primary natural EBV infection in a cohort of prospectively studied volunteers. Particular attention was paid to the previously uncharacterized incubation period. Our findings have focused on understanding the immune response that occurs in young adults presenting with infectious mononucleosis, via gene expression changes as observed in peripheral blood mononuclear cells and innate and adaptive immune cells. Using a systems biology approach we discovered that important gene expression changes occur during the immune response to primary EBV infection. A typical antiviral type I interferon response was not observed at onset of infectious mononucleosis symptoms, but rather up to two weeks prior. The gene expression signature at symptom onset was dominated by cell cycle related genes, probably due to the CD8 T cell lymphocytosis, and type II interferon regulated genes. Interestingly, comparison of the EBV signature with other acute viral infections revealed very little similarity. The EBV signature showed the greatest similarity with hemophagocytic syndromes. This result is consistent with the view that infectious mononucleosis is an immunopathologic disease, and is supported by evidence that EBV can cause hemophagocytic lymphohistiocytosis. As an extension of this work, we carefully examined changes in cellular phenotypes and population frequencies to determine if there were significant alterations to certain compartments during the response to primary EBV infection. We observed a type I interferon signature in a larger subset of study participants during the incubation period. This response was concurrent with the transition of virally infected B cells from the oral cavity to the blood, a decline in plasmacytoid dendritic cells from the circulation, and a polyclonal CD8 T cell activation. No EBV specific CD8 T cells activation was observed until the onset of infectious mononucleosis symptoms. A major obstacle to understanding EBV related sequelae has been the lack of an efficient animal model for EBV infection, although progress in primate and mouse models has recently been made. Taken together, the data compiled in this thesis provide important first descriptions of the immune responses that occur during the establishment of a natural persistent infection in humans. Key future challenges are to develop protective vaccines and effective treatment regimens.