Abstract 3969: Blocking colony stimulating factor 1 receptor (CSF1R) improves anti-tumor effects of adoptive T cell transfer therapy.
Authors: Mok Stephen , Tsui Christopher , Xu Jingying , Comin-Anduix Begonya , Chodon Thinle , Ribas Antoni , Koya Richard C. 
Published: August 2014

Tumors can escape immune responses by creating an inflammatory milieu that supports and recruits tumor-infiltrating myeloid cells (TIMs) with immune suppressive functions, such as myeloid derived suppressor cells (MDSC) and M2-polarized macrophages. Therapies that block recruitment of suppressor TIMs have the potential to enhance adoptive cell transfer (ACT) of T cell based-immunotherapies.

We originally established a BRAFV600E mutant murine melanoma cell line (SM1) for which ACT of melanoma-targeted T cells induced anti-tumor responses, but not complete eradication in vivo. By gene expression profiling and single-nucleotide polymorphism (SNP) arrays, we found that, out of 108 cytokines produced by SM1, colony stimulating factor 1 (CSF1) was highly expressed. CSF1 induces proliferation of immune suppressors Gr-1(+) CD11b(+) MDSC and F4/80(+) CD11b(+) macrophages. In order to block the recruitment of TIMs, we tested a potent CSF1R inhibitor, PLX3397. Cultured primary murine T cells exposed to PLX3397 with a broad range of concentrations (0.1 to 50 μM) showed no evidence of cytotoxicity by MTS assay. We then tested the combination of PLX3397 and ACT in vivo using two models. In the first one, SM1 cells stably expressing the chicken ovalbumin (OVA) antigen were implanted in C57BL/6 mice. Daily oral gavage with PLX3397 (50mg/kg) combined with ACT of OVA-specific TCR transgenic cells (OT-1) demonstrated superior effects of the combined treatment (tumor size on day 14_vehicle: 1611.2±22.5 mm2, PLX3397: 1028.0±24.0 mm2, OT-1: 1112.5±35.7 mm2, combined PLX3397+OT-1: 347.3±15.5 mm2, p&lt;0.001). We have observed a dramatic reduction of the number of macrophages in the tumor tissue (vehicle: 10.3±1.6%, PLX3397: 1.35±0.5%, OT-1: 12.0±0.6%, combined: 2.33±0.3%, p&lt;0.001) and a skewing of type M2 to type M1 macrophages. In vivo real-time T cell tracking by luciferase bioluminescence imaging demonstrated an increased number of tumor infiltrating lymphocytes (TILs) in the combined therapy group (photon counts/pixel on day 5 post ACT: OT-1: 4424.5±412, combined: 9176±1780, p&lt;0.001). The second in vivo model was based on pmel-1 transgenic T cells, which targeted the endogenous melanoma antigen (gp100) expressed by SM1. The combined treatment of PLX3397 and ACT of pmel-1 also resulted in significant tumor shrinkage, reduction of M2 macrophages, and increased number of TILs. TILs were also collected and analyzed for function by intracellular staining of IFNγ. TILs from the combined treatment group released significantly higher levels of IFNγ (ACT: 48.2±8.7%, combined: 78.8±3.5%, p&lt;0.001).

In conclusion, our data derived from two independent in vivo models demonstrated dramatic potentiation of anti-tumor effects with the combination of CSF1R blockade and ACT immunotherapy, supporting the rationale for further testing in patients with metastatic melanoma.

Citation Format: Stephen Mok, Christopher Tsui, Jingying Xu, Begonya Comin-Anduix, Thinle Chodon, Antoni Ribas, Richard C. Koya. Blocking colony stimulating factor 1 receptor (CSF1R) improves anti-tumor effects of adoptive T cell transfer therapy. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3969. doi:10.1158/1538-7445.AM2013-3969