Development of Chronic Lymphocytic Leukemia (CLL) Reactive Cytotoxic T Lymphocytes after Non-Myeloablative Hematopoietic Stem Cell Transplant Correlates with Anti-Leukemia Response.

Allogeneic hematopoietic stem cell transplantation (HSCT) following non-myeloablative (NM) conditioning is a promising approach for treating patients with advanced fludarabine refractory CLL. In this setting, a graft versus leukemia (GVL) effect mediated by donor T cells is critical for tumor eradication. We have evaluated the development of alloreactive and CLL-reactive cytotoxic T lymphocyte (CTL) responses in patients after NM-HSCT to determine if the generation of detectable T cell responses was associated with an antitumor response. Seven patients with fludarabine refractory CLL were conditioned with fludarabine (30mg/m2 x 3 doses) and total body irradiation (2 Gy) prior to receiving G-CSF mobilized peripheral blood stem cells from an HLA matched donor. Peripheral blood mononuclear cells (PBMC) were obtained from the recipient pretransplant and at intervals after NM-HSCT. When chimerism showed a major proportion of donor CD3+ T cells, the postransplant PBMC were stimulated in vitro with recipient CLL cells from the pretransplant collections. CLL cells lack or express low levels of co-stimulatory and adhesion molecules, and are poor stimulators of T cells in vitro. Thus, prior to their use as stimulators and targets, the CLL cells were activated with CD40 ligand (CD40L), which upregulates costimulatory, adhesion, and MHC molecule expression, and turns CLL cells into effective antigen presenting cells. The cultures were stimulated weekly and supplemented with IL2 and IL7. After two stimulations, the T cell lines were tested for cytotoxicity against donor and recipient target cells including recipient CLL. T cell lines generated from four patients with a good antitumor response after NM-HSCT exhibited cytotoxicity against recipient CLL and EBV transformed B cells (B-LCL), but not against donor B-LCL. By contrast, T cell lines generated from three patients with persistent or progressive disease after NM-HSCT did not have cytotoxicity against recipient CLL, despite the development of GVHD in all patients. Multiparameter flow cytometry and IFN-g secretion assay of T cell lines from patients with an antitumor response showed that both CD8+ and CD4+ T cells produced INF-g in response to recipient CLL. We sorted and expanded CD8+ INF-g+ and CD4+ IFN-g+ T cells and both subsets were able to lyse CLL cells. The cytotoxicity of CD4+ and CD8+ T cells was inhibited completely by concanamycin A, suggesting perforin is the major mechanism for leukemia cell lysis. Twenty-one CD8+ T cell clones specific for distinct minor histocompatibility antigens expressed on CLL were isolated from T cell lines of the four responding patients. Multiple specificities were recognized in three of the four patients. Screening a cDNA expression library has identified the genes encoding two minor histocompatibility antigens recognized by CD8+ T cells, and their characterization is in progress. These findings suggest that the development after NM-HSCT of early, diverse, alloreactive T cell responses specific for antigens expressed by CLL may be an important predictor of outcome. The identification of the antigens recognized may facilitate the development of strategies to evoke an effective antitumor response in a larger fraction of patients.