Maintenance of Naïve T Cells and a Diverse TCR Repertoire Are Critical for Reconstitution of EBV-Specific Immunity after Double Cord Blood Transplantation

Poor reconstitution of T cell immunity after umbilical cord blood transplantation (UCBT) results in susceptibility to viral infections that require intact T cell immunity. Epstein Barr virus (EBV) is a gammaherpes virus that is reactivated from latent to lytic cycle and can cause aggressive lymphomas known as post transplant lymphoproliferative disorder (PTLD) in immunocompromised hosts. We attempted to distinguish immune reconstitution profiles in double UCBT recipients who developed EBV viremia from those who did not. Thirty-one patients with hematologic malignancies received dUCBT with melphalan, fludarabine, ATG conditioning and tacrolimus plus sirolimus for GvHD prophylaxis. During the first 12 months after dUCBT, 14 of 31 (45%) patients developed EBV viremia and four (13%) developed PTLD. At one month after dUCBT, patients with EBV viremia displayed higher numbers of CD19+ B cells (p=0.04) and CD4+CD25+ Treg cells (p=0.03) compared with patients who never became viremic. Surprisingly, EBV viremia correlated with increased numbers of CD3+ (p=0.04), CD4+ (p=0.015) and CD8+ (p=0.021) T cells, but this response was ineffective at controlling the virus. One explanation for this unexpected result might be skewing towards a late effector memory T cell phenotype, a stage in which T cells are incapable of mounting protective immune responses. We examined naïve vs. memory T cell distribution and determined that patients who developed EBV reactivation had higher numbers of memory cell subsets (CD4+CD45RO+, p=0.023; CD8+CD45RO+, p=0.019) at two months after dUCBT. Analysis of repertoire diversity by deep-sequencing on PCR-amplified CDR3 regions of the TCRb gene using the ImmunoSEQ assay showed a more diverse TCR repertoire, as determined by higher entropy (p=0.03) and lower clonality (p=0.03), among patients who did not develop EBV reactivation. Assessment of SCF and IL-7, critical regulators of hematopoietic stem cell and naïve T cell homeostasis, respectively, showed that patients without EBV viremia had higher levels of SCF (p=0.0016) and IL-7 (p=0.046) compared with patients who developed viremia and PTLD. Our findings suggest that control of EBV reactivation after dUCBT might be linked to the support of naïve T cell homeostasis, which enables maintenance of a diverse TCR repertoire.