Memory B-cell reconstitution following allogeneic hematopoietic stem cell transplantation is an EBV-associated transformation event.

Allogeneic stem cell transplantation (allo-HSCT) provides a unique opportunity to track Epstein-Barr virus (EBV) infection in the context of the reconstituting B-cell system. Although many allo-HSCT recipients maintain low or undetectable levels of EBV DNA posttransplant, a significant proportion exhibit elevated and rapidly increasing EBV loads which, if left untreated, may lead to potentially fatal EBV-associated posttransplant lymphoproliferative disease. Intriguingly, this high-level EBV reactivation typically arises in the first 3 months posttransplant, at a time when the peripheral blood contains low numbers of CD27+ memory cells which are the site of EBV persistence in healthy immunocompetent donors. To investigate this apparent paradox, we prospectively monitored EBV levels and B-cell reconstitution in a cohort of allo-HSCT patients for up to 12 months posttransplant. In patients with low or undetectable levels of EBV, the circulating B-cell pool consisted predominantly of transitional and naive cells, with a marked deficiency of CD27+ memory cells which lasted >12 months. However, among patients with high EBV loads, there was a significant increase in both the proportion and number of CD27+ memory B cells. Analysis of sorted CD27+ memory B cells from these patients revealed that this population was preferentially infected with EBV, expressed EBV latent transcripts associated with B-cell growth transformation, had a plasmablastic phenotype, and frequently expressed the proliferation marker Ki-67. These findings suggest that high-level EBV reactivation following allo-HSCT may drive the expansion of latently infected CD27+ B lymphoblasts in the peripheral blood.