In general, PTLD occurs after EBV infection of the nasopharynx and epithelial cells, with subsequent B cell transformation due to altered host immunity after transplantation. Initial EBV infection results in a cellular program aimed at viral production through ultimately lytic infection of tonsillar B cells and establishment of a latent infection which is a life-long one. During type III latency, a period of growth and survival of infected B cells, genes expressed include Epstein Barr nuclear antigens (EBNAs 1-3C), latent membrane proteins (LMPs 1-2B), and nuclear RNAs (EBERs). Subsequently, during type II latency, LMP 1 and LMP 2 provide differentiation within germinal centers, through CD40, a common pathway for T helper signaling of B cells. Lastly, during type I latency, no genes are expressed, thus evading cytotoxic T cell responses [14
]. Latent proteins which promote B cell immortalization include EBNA-1, EBNA-2, EBNA-3A, EBNA-3C, EBNA-LP, and LMP-1. Functions associated with these proteins include replication of the EBV genome, upregulation of c-myc, cell cycle checkpoint inhibition, binding of CD40, anti-apoptosis pathways through Bcl-2, and modulation of intracellular signaling pathways including NFκ
Host immunity to EBV is inhibited by EBV-derived cytokines which downregulate cytotoxic T cell responses and induction of anti-apoptosis pathways which prevent cell death by EBV latency proteins. During lytic infection, viral interleukin-10 (vIL-10), bearing homology to human IL-10, reduces IL-12 and γ-interferon release essential for cytotoxic T cell activity. Similarly, an EBV-secreted soluble receptor causes inhibition of colony-stimulating factor-1 (CSF-1) necessary for monocyte-associated antiviral cytokine production. Stimulation of anti-apoptosis results from blockade of death receptor signals at the cell surface (Fas, TNF-related apoptosis inducing ligand), Bcl-2 amplification within mitochondria (intrinsic pathway), and NFκB activation within the cell. Please see for a diagram of EBV proliferation.
Figure 1 This proposed model of pathogenesis shows EBV infection of B cells (1), concurrent immunosuppression of plasmacytoid dendritic cells, PDCs, (2), net increase in IL-10, release of α-interferon and γ-interferon, and stimulation of cytotoxic (more ...)
Antiviral responses are predominantly mediated by cytotoxic T cells, primed by prior immunological stimuli, with specific antigen-specific memory. Mifsud and colleagues described EBV-specific CD8+ T cells in a longitudinal study cohort of lung transplant recipients, focusing upon an HLA-B8 restricted cohort with reactivity directed towards epitope FLRGRAYGL on the EBV protein EBNA3A. The authors found the frequency of EBV-specific T cells in immunosuppressed lung transplant patients to be 4-5-fold greater compared to healthy controls. Although ex vivo stimulation of EBV specific T cells revealed alloreactive responses, γ
-interferon production was blunted and variable among the patient's peripheral T cells tested, compared to healthy controls, suggesting limited cytotoxic responses in EBV seropositive lung transplant patients likely persisting after initial EBV infection [17
]. In a pediatric thoracic transplant population, children with high EBV viral loads demonstrated significantly higher number of CD8+ T cells and EBV-specific CD8+ cells compared to those with undetectable viral loads, although they showed no difference compared to healthy controls. Examination of γ
-interferon production as related to level of EBV viral load showed that those children with high viral loads had significantly lower levels of γ
-interferon production compared to those with low viral loads, consistent with depletion and reflecting a possible mechanism for the 45% rate of progression to PTLD in these patients [18
]. In addition to immunological exhaustion demonstrated by these EBV-specific T cells, a subsequent study of pediatric thoracic transplant recipients with PTLD by Wiesmayr and coworkers also identified impaired NK cell responses, as evidenced by decreased expression of NKp46 and NKG2D and increased PD-1 expression, thus limiting T cell responses [19
]. Taken together, these studies provide significant insight into pathogenic mechanisms which amplify EBV-infected B cell immortalization, constrain maximal host immunity, and facilitate PTLD.