Abstract A02: Imprime PGG modulates the myeloid component of the tumor microenvironment to coordinate an antitumor immune response
Authors: Fraser Kathryn , Ottoson Nadine , Qiu Xiaohong , Chan Anissa SH , Jonas Adria , Kangas Takashi , Graff Jeremy , Bose Nandita 
Published: May 2016

Imprime PGG (Imprime) is a soluble, yeast-derived 1,3-1,6 β-glucan that acts as a pathogen associated molecular pattern (PAMP), effectively binding to and activating cells of innate immune system to drive a coordinated anti-cancer immune attack. In a randomized phase 2 clinical study, stage IV NSCLC patients treated with Imprime plus the anti-VEGF antibody bevacizumab (bev), carboplatin and paclitaxel showed a median overall survival of 16.1 months versus 11.6 months in patients not receiving Imprime. We sought to explore a mechanistic understanding for this promising clinical activity.

In vitro mechanistic studies using human whole blood from healthy volunteers have demonstrated that Imprime has the potential to alter the immunosuppressive nature of myeloid cells, including macrophages, myeloid-derived suppressor cells (MDSC) and neutrophils. Utilizing whole blood from human volunteers, we found that monocyte-derived M2 macrophages displayed M1-like characteristics, including reduced surface expression of CD163 and increased expression of CD86 and PD-L1. In mixed lymphocyte reactions, these Imprime treated myeloid cells triggered enhanced expansion of both CD4 and CD8 T cells and increased production of the potent anti-tumor cytokine interferon gamma indicating a shift toward Th1 cell polarization. We also found that Imprime treatment significantly reduced the ability of myeloid derived suppressor cells (MDSC) cultured from human cord blood to inhibit T cell proliferation. Furthermore, Imprime treatment enhanced neutrophil activity as evidenced by modulation of activation markers CD11b, CD62L, CD88, as well as enhanced reactive oxygen species production specifically in response to anti-CD20 antibody coated B cell lymphomas.

We sought to evaluate whether Imprime treatment would affect the same myeloid subsets in vivo. Athymic nude mice were injected with the H1299 NSCLC cells. Once tumors reached a group mean of ~100mm3, mice were treated with vehicle, bev (5 mg/kg twice weekly IP), Imprime (1.2 mg/ mouse twice weekly IV) or bev+ Imprime. Spleen and tumor tissue were harvested at the end of the study and single cell suspensions were analyzed by flow cytometry, RT-PCR and ELISA. Murine macrophages harvested from spleens of mice treated with Imprime + bev showed a shift, when compared to splenic macrophages from bev-treated mice, to an M1-like phenotype- increased expression of the activation marker CD86 and inducible nitric oxide synthase (iNOS2) in concert with a significantly decreased Arginase-1 (Arg-1). Splenic MDSCs showed a similar increase in iNOS2 expression with significantly reduced Arg-1 expression. Remarkably, CD11b+ cells harvested from the tumor tissue of mice treated with both Imprime and bev showed a similar shift toward an activated phenotype, demonstrating significant increases in iNOS2 and PD-L1 expression, and significantly reduced expression of Arg-1, compared to cells examined from bev group. Tumor associated macrophages and neutrophils from Imprime + bev treated mice showed a significant upregulation of the activation marker CD86 compared to bev treatment alone, indicating a shift in polarization to a M1 and N1 activated phenotype within the tumor microenvironment. Furthermore, ELISA data from these tumors showed that Imprime treatment significantly reduced expression of the potent immunosuppressor, TGFβ, when compared to tumors from mice treated only with bev. Importantly, the greatest reduction of TGFβ was evident in the tumors most profoundly inhibited by Imprime plus bev treatment. Collectively, these data show for the first time that, in vivo, Imprime PGG treatment can activate myeloid cells within the tumor microenvironment to orchestrate a profound shift in this microenvironment that promotes tumor recognition and eradication.

Citation Format: Kathryn Fraser, Nadine Ottoson, Xiaohong Qiu, Anissa SH Chan, Adria Jonas, Takashi Kangas, Jeremy Graff, Nandita Bose. Imprime PGG modulates the myeloid component of the tumor microenvironment to coordinate an antitumor immune response. [abstract]. In: Proceedings of the Fourth AACR International Conference on Frontiers in Basic Cancer Research; 2015 Oct 23-26; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2016;76(3 Suppl):Abstract nr A02.