Immune Reconstitution after Allogeneic Hematopoietic Cell Transplantation: Comparing Post-Transplant Cyclophosphamide Versus Anti-T-Lymphocyte Globulin As Graft-Versus-Host Disease Prophylaxis in a Retrospective Cohort Study

Introduction: Both post-transplant cyclophosphamide (PT-Cy) and anti-T-lymphocyte globulin (ATLG) eliminate proliferating allo-reactive T cells after allogeneic hematopoietic cell transplantation (HCT) and therefore contribute to reduce the incidence of graft-versus-host disease (GVHD). Exposure to ATLG has been previously associated with delayed T cell reconstitution (Gooptu et al. BBMT 2018). Yet, no study has compared PT-Cy to ATLG for its effect on cellular immune reconstitution and only one small study compared it to anti-thymocyte globulin (Retiere et al. Oncotarget 2018). Hence, we analyzed the dynamics of immune reconstitution after HCT in patients that received either PT-Cy or ATLG as additional GVHD prophylaxis. Methods: We retrospectively analyzed 247 patients (138 male, 109 female) from a single-center, who received HCT from HLA-identical siblings (n=29), haploidentical family donors (n=21), or matched unrelated donors (n=197) between January 2017 and December 2018. All patients were transplanted for hematologic malignancies (49% acute myeloid leukemia). Median age was 56 (range, 18-76) years. Myeloablative conditioning regimen was performed in 119 patients and reduced intensity conditioning in 128 patients. PT-Cy (n=59) was dosed 50 mg/kg/day intravenously (i.v.) on days HCT +3 and +4, followed by tacrolimus in combination with mycophenolate mofetil from day +5. In 188 patients, ATLG was administered at 10 mg/kg bodyweight i.v. on days -3, -2 and -1 in combination with cyclosporine 3 mg/kg i.v. from day -1 and methotrexate (15 mg/m2 on day +1 and 10mg/m2 on days +3, +6, and +11 i.v.). All patients received HCT using peripheral blood stem cells with amedian dose of 6.3x106CD34+ cells/kg (range, 1.3 to 25). Blood samples were collected on days +30, +90, +180, +270 and +365 and analyzed by multiparametric flow cytometry for the following cell subsets: T lymphocytes (CD3+), T helper cells (CD3+/CD4+); cytotoxic T cells (CD3+/CD8+), regulatory T cells (CD3+/CD4+/CD25+/CD127+), T cell receptor αβ(CD3+/TCRαβ), T cell receptor γδ(CD3+/TCRγδ), NK T-cells (CD3+/CD16+/CD56+), NK-cells (CD3-/CD16+/CD56+), naïve helper T cell (CD4+/CD45RA), memory helper T cells (CD4+/CD45RO) and B cells (CD19+). Results: Immune cell reconstitution differed significantly between the PT-Cy and the ATLG cohorts. The use of PT-Cy associated with significantly higher median counts of helper T cells during the first 6 months after HCT (p<0.0001, Fig. 1A). In particular, naïve helper T cells (Fig. 1B; median absolute (abs.) cell counts of PT-Cy versus (vs) ATLG cohort: month 1, 15 cells/µL vs 12 cells/µL , p<0.0001; month 3, 13 vs 3 cells/µL, p<0.0001; month 6, 25 vs 4 cells/µL, p<0.0001) and memory helper T cells (median abs. counts month 1, 94 vs 3 cells/µL, p<0.0001; month 3, 116 vs 64 cells/µL, p<0.0001; month 6, 189 vs 89 cells/µL, p =0.004) were significantly higher in the PT-Cy cohort. Cytotoxic T cells (Fig. 1C) and NK cells did not differ between PT-Cy and ATLG cohorts. Interestingly, γδ T cells were significantly higher in the ATLG cohort (Fig. 1D; median abs. counts month 1, 14 cells/µL vs 3 cells/µL; p =0.019). For B cells or NKT cells the use of PT-Cy associated with earlier immune reconstitution with significant differences only at month 1 after HCT (median abs. cell counts 10 cells/µL vs 1 cell/µL, p=0.007 and 11 vs 2 cells/µL, p=0.03, respectively), regulatory T cells differed significantly in months 3 and 6 (median abs. count 9 vs 2 cells/µL, p<0.0001; 10 vs 4 cells/µL, p<0.0001). The incidence of grade II to IV acute GVHD was significantly lower in the PT-Cy cohort as compared to the ATLG cohort (Hazard ratio 0.48, 95% Confidence interval, 0.30-0.78, p=0.003). Within a median follow up of 11 months, no significant differences in overall survival, relapse incidence and non-relapse mortality were observed between the PT-Cy and ATLG cohorts. Conclusions: Our data suggest that the choice of the additional T cell depleting regimen using either ATLG or PT-Cy significantly affects immune reconstitution after HCT. Knowledge of the distinct immune reconstitution profiles should assist clinical decision-making and help optimizing GVHD prophylaxis. Disclosures Bogdanov: Jazz Pharmaceuticals, MSD.: Other: Travel subsidies. Beelen:Medac GmbH Wedel Germany: Consultancy, Honoraria. Turki:Jazz Pharmaceuticals, CSL Behring, MSD.: Consultancy; Neovii Biotech, all outside the submitted work: Other: Travel subsidies.