A multidimensional analysis reveals distinct immune phenotypes and the composition of immune aggregates in pediatric acute myeloid leukemia.

Because of the low mutational burden and consequently, fewer potential neoantigens, children with acute myeloid leukemia (AML) are thought to have a T cell-depleted or 'cold' tumor microenvironment and may have a low likelihood of response to T cell-directed immunotherapies. Understanding the composition, phenotype, and spatial organization of T cells and other microenvironmental populations in the pediatric AML bone marrow (BM) is essential for informing future immunotherapeutic trials about targetable immune-evasion mechanisms specific to pediatric AML. Here, we conducted a multidimensional analysis of the tumor immune microenvironment in pediatric AML and non-leukemic controls. We demonstrated that nearly one-third of pediatric AML cases has an immune-infiltrated BM, which is characterized by a decreased ratio of M2- to M1-like macrophages. Furthermore, we detected the presence of large T cell networks, both with and without colocalizing B cells, in the BM and dissected the cellular composition of T- and B cell-rich aggregates using spatial transcriptomics. These analyses revealed that these aggregates are hotspots of CD8+ T cells, memory B cells, plasma cells and/or plasmablasts, and M1-like macrophages. Collectively, our study provides a multidimensional characterization of the BM immune microenvironment in pediatric AML and indicates starting points for further investigations into immunomodulatory mechanisms in this devastating disease.

A multidimensional analysis reveals distinct immune phenotypes and tertiary lymphoid structure-like aggregates in the bone marrow of pediatric acute myeloid leukemia.

Because of the low mutational burden, children with acute myeloid leukemia (AML) are thought to have a 'cold' tumor microenvironment and consequently, a low likelihood of response to T cell-directed immunotherapies. Here, we provide a multidimensional overview of the tumor immune microenvironment in newly diagnosed pediatric AML. On a cohort level, we demonstrate wide variation in T cell infiltration with nearly one-third of cases harboring an immune-infiltrated bone marrow. These immune-infiltrated cases are characterized by a decreased abundance of M2-like macrophages, which we find to be associated with response to T cell-directed immunotherapy in adult AML. On an organizational level, we reveal the composition of spatially organized immune aggregates in pediatric AML, and show that in the adult setting such aggregates in post-treatment bone marrow and extramedullary sites associate with response to ipilimumab-based therapy. Altogether, our study provides immune correlates of response to T cell-directed immunotherapies and indicates starting points for further investigations into immunomodulatory mechanisms in AML.