Abstract A015: Imprime PGG treatment enhances antibody-dependent cellular phagocytosis (ADCP) of tumor cells by monocyte-derived macrophages

Imprime PGG (Imprime), a soluble, yeast β-1,3/1,6 glucan is currently in clinical development for the treatment of cancer in conjunction with several therapeutic monoclonal antibodies (MAbs). Some of the MAbs that are being evaluated in combination with Imprime include, rituximab (anti-CD20) in an investigator initiated study for chronic lymphocytic leukemia, bevacizumab in a phase 2 study for non small cell lung cancer, and erbitux in a phase 3 study for metastatic colorectal cancer. Previous studies have demonstrated that Imprime, a pathogen- associated molecular pattern (PAMP) forms an immune complex with endogenous anti-β-glucan antibodies, then binds and primes innate immune cells (including macrophages, monocytes and neutrophils) to kill antibody-targeted cancer cells via a complement-dependent cellular mechanism (CDCC). The objective of this study was to evaluate the effect of Imprime on other innate immune effector mechanisms that are employed by the tumor-targeting MAbs, including antibody dependent cellular phagocytosis (ADCP)- a function largely attributed to macrophages. We therefore sought to investigate whether Imprime PGG treatment might enhance ADCP. Monocytes enriched from Imprime PGG- or vehicle-treated whole blood were cultured in media containing the appropriate cytokines for differentiation of different macrophage subtypes: GM-CSF for M1 macrophages; M-CSF for M2 macrophages; M-CSF plus IL-4 for M2a macrophages; and M-CSF plus IL-10 for M2c macrophages. ADCP was evaluated by using specific macrophage subtypes with various tumor cell lines, including Raji (Burkitt's lymphoma) and Z138 (Mantle cell lymphoma) in the presence of rituximab, ofatumumab, and obinutuzumab (anti-CD20 MAbs), or SKBR3 (Her2 positive breast cancer) and MDA-MB-231 (Her2 negative breast cancer) in the presence of trastuzumab (anti-Her2). A flow cytometric phagocytosis assay was performed by incubating macrophages and tumor cell lines that are labeled with different flourochromes, and subsequently quantitating the phagocytosed cells that are rendered double positive. The results demonstrated that among all the different sub-types of macrophages, M1 and M2c macrophages have higher phagocytic capacity. In comparison to the vehicle control, Imprime treatment enhanced the ability of M2c macrophages to phagocytose both Raji and Z138 tumor cells in the presence of each of the anti-CD20 antibodies- rituximab, ofatumumab, and obinutuzumab. For the breast cancer cell lines, I