Imprime PGG (Imprime), a soluble yeast 1,3/1,6 β-glucan, is being developed as a novel cancer immunotherapy in conjunction with anti-tumor antibodies in several cancers. In clinical studies, including randomized Phase 2 clinical trials in the 1st-line treatment of stage IV non-small cell lung cancer with bevacizumab, Imprime treatment has shown promising efficacy in both objective tumor response rates and survival. In numerous syngeneic and xenogeneic mouse tumor models, Imprime treatment in combination with an anti-tumor antibody reduced tumor growth and prolonged survival beyond that observed with antibody alone. Mechanistic studies have now demonstrated that, with endogenous anti-β-glucan antibodies (ABA), Imprime forms an immune complex. This Imprime-ABA complex then activates the classical complement pathway and is subsequently opsonized by iC3b. This immune complex then binds to and primes innate immune cells, including macrophages, monocytes and neutrophils, triggering a coordinated immune attack against antibody-targeted cancer cells. In this study, we sought to explore more fully the functional consequences of Imprime-ABA immune complex binding to, and priming of, neutrophils.

Neutrophils are the first line of defense against fungal infections. Neutrophils detect fungal infections through recognition of β-glucans on the surface of yeast and fungi. These β-glucans serve as fungal pathogen associated molecular patterns (PAMPs) that are efficiently and effectively recognized by receptors on innate immune cells, initiating a coordinated immune response to infection. Here we demonstrate that Imprime, in individuals with high ABA levels, is recognized as a PAMP, directly interacting with neutrophils via multiple receptors, priming these cells and ultimately leading to enhanced effector responses including reactive oxygen species (ROS) generation and tumor cell killing. Our data now show that Imprime elicits enhanced neutrophil survival as measured by cell viability dye exclusion using flow cytometry. Imprime treatment also enhances the responsiveness of neutrophils to Fc receptor activation. As measured by luminol-based read-outs, ROS production in response to anti-Fc receptor antibody-coated beads is profoundly enhanced by Imprime treatment. Moreover, Imprime treated neutrophils show a remarkable surge in ROS production compared to vehicle–treated neutrophils when exposed to B cell lymphoma cells decorated with anti-CD20 monoclonal antibodies (i.e. rituximab). These Imprime-treated neutrophils also exhibit a robust increase in cytotoxicity against these B cell lymphoma cells coated with rituximab using in vitro Calcein AM dye release assays. We next sought to delineate which receptors might be most critical for Imprime binding and priming of neutrophils. As demonstrated by antibody blocking studies, the Imprime-ABA complex can bind to both complement receptors (notably CR3) and Fc receptors. Imprime can also block binding of a dectin-1 agonizing antibody to dectin-1 and can bind directly to HL-60 and HEK cells lines engineered to overexpress dectin-1 by retroviral transduction. Collectively, these data show for the first time that Imprime directly primes neutrophils to recognize and kill antibody-decorated tumor cells and interacts with the Fc, complement and dectin-1 receptors on the surface of neutrophils.

Citation Format: Steven M. Leonardo, Ross B. Fulton, Lindsay R. Wurst, Keith B. Gorden, Adria Bykowski Jonas, Xiaohong Qui, Anissa SH Chan, Jeremy R. Graff. Imprime PGG binds to neutrophils through complement, Fc, and dectin-1 receptors, priming these cells for enhanced ROS production and tumor cell cytotoxicity. [abstract]. In: Proceedings of the CRI-CIMT-EATI-AACR Inaugural International Cancer Immunotherapy Conference: Translating Science into Survival; September 16-19, 2015; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(1 Suppl):Abstract nr A160.