ATIM-34. SINGLE-CELL RNA SEQUENCING REVEALS DYNAMIC IMMUNE RESPONSE CHANGES IN GLIOBLASTOMA PATIENT WITH DURABLE COMPLETE RESPONSE TO CMV PP65-LAMP RNA-PULSED DENDRITIC VACCINES

Abstract BACKGROUND The application of single cell sequencing as a novel immune monitoring platform can enhance the ability to interrogate at unprecedented depth the single-cell level dynamic phenotypic and functional attributes of immune cells in patients undergoing cancer immunotherapy treatment. We applied single-cell sequencing analysis of PBMCs in a patient with newly-diagnosed GBM enrolled on the ATTAC II clinical trial (FDA IND BB-16530) who experienced a sustained complete radiographic response to autologous CMV pp65-LAMP RNA-pulsed DC vaccines plus GM-CSF and tetanus-diphtheria booster administered during adjuvant cycles of dose-intensified TMZ. METHODS We constructed 5’ gene expression libraries and T cell receptor enriched libraries for 10x Genomics single-cell sequencing, generated from PBMCs collected prior to and during patient immunization using dendritic cells loaded with RNA encoding the CMV matrix protein pp65 conjugated with the lysosomal associated membrane protein (LAMP) sequence. RESULTS RNA-Seq revealed dynamic changes in immune cell subsets over course of first three vaccines, including increases in cytotoxic T cells and memory T cell subsets. Increased markers of T cell activation were observed during vaccination including enhanced T cell signaling pathways, proliferative signals, and cytokine production. We found that proportion of cytotoxic T cells increased from 3.42% to 11.74% in whole PBMCs after immunization. Surprisingly, we observed very high level of frequency natural killer T (NKT) cells comprising 4.75 % of this patient’s PBMCs at baseline. After three DC vaccines, the level of NKT cells in PBMC increased up to 10%. CONCLUSIONS Single cell RNA sequencing of a patient with sustained complete response to DC vaccination during cycles of dose-intensified TMZ reveals dynamic immune activation and expansion within the cytotoxic T cells and NKT compartments. These results emphasize the importance of subset specific profiling to achieve higher resolution in monitoring immune responses compared with bulk expression profiling in patients receiving immunotherapeutic treatment.