Favorable Outcomes in Patients with High Donor-Derived T Cell Count Following In Vivo T Cell Depleted Reduced Intensity Allogeneic Stem Cell Transplantation
Authors: Toor Amir A , Sabo Roy T , Chung Harold M , Roberts Catherine H , Manjili Rose H , Song Shiyu , Williams David C. , Clark William , Manjili Masoud H , McCarty John M 
Published: October 2019

Abstract
Abstract 3043
In patients undergoing T cell depleted stem cell transplantation (SCT) a high rate of mixed chimerism is seen. Further, chimerism analysis does not consistently predict risks of GVHD or relapse upon withdrawal of immunosuppression (IS) in these patients. Thus, a reliable predictor for the expected evolution of mixed T cell chimerism is needed to help in clinical decision-making for withdrawal of IS and donor lymphocyte infusion (DLI). An alternative immune parameter is T cell recovery post-transplant. We combined this measure with T cell chimerism and examined the predictive value of a calculated donor-derived T cell count for clinical outcomes following SCT.
Patients were enrolled in a randomized clinical trial examining two different doses of rabbit anti-thymocyte globulin (ATG 2.5 or 1.7 mg/kg/day; Thymoglobulin®, Genzyme, Cambridge, MA) given IV on day –9 through –7, followed by total body irradiation (4.5 Gray), administered in 3 doses on day –1 and 0 (NCT00709592). Donor engraftment was measured by PCR of short tandem repeat alleles in each donor and patient at 4, 8, 12, and 24 weeks following SCT on whole blood, granulocytes, and total T cells. Between 2008 and 2011, 25 patients were enrolled in this trial, and 22 patients were eligible for this analysis. Nine of the 22 patients had mixed T cell chimerism (≥5% recipient DNA) at 8 weeks post-transplant, and of these 33% (n=3) became fully donor T cell chimeric after withdrawal of IS over the ensuing weeks. One of the 9 patients had improved donor chimerism after DLI. Of the 13 patients who were full donor chimeric in T cells, one patient reverted from full donor to mixed chimerism.
Donor-derived CD3+ T cell count (dd CD3) was calculated from T cell chimerism and absolute blood CD3+ T cell count. Median dd CD3+ cell count at 8 weeks following SCT was 433 ƒýL (range: 3–2464). After calculating the sum of receiver operating characteristic area under the curves (AUC), a dd CD3 of 110ƒýL was found to be the optimal cut-off value with both the highest AUC sum and the highest AUC for each measure (cumulative acute and chronic GVHD: 0.79, remission: 0.74, whole blood chimerism: 0.88), and was subsequently used to distinguish between low (<110; n=8) and high (>110; n=14) values of dd CD3. A significant correlation was found between this single dd CD3 level discriminator and higher rates of cumulative GVHD (Fishers Exact Test: p = 0.024), remission (p = 0.0524) and full donor whole blood chimerism at ≥12 weeks following SCT (p < 0.0001) in patients with high-dd CD3 cell counts. Furthermore, Bayesian analysis showed higher whole blood mixed chimerism rates (Posterior Probability = 0.99), lower GVHD rates (PP = 0.99), and lower remission rates (PP = 0.99) in the low-dd CD3 group. Both overall survival and time to relapse favored the high-dd CD3 group (vs. low-dd CD3 group). However, while the relationship was significant for relapse (p = 0.028), it was only marginally significant for overall survival (p = 0.07). When measured after withdrawal of IS, declining donor or persistent mixed T cell chimerism was observed in patients who had mixed T cell chimerism and low- dd CD3 at eight weeks post SCT (Fig 1A), whereas those with high- dd CD3 had increasing donor T cell chimerism (Fig 1B). Granulocyte chimerism too was modulated by dd CD3 in patients with mixed chimerism and early DLI arrested the decline of donor hematopoiesis in a patient with low-dd CD3. NK cell counts were significantly higher in the high-dd CD3 group than in the low-dd CD3 group at 8 weeks (p = 0.044; PP = 0.99). No significant relationships were found between dd CD3 levels and donor age, donor type, total ATG dose, and infused graft CD34+ or CD3+ cell dose.Figure 1AFigure 1A.
In conclusion, we report the effect of an easily calculable measure of donor T cell reconstitution on clinical outcomes following SCT (particularly GVHD), stability of engraftment and disease relapse. Donor-derived CD3+ cell count may help in the decision-making regarding IS withdrawal and DLI timing in allogeneic SCT recipients. We will be confirming the optimal dd CD3 cell count parameter in larger patient cohorts with comparable disease biology for validation in regimens of varying myeloablative intensities.

Disclosures:
Toor: Genzyme: Research Funding. Off Label Use: ATG in stem cell transplantation.