Differential T Cell Suppression by Two Different Pentostatin Conditioning Protocols for Nonmyeloablative Allogeneic Stem Cell Transplantation (NST).
Authors: Coskun Ugur , Bociek Robert G. , Pavletic Zivko S. , Kuszynski Charlie , Devetten Marcel , Shafer Laura , Zobel Lori , Talmadge James E. 
Published: October 2019

Abstract
NST is increasingly being used to introduce a graft-versus-malignancy (GVM) effect with reduced regimen related toxicity. The first NST protocol, which accrued patients (pts) between 9/01 and 7/04, 39 pts with high risk/relapsed/refractory hematopoietic malignancies who were not candidates for full intensity allogeneic SCT underwent NST using Pentostatin/TBI. Their median age was 52 years (range 22–70). The median number of prior therapies was 4 (range 0–8) including prior autologous SCT in 22 pts. Conditioning consisted of Pentostatin 4 mg/m2 daily on day -21, -20, and -19, followed by 200 cGy TBI on day -1. The second protocol, between 10/04 and 6/06, 24 pts received the same conditioning regime on days -10, -9, and -8, and TBI on day -1. The median age of these pts was 59.5 years (range 36–70) and the median number of prior therapies was 2 (range 0–6). Post-grafting immunosuppression was cyclosporine (CsA)/mycophenolate mofetil (MMF). In the both protocols CsA 2.0 mg/kg IV q12 hrs was administered on days -1, 0, and +1 and then converted to oral 5 mg/kg PO BID with a taper beginning on day +70. In the second protocol pts with unrelated donor transplants remain on CsA until day + 100. MMF at 15 mg/kg PO BID was administered on days 0–27 for related donor transplants in both protocols and in the second protocol this was extended until day +40 for the unrelated donor transplants.
Results: In the day -21 protocol SCTs were from matched related (n=14) or unrelated (n=25) donors. The cumulative incidence of all grades of acute graft-versus-host disease (GVHD) at day 100 was 40% and was more common in unrelated donor transplants (60% vs. 15%). In the day -10 protocol, SCT was performed with products from matched related (n=8) or unrelated (n=16) donors. The cumulative incidence of grade II–IV acute GVHD was approximately XX% (XX% in related versus XX% in unrelated donors). In both protocols, the median chimerism values for CD3+ cells and WBC at day 28 were >80% and 90% donor cells, respectively and T, B and NK lymphocyte frequency and numbers were identical at protocol entry. One day following TBI conditioning there was a 32% depression in CD3+ cells in the day -21 protocol that was partially recovered on days 28 and 70 post transplant (27% and 16% respectively). In the day -10 protocol there was a 45% depression in the frequency of CD3+ cells (p<0.05) on day following TBI, which was depressed further (72%) on day 28 post transplant with partial recovery on day +70 (24% depression). In both protocols the CD8+ T cells were depressed at these time points; however, the CD4+ cells were only depressed in the second protocol. Similarly, by day 28 post transplant the frequency of monocytes, granulocytes and NK cells had recovered to pre-transplant levels. The DCs, both CD123+ and CD11c+ were not suppressed by the conditioning regime, although 70 days following transplant in the day -10 protocol the CD123+ cell frequency was significantly.
Conclusions: There appears to be a trend for a lower incidence of acute GVHD in the day -10 protocol, which can be associated with a reduction in the frequency of circulating T cells. This significantly greater reduction in T cell frequency, observed one day following TBI and 28 days post transplant, is directly associated with the timing of transplant relative to pentostatin conditioning as the post-grafting immunosuppression was similar between the two protocols at these time points.