Development of CMV-SPECIFIC Immunity after Cord Blood Transplantation in Adults Depends on Reconstitution of Thymopoiesis and Regeneration of NAÏVE CD8+ T Cells

The treatment of hematological malignancies with umbilical cord blood transplantation (UCBT) is rapidly increasing for adult patients. Disadvantages of UCBT include insufficient cell numbers for adult patient reconstitution, a lack of antigen experienced cells, and deficits in T cell signal transduction mechanisms. Consequently, UCBT is frequently associated with impaired immune function and high infection-related mortality. To counter these difficulties, transplantation with two UCB units has been employed to improve immune reconstitution in adult patients. We evaluated both the quantitative and functional reconstitution of cellular immunity in a group of adult patients undergoing UCBT. Thirty-two patients with a median age of 50 years with hematopoietic malignancies were treated with reduced intensity conditioning (Flu/Mel/rATG) followed by infusion with two sequential UCB grafts and GvHD prophylaxis with tacrolimus and sirolimus. The grafts were at least a 4/6 match with each other and the recipient. Here we report the results of 27 patients who have completed at least one year of follow up. Assessments were done prior to transplant and at various time intervals until 12 months post UCBT. Neutrophil and platelet engraftment occurred at a median of 21 days and 42 days, respectively. CD3+ populations remained severely depressed until 8 wks post-transplant when they gradually began to re-emerge. However the CD4+ and CD8+ populations demonstrated distinctly different reconstitution kinetics. At 6 months the median value of absolute numbers of CD4+ lymphocytes was 35% of pre-transplant levels increasing to 42% at 1 yr post-transplant, a median value far below the normal range for adults. In contrast, at 6 months post-transplant CD8+ lymphocytes remained severely depressed to 12% of pre-transplant levels, but dramatically increased and reached normal levels by 1 yr after UCBT. Interestingly, both the CD14+ monocyte and the CD16+CD56+ NK cell populations expanded dramatically at 4 wks post-transplant and reached pre-transplant levels and were within the normal range by 6 months. CD20+ B cell repopulation began at 8 wks post-transplant, displayed a striking expansion leading to a 17-fold increase in the median value for B cell numbers over pre-transplant values at 1 year and resulting in a median value of absolute numbers near the top of the normal range. To evaluate functional T cell immune reconstitution in vivo, we performed IFN-γ ELISpot analysis on CMV stimulated PBLs and compared the results to a PCR-based assay for CMV viremia. Additionally, we assessed the reconstitution of thymopoiesis with the T cell receptor excision circle (TREC) assay and real-time PCR. 16/27 patients and 26/52 UCB products were CMV seropositive prior to transplant. In the post-UCB period, development of CMV-specific effectors as determined by ELISpot did not always correlate with clearance of CMV viremia. Specifically, prior to 8 weeks post-UCBT, 8 out of 12 (67%) patients with CMV-positive ELISpot displayed CMV viremia, between 8 weeks and 100 days post-UCBT 4 out of 11 (36%) patients with CMV-positive ELISpot displayed CMV viremia and after 100 days post-UCBT only 3 out of 10 patients (30%) who developed CMV-positive ELISpot remained positive for CMV viremia. Identification of functional CMV effectors was only associated with the numbers of CD8+CD45RA+ cells (p=0.01) and the development of high TREC concentrations (p=0.01) that were detected after 6 months of UCBT, and was independent of GvHD or mixed chimerism. Taken together these results indicate that reconstitution of T cell immunity after UCBT is characterized by delayed recovery of CD4+ and CD8+ T cells and correlates with reconstitution of thymopoiesis and increase of naïve CD8+CD45RA+ T cells that can develop into efficient pathogen-specific effectors.