Epstein-Barr Virus (EBV) is a gamma herpesvirus that manipulates B cell signaling pathways in order to establish a latent infection during which it resides in memory B cells. Two viral latency proteins LMP-1 and LMP2A simulate the activation of B cells by mimicking the signaling of an antigen bound BCR and the survival signal given by a ligand bound CD40. Recently, a third pathway has been reported to contribute to the activation of B cells, the toll-like receptor (TLR) signaling pathway. Thus, this pathway is also potentially a target for EBV modulation.

Toll-like receptors are known for their stimulation of the innate immune response as well as for mediating the interface between innate and adaptive immunity. TLRs recognize patterns on pathogens and begin signaling cascades that activate IRFs as well as several pathways including the MAPK and NF-κB pathways. As a result of TLR signaling, interferon and pro-inflammatory cytokines may be produced from cells, depending on the TLR expression and signaling protein availability within the cell type. In addition to the effects of TLR signaling described above, TLR stimulation can induce B cell proliferation.

Here it is shown that infection of primary naïve B cells can upregulate TLR 7 expression, as well as MyD88, the key adaptor for most TLR pathways. Other TLRs that are commonly expressed on B cells were either repressed (TLR 9) or did not change (TLR 1, 6, 10). This pattern of expression was also observed in a latency type III lymphoblastoid cell line (LCL), which expresses all 11 EBV latency genes. While TLR 7 is located in the endosome, evidence is provided that stimulation of the innate immune response can take place by virus binding alone. UV-inactivated (and thus replication incompetent) EBV was able to induce the expression of several interferon stimulated genes (ISGs).

IRF-5 is a key mediator of TLR responses, playing a role in the induction of proinflammatory cytokines after TLR stimulation. In addition, IRF-5 is reported to have tumor-suppressor characteristics. As described in this thesis, EBV modulates IRF-5 expression and activity at many levels. IRF-5 transcripts are induced during primary infection and type III latency cell lines have higher levels of IRF-5 expression than latency type I cell lines. IRF-5 activity may be modulated through EBV stimulated IRF-5 alternative splicing and through the inhibition of IRF-5 activation via IRF-4 expression induced by LMP-1. This modulation of IRF-5 activity may be important to EBV induced tumorigenesis by allowing IRF-5 to induce pro-inflammatory cytokines very early in infection, while blocking its function (and thus possibly its anti-tumor properties) after the pro-proliferative EBV latency genes are expressed. Indeed, it is also shown here that EBV infection can rescue cells from the anti-proliferative effects of a treatment with a high dose of the TLR 7 ligand R837, but only after the expression of latency genes has taken place. This result shows that EBV can modulate the TLR pathways and the outcome of TLR signaling.