Immunogenomic Exploration of the Acute Myeloid Leukemia Microenvironment Identifies Determinants of T-Cell Fitness

Advances in Acute Myeloid Leukemia (AML) research have shown that the bone marrow microenvironment may distinctly vary across disease subtypes, and that this variation is associated with disease risk and response to conventional therapies. Novel therapies aimed at altering the tumor microenvironment, such as T-cell redirection, CAR-T and checkpoint inhibition, are emerging as promising treatment options for AML patients; however, there remains a critical need to determine how response to immune modulation may vary within different subsets of AML. Thus, in collaboration with the Beat AML Consortium, we carried out comprehensive mass cytometry profiling of patient bone marrow samples of nearly 100 Beat AML subjects and characterized their ex vivo response to several immune modulators.

As a complement to this study, we leveraged the Beat AML Consortium dataset (including next-generation sequencing, functional cell-based assays, small molecule screening and clinical information) to investigate connections between disease subtype, immune function and clinical outcome. The mass cytometry time of flight (CyTOF) immune profiling, combined with matched genomic, cytogenetic, and outcome data from the same subjects, provided a unique opportunity to investigate features of the immune environment at single-cell resolution and test for their association with clinical covariates in a large treatment-naïve cohort.

Interestingly, flow cytometry analysis of T-cells isolated from patient bone marrow showed a distinct subset of AML subjects with highly proliferative T-cells and a group of AML subjects with non-proliferative T-cells. To characterize molecular determinants of T-cell function in the AML microenvironment, we compared the transcriptional profiles of tumor specimens from subjects within these two groups. The data revealed a distinct set of differentially expressed genes associated with T-cell proliferation; pathway enrichment analysis indicated that these genes were involved in leukocyte migration, inflammation and response to hypoxia. Genes related to immune function were also enriched, likely due to the presence of immune cell infiltrates and stromal cells in addition to tumor cells from the AML specimens used for RNA-Seq. To estimate the extent of immune and stromal cells in the AML bone marrow, we next computed the approximate cellularity of the RNA-Seq samples using the xCell algorithm. The results of this analysis indicated enrichment of several types of immune cells in the RNA-Seq specimens from the proliferator group, including monocytes, neutrophils and activated dendritic cells. These observations were validated by preliminary results of the CyTOF immune cell profiling of the same subjects.

Ongoing work is focused on the biological interpretation of CyTOF data collected for these subjects, including evaluating the association of functional marker expression on T-cell and myeloid cell populations with T-cell proliferation. Furthermore, we are exploring the functional impact of variation in T-cell fitness and immune cell composition on response to several immune modulators in a series of ex vivo experiments using Beat AML patient samples. Initial findings suggest that for a subset of patients, low baseline levels of T-cell proliferation did not prevent response to immune modulation. We are interrogating the Beat AML dataset for common molecular features of patients in this responder group. Overall, this study evaluates determinants of immune function and variation within the tumor microenvironment of AML patients to advance current knowledge of AML disease biology and to assess the impact of immune fitness on response to immune modulation. These results will contribute to early target identification and development, and importantly shed light on features of the AML bone marrow environment associated with response to therapy.