Mass Cytometric Analysis Revealed Dynamic Alteration of the Tumor Immune Environment in Bone Marrow from Children with Recurrent B Cell Precursor Acute Lymphoblastic Leukemia

Introduction

Due to the considerable success of cancer immunotherapy for leukemia, the tumor immune environment (TIE) has become a focus of intense research; however, there are few reports on the dynamics of the TIE in leukemia, especially in bone marrow (BM), the primary site of leukemia. Mass cytometry, which allows high-dimensional analysis with single-cell resolution, is a powerful tool for the characterization of the TIE, and enables us to examine pediatric BM samples containing only a few numbers of immune cells.

Methods

Primary and recurrent BM samples were collected serially from pediatric patients with BCP-ALL at Kyoto University Hospital from 2006 to 2013. Mononuclear cells were isolated by density centrifugation and viably preserved until they were used. We examined the TIE of pediatric B cell precursor acute lymphoblastic leukemia (BCP-ALL) by analyzing serial BM samples from patients in primary and recurrent disease phases by mass cytometry, using 39 immunophenotype markers, and transcriptome analysis.

Results and Discussion

The proportion of BCP-ALL cells in the samples was 87.3-97.8% (mean: 94.0%) at onset, and 81.3-98.8% (mean: 92.2%) at relapse. High-dimensional single-cell analysis by mass cytometry elucidated a dynamic shift of T cells from naïve to effector subsets, and clarified that the TIE during relapse comprised a T helper 1 (Th1)-polarized immune profile, together with the increase of effector regulatory T cells (Tregs). Th1 cells are typically known as supporters of cytotoxic T lymphocytes which would eliminate leukemia cells and work against relapse, but recently, it is reported that Th1 cells directly support ALL proliferation in vitro (Traxel et al. Oncogene. 2019; 38:2420-2431), and that the concentrations of pro-inflammatory cytokines and Th1 cytokines (IFN-γ and IL-12) were elevated in patients with ALL, which could create favorable conditions for ALL (Perez-Figueroa et al. Oncol Rep. 2016; 35: 2699-2706, Vilchis-Ordonez et al. Biomed Res Int. 2015; 2015: 386165). Therefore, there is a possibility that the Th1-polarized immune profile would work in favor of leukemia survival for relapse.

Furthermore, Gene set enrichment analysis based on RNA expression identified the enrichment of six immune-related pathways were enriched at the time of relapse; chemokine activity, chemokine production, complement activation, positive regulation of cytokine production involved in immune response, positive regulation of lymphocyte chemotaxis, and positive regulation of lymphocyte migration. These expression signatures suggest that BCP-ALL cells attract lymphocytes, and upregulate immune activities at relapse.

Conclusion

In summary, a TIE characterized by a Th1-polarized immune profile, with the increase of effector Tregs, may be involved in the pathophysiology of recurrent ALL, and BCP-ALL cells at relapse were enriched with gene expressions related to lymphocyte attraction and activities. This information could contribute to the development of effective immunotherapeutic approaches against BCP-ALL relapse.