Functional Analysis of the CD4 + T-Cell Response to Epstein-Barr Virus: T-Cell-Mediated Activation of Resting B Cells and Induction of Viral BZLF1 Expression

ABSTRACT In contrast to the major role played by Epstein-Barr virus (EBV)-specific CD8 + cytotoxic T-cell responses in immunosurveillance, recent studies have offered the apparently paradoxical suggestion that development of EBV-driven human B-cell lymphoproliferative disorders and tumors in SCID/hu mice is dependent on the presence of T cells, in particular CD4 + T cells. This study presents a functional analysis of the CD4 + T-cell response to EBV and shows that while CD4 + T cells may be cytotoxic, they also express Th2 cytokines and CD40 ligand (gp39) and possess B-cell helper function. We show that EBV-specific CD4 + T cells can provide non-HLA-restricted help for activation of resting B cells via a gp39-CD40-dependent pathway and are able to induce expression of BZLF1, a viral lytic cycle transactivator in latently infected resting B cells, ultimately resulting in rapid outgrowth of transformed B-cell colonies. These results support the proposal that CD4 + T cells may play a key role in reactivation of latent EBV infection and may thus contribute to the pathogenesis of EBV-driven lymphoproliferative disorders.