Abstract LB-224: Nab-paclitaxel and agonist CD40 mAb combination therapy induces tumor-associated macrophage polarization switching in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is characterized by an extensive tumor stroma that is composed of immune cells, vascular cells, fibroblasts and extracellular matrix components (Erkan et al., 2012). This increased desmoplasia correlates with decreased survival and has been shown to mediate chemoresistance (von Hoff et al., 2009). Macrophages within the stromal compartment have been shown to play tumor-promoting roles by switching from their classical immunostimulatory function (‘M1′) to an immunosuppressive state (‘M2′) (Yoshikawa et al., 2012). Nab-Paclitaxel (nab-Ptx) is an albumin-bound form of Paclitaxel that, in combination with Gemcitabine, is currently accepted as the standard chemotherapeutic regimen for pancreatic cancer. However, pre-clinical and clinical studies have suggested that nab-Ptx may exert its effects by altering the tumor stroma (Desai et al., 2006, von Hoff et al., 2011). Here, we show that tumor-associated macrophages uptake high levels of nab-Ptx in an orthotopic model of PDAC via macropinocytosis. Eighty to ninety percent of macrophages within the tumor internalize fluorescently labeled nab-Ptx ex vivo, as compared to thirty to forty percent of macrophages from the spleens of the same animals. These data suggest that M2-like macrophages within the tumor microenvironment preferentially macropinocytose nab-Ptx. To analyze the potential consequence of nab-Ptx internalization on macrophages, we treated the RAW macrophage cell line with Ptx alone or in combination with the immunostimulatory cytokine IFN gamma (IFNγ). Prolonged exposure (48h) of RAW cells to Ptx induced an increase in the M1 marker iNOS, with the combination of low dose IFNγ and Ptx resulting in a synergistic increase in iNOS expression with accelerated kinetics (12h). Moreover, Ptx alone or in combination with IFNγ was able to revert IL-4-induced expression of the M2 marker Arginase 1. These findings suggest that high levels of nab-Ptx internalization by M2 macrophages may re-polarize them to an M1, immunostimulatory state. Our in vitro studies suggest that the potential effect of nab-Ptx and Gemcitabine on macrophage polarization in vivo may be enhanced by the presence of an additional immunostimulatory signal. The agonist CD40 monoclonal antibody (mAb) is a member on the TNFα receptor superfamily that has been shown to induce immune cell activation and therapeutic efficacy in human and mouse models of PDAC (Beatty et al., 2013). We therefore examined the effect of combining CD40 mAb with nab-Ptx and Gemcitabine treatment on macrophage phenotype in an orthotopic model of PDAC. Our studies to date show that the induction of both an increase in M1 marker (MHCII and CD86) expression and a decrease in M2 marker expression (CD206) in pancreas tumor-associated macrophages requires the triple combination therapy. Together, these data suggest that the internalization of nab-Ptx by tumor-associated macrophages in combination with immune activating signals like CD40 mAb may be required to restore their M1-like, tumor cell cytotoxic functions. CD40 mAb and nab-Ptx combination therapy may therefore enable the effective targeting of the pancreatic tumor stroma, resulting in enhanced therapeutic benefit in PDAC. Citation Format: Jane E. Cullis, Despina Siolas, Anirban Maitra, Dafna Bar-Sagi. Nab-paclitaxel and agonist CD40 mAb combination therapy induces tumor-associated macrophage polarization switching in pancreatic cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr LB-224. doi:10.1158/1538-7445.AM2015-LB-224