Regulatory and Activated T Cells in Stem Cell Autografts Collected by Mobilization with High-Dose Cyclophosphamide and Granulocyte Colony Stimulating Factor.

High-dose chemotherapy (HDC) supported by autograft of hematopoietic progenitors (HP) is a standard therapy for patients with multiple myeloma (MM). High-dose cyclophosphamide (CTX) and G-CSF are widely used to collect HP. As the number of lymphocytes in the autograft is a powerful prognostic factor in patients with MM, our purpose was to study how CTX-G-CSF treatment affects the phenotype and function of T cells, in particular regulatory T cells (Treg), in 15 patients with MM. CTX induced severe T cell immunosuppression with a slow and partial T-cell recovery (a threefold decrease) at the time of HP collection. CTX-G-CSF treatment did not affect the percentages of central memory (CD45RA⁻, CCR7⁺), effector memory (CD45RA⁻, CCR7⁻), and late effector (CD45RA⁺, CCR7⁻) CD4 or CD8 T cells but a decrease of naïve CD4 cells (CD45RA⁺, CCR7⁺) was found. The percentages of CD25⁺ cells increased two- to threefold in CD4 or CD8 T cells, respectively. Post-CTX treatment CD4CD25⁺ cells included both activated CD4CD25^low cells and CD4CD25^high T cells. The latter were Treg because they expressed high level of FOXP3 and membrane CTLA-4 mRNA and protein and displayed functional suppressor function. In CTX-G-CSF leukaphereses from 15 patients with MM, the mean Treg number was one fifth that of CD34 and the CD3, CD4 and CD8 numbers respectively 3 fold, 2 fold and equal that of CD34. Post-CTX-G-CSF treatment CD3 cells did not cell cycle in vivo and died in short-term culture in vitro. Adding IL-2 or IL-15 induced their survival and cell cycle, and stimulation with anti-CD3 MoAb led to efficient growth in vitro. These results suggest that following CTX-G-CSF treatment, CD3 cells are preactivated in vivo and do not cell cycle, likely due to a lack of T cell growth factors in vivo. The current data indicate that CTX-G-CSF treatment profoundly affects T cell function without eliminating Treg. The persistence of Treg could be explained by an opposite effect of CTX known to kill Treg and of G-CSF amplifying Treg. Given the major impact of lymphocyte count on patients’ survival post HDC and HP and T cell graft, the present data invite to define novel therapeutic strategies to improve T cell recovery in vivo while limiting Treg expansion.