Modeling the dynamics of virus shedding into the saliva of Epstein-Barr virus positive individuals
Highlights
► We model virus shedding into saliva of Epstein-Barr virus (EBV) positive individuals. ► Target cells are susceptible to virus released from B cells and epithelial cells. ► Stochastic variation in the number of target cells susceptible to B-cell virus can explain the data. ► Infection of epithelial cells can account for high level of virus shedding. ► Clearance of virus shedding is possible only when there is no virus reactivation from B cells.
Introduction
Epstein-Barr virus (EBV) is a human herpesvirus, infecting over 90% of humans worldwide (Rickinson and Kieff, 2001). EBV is transmitted by intimate contact, mainly through saliva (Andiman, 2006). Most people infected with EBV are asymptomatic (i.e. healthy carriers), especially if infected in early childhood (Cohen, 2005). However, EBV can persist inside memory B cells for the lifetime of an infected person. Persistent infection with EBV has been associated with many types of cancers including Burkitt's lymphoma, Hodgkin's disease, and nasopharyngeal carcinoma (NPC) (Kieff et al., 2010).
Within a host, EBV can infect both B cells and epithelial cells. Infection of B cells helps the virus to persist within a host (Robertson, 2005). Infection of epithelial cells helps EBV to amplify its population of shedding into the saliva (Hutt-Fletcher, 2005). It has been usually believed that the majority of infected people shed virus into the saliva intermittently, which possibly results from viral suppression by the immune response, and that individuals can be classified into different groups of high, intermediate, or low shedder (Niederman et al., 1976, Yao et al., 1985).
The dynamics of virus shedding in EBV positive individuals are poorly understood, partially due to the lack of a sensitive biological assay to quantify the virus population. Recently, Hadinoto et al. (2009) have shown that high levels of virus shedding have been constantly detected in the saliva of eight healthy carriers using sensitive and quantitative assays. They found that the level of virus shedding was quite stable over short periods (days) but varied significantly over longer periods (months/years). They also observed that the levels of latently infected memory B cells were low and remained stable in all subjects. This posed a challenge to an early view that the level of virus shedding is positively correlated with the level of latently infected memory B cells and that individuals could be classified into groups of high, intermediate, or low shedders (Niederman et al., 1976, Yao et al., 1985). A natural question is what regulates the level of virus shedding? Hadinoto et al. proposed a biological mechanism to explain the virus shedding into the saliva. Latently infected memory B cells can differentiate into infected plasma-like B cells within which virions replicate and burst out (lytic infection). Virus released from plasma B cells can infect epithelial cells of the tonsil epithelium. Virus released from infected epithelial cells go through more rounds of epithelial cell infections before being shed into saliva. The amount of virus that is lost due to swallowing can be replenished quickly by virus released from infected epithelial cells (Hadinoto et al., 2009).
Despite the success in modeling HIV, hepatitis B and C, and influenza virus dynamics with/without treatments, very few models have been developed to study EBV dynamics. Mathematical models have been used to study the dynamics of other herpesvirus infection like cytomegalovirus (CMV) (Emery and Griffiths, 2000, Wodarz et al., 2007), HHV-6 (Wang et al., 2003) and HSV-2 (Schiffer et al., 2009). In these studies, both deterministic and stochastic approaches have been used to model reactivation of herpesviruses. The dynamics of EBV infection of B cells have been studied using agent-based models (Castiglione et al., 2007, Shapiro et al., 2008). A mathematical model using ordinary differential equations has been developed by Davenport et al. (2002) to describe the within-host dynamics of EBV infection. This model was used to study the T-cell responses to persistent virus infection. Infection of epithelial cells, which was shown to play an important role in virus shedding, has not been included in these models. In our previous work, we developed mathematical models of within-host dynamics to study EBV long-term infection, the effect of switching infection between B cells and epithelial cells, the evolution of infection, and the age dependence of EBV associated infectious mononucleosis (Huynh and Adler, 2011a, Huynh and Adler, 2011b).
In this paper, we develop a mathematical model to study the dynamics of virus shedding in EBV positive individuals. The model is constructed based on the biological mechanism proposed by Hadinoto et al. that involves reactivation of virus from B cells, infection of epithelial cells and virus shedding into saliva (Hadinoto et al., 2009). We focus the model on the epithelium of the tonsils where virus is replicated and shed into saliva of an infected host. Human tonsil epithelium is heterogenous and contains different cell lines (Perry, 1994). It has been observed that different epithelial-cell lines can have differential susceptibility to EBV in vitro (Imai et al., 1998, Pegtel et al., 2004). Moreover, the structure of the tonsil epithelium is complex and the degree of infiltrated lymphocytes like T cells and B cells varies (Perry, 1994). We thus hypothesize that the number of epithelial cells susceptible to EBV may vary locally depending on the site of the tonsil where the reactivation of lytic infection in B cells occurs. We further discriminate the infection of epithelial cells by virus released from B cells (B-cell virus) from that released from epithelial cells (epithelial-cell virus) in our model since epithelial cells can have different susceptibilities to EBV of divergent cellular origin (Borza et al., 2004). We also include the control of virus shedding by the immune response in the model. By comparing modeling predictions with the data published in the Hadinoto study, we investigate which factor(s) can explain both the long-term variation and short-term stability of virus shedding into the saliva of EBV positive individuals.
Section snippets
Description of data
The experimental data on the levels of virus shedding that we used to compare with our model predictions were published in Hadinoto et al. (2009). In their study, saliva and blood samples were obtained from eight EBV positive subjects over months/years. These eight subjects are healthy volunteers. To study virus shedding in the short term, saliva samples were collected hourly and daily from two subjects, 1 and 2. Saliva samples were collected by mouth rinse with 5 ml of water. Viral DNA was
Model
Our model of the dynamics of EBV shedding (Fig. 1) tracks the number of infected epithelial cells in the tonsil epithelium (I), the number of cytotoxic T cells (T) responding to EBV infection, and the level of virus shedding (VE). Viruses produced in the tonsil are shed into saliva. In this model, we only track the virus shedding from epithelial cells since it has been shown that most of virus shedding found in the saliva has the characteristics of being produced by infected epithelial cells (
Clearance of virus shedding is possible only when there is no virus reactivation from B cells
Analyzing the system of Eq. (3.1) shows that the free-shedding equilibrium, where all three state variables equal zero (, , and ), exists if and only if there is no epithelial cell infection by virus released from B cells (IB=0). This equilibrium is stable when
Existence of the free-shedding equilibrium is possible only when there is no virus reactivation from latently infected memory B cells () or when the tonsil epithelium is not susceptible to infection by B-cell
Discussion
We have developed a mathematical model to study the dynamics of virus shedding in EBV positive individuals. The model focuses on the tonsil epithelium, and tracks infected epithelial cells, CD8+ T-cell response and virus shedding. EBV positive individuals shed high level of virus into saliva. In consistence with Hadinoto et al. (2009), our result shows that infection of epithelial cells can account for the high level of shedding detected in saliva of infected hosts. Despite the high level of
Acknowledgments
We would like to thank Dr. Thorley-Lawson and other members of his laboratory for the opportunity to visit their lab and study the biology of EBV infection of B cells, and Dr. Hutt-Fletcher for insightful discussions on EBV infection of epithelial cells. We also thank two reviewers whose comments and suggestions improved the manuscript. This work is supported by the NIH grant P30-EB011339 and NSF DMS-1122290(LR).
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