Date Published: December 1, 2015
Publisher: Public Library of Science
Author(s): Samantha K. Dunmire, Jennifer M. Grimm, David O. Schmeling, Henry H. Balfour, Kristin A. Hogquist, Paul D. Ling.
Epstein-Barr virus (EBV) is a human herpesvirus that causes acute infectious mononucleosis and is associated with cancer and autoimmune disease. While many studies have been performed examining acute disease in adults following primary infection, little is known about the virological and immunological events during EBV’s lengthy 6 week incubation period owing to the challenge of collecting samples from this stage of infection. We conducted a prospective study in college students with special emphasis on frequent screening to capture blood and oral wash samples during the incubation period. Here we describe the viral dissemination and immune response in the 6 weeks prior to onset of acute infectious mononucleosis symptoms. While virus is presumed to be present in the oral cavity from time of transmission, we did not detect viral genomes in the oral wash until one week before symptom onset, at which time viral genomes were present in high copy numbers, suggesting loss of initial viral replication control. In contrast, using a sensitive nested PCR method, we detected viral genomes at low levels in blood about 3 weeks before symptoms. However, high levels of EBV in the blood were only observed close to symptom onset–coincident with or just after increased viral detection in the oral cavity. These data imply that B cells are the major reservoir of virus in the oral cavity prior to infectious mononucleosis. The early presence of viral genomes in the blood, even at low levels, correlated with a striking decrease in the number of circulating plasmacytoid dendritic cells well before symptom onset, which remained depressed throughout convalescence. On the other hand, natural killer cells expanded only after symptom onset. Likewise, CD4+ Foxp3+ regulatory T cells decreased two fold, but only after symptom onset. We observed no substantial virus specific CD8 T cell expansion during the incubation period, although polyclonal CD8 activation was detected in concert with viral genomes increasing in the blood and oral cavity, possibly due to a systemic type I interferon response. This study provides the first description of events during the incubation period of natural EBV infection in humans and definitive data upon which to formulate theories of viral control and disease pathogenesis.
Epstein-Barr virus (EBV) is a ubiquitous human herpesvirus. As with all herpesviruses, EBV causes lifelong infection in its host. Infection is associated with autoimmune diseases [1, 2] and is known to cause several types of cancer, representing 1% of the worldwide cancer burden . Primary infection in children is either asymptomatic or causes mild symptoms not readily associated with EBV. In adolescents and young adults, however, EBV is most commonly recognized as the etiologic agent of acute infectious mononucleosis (AIM) . The virus is chiefly transmitted from person to person in oral secretions, although it can be acquired from blood transfusions or from receipt of allogeneic donor cells or tissue. There is currently no vaccine or effective treatment for AIM or other EBV related diseases. Because EBV infection is limited to primates, there are no small animal models of infection except humanized mice . Neither humanized mice nor mice infected with the related gamma herpesvirus MHV68 exhibit true AIM. Therefore a detailed and accurate understanding of primary infection in humans is critical for developing therapeutic tools to treat EBV related diseases.
Our findings have important implications regarding how EBV infection progresses through natural routes in its native host. Despite an oral transmission mode, viral genomes were not detected in the oral cavity in appreciable quantities until subjects had presumably been infected five to six weeks. The lack of detectable viral genomes in oral wash argues against substantial lytic replication within squamous epithelial cells early during infection. Rather, it is consistent with the idea that B cells are a major cell type initially infected in the nasopharyngeum (S2 Fig). EBV efficiently infects B cells, particularly when virus is derived from epithelial sources, which it likely would be during transmission, since virus produced by epithelial cells packages more gp42 into virions than virus produced by B cells . Infected B cells are known to divide and differentiate, replicating the viral genome as an episome along with cellular division . This “vertical” replication would be expected to expand viral load relatively slowly, compared to active viral replication in lytically infected cells. Starting approximately 1 week before symptom onset, viral genomes became detectable at high copy number in the oral wash. It is unclear what event precipitates this sharp increase, but it was not gradual like the decline in viral loads in the oral cavity during latency. It has been postulated that undefined signals may trigger viral reactivation in latently infected B cells , which could then lead to high local production of virus and large-scale infection of epithelial cells.