Dynamical Systems Modeling of Early-Term Immune Reconstitution with Different Anti-Thymocyte Globulin (ATG) Administration Schedules in Allogeneic Stem Cell Transplantation

Alloreactivity forms the basis of allogeneic hematopoietic cell transplantation (HCT), with donor derived T cell response to recipient antigens mediating clinical responses either in-part or entirely. These encompass the different manifestations of graft vs. host disease (GVHD), infection risk as well as disease response. Whilst the latter is contingent upon disease biology and thus may be less predictable, the former two are more likely to be directly proportional to the magnitude of donor derived T cell recovery. Herein we explore the quantitative aspects of immune cell recovery following allogeneic HCT and clinical outcomes in two cohorts of HLA matched allograft recipients who received rabbit anti-thymocyte globulin (ATG) on different schedules (days -9 to -7 vs. -3 to -1). Monocyte as well as donor derived T cell (ddCD3) recovery was superior in those given ATG early in their course (days -9/-7). This difference was related to a more rapid rate of ddCD3 recovery, largely driven by CD3+/8+ cells in the first month following transplantation. Early monocyte recovery was associated with later T cell recovery and improved survival. In contrast rapid and early ddCD3 expansion out of proportion to monocyte recovery was associated with a high likelihood of acute GVHD and poor survival. This analytic methodology demonstrates that modelling ‘early-term immune reconstitution' following HCT yields insights that may be useful in management of post-transplant immunosuppression and adaptive cellular therapy to optimize clinical outcomes.