Abstract :
[en] Background
Post-transplant administration of high-dose cyclophosphamide (PTCy) is one of the most efficient way to prevent graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (allo-HCT). However, the impact of PTCy on immune-mediated graft-versus-leukemia (GvL) effects has remained debated. Here, we investigated the impact of PTCy administration on xenogeneic GVHD and allogeneic GvL effects in two humanized mouse models.
Impact of PTCy on xGVHD
A single i.p. injection of cyclophosphamide (100 mg/kg) 3 days after infusion of 2 × 107 human PBMC in NSG mice significantly delayed xGVHD (64 vs 43 days, P < 0.0001). However, xGVHD was not abrogated and the majority of treated mice eventually succumbed from xGVHD. On day 21 after PBMC infusion, (human) blood T-cell counts were significantly lower in PTCy than in control mice (P = 0.0028) while the proportions of KI67+ CD4+ and CD4+ T cells were similar. Interestingly, at that time point the frequency of Treg in the blood was significantly higher in PTCy than in control mice (12.9 vs 4.3 %, P = 0.03). In further analyses, we compared human cells recovered from mouse blood and organs on day 6 after PBMC infusion (60 hours after PTCy in PTCy-treated mice). In comparison to untreated mice, PTCy mice had a dramatically lower proportion of KI67+ T cells as well as a much higher proportion of CD4+ apoptotic T cells. Treg numbers in the peripheral blood were also significantly lower in PTCy than in control mice. Looking at the bone marrow and other xGVHD target organs, PTCy-treated mice had a lower proportion of proliferative (i.e. KI67+) CD4+ T cells as well as of proliferative Tregs.
Impact of PTCy on GvL effects
The impact of PTCy on GvL effects was assessed in NSG-HLA-A2/HHD mice (a strain of NSG mice that express in addition to mouse MHC, the human HLA-A0201). All mice were i.v. injected with the human (HLA-A0201) AML cell line THP-1 transfected with the luciferase gene on day 0, while a second injection of THP-1 cells was administered on day 5. Four groups of mice were compared, a group of mice given only THP-1 cells, a second group given THP-1 cells and PTCy on day 3, a third group given THP-1 cells and human PBMC from a non HLA-A2 donor, and a last group of mice given THP-1 cells, PTCy and human PBMC from the same non HLA-A2 donor. On day 34, none of the THP-1+ PBMC mice had detectable tumors while THP-1 + PTCy + PBMC mice had detectable tumors but to a lesser extent that THP-1 only mice or THP-1+PTCy mice. The best survival was observed in THP-1+PTCy+PBMC mice while THP-1+PBMC mice died from xGVHD whereas THP-1+PTCy as well as THP-1 mice died from leukemia.
Conclusions
These results suggest that PTCy prevents xGVHD by depleting proliferative T cells and favoring the recovery of Treg. Further, GvL effects (although decreased) were not abrogated by PTCy.
