Introduction: Gastrointestinal stromal tumors (GISTs) are densely infiltrated with tumor-associated macrophages (TAMs), which we have previously shown to exhibit an M1, inflammatory phenotype in established, untreated tumors in mice and humans. Since TAMs in most tumors are reported to be M2, we sought to determine the role of TAMs in early GIST development and the mechanism of TAM recruitment.

Methods: Genetically engineered KitV558del/+ mice that develop GIST were treated as adults and from birth with PLX5622, a small molecule inhibitor of colony stimulating factor 1 receptor (CSF1R). Tumors were weighed and analyzed by flow cytometry, immunohistochemistry, and immunofluorescence. Adoptive transfer experiments utilized tail vein injection of freshly harvested CD45.1+, CFSE-labeled bone marrow cells 2 days following withdrawal of CSF1R inhibition. Cell proliferation was determined by Ki67 staining.

Results: Tumors from KitV558del/+ mice treated with PLX5622 from birth for 4 weeks showed an 85% reduction of TAMs and a 40% decrease in size (n=8/group, p<0.05). TAMs from untreated 2 week old developing GIST mouse tumors tended to be more M2-like with 2.5x lower CD11c and 1.25x lower MHCII, while the M2 markers CD206 and MSR1 were increased 1.5x and 3x, respectively. Likewise, 3 days following withdrawal of PLX5622, newly recruited TAMs displayed a 20% reduction of MHCII+ cells (p<0.05). Furthermore, following PLX5622 withdrawal, flow cytometry revealed that TAM repopulation was preceded by an initial surge in Ly6Chi monocytes in the blood and the tumor. Subsequently, there was a reciprocal decrease of blood monocytes as the F4/80hi TAM population expanded to baseline levels of 35-40% of leukocytes over the course of 7 days. TAMs exhibited increased proliferation (55% Ki67+) compared to monocytes (17% Ki67+) and Kit+ tumor cells (2.5% Ki67+) 4 days after withdrawal of CSF1R inhibition. Following adoptive transfer of CD45.1+, CFSE-labeled bone marrow, the labeled cells showed an increasing TAM:monocyte ratio over time (2 on day 3, 7 on day 5, and 68 on day 10).

Conclusion: Although TAMs infiltrating established GISTs in adult mice exhibit an M1-like, anti-tumor phenotype, newly recruited TAMs in tumor development or TAMs after CSF1R inhibition are relatively M2-like. Within the tumor, bone marrow-derived monocytes differentiate into CSF1R-dependent TAMs, which proliferate to fully repopulate the tumor.

Citation Format: Michael J. Beckman, Michael Cavnar, Adrian Seifert, Juan Santamaria-Barria, Jennifer Zhang, Adam Levy, Ferdinand Rossi, Shan Zeng, Ronald P. DeMatteo. CSF1R-dependent tumor-associated macrophages arise from bone marrow-derived monocytes and promote gastrointestinal stromal tumor development. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr B64.