A Dendritic Cell Based Vaccination Strategy Geared towards Eradication of Leukemic Stem Cells.
Authors: Bontkes Hetty J , Ruben Jurjen , van den Ancker Willemijn , Westers Theresia M , Ossenkoppele G. , van de Loosdrecht A. A 
Published: October 2019

Abstract
Abstract 2046
Poster Board II-23

Introduction:
In the majority of cases, initial remission of acute myeloid leukemia (AML) is reached but unfortunately relapse rates remain high and therefore novel treatments are needed. It is thought that recurrent AML originates from chemotherapy resistant quiescent leukemic stem cells (LSC). The application of immunotherapeutic approaches to eradicate LSC remaining after first line chemotherapy may contribute to improved disease outcome. Vaccination strategies have often used dendritic cells (DC) ex vivo pulsed with tumor-derived whole lysates or peptides as modalities to present a broad range of tumor antigens to T cells to stimulate effective anti-tumor T-cell immunity in vivo. It is likely that certain proteins expressed by LSC have a distinct antigenicity as compared to more mature AML blasts and thus provide targets for specific T-cells. Even without identification of specific antigens, LSC can be a useful source of tumor antigens in DC vaccination-based immunotherapy. CD34+CD38- LSC can be identified using malignant stem cell associated cell surface markers including CLL-1 and lineage markers such as CD7, CD19 and CD56. However, the low frequency of these cells precludes the use of LSC derived apoptotic cells or lysates for DC loading. Alternatively, mRNA isolated from LSC can be amplified and subsequently transfected into DC.

Materials and Methods:
We have made use of the CD38- AML derived cell line MUTZ-3 which contains a subpopulation of CD34+CLL1+ cells which resembles the phenotype of a putative LSC. CLL1+CD34+ and CLL1-CD34- cells were isolated by FACS sorting and total RNA was isolated. mRNA was converted to cDNA and amplified by PCR using the SMART system. Subsequently, mRNA was in vitro transcribed from the amplified cDNA. Mature monocyte derived DC (MoDC) were generated from healthy donor blood and transfected with amplified CLL1+CD34+ derived mRNA and used to stimulate autologous CD8β+ T-cells. After three weekly re-stimulations with CLL1+CD34+ mRNA transfected DC, specificity of the T-cells was analyzed by intracellular IFNγ staining upon 5 hour stimulation with autologous immature MoDC transfected with GFP mRNA, mRNA amplified from unsorted, CLL1+CD34+ or CLL1-CD34- MUTZ-3 subpopulations.

Results:
Amplification of CLL1 and survivin (also expressed by MUTZ-3) transcripts was confirmed by RT-PCR. After 3 weekly re-stimulations with CLL1+CD34+ amplified RNA transfected DC, 0.04% (range 0.01-0.12%) of the T-cells were positive for IFNγ upon a 5 hr re-stimulation with GFP transfected DC. 0.44% (range 0.04-0.69%) of the T-cells responded to DC transfected with unsorted MUTZ-3 amplified mRNA (p<0.00005 versus GFP control, 2-sided student's T-test), 0.51% (range 0.24-1.35%) responded to DC transfected with CLL1+CD34+ amplified mRNA (p<0.005 versus GFP control) and 0.46% (range 0.24-0.94%) responded to DC transfected with CLL1-CD34- amplified mRNA (p<0.0001 versus GFP control).

Conclusion:
We show that MoDC transfected with RNA amplified from one MUTZ-3 sub-population resembling the phenotype of LCS cells are capable of inducing T-cells which recognize both cells transfected with mRNA from the LSC resembling MUTZ-3 subset as well as the CLL1-CD34- subset. We are currently testing the efficacy and feasibility of this approach in an autologous setting in vitro. CD8β+ T-cells are stimulated with autologous MoDC from AML patients transfected with amplified mRNA isolated from their own LSC enriched populations. The capacity of these T-cells to kill autologous AML blasts and LSC is subsequently analysed in a 6-colour FACS based cytotoxicity assay.

Disclosures:
No relevant conflicts of interest to declare.