Dynamics of Immune Cell Reconstitution in Allogeneic Hematopoietic Cell Transplant Patients Receiving Post-Transplant Cyclophosphamide (PTCy)

Abstract

In the setting of haploidentical hematopoietic cell transplantation (haplo-HCT), post-transplant cyclophosphamide (PTCy) selectively eliminates alloreactive T cells in-vivo, resulting in favorable graft versus host disease (GVHD), non-relapse mortality (NRM) and relapse outcomes. However, few studies have examined the impact of PTCy on immune reconstitution (IR). We quantified IR in 63 patients after haplo-HCT with PTCy, mofetil mycophenolate and tacrolimus (TAC) and compared results to 93 patients with 8/8 HLA matched related or unrelated donors (MD) receiving TAC, methotrexate and sirolimus for GVHD prophylaxis. Both groups received reduced intensity conditioning for hematologic malignancies. The median age of the Haplo-PTCy and MD cohorts was 55 and 57 years. Patient samples were analyzed using multi-color flow cytometry panels to characterize distinct lymphocyte populations. All IR values are expressed as median absolute cell count per μL. One month after HCT, recovery of all T cell subsets (CD3, CD4Tcon, Treg, CD8) was significantly reduced in the PTCy cohort compared to MD (Figure A, B, C). Recovery of CD4Tcon was also reduced at 2 and 3 months after PTCy (p NK cells were lower 1 month after PTCy (52.7 vs 91.1, p=0.08), but were significantly higher at 2, 3 and 6 months (153.4 vs 94.8, p=0.001, 153.7 vs 87.5, p=0.008, 180 vs 102, p=0.01, respectively, Figure D) compared to the MD cohort. Delayed NK cell recovery at 1 month after PTCy was due entirely to reduced numbers of CD56dim NK cells (Figure E). Subsequently recovery of CD56dim NK cells was similar in both cohorts. Recovery of CD56bright NK cells was significantly increased in the PTCy cohort (p Consistent with prior reports, 1 year cumulative incidence of extensive cGvHD was lower in the PTCy cohort compared to the MD cohort, 13% (5-26%, 95% CI) and 40% (30-50%, 95% CI) respectively, p=0.003, without increased NRM (p=0.28) or relapse (p=0.17). In summary, the effect of PTCy on IR was most pronounced 1 month after transplant with significantly delayed recovery of CD3, CD4Tcon, Treg, CD8 and CD56dim NK cells. Slow recovery of CD4Tcon persisted for 3 months and delayed recovery of Treg persisted for 1 year. Beginning 2 months after HCT, recovery of both CD56dim and CD56bright NK cells was more rapid in the PTCy cohort. Further studies will examine the effects of these differences in IR on clinical outcomes such as relapse, infections and GVHD.