CD45RO+ T-Cell Add Back and Prophylactic Blinatumomab Administration Post Tcrαβ/CD19-Depleted Haploidentical Transplantation in Pediatric Patients with High Risk Acute Leukemia

For patients receiving haploidentical donor (haplo) hematopoietic cell transplant (HCT), depletion of TCRαβ T cells from the haplo-graft allows for excellent outcomes and low rates of graft-versus-host disease (GVHD), albeit with a significant delay in immune reconstitution (IR). The adoptive transfer of donor T-cells has been used to optimize IR but increases the risk of GVHD. CD45RA-depleted (memory) T-cells are associated with low rates of alloreactivity and thereby GVHD but retain specificity for leukemic and viral antigens. We implemented a prospective trial utilizing escalating doses of CD45RA-depeleted T-cell as addback following TCRαβ/CD19-depleted haploHCT to improve IR. Patients with acute lymphoblastic leukemia (ALL) also received prophylactic Blinatumomab (Blina) following infusion of CD45RA-depeleted T-cell to overcome the risk immune escape secondary to HLA-loss and relapse (NCT03849651).

Between 2019 to 2020, 30 pediatric patients (9 males, 21 females) with high-risk acute leukemia were enrolled. Median age at HCT was 8.7 years (range 0.9-18.8). Nineteen patients had ALL, 11 patients had AML. Ten patients were in CR1, 13 in CR2 and 7 in CR3/>. Five patients received prior CD19-CAR T-cell therapy. The donors used were mothers (n=15), fathers (n=13), or sibling/others (n=2). All patients received a reduced intensity preparative regimen consisting of Fludarabine, Melphalan, Cyclophosphamide and Thiotepa. ATG was given on days -5, -4 and -3. Mobilized peripheral blood graft were infused on day 0 with a median number of CD34+ cells, γδ+ T-cells, αβ+ T cells, and B cells of 14.8, 22.7, 0, and 0.09x 10 ⁶/kg, respectively. No GVHD prophylaxis was used post-HCT. Two weeks following engraftment, patients received CD45RA-depleted T-cell addback in 3 escalating doses (DL1: 1x10 ⁵cells/kg, DL2: 1x10 ⁶ cells/kg, DL3:1x10 ⁷ cells/kg).

All 30 patients engrafted with a median time for neutrophil and platelet engraftment of 10 (range: 9-11) and 15 (range: 13-20) days, respectively. Infusion of escalating doses of CD45RA-depleted T-cells was well tolerated. One month post infusion, there was a significant increase in the median number of CD3 T-cells, including CD8 and CD45RO+ T-cell subsets (**p<0.01, ***p<0.001, Fig 1A). There was also significant expansion of virus-specific T-cells (VSTs) directed towards Cytomegalovirus (CMV), Adenovirus (AdV), BK, or HHV-6 as shown by Elispot assays (**p<0.01, Fig. 1B). TCR repertoire, as assessed by Vb spectratyping, was broad and comparable to the donor by month 6 post-HCT. The incidence of CMV, AdV, and HHV-6 viremia was 60%, 6.7%, and 16.7% respectively. The median duration of viremia was 4 weeks for CMV (range: 1-13), 3 weeks for AdV (range: 2-13) and 2.5 weeks for HHV-6 (range: 2-20). There were 11 episodes of viral disease (7 colitis, 3 pneumonitis, 1 lymphadenitis). All episodes of viral disease resolved, except 2 that were ongoing at the time of death. The incidence of acute GVHD within 28 days post-infusion after dose level 1, 2 and 3 was 0%, 20% and 10% respectively (p=NS). The cumulative incidence of aGVHD and grade III-IV aGVHD for the entire cohort was 26.7% (12.4-43.3%) and 13.3% (4.1-28.1%) respectively. There was no chronic GVHD; however, follow up is short. Sixteen patients received and tolerated prophylactic Blina infusions. The median time to receiving Blina after CD45RO+ infusion was 29 days (range 15-56). Four of the 16 patients who received Blina relapsed: 3 with CD19+ disease and 1 with CD19-negative disease. With a median follow up of 12.7 months (range 3.5-24.5), 1 year OS and LFS for the cohort was 86.3% (74.6-99.7%) and 69.8% (55.2-88.4%) respectively. The cumulative incidence of relapse was 31.8% (15.3-49.8%) and of non-relapse mortality was 3.3% (0.2-14.8%) respectively.

In this interim analysis, addback of CD45RA- depleted T-cells following TCRαβ/CD19-depleted haplo HCT was safe and led to enhanced functional immune reconstitution. Prophylactic infusion of Blina is well tolerated and its use post-transplant warrants further investigation. Analyses into the effect of ATG on immune reconstitution are underway.

Figure 1

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Figure 1

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