Does Early T Cell Chimerism Predict Outcomes after Allogeneic Hematopoietic Cell Transplantation ?
Authors: Jain Tania , Mountjoy Luke , Kunze Katie L. , Noel Pierre , Khera Nandita , Leis Jose F. , Slack James L. , Sproat Lisa Z. , Palmer Jeanne 
Published: October 2019

Abstract
Introduction: Donor T cell chimerism is tested frequently early after allogeneic hematopoietic cell transplantation (HCT). However, the predictive ability of testing this early after HCT, i.e. at day +30 or day +60, has not been well described, outside of smaller single center studies. Herein, we evaluate the correlation of early donor T cell chimerism with clinical outcomes of overall survival (OS) and relapse in patients undergoing HCT for various hematological malignancies.
Methods: We identified patients who received HCT with 8/8 matched unrelated donors at our center between 2007 and 2016 and obtained the clinical data retrospectively. Chimerism studies were included for patients who had testing done at day+30 and/ or day +60. Chimerism is tested using PCR based technique. Due to limited variability in chimerism levels <100% donor chimerism, a cut-off of 100% was used for analysis. Hence, patients were divided into those who achieved full donor chimerism in CD3 fraction (100%) and those in whom CD3 donor chimerism was < 100% at the respective time points. Clinical outcomes evaluate for this abstract were OS and time to relapse. Univariate and multivariate Cox proportional hazard regression models were used using variables of interest, the later adjusted for age at HCT, use of ATG, conditioning regimen and disease risk index (DRI).
Results:
Baseline patient and HCT data: There were 209 patients who met the selection criteria of which 4 died within 30 days from HCT and hence, were not included in the analysis. Among the 205, 87 (42%) were women and median age at HCT was 55 (range, 18 - 74) years. The hematological malignancy diagnosis was acute myeloid leukemia (AML)/ myelodysplastic syndrome (MDS) in 112 (55%), acute lymphoblastic leukemia (ALL) in 35 (17%), chronic leukemias in 13 (6%), lymphomas in 13 (6%) and myeloproliferative neoplasm in 20 (10%); with DRI being very high in 9 (5%), high in 53 (27%), intermediate in 120 (60%) and low in 17 (9%) patients. Myeloablative conditioning was used in 81 (40%) and reduced intensity conditioning in 124 (60%). Graft versus host disease prophylaxis used was calcineurin inhibitor with methotrexate of mycophenolate in all patients while anti-thymocyte globulin was used in 125 (61%) patients. All patients received a peripheral blood graft.
Outcomes: Median follow-up for all included patients was 615 (range, 30 - 3147) days. At day +30, 138 patients had achieved 100% donor CD3 chimerism of 189 patients in whom this data was available (73%) while at day +60, 131 (76%) out of 173 had 100% donor CD3 chimerism. In the univariate model, CD3 donor chimerism level at day +30 as well as day +60, was not predictive of OS (HR 1.5, 95% CI 0.82 - 2.9; p = 0.18 for day +30, HR 1.8, 95% CI 0.81 - 4.0, p = 0.15 for day +60). As for time to relapse, CD3 chimerism did not significantly affect the risk of relapse at these time points (HR 0.43, 95% CI 0.17 - 1.1, p = 0.08 for day +30; HR 0.55, 95% CI 0.18 - 1.6, p = 0.28 for day +60). In the multivariate model also, after adjustment as described above, CD3 at day +30 or at day +60 was not significantly associated with the risk of OS or time to relapse (Figure 1). Kaplan Meier curves for OS for CD3 chimerism at day +30 and day +60 are shown in Figure 2.
Conclusions: In this study, T cell chimerism as early as day +30 or day +60 does not seem to predict OS or relapse in patients undergoing HCT for various hematological malignancies. Given the limitations of a smaller retrospective design, this must be validated in a larger study, possibly using registry data. If these findings are indeed confirmed, then the practice of testing chimerism early and frequently, that adds additional expense to the overall cost of HCT, must be reconsidered.

Disclosures
Palmer: Novartis: Research Funding.