Early Post-Transplant Whole Tumor Cell Vaccination Elicits Anti-Tumor T Cell Responses in Patients With Advanced Chronic Lymphocytic Leukemia (CLL)

Abstract

Vaccination with whole leukemia cells early after allogeneic hematopoietic stem cell transplantation (HSCT) may facilitate expansion of anti-tumor responses. We tested this hypothesis by initiating a phase I study to vaccinate CLL patients between days 30-100 following reduced-intensity (RIC)-HSCT. One cycle of 6 vaccines consisted of 1x107 irradiated autologous CLL cells admixed with irradiated K562 bystander cells secreting the cytokine adjuvant GM-CSF. Of 20 enrolled patients, 15 initiated vaccination. Four of 20 developed GvHD before day 45, precluding vaccination. At a median of 21 months follow-up, all vaccinated patients remain disease-free--an improvement, compared with a historic relapse rate of 50% at 2 years for CLL patients undergoing RIC-HSCT at our center. We thus compared T cell immune reconstitution of study subjects with CLL patients who had undergone RIC-HSCT without vaccines. Recovery of peripheral CD3+ T cell numbers until d100 was similar between vaccinated subjects and 15 transplanted/non-vaccinated CLL controls. Specificity of the reconstituted T cells in vaccinated patients, however, was consistently directed against autologous tumor cells. Five of 5 vaccinated patients demonstrated increased circulating IFN-γ secreting CD8+ T cells by ELISPOT against autologous tumor (mean of 516 vs 184 spots/106 CD8+ T cells at d60 (post-vaccine) vs d30 (pre-vaccine)), but not against autologous hematopoietic cells (PHA blasts, 57 spots/106 CD8+ cells, d60) nor autologous non-hematopoietic cells (fibroblasts, 108 spots/106 CD8+ cells, d60). In contrast, 5 of 5 controls (+HSCT, no vaccine) did not have expanded anti-tumor responses at d60 (mean 51 spots/106 CD8+ cells). Moreover, with GvHD, T cell reactivity developed against autologous PHA blasts and fibroblasts (mean 438 and 434 spots/106 CD8+ cells at d90, respectively). After vaccination, tumor-reactive CD8+ T cells were consistently polyfunctional, secreting high levels of GM-CSF, TNF-α, and IL-2. By limiting dilution, we isolated T cell clones from PBMC of 4 vaccinated subjects, and confirmed that 30-50% of T cell clones demonstrated specific recognition of autologous CLL, but not of alloantigen-bearing cells. Our studies reveal that early post-transplant CLL/GM-K562 vaccination is associated with induction of immunity against recipient CLL cells, and suggest that this is an effective strategy for tipping the balance of immunity in favor of GvL following nonmyeloablative HSCT.