Efficient replication of Epstein-Barr virus-derived plasmids requires tethering by EBNA1 to host chromosomes.

The EBNA1 protein of Epstein-Barr virus enables plasmids carrying oriP both to duplicate and to segregate efficiently in proliferating cells. EBNA1 recruits the origin recognition complex (ORC) to establish a replication origin at one element of oriP, DS (dyad symmetry); at another element, FR (family of repeats), EBNA1 binds to an array of sites from which it tethers plasmids to host chromosomes for mitotic stability. We report experiments leading to the conclusion that tethering by EBNA1 to host chromosomes is also needed within interphase nuclei in order for plasmids to be replicated efficiently from oriP. The DNA-binding domain of EBNA1, which lacks chromosome-binding ability, was found to support weak, DS-specific replication in HEK293 cells after transient transfection, being 17% as active as wild-type EBNA1. The low efficiency of replication was not due to the failure of the DNA-binding domain to retain plasmids within nuclei, because plasmids were recovered in similar amounts and entirely from the nuclear fraction of these transiently transfected cells. A derivative of EBNA1 with its chromosome-tethering domains replaced by a 22-amino-acid nucleosome-binding domain was fully active in supporting oriP functions. The implication is that EBNA1's DNA-binding domain is able to recruit ORC to DS, but either this step or subsequent replication is only efficient if the plasmid is tethered to a host chromosome. Finally, with some cell lines, DS can hardly support even transient plasmid replication without FR. A loss of plasmids lacking FR from nuclei cannot account for this requirement, suggesting that the stronger tethering to chromosomes by FR is needed for plasmid replication within the nuclei of such cells.

The BAH domain facilitates the ability of human Orc1 protein to activate replication origins in vivo.

Selection of initiation sites for DNA replication in eukaryotes is determined by the interaction between the origin recognition complex (ORC) and genomic DNA. In mammalian cells, this interaction appears to be regulated by Orc1, the only ORC subunit that contains a bromo-adjacent homology (BAH) domain. Since BAH domains mediate protein-protein interactions, the human Orc1 BAH domain was mutated, and the mutant proteins expressed in human cells to determine their affects on ORC function. The BAH domain was not required for nuclear localization of Orc1, association of Orc1 with other ORC subunits, or selective degradation of Orc1 during S-phase. It did, however, facilitate reassociation of Orc1 with chromosomes during the M to G1-phase transition, and it was required for binding Orc1 to the Epstein-Barr virus oriP and stimulating oriP-dependent plasmid DNA replication. Moreover, the BAH domain affected Orc1's ability to promote binding of Orc2 to chromatin as cells exit mitosis. Thus, the BAH domain in human Orc1 facilitates its ability to activate replication origins in vivo by promoting association of ORC with chromatin.

CD8 T cell recognition of endogenously expressed epstein-barr virus nuclear antigen 1.

The Epstein-Barr virus (EBV) nuclear antigen (EBNA)1 contains a glycine-alanine repeat (GAr) domain that appears to protect the antigen from proteasomal breakdown and, as measured in cytotoxicity assays, from major histocompatibility complex (MHC) class I-restricted presentation to CD8+ T cells. This led to the concept of EBNA1 as an immunologically silent protein that although unique in being expressed in all EBV malignancies, could not be exploited as a CD8 target. Here, using CD8+ T cell clones to native EBNA1 epitopes upstream and downstream of the GAr domain and assaying recognition by interferon gamma release, we show that the EBNA1 naturally expressed in EBV-transformed lymphoblastoid cell lines (LCLs) is in fact presented to CD8+ T cells via a proteasome/peptide transporter-dependent pathway. Furthermore, LCL recognition by such CD8+ T cells, although slightly lower than seen with paired lines expressing a GAr-deleted EBNA1 protein, leads to strong and specific inhibition of LCL outgrowth in vitro. Endogenously expressed EBNA1 is therefore accessible to the MHC class I pathway despite GAr-mediated stabilization of the mature protein. We infer that EBNA1-specific CD8+ T cells do play a role in control of EBV infection in vivo and might be exploitable in the control of EBV+ malignancies.

Direct killing of Epstein-Barr virus (EBV)-infected B cells by CD4 T cells directed against the EBV lytic protein BHRF1.

Due to their low frequency, CD4 T-cell responses to Epstein-Barr virus (EBV) lytic antigens are, so far, poorly characterized. Human peptide major histocompatibility complex (MHC) class II multimers provide a means to detect and characterize such rare T cells. Along a screening of T-cell responses to lytic or latent EBV antigens within peripheral blood leukocyte (PBL)- or synovial-derived CD4 T-cell lines, we identified an human leukocyte antigen-DR*0401 (HLA-DR*0401)-restricted epitope derived from BHRF1 (BamHI fragment H rightward open reading frame 1), a viral protein produced during the early stages of the lytic cycle. We show here that T-cell responses to this particular BHRF1 epitope are shared by most EBV-infected DR*0401(+) individuals, as BHRF1-specific CD4 T cells could be sorted out from all the DRB*0401 T-cell lines analyzed, using magnetic beads coated with recombinant BHRF1/DR*0401 complexes. Sorting with these peptide MHC class II multimers was very efficient, as the yield of recovery of BHRF1-specific T cells was nearly 100%. Functional analysis of a large number of clones responding to BHRF1/DR*0401 demonstrated their cytolytic action against autologous and allogeneic DR*0401(+) EBV-transformed B-lymphoblastoid cell lines (B-LCLs), with 40% to 80% killing efficiency and potent interferon gamma production, thus suggesting that this CD4 T-cell population contributes to the control of EBV replication. B-LCL lysis by these T-cell clones was DR*0401 dependent, EBV dependent, and was not merely due to bystander killing. Taken together, these data provide the first demonstration that a lytic antigen can induce a direct cytolytic response against EBV-infected cells.

Methylation of the EBV genome and establishment of restricted latency in low-passage EBV-infected 293 epithelial cells.

Epstein-Barr virus (EBV) encodes multiple latency programs: a growth-transforming program (type III) latency program and restricted-latency (types I and II) programs. During type III latency, EBV expresses six nuclear antigens, all of which are encoded by a single complex transcriptional unit driven by two linked promoters, Cp and Wp, while restricted viral latency is characterized by the expression of a single nuclear antigen, EBNA1, whose expression is driven from a distinct transcription unit under the control of the Qp promoter. EBV infection of the 293 epithelial cell line frequently leads to the establishment of a type I/II latent infection. Here we report that during the initial stages of virus infection of the 293 cell line, both Cp and Wp are active. However, analysis of four established, low-passage EBV-infected 293 cell lines revealed that three of these exhibited Qp-driven transcription of the EBNA 1 gene and little or no detectable Cp and Wp activity, while the fourth cell line exhibited Cp activity. Notably, all four cell lines contained the necessary transcription factors to drive transcription initiation from Cp and Wp when transiently transfected with unmethylated reporter constructs. Furthermore, in the cell lines exhibiting restricted EBV latency the viral genomes were extensively methylated around Cp and Wp, but not Qp. In contrast, in the cell line exhibiting Cp activity the viral genomes were hypomethylated around Cp, Wp, and Qp. Taken together, these results provide evidence that the establishment of a restricted latent infection in the 293 epithelial cell line is not due to a failure to initiate the growth-transforming (type III) latency program, but rather may arise from a selection against the type III latency program. Furthermore, these results are consistent with the hypothesis that methylation of Cp and Wp is required for entry into the type I or II latency programs.