Prognostic Value of TREC, IL-7 and SCF Levels on Clinical Outcomes after Double Umbilical Cord Blood Transplantation in Adults

Umbilical cord blood (UCB) is an alternative source of hematopoietic stem cells (HSC) for allogeneic HSC transplantation but reconstitution of T cell immunity remains problematic even with double unit UCB transplantation (dUCBT). We previously reported that reconstitution of thymopoiesis plays a critical role in the clearance of CMV viremia, and is associated with improved overall survival in dUCBT recipients. Furthermore, we determined that T cell subset recovery after dUCBT depends on the production of IL-7 and SCF. In the present study we investigated whether thymic reconstitution after dUCBT depends on IL-7 and SCF and examined the prognostic role of T-cell receptor excision circles (TRECs), IL-7 and SCF on clinical outcomes after dUCBT. We analyzed 52 adult patients with hematologic malignancies who enrolled in parallel Phase I/II clinical trials of dUCBT at the Dana-Farber/Harvard Cancer Center. Forty three patients received reduced intensity conditioning with fludarabine, melphalan and antithymocyte globulin; nine patients received myeloablative conditioning with fludarabine, cyclophosphamide and TBI. Thirteen of these patients received human PTH and 13 patients received one untreated and one ex-vivo PGE₂ treated UCB unit, per study protocols. GvHD prophylaxis was tacrolimus in combination with sirolimus or mycophenolate mofetil. Assessments were performed at baseline (before conditioning) and at 1, 2, 3, 6 and 12 months after dUCBT. The median follow-up time among survivors was 71 months (range 24-96). The incidence of grade II-IV aGvHD was 15.4% and 14 out of 48 patients (29%) alive after 3 months developed cGvHD. The cumulative incidence of relapse at 5 years was 43%. The 5-year non-relapse mortality (NRM), progression-free survival (PFS) and overall survival (OS) were 31%, 26%, and 41% respectively. Plasma levels of IL-7 increased 3-fold from baseline at 1 month after dUCBT (median 26.4 pg/mL), remained elevated during the first three months, and gradually declined to pre-transplant levels by 1 year. SCF increased more slowly reaching peak levels at 2 months after dUCBT (median 1335.7 pg/mL), and gradually decreased thereafter. All but one evaluable patient had detectable TRECs at baseline with a median value of 3304 copies/ug DNA. TRECs were absent or extremely low during the first 3 months after dUCBT. At 6 months, 69.7% of patients had detectable TRECs, albeit below normal range (median value of 248 copies/ug DNA), but at 12 months 86.6% of the patients had detectable levels with a normal median value of 2404 copies/ug. We observed a statistically significant inverse correlation between IL-7 and TREC levels at 2 months after UCBT (r=-0.48, p=0.036). Similarly, SCF inversely correlated with TRECs at 6 months (r=-0.5, p=.004) and 12 months after dUCBT (r=-0.48, p=.02). We hypothesize that this inverse correlation is likely due to the uptake of IL-7 and SCF by thymocytes leading to their differentiation into TREC-containing Recent Thymic Emigrants (RTE). In support of this hypothesis, IL-7 and SCF inversely correlated with naïve CD4CD45RA+ T cells at various time points after dUCBT. Next, we examined whether TREC, IL-7 and SCF levels might have a prognostic value for clinical outcomes after UCBT. In multivariable models, higher TREC levels were found to be independently associated with improved progression-free survival (PFS) (p=0.02) and overall survival (OS) (p=0.02), and with less non-relapse mortality (NRM) (p=0.002). In contrast, higher levels of IL-7 and SCF were associated with lower OS (p=0.005 and p<.0001 respectively) and a trend towards lower PFS. Furthermore, higher levels of IL-7 and SCF were adverse prognostic factors for NRM (p=0.07 and p=0.007) but not for relapse. Taken together, our findings suggest that adult dUCBT recipients develop thymic reconstitution as early as 6 months after transplantation and TREC values correlate with improved clinical outcomes of dUCBT. In contrast, higher levels of IL-7 and SCF are associated with poor quantitative T cell and thymic reconstitution and they are negative prognostic factors for OS and NRM, which mainly reflects mortality due to infections and GvHD. These findings emphasize the link between cytokine-mediated immune reconstitution of post-thymic T cells and clinical outcomes of transplantation in adult dUCBT recipients.