Abstract B009: Imprime PGG, a soluble yeast b-glucan PAMP, activates both innate and adaptive immune effector cells, resulting in enhanced antitumor responses that synergize with anti-PD-1 antibody therapy
Authors: Leonardo Steven , Fulton Ross B. , Fraser Kathryn A. , Kangas Takashi O. , Gorden Keith B. , Harrison Ben , Jonas Adria LB , Chan Anissa SH , Yokayama Yumi , Bose Nandita , Graff Jeremy R. , Uhlik Mark 
Published: August 2018

Abstract
Immune checkpoint therapy has made a splash in the cancer world by providing long-term durable responses in patients across multiple cancer indications. However, not all patients respond to checkpoint intervention as this therapeutic relies on an underlying recognition and activation of T cells against the tumor. Furthermore, recent evidence suggest that acquired resistance to CPIs may occur through beta-2 microglobulin mutations and loss of MHC class I-restricted T-cell responses, in which case other cytotoxic effector cells like natural killer (NK) cells could play a critical role. Therefore, combination therapies utilizing checkpoints and innate immune modulators such as PAMPs (pathogen-associated molecular patterns) are being evaluated to expand both the magnitude and duration of the response, as well as the responding patient population. Imprime PGG (Imprime) is a soluble, systemically delivered yeast 1,3/1,6 β-glucan PAMP capable of triggering innate immune cell function leading to a cascade of immune activation and enhanced antitumor killing. Imprime is currently being evaluated in combination with anti-PD1 therapy in multiple clinical trials. Here we employ a combination of experimental techniques, including both flow cytometry and multiplex immunofluorescence (IFC), to examine how Imprime modulates the antitumor response in the MC38 murine adenocarcinoma model. C57BL/6 mice were injected with MC38 cells subcutaneously and randomized into 4 treatment groups (vehicle, Imprime, anti-PD1, Imprime+anti-PD1) once tumors reached ~50 mm3. After 2 weeks of treatment, mice were evaluated for inhibition of tumor growth, presence and activation of tumor-infiltrating immune cells by IFC, and the induction of polyfunctional T cells by flow cytometry. Mice treated with the combination of Imprime and anti-PD1 had significantly smaller tumors than all other groups, including those treated with anti-PD1 alone. IFC-based analysis of these tumors showed that these mice had increased frequencies of CD8+GranzymeB+ cells, indicative of an enhanced antitumor T-cell response. TILs from these tumors also showed increased frequency of IL-2 production after stimulation ex vivo with a pool of known MC38 tumor antigens. Interestingly, there was also an increase in a CD3-GranzymeB+ population (also negative for B220, F4/80, Ly6C, Ly6G, MHCII) in the tumor of mice treated with the combination. We classified these as most likely to be NK cells. In peripheral blood, flow cytometric analysis of NK1.1+ NK cells also showed increased GranzymeB expression. To explore the impact of Imprime on NK-mediated tumor killing, we further evaluated in vivo killing of MHCI-deficient cancer cell targets after intravenous administration of Imprime. In these experiments Imprime was able to enhance the NK cytotoxic killing of the targets, providing further evidence that Imprime effects innate effector function. These data demonstrate a unique synergy between anti-PD1 and Imprime treatment. The enhanced cytotoxic T-cell response coupled with enhanced effector NK function represents an exciting novel mechanism that may provide enhanced therapeutic benefit to patients in our ongoing clinical trials.
Citation Format: Steven Leonardo, Ross B. Fulton, Kathryn A. Fraser, Takashi O. Kangas, Keith B. Gorden, Ben Harrison, Adria LB Jonas, Anissa SH Chan, Yumi Yokayama, Nandita Bose, Jeremy R. Graff, Mark Uhlik. Imprime PGG, a soluble yeast b-glucan PAMP, activates both innate and adaptive immune effector cells, resulting in enhanced antitumor responses that synergize with anti-PD-1 antibody therapy [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2017 Oct 26-30; Philadelphia, PA. Philadelphia (PA): AACR; Mol Cancer Ther 2018;17(1 Suppl):Abstract nr B009.