Characterization of lymphocytes emerging early after nonmyeloablative conditioning and hematopoietic stem cell transplant supported with sirolimus (P2186)

Abstract Hematopoietic stem cell transplant is a curative option for patients with severe congenital anemias such as Sickle Cell Disease. Due to the higher transplant-related mortality associated with myeloablative conditioning and the reversal of the SCD phenotype achieved with as low as 11% stable donor chimerism, we developed a successful non-myeloablative regimen for patients with matched-sibling donors consisting of pre-transplant alemtuzumab, 300cGy TBI, and post-transplant sirolimus. However, mechanisms that promote tolerance in these patients remain unclear. We hypothesized that lymphocytes emerging early post-transplant would display a toleragenic phenotype. Samples were thawed from time points near day 60 or day 100 post-transplant. 17-parameter flow cytometry was performed, revealing dramatically high proportion of Tregs (6.3%-32.8%) and T-helper-17 (Th17) (8.2-21.2%) cells, elevated markers of proliferation, activation, and B cell maturity in engrafting patients. Using multiplex cytokine immunoassay, 67 analytes were measured, revealing high expression of IL-17A in the engrafting patients, MCP-1 in the rejecting patients, but little difference in Treg/Th1/Th2-associated cytokines. These unexpected observations have provided insight into reconstitution following nonmyeloablative conditioning and sirolimus-based immunosupporession and suggest targets for further investigation into both the long-term, stable mixed chimerism and the lack of GvHD observed in our patients.