TCRα/β Depleted Mobilized Pbscs without Calcineurin Inhibitors in Patients with Fanconi Anemia

Background: Over the past 2 decades, sequential improvements in conditioning regimens for patients with Fanconi anemia (FA) undergoing hematopoietic cell transplantation (HCT), including T cell depletion (TCD), have led to marked improvements in outcomes (MacMillan, Blood 2015). We performed a single center phase II prospective clinical trial (NCT03579875) to test the hypothesis that TCRa/b depletion would sufficiently deplete graft-versus-host disease (GVHD) causing a/b T cells to eliminate the need for prolonged immunosuppression, and reduce the risk for opportunistic infections after related or unrelated donor peripheral blood stem cell (PBSC) transplantation in patients with FA.

Methods: Patients received our previously published conditioning regimen with fludarabine, cyclophosphamide and methylprednisolone alone if the patient had marrow failure only and an HLA matched sibling donor (n=2), or in combination with total body irradiation with thymic shielding (n=12) or busulfan (n=17) if there was clonal disease, or if the donor was a haploidentical related or HLA matched or mismatched unrelated donor. Additionally, all but the first patient received one dose of rituximab the day before transplant for B cell depletion. PBSCs collected after Neupogen mobilization were depleted of TCRa/b cells prior to transplant. Mycophenolate mofetil was given after transplant if the TCRa/b dose ≥2 x 10⁵ TCR α/β T cells/kg recipient weight (n=4). Mass cytometry (CyTOF) was used for immune reconstitution studies, targeting markers for delineating naïve, effector and effector memory CD4 cells and naïve, stem memory, central memory and effector memory CD8 cells, as well as T regulatory, B and gd T cell subpopulations.

Results: 31 patients with FA, median age 9.5 years (range 1.7-43.8) were enrolled. Genotypes included 17 FANCA, 4 FANCC, 6 BRCA2, and one of each FANCB, FANCF, FANCG and FANCJ. Patients received TCRa/b depleted PBSCs for marrow failure (n=18), myelodysplastic syndrome MDS (n=4), relapsed MDS (n=1), APML (n=1), ALL (n=1), immune deficiency (n=1), BRCA2 with clonal abnormality (n=2) or pre-emptively for BRCA2 (n=3), with 2 patients having had a prior transplant. Median CD34+ cells/kg was 1.14 x 10⁷/kg and median TCRa/b dose was 6.2 x 10⁴/kg. Neutrophil engraftment occurred in all patients at a median of 9 days. Platelet recovery ≥20,000/uL occurred in 30 patients at a median of 15 days. Complete donor myeloid engraftment was achieved by day 21 in all patients with sustained neutrophil engraftment who did not relapse. Donor lymphoid engraftment was slower with complete CD3+ chimerism achieved in only 15 patients at 1 year after PBSCT. To date, 3 patients developed grade II-IV acute GVHD. No patient has developed chronic GVHD requiring systemic therapy.

With a median follow-up of 27 months, probability of survival at 2 years is 90% (95% CI, 71-97%), including 5 of 6 BRCA2 patients and all 7 adult patients. Secondary graft failure occurred in 2 patients, both of whom were successfully retransplanted. Relapse was observed in 3 patients, two of whom were successfully retransplanted and remain in remission.

By 1 year, 6 patients developed viral infections requiring systemic therapy: CMV (n=2), 1 adenovirus (n=1), adenovirus and BK (n=1), HHV6 (n=1) BK, HHV6 and EBV-PTLD (n=1).

Fifteen patients had sufficient follow up for evaluating immune recovery to 1 year. Compared to 10 historical FA patients similarly treated except for GVHD prophylaxis (TCD by CD34 selection in combination with calcineurin inhibitor, MacMillan, Blood 2015), the significant finding were: lower proportions of total CD4+ cells (43.0% vs 76.7%, p <0.0002), lower proportions of memory Tregs (4.8% vs 14.6%, p<0.01), and higher proportions of gd T cells (30.8% vs 4.8%, p<0.0003) in recipients of TCRa/b PBSC at day 28. Analysis of gd T cell proportions at 6 months (16.2% vs 3.2%, p = 0.0106; TCRa/b PBSC vs. CD34 selection) indicated that TCRa/b depletion resulted in a lasting immune imprint, underpinning the differences in reconstitution between these two transplant methodologies.

Conclusions: TCRa/b depleted PBSC transplantation without CNI results in excellent engraftment, minimal GVHD, few viral infections and excellent survival in FA patients with promising outcomes in those with hematologic malignancy, biallelic BRCA2 genotype and older age who historically often did poorly.

MacMillan:Incyte: Research Funding; Equillium: Membership on an entity's Board of Directors or advisory committees; NMDP: Membership on an entity's Board of Directors or advisory committees. Wagner:Vertex Pharmaceuticals: Consultancy; Magenta Therapeutics: Current equity holder in private company; Garuda Therapeutics: Membership on an entity's Board of Directors or advisory committees; Rocket Pharma: Consultancy, Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees; ASC Therapeutics: Membership on an entity's Board of Directors or advisory committees.