Marrow Environment Post-Transplant Day (+) 7-9 Following Cyclophosphamide and Fludarabine Minimal Intensity Allotransplantation

Introduction: Cyclophosphamide/Fludarabine (Cy/Flu) nonmyeloablative conditioning for the treatment of acute myeloid leukemia (AML) in remission and myelodysplastic syndrome (MDS) permits engraftment with minimal tissue injury. Unrelated donor (URD) recipients experience rapid neutrophil engraftment, quicker rates to full donor T cell and peripheral blood mononuclear cell (PBMC) chimerism and significantly more effective disease control. Following allogeneic PBMC infusion on day 0, peripheral blood leukocyte counts on day (+) 1 are uniformly less < 200/mm³. Mixed chimeric, predominately donor T and NK lymphocytes are detectable on day (+) 6-8 prior to neutrophil engraftment. The lack of respiratory symptoms or splenic enlargement post-infusion suggested that cells may be detectable in the marrow at a much earlier time point post-transplant than is classically recognized after myeloablative allotransplant. We hypothesized that mature lymphocytes would be detectable in a cellular marrow environment by day (+) 8. To begin to test this hypothesis, we collected marrow between day (+) 7-9 (avoiding weekends) from 6 consenting participants with AML, MDS or chronic lymphocytic leukemia (CLL) who participated in a single institution, phase II clinical trial at Indiana University and the IU Melvin and Bren Simon Cancer Center.

Methods: Marrow aspirate H and E stains, biopsies, and flow cytometric analyses (including CD2, CD3, CD56 and CD19) were obtained pre-transplant and on day (+) 7-9 following PBMC infusion in the setting of minimal intensity cyclophosphamide/fludarabine conditioning. PMBC chimerism was analyzed on day (+) 7-9. Clinical characteristics and peripheral blood cell counts were recorded.

Results: Three patients had AML or MDS and 3 patients had CLL. All patients achieved full engraftment. In the AML/MDS subgroup day (+) 7-9 cellularity ranged from less than 5 to 70% with PBMC chimerism from 39.2 to 93.4% donor. In the CLL subgroup day (+) 7-9 cellularity ranged from 50 to 70% with PBMC chimerism from 0.9 to 5.6% donor. Baseline cellularity, corresponding peripheral cell counts, and marrow flow cytometric analyses from day (+) 7-9 are included in Table 1.

Conclusion: Cy/Flu conditioning appears to be associated with early marrow cellularity at a time when peripheral blood chimerism is mixed. Day (+) 7-9 marrow examinations after Cy/Flu are generally quite cellular. Patients with AML/MDS and CLL appear to differ substantially in histological appearance, cell differentials and flow cytometric analyses. This suggests a unique engraftment kinetic that has yet to be appreciated and may be a useful platform for the study of cellular immune anti-cancer activity.