Tcrαβ+/CD19+-Depletion in Hematopoietic Stem Cells Transplantation from Matched Unrelated and Haploidentical Donors in Pediatric Acute Lymphoblastic Leukemia Patients in Complete Remission

Introduction Relapse, graft-versus-host disease (GvHD) and GvHD-associated mortality are major obstacles to success of transplantation from unrelated and haploidentical donors in children with acute lymphoblastic leukemia (ALL). Depletion of α/β (+) T cells and CD19+ B lymphocytes is used to prevent GvHD, improve immune reconstitution and maintain the control of leukemia.

Patients and methods: A total of 236 children with ALL (T-ALL- 83, PB-ALL-153, 82 female, 154 male, median age 9,4 years) in CR underwent allo HSCT between May 2012 and March 2021. Two hundred and two patients received haplo graft, 34 - a graft from MUD. Disease status at transplant was a CR 1 in 81 pts, CR2 in 124 pts and CR≥2 in 31 pts. Transplantation in CR1 was performed according to risk stratification scheme in the current institutional ALL protocol. Flow cytometry-based MRD detection in bone marrow prior to НSСТ was performed in 198 patients (84% of the total cohort). 143 patients were MRD-negative before HSCT, 55 were MRD-positive. 93 pts received treosulfan-based myeloablative preparative regimen, while TBI-based regimen was used in 143 pts.

Three regimens of GvHD prophylaxis were used: regimen 1 included horse ATG at 25 mg/kg/day and CsA+/-Mtx, regimen 2 (n=73): thymoglobulin 5mg/kg, rituximab 200 mg/m² on d-1 and bortezomib on day +2, +5 and regimen 2 (n=163): rituximab 200 mg/m² and tocilizumab at 8 mg/kg on day -1, abatacept at 10 mg/kg on day +2, +7, +14, +28 and post-transplant bortezomib on d+2, +5.

TCRαβ+/CD19+ depletion of HSCT with CliniMACS technology was implemented in all cases according to manufacturer's recommendations. The median dose of CD34+ cells in transplant was 10 x10⁶/kg (range 3,9-18,8), TCRα/β - 19x10³/kg (range 0,2-361).

Results Four patients died before engraftment due to bacterial infection. Primary engraftment was achieved in 227 (98%) patients, surviving till day +30, with median time to engraftment of 13 days for neutrophils and 14 days for platelets. All evaluable patients achieved sustained complete donor chimerism by day +30. Five patients failed to engraft and required rescue with a second HSCT from the alternative donors.

Day +100 mortality was 2% (95%CI:1-5). The CI of TRM among all patients was 9% (95%CI:6-14), 9% (95%CI:6-14) for haplo and 12% (95%CI:5-29) for MUD, p = 0,64. The only factor, significantly associated with non-relapse mortality, was serotherapy, with TRM of 15% (95%CI: 9-26) among patients who received ATG versus 7% (95%CI: 2-13) in those without serotherapy, p = 0,05. The direct causes of non-relapse death included bacterial and viral infections.

CI of aGvHD grade II-IV was 15% (95% CI, 11 - 21), grade 3-4 was 6% (95% CI, 3 - 11). CI of cGvHD at 2 years was 15%(95%CI:11-21). The CI of cGVHD was 24% (95%CI:16-36) with serotherapy and 11% (95%CI: 7-17) without serotherapy, p = 0,01.

The CI of relapse at 4 years was 31% (95%CI:26-38) in the entire cohort. A pre-HSCT MRD level above 0.01% or unknown MRD status, non-TBI regimen, CR>1 were associated with increased risk for relapse in the univariable analysis (p = 0.000, p = 0.056 and p = 0.015 respectively).

EFS and OS at 4-years were 57% (95%:51-64) and 67% (95%:61-73), respectively. The 4-year GRFS in whole group was 55% (95%CI:49-62). Use of TBI regimen had a significant positive correlation with EFS outcomes: EFS for TBI 64% (95%CI:56-72) and for Treo 46%(95%CI:36-56), p=0,019, without effect on OS. In MRD (-) group EFS was 64% (95%CI: 56-72), as compared to 47%(95%CI:34-61) in the MRD (+) group, p=0,014 and OS was 75% (95%CI: 67-82), as compared to 58% (95%CI:45-71) respectively, p=0,007. In MVA in Haplo group number of CR (>1), MRD positive level pre HSCT reached statistical significance for OS (HR, 2.96 (1.42-6.17); P .004 and 1.94 (1.10-3.45); P .0023), for EFS (HR, 2.08 (1.17-3.69); P .012 and 1.91 (1.16-3.13); P .0011), and relapse rate (HR, 3.92 (1.93-7.94); P < .0001 and 3.80 (2.17-6.65); P <0.001). EFS was high (HR, 0.51 (0.30-0.89); P.017) and CIR was low (HR, 0.35 (0.20-0.62); P<0.001) for patients transplanted with TBI. Median time of follow-up for survivors was 4 years (range, 0,7 - 9,4).

Discussion: We confirm that the depletion of TCR-alpha/beta and CD19 lymphocytes from the graft ensures high engraftment rate and acceptable transplant-related mortality in pediatric ALL patients. Use of TBI and MRD negative status before HSCT had a significant correlation with good outcome.

No relevant conflicts of interest to declare.