T-allo10 Immunotherapy Results in Enhanced Early Immune Reconstitution and Reduced GvHD with Excellent Clinical Outcomes Post Αβ-Depleted HSCT in Pediatric and Young Adult Hematologic Malignancy Patients

We completed the first phase of a Phase Ia/Ib trial at Stanford Children’s Health involving T-allo10 cells. Fourteen pediatric and young adult patients with hematologic malignancies were enrolled (NCT 04640987), and 13/14 received T-allo10 infusion on Day 35 ±7 days post-TCRαβ+ T-cell and CD19+ B-cell-depleted (αβdepleted) HSCT. T-allo10 cells, generated from donor CD4+ T cells and enriched in host alloantigen-specific type 1 regulatory T (Tr1) cells, suppress host-reactive TCRαβ+ T cells causing acute (a)GvHD. They also contain polyclonal naive and memory TCRαβ+ T cells crucial for pathogen and tumor defense.

We hypothesized T-allo10 infusion post-αβdepleted-HSCT would expedite immune reconstitution (IR) by providing polyclonal TCRαβ+ T cells and modulate anti-host immune responses via Tr1 cells, aiming to enhance IR, reduce infections and leukemic relapse without severe GvHD. Achieving these goals would address αβdepleted-HSCT challenges like viral reactivations (~50%) and leukemic relapse (25-30%).

In the Phase Ia portion of this trial, T-allo10 cells were administered at three escalating doses (Cohort 1: 10^5/kg; Cohort 2: 3x10^5/kg; Cohort 3: 1x10^6/kg). No dose-limiting toxicities (DLTs) were observed. Across cohorts, the patients' ages ranged from 1.7 to 24 years (7 females, 6 males). Diagnoses included acute myeloid leukemia (AML, 4 patients), acute lymphoblastic leukemia (ALL, B-cell, 6 patients), myelodysplastic syndromes (MDS, 2 patients), and mixed phenotype acute leukemia (MPAL, 1 patient). Patients received peripheral blood stem cells (PBSC) with doses ranging from 8 to 35 x 10^6 CD34+ cells/kg and αβ+/CD3+ cells ranging from 0.04 to 0.63 x 10^5 cells/kg. Tr1 cells in T-allo10 ranged from 0.9% to 14.7%.

No cases of non-relapsed mortality (NRM) occurred, with 0% NRM at 6 months and 1 year, favorably compared to historical controls (9%). Relapse was observed in 2 patients (15.4%) in Cohort 1, at +307 and +90 days post-HSCT. No relapses were reported in Cohorts 2 and 3. Patients treated with intermediate and high doses of T-allo10 achieved 100% 2-year leukemia-free survival (LFS) and graft-versus-host disease-free relapse-free survival (GRFS).

Steroid-responsive grade II aGvHD developed in 4 patients (30.8%). No grade III-IV aGvHD was observed. Remarkably, 66.7% of evaluable patients (6/9) achieved the IR efficacy endpoint, doubling the historical control rate of 31%. This endpoint is defined as ≥50 CD3+CD4+ T cells/µL by Day +60 post-αβdepleted-HSCT.

T-allo10 infusion increased the frequency of TCRαβ+ CD3+CD4+ T cells with a memory phenotype in all patients. Incidence and severity of viral reactivations decreased with increasing dose of T-allo10 cells. Adenovirus reactivation decreased from 60% in Cohort 1 to 0% in Cohort 3, and BK virus reactivation was not observed in Cohorts 2 and 3 compared to 60% in Cohort 1. These results are paralleled by an increased in vitro response to viral antigens and mitogens at Day +180 post-HSCT, irrespective of infection status. Two-thirds of patients in Cohorts 2 and 3 responded positively to mitogens at six months, with viral antigen-specific responses comparable to the donor's baseline.

Tr1 cells were detectable in peripheral blood by flow cytometry shortly after T-allo10 infusion, peaking at 20.4% at day +7, then remained stable and within the normal range. Their persistence over time was confirmed by tracking the TCRα and TCRβ Tr1 cell clonotypes identified in each T-allo10 product.

These findings indicate that T-allo10 promotes phenotypic and functional IR without escalating GvHD risk. We are currently enrolling patients in the Phase Ib at dose levels of Cohorts 2 and 3 to define the optimal dose of T-allo10 cells.

In conclusion, Phase I trial results show that T-allo10 infusion is safe and significantly improves early IR and reduces GvHD risk in pediatric and young adult patients post-αβdepleted-HSCT. No NRM was observed at 6 months and 1 year. Cohorts 2 and 3 achieved 100% 2-year LFS and GRFS. Tr1 cells were detected shortly after infusion, reaching up to 20% within days 1-7, showing a dose-dependent trend. T-allo10 infusion enhanced CD4+ T-cell IR, particularly expanding CD4+ memory T cells. These findings support T-allo10 as a promising immunotherapy to enhance IR and improve survival outcomes, warranting farther investigation.