Impact of HLA of Winning Cord Blood Unit on Outcomes after Double Umbilical Cord Blood Transplantation in Adults with Acute Leukemia: A Retrospective Study on Behalf of Eurocord, the Cord Blood Committee Cellular Therapy and Immunobiology Working Party and the Acute Leukemia Working Party of the EBMT

Umbilical cord blood transplantation (UCBT) from unrelated donor is a valid alternative for patients (pts) with acute leukemia (AL) who lack an HLA matched donor. Double UCBT (dUCBT) has extended the use of UCBT to adults. In the majority of the cases, chimerism analysis shows that one unit emerges as the sole source of long term hematopoiesis in the recipient (rcp) following dUCBT. However, no clear factor has yet been identified to reliably predict which unit will emerge as the predominant one.

With the aim of analyzing factors that may predict cord blood unit (CBU) predominance and the impact of the winning CBU characteristics on outcomes after dUCBT, we studied adults with AL who underwent dUCBT as first transplant between 2004 and 2013 at EBMT centres. We selected pts who achieved engraftment and who had available chimerism data assessed within day 130 after dUCBT, with one of the CBUs representing at least 50% of the rcp hematopoiesis (winCBU). According to these criteria, 347 pts were included: 323 had full donor (18 being dual chimera) and 24 had mixed chimerism (>5% of donor cells). At diagnosis, 35% had ALL and 65% AML. At dUCBT 45% were in first complete remission (CR), 44% in second CR and 11% had more advanced disease. Overall, 33% had high risk cytogenetic features. Median time from diagnosis to dUCBT was 12 months (range, 2-202), median age was 40 years (y) (18-76). Conditioning regimen (CT) was reduced intensity (RIC) in 52%. Cyclophosphamide+fludarabine+total body irradiation was the most frequent CT (63%). Anti-thymoglobulin (ATG) was administrated in 25% of the transplants. Median number of total nucleated cells (TNC) and CD34+ cells at cryopreservation were 5.1x10⁷/Kg (range, 2.3-13.7) and 1.3x10⁵/Kg (range, 0.4-10.7), respectively. Considering the unit with the highest number of HLA mismatches (MMs) with the rcp, 73% of the donors were matched at 4 or less loci (≤4/6). For winCBUs, median number of TNC and CD34 cells at cryopreservation were 2.5x10⁷/Kg (range, 0.95-6) and 1x10⁵/Kg (range, 0.06-10), respectively. Only 4% of the winCBUs were 6/6 HLA-matched to the rcp, while 37% were 5/6, 55% were 4/6 and 4% were 3/6 HLA-matched. Overall, 54% of the winCBUs were gender matched and 38% were ABO compatible with the rcp. WinCBUs median age was 3.4 years (range, 0.2-14). As for inter unit characteristics, 50% of the pts received gender matched units. Units were ABO compatible in 40% of the cases, while 30% had minor and 30% had major ABO incompatibility.

In our study population, median follow-up for survivors was 35 months (range, 3-99). All pts engrafted with neutrophil >0.5x10⁹/L in a median time of 25 days (range, 6-66) post DCBT. No secondary graft failure was reported. The cumulative incidence (CI) of acute GvHD grade II-IV at 100 days was 41% with a median time of onset of 29 days (range, 7-106). The 3y-CI of chronic GvHD was 41% (50% of the pts had extensive). The 3y-CI of relapse (RI) and transplant related mortality (TRM) were 27% and 24%, respectively. At 3y, leukemia free survival (LFS) was 49% and overall survival (OS) was 54%.

In multivariate analysis (MVA) including unit characteristics (HLA and gender matching, type of HLA MMs, number of TNC and CD34 cells at cryopreservation, ABO compatibility and unit age) and inter unit features (gender match, ABO compatibility), no significant factor predicting the winCBU was identified.

Analyzing the impact of winCBU on dUCBT outcomes, the HLA matching between CBU and the rcp was the only characteristic related to the winCBU significantly affecting results. The 3y-LFS for pts with 6/6 or 5/6 HLA-matched winCBUs was 56% as compared to 44% for those with 4/6 (p=0.03), while OS was 62% versus 49% (p=0.01), respectively. Acute GvHD was increased in pts receiving a 4/6 HLA-matched winCBU (46% versus 35% for those 6/6 or 5/6, p=0.04). In MVA, 4/6 HLA-matched winCBU was associated with decreased LFS (HR 1.5, p=0.03) and OS (HR 1.5, p=0.03), and with increased TRM (HR 1.9, p=0.03) and acute GvHD (HR 1.7, p=0.01). Notably, older winCBUs (>3.4y) were associated with higher acute GvHD (48% versus 35%; HR 1.6, p=0.02). Other factors associated with poor outcomes were advanced disease status and the use of ATG.

Although we failed to identified any factor predicting CBU predominance, we were able to demonstrate that a 4/6 HLA matched winCBU is associated with poor outcomes. Therefore, selecting units with lower number of HLA MMs for dUCBT may improve final outcomes.