Three Restricted Forms of Epstein-Barr Virus Latency Counteracting Apoptosis in c-Myc Expressing Burkitt Lymphoma Cells.

Epstein-Barr virus (EBV) is aetiologically linked with Burkitt Lymphoma (BL) but its contribution to lymphomagenesis, versus that of the chromosomal translocation activating c-myc expression, remains unclear. This is in part because the full virus growth transforming programme that is expressed when EBV infects normal resting B cells, is not expressed in BL. Instead EBV in BL normally exhibits a restricted Latency I form of infection characterised by expression of only one latent antigen EBNA1 from the BamHI Q promoter. Here we describe an endemic BL, Awia, which uniquely is heterogeneous at the single cell level for EBV gene expression. Analysis of single cell clones of Awia-BL revealed cells displaying three forms of restricted EBV latency:

• classical Latency I,

• ’Wp-restricted latency’ expressing EBNAs 1, 3A, 3B, 3C and -LP, and

• a novel ’EBNA2⁺/LMP1^- latency’ in which all EBNAs including EBNA2 are expressed without the latent membrane proteins LMPs 1 and 2.

Comparison with rare EBV-negative clones from the same tumour showed that each form of infection provides the c-myc-expressing BL cells with a specific degree of protection from apoptosis. Microarray analysis was carried out on the isogenic Awia-BL clones to determine the influence of EBV gene expression on cellular transcription. Interestingly it was found that expression of the pro-apoptotic Bcl2 family protein BIM, which has previously been implicated in c-myc induced apoptosis, was downregulated in the apoptosis resistant BL cells. Our work suggests that EBV acts as an anti-apoptotic rather than a growth-promoting agent in BL by downregulating expression of the cellular BIM protein. In addition the microarray analysis on these isogenic Awia-BL clones showed that the EBNA profile influences the differentiation status of the BL cell on the germinal centre to plasmacytoid differentiation pathway. These findings may reflect viral functions that are important not just for BL pathogenesis but also for EBV’s normal strategy of persistence in the B cell system.