4-1bb Enrichment Enhances Adoptive T Cell Therapy for Hematological Malignancies Using a Novel Approach for Ex Vivo Immune Cell Priming with DC/Tumor Fusion Vaccine

Introduction: While CAR T cells have transformed outcomes for patients with hematologic malignancies, application to patients with acute myeloid leukemia (AML) is limited by lack of surface antigens that differentiate between malignant and normal hematopoietic precursors. We have developed a personalized vaccine created by fusion of patient-derived tumor with autologous dendritic cells (DCs). The vaccine presents an array of tumor antigens stimulating polyclonal immunity. In a phase II clinical trial of patients with AML in first remission, vaccination with DC/AML fusions resulted in expansion of leukemia specific T cells associated with durable disease remission. The ex vivo generation of vaccine educated T cells provides a powerful substrate of effector cells for adoptive immunotherapy that are selective and capture tumor heterogeneity. This novel adoptive cell therapy enhances T-cell potency with potential to mediate disease regression. Enrichment of vaccine-educated T cells for activated antigen-specific effector cells via agonistic 4-1bb antibody-based selection further enhances for activation and memory cell phenotype; superior T cell efficacy was observed in AML and MM models.

Methods: DC/tumor fusion vaccines were generated from C57BL/6J mice DCs and syngeneic C1498 mCh/luc⁺ AML cells or C57BL/KaLwRij and syngeneic 5TGM1 mCh/luc⁺ MM cells, respectively. T cells isolated from spleen were co-cultured with autologous irradiated DC/tumor fusions in presence of IL-2/7/15. Vaccine-educated T cells underwent selection with agonistic 4-1bb (3H3) followed by expansion with anti-CD3/CD28 activation beads. T cells were phenotyped for markers of activation (CD25/CD69), immune checkpoint (PD1/LAG3/TIM3), and memory (CD44+CD62L-) and for enrichment (anti-rat H&L). Cytotoxicity was evaluated on luminescent assay. Mice engrafted with C1498 mCh/luc⁺ received the T cell therapy 7 days later. Disease progression and survival was monitored for 100 days by BLI.

Results: In the C1498 model, vaccine-educated T cells demonstrated evidence of immune activation (CD25+CD69+) and memory (CD44+CD62L-) phenotype compared to unstimulated naïve T-cell controls (TN) (4-fold, p=0.1143; 1.3-fold, p=0.0195, respectively). Vaccine-educated T cells selected based on 4-1bb expression showed 9-fold higher expression of activation markers (p=0.0034) while memory markers were 4.4-fold higher compared to TN (p=0.0274). Selection enriched for 4-1bb⁺ vaccine-educated T cells resulting in enhanced antigen-specific recognition as measured by induction of IFNg expression. Tumor specificity and activation was maintained following CD3/CD28-mediated expansion. The 4-1bb positive vaccine-educated T cells showed 11-fold enhanced cytotoxicity compared to TN at 10:1 E:T (P=0.0005), while vaccine-educated T cells showed 4-fold increase (P=0.0226). Phenotypic and functional analysis support 4 days as the optimal duration of time for T-cell vaccine education. Cell fold-expansion supports 3-5 days as the optimal duration for anti-CD3/CD28 expansion. In vivo, 60% of mice treated with 4-1bb⁺ vaccine-educated cells were alive at 60 days vs 20% treated with unselected vaccine-educated cells. In the 5TGM1 model, activation, memory, immune checkpoint phenotype and cytotoxicity were increased in vaccine-educated T cells selected for 4-1bb expression.

Conclusion: Vaccine-educated T cells represent a unique platform for adoptive immunotherapy for AML further enhanced by selection for 4-1bb expression by agonistic antibody. T-cell stimulation by fusion vaccine and subsequent enrichment by agonistic 4-1bb selection enhances cytotoxicity, activation and memory cell phenotype. Thus, ex vivo vaccine stimulation and 4-1bb selection presents a novel approach for cell-based immunotherapy for AML.