Replacement of Polyclonal Anti-Thymocyte Globulin By Targeted Immunomodulation Is Associated with Improved Outcome of Alfa\Beta T Cell-Depleted Hematopoietic Stem Cells Transplantation in Children with Acute Leukemia

Introduction

Relapse, graft-versus-host disease (GvHD) and associated non-relapse mortality are the main obstacles to successful hematopoietic stem cell transplantation in children with leukemia. αβ T cell depletion was developed to prevent GvHD and improve immune reconstitution in recipients of mismatched grafts. Most current protocols use rabbit anti-thymocyte globulin (ATG) as an essential component of preparative regimen to secure engraftment and GVHD control. In order to avoid damaging effects of circulating ATG on graft NK and gd T cells, we have replaced ATG with pharmacologic blockade of IL-6 and CD80/CD28 co-stimulation axis in our ongoing study.

Patients and methods

Major transplantation outcomes were compared between participants of the current prospective trial (ATG-) and a retrospective control group (ATG+). A total of 165 children with acute leukemia (67 AML, 98 ALL, 68 female, 97 male, median age 8,7 y) underwent allo HSCT between 01.11.2013 and 01.03.2018. Of them 134 - from haploidentical donor and 31 from unrelated donor. All pts were in complete remission (CR1=80, CR2=67, CR>2=18).

Ninety-two pts received treosulfan-based conditioning, 73 - TBI-based (all ALL). Either melphalan (n=46) or thiophosphamide (n=98) or etoposide (n=21) were added as a second agent. Fludarabine was used in all pts.

Two types of GVHD prophylaxis were used: type 1 (ATG+), (n=98): thymoglobulin 5mg/kg, rituximab 200mg/m² with either bortezomib on days +2, +5 (n=72) or tacro (n=9) or without any additional agents (n=17); Type 2 (ATG-) (n=67): tocilizumab at 8 mg/kg on day -1, bortezomib on day +2, +5 with abatacept at 10 mg/kg on day -1, +7, +14, +28 (n=63) or without added agent (n=4).

αβ T cell depletion with CliniMACS was used in all cases. The median dose of CD34+ cells was 9x10⁶/kg, αβ T cells - 16 x10³/kg.

Modified (CD45RA-depleted) donor lymphocyte infusions (DLI) were administered to 113 pts. Twenty-five patients received DLI on day 0 and 88 pts received DLI after engraftment. Median time of follow-up for survivors was 2 years (range, 0,3 - 4,5).

Results

Three patients died before engraftment due to septic event. Primary engraftment was achieved in 161 of 162 evaluable pts (99,3%), the median time to neutrophil and platelet recovery was 16 and 15 days.

Among the whole cohort the cumulative incidence of acute graft-versus-host disease (GvHD) grades II - IV and III - IV was 11,5% (95% CI: 7,5-17,6) and 4,8% (95% CI: 2,5-9,5) respectively. The cumulative incidence of cGvHD was 10 % (95% CI: 6,3-15,9). The incidence of aGvHD and cGvHD was not different between ATG (+) and ATG (-).

Among the whole cohort 2-year pTRM was 8% (95%CI: 4,8-13,5). pTRM was significantly lower among ATG (-) group - 1,5% (95%CI:0,2-10,4) versus 12,2% (95%CI:7,2-20,8) among ATG (+) group, p = 0,015.

The cumulative incidence of relapse at 2 years was 21% (95%CI: 15,5-29), 24% (95%CI: 16-35), among ATG (+) and 19% (95%CI: 11-32), among ATG (-), p = 0,8. Two-year pEFS was 70% (95%CI: 62-77), 2-year pOS - 78% (95%CI: 71-85). Among patients, who received ATG (-) regimen, pEFS was 76% (95%CI: 68-89), as compared to 65% (95%CI: 56-75) among ATG (+), p=0,1 and pOS was 89% (95%CI: 81-97) versus 72% (95%CI: 63-81), p=0,032, respectively.

αβ T cell recovery at day +30 was associated with a trend to decreased incidence of relapse, CI of relapse was 32% (95% CI:22 - 47) in those with αβ-T cell count < median vs 18 % (95% CI: 11-32) in those with αβ-cell count >median, p=0,08. EFS among αβ T" high" was 81% (+/-10) vs 56% (+/-14) among αβ T"low", p=0,002.

Discussion

We confirm that the depletion of αβ T cells from the unrelated and haploidentical graft in combination with intensive conditioning regimen ensures high engraftment rate and low transplant-related mortality. Our analysis suggests that polyclonal ATG serotherapy is not an essential part of the transplant regimen in αβ T-depleted transplantation. Combined administration of tocilizumab and abatacept after αβ T cell-depleted grafting effectively prevents GVHD, does not compromise engraftment, appears to decrease non-relapse mortality and improve survival.