Abstract 1353: CD71 and CD117 identifies an early cancer associated macrophage progenitor population in leukemic mouse bone marrow (2023)

Abstract Tumors emerge and evolve in complex and dynamic microenvironments that influence their growth, invasion, and metastasis. In the tumor microenvironment, the immune system plays a critical role in the prevention and elimination of tumors. Despite immune surveillance, tumors still manage to develop through evolution facilitating the establishment of an immune suppressive microenvironment. There is emerging evidence showing that the antitumor immunity of immune effector cells can be obstructed by immune suppressive cell populations such as MDSCs or Tregs. Hematologic malignancies associated with FGFR1 translocations, also known as Stem Cell Leukemia/Lymphoma (SCLL) syndrome, show a complex presentation including myeloproliferative neoplasm, AML, and T- or B-cell lineage lymphoblastic leukemia/lymphoma, providing an ideal, homogeneous model to investigate mechanisms underlying leukemogenesis. Our recent investigation in mouse models of SCLL revealed that mice xenografted with SCLL cells showed increased CD11b+Ly6CHigh M-MDSC and CD11b+Ly6CInt PMN-MDSC levels in the peripheral blood (PB) during leukemia development, accompanied by significantly decreased levels of T cells. To further confirm this MDSC-mediated suppression of T cell function, MDSC depletion experiments were performed in the SCLL cell engraftment model using either the anti-Ly6G antibody (for PMN-MDSCs) or the anti-Gr-1 (aka Ly6G/Ly6C; for total MDSCs) antibody. In both cases, significantly increased levels of CD4+ and CD8+ cells were observed, along with significantly prolonged mouse survival. These observations were consistent in multiple SCLL cell line models, supporting the idea that SCLL cells promote the accumulation of MDSCs, which in turn suppresses the antitumor activity of cytotoxic T cells, allowing SCLL progression. To further characterize these leukemia cell induced MDSCs, CyTOF analysis was performed with the PBMC and bone marrow (BM) samples from leukemic and naïve mice. In peripheral blood leukemia cell induced M-MDSCs also express macrophage markers, including F4-80 and CD64, implying a potential overlap of MDSC and tumor-associated macrophage (TAM). These TAMs also express high CD71 and CD184. Most importantly, we can also identify a CD64/CD71/CD184 expressing cluster in the BM samples. Different from the PB, these cells in the BM express stem cell marker CD117, suggesting there are potential progenitor cells of TAMs. Taken together, our studies demonstrated that SCLL cells establish a tumor favorable microenvironment by reshaping hematopoiesis toward MDSC generation in the BM. A comprehensive understanding of these immune modulation processes holds the promise of improving patient outcomes by reversing the generation of MDSC/TAM in the BM and overturning the establishment of the immunosuppressive microenvironment at the outset. Citation Format: Xiaocui Lu, Ting Zhang, Atsuko Matsunaga, Xuexiu Fang, Stephanie F. Mori, John Cowell, Tianxiang Hu. CD71 and CD117 identifies an early cancer associated macrophage progenitor population in leukemic mouse bone marrow [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1353.