B lymphocytes and Epstein-Barr virus: the lesson of post-transplant lymphoproliferative disorders.

Epstein-Barr virus (EBV) is a ubiquitous human gamma-herpes virus that establishes a life-long asymptomatic infection in immunocompetent hosts by colonizing memory B lymphocytes and hijacking cellular signaling pathways that regulate antigen-dependent B-cell activation and differentiation. In patients with solid organ or hematopoietic stem cell transplantation, the defect in EBV-specific immune responses may allow the outgrowth of EBV-carrying B lymphocytes that may give rise to a spectrum of different clinico-pathologic entities encompassed by the term post-transplantation lymphoproliferative disorders (PTLD). EBV-driven immortalization of B-cells is mediated by the cooperative activity of viral proteins that derange critical cellular pathways controlling growth and/or survival of B lymphocytes. Full transformation of infected B-cells is achieved by the contribution of poorly defined additional co-factors, including microenvironmental stimuli, genetic and epigenetic alterations. The quantification of circulating EBV DNA and EBV-specific T cells are valuable tools in the clinical monitoring of EBV-associated PTLD. The recent advances in elucidation of the mechanisms underlying EBV-induced growth transformation will be instrumental in guiding the design of novel approaches for the treatment of these often life-threatening lymphoproliferative disorders.