CD4 and post-transplant lymphoproliferative disease: Polymorphisms of the EBNA-1 sequence have been reported in EBV+PTLD patients and healthy controls who have detectable CD8+ T cell responses against the viral variant but not against the prototype EBV laboratory strain B95-8[34]; thus, protein sequence variations might explain low detection of specific-CD4+ T cells, except the class II MHC-restricted peptide pools we used were composed of 6 different latent EBV proteins and all of tested HCs strongly recognized at least one peptide pool.