A Blood Dendritic Cell Vaccine for Acute Myeloid Leukemia

Many acute myeloid leukemia (AML) patients achieve a complete remission (CR) with chemotherapy but relapse is common. Removal of residual disease remains the greatest challenge. Allogeneic transplantation (alloHCT) addresses this through an immune-mediated graft versus leukemia effect (GVL), but has high morbidity and mortality. Therapeutic dendritic cell (DC) vaccination has the potential to provide immune control with limited toxicity. Previous trials using monocyte-derived DC (Mo-DC) have demonstrated modest clinical effects. This is understandable as Mo-DC have demonstrably poor migration in vivo and relatively inferior antigen processing and presentation compared to blood dendritic cells (BDC). We have developed a more practical, functionally superior vaccine composed of natural blood DC (BDC). This is achieved using the human-mouse chimeric monoclonal antibody CMRF-56 to enrich BDC from patient peripheral blood after a short incubation.We assessed the potential for preparing a CMRF-56+ BDC vaccine from AML patients in CR.

We developed an extended flow cytometry panel to distinguish different BDC subsets from blasts in AML, sorted them to confirm morphology, then used TruCount methodology to enumerate them at diagnosis, post-chemotherapy (5-28 weeks) and post alloHCT. We correct previous reports that suggested BDC numbers are normal at AML diagnosis by demonstrating that the Lineage^- HLA-DR⁺ CD11c⁺ cells commonly classified as myeloid DC contain myeloblasts. Exclusion of myeloblasts, revealed that CD1c and CD141 BDC are grossly depleted at AML diagnosis to 567/mL and 24/mL, 4% and 3% respectively of the the levels of healthy aged-matched controls (HC) (n=9; n=13), but recovered to 7323/mL and 294/mL, representing 57% and 39% HC levels (n=12) during CR1, and to 10282/mL and 299/mL, representing 80% and 40% of HC after alloHCT (n=6). In contrast, plasmacytoid dendritic cells (pDC) levels were 2229/mL and 27% of HC at diagnosis, but failed to recover further remaining at 1453/mL or 18% of HC at CR1 and at 1986/mL and 24% of HC post alloHCT. CD1c BDC from AML patients in CR upregulated the CMRF-56 antigen,. similarly to HC (n=5, p=0.4) but primary AML blasts did not, enabling myeloblast free, CMRF-56+ BDC purifications. CMRF-56+ BDC isolated from AML patients in CR expanded anti-viral and Wilms' Tumour 1-specific autologous CD8+ T cells in vitro. However, patients who failed standard induction chemotherapy and required fludarabine-containing salvage regimens produced good CMRF-56+ BDC preparations but did not expand functional T cells.

These data support the feasabillity of preparing a functional BDC vaccine from AML patients in CR using CMRF-56 immune selection and highlight the potential detrimental effects of specific chemotherapeutics on cellular therapy. BDC vaccination may consolidate chemotherapy induced CR in AML, or enhance GVL post alloHCT, by stimulating specific immune responses to control residual disease.