Abstract LB-236: Imprime PGG conjugated directly to protein enables cross-presentation of antigen that generates multifunctional cytotoxic T cells
Authors: Fulton Ross B. , Leonardo Steven , Michel Kyle , Wurst Lindsay , Phelon Trinda , Danielson Mike , Gorden Keith 
Published: August 2015

Abstract
Imprime PGG is a soluble yeast-derived β-1,3/1,6 glucan innate immune cell modulator that is in phase 3 and multiple phase 2 clinical trials in combination with complement activating monoclonal antibodies (e.g. bevacizumab, cetuximab). We have previously shown that Imprime binds to complement receptors and modulates the function of a variety of immune cells, including monocytes, neutrophils, and B cells. We have also demonstrated that Imprime PGG binds to various subsets of dendritic cells (DCs) and can cause intermediate upregulation of MHC class II and the co-stimulatory molecules CD80/86 critical for antigen presentation and T cell activation. Based on these findings, we hypothesized that Imprime PGG conjugated to a protein would efficiently deliver antigen to DCs and prime a cytotoxic CD8+ T cell response.
To test this hypothesis, we employed the model antigen chicken ovalbumin (OVA) in a C57BL/6 mouse model to examine the generation of OVA-specific CD8+ T cell responses. We covalently linked OVA to Imprime PGG to generate a β-glucan/protein conjugate (Imprime-OVA). Using T cell receptor transgenic OT-I CD8+ T cells to track responses to OVA, we treated mice with Imprime-OVA intravenously and examined the expansion and functional quality of the T cell response 7 days later at the peak of expansion. Following Imprime-OVA treatment, OVA-specific CD8 T cells underwent vigorous expansion, upregulated the transcription factor Tbet, which is central to developing effector functions, and gained the ability to produce the cytokines IFN-γ, TNF-α and IL-2. By comparison, OVA alone did not generate a functional CD8+ T cell response and instead induced anergy. To determine if cross-presenting DCs are required for CD8+ T cell activation, we used mice deficient in the transcription factor Batf3, which selectively eliminates CD8α+ cross-presenting DCs. Following treatment with the Imprime-OVA conjugate in these Batf3-/- mice, the expansion of OVA-specific CD8+ T cells was more than 10-fold reduced compared to that in wild-type mice. Further, these cells failed to develop effector functions, indicating that CD8α+ cross-presenting DCs are crucial for this response. Together, these data show that an Imprime PGG-protein conjugate can effectively elicit the expansion and functional activation of cytotoxic T cells and may have utility as a potential cancer vaccine platform.
Citation Format: Ross B. Fulton, Steven Leonardo, Kyle Michel, Lindsay Wurst, Trinda Phelon, Mike Danielson, Keith Gorden. Imprime PGG conjugated directly to protein enables cross-presentation of antigen that generates multifunctional cytotoxic T cells. [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-236. doi:10.1158/1538-7445.AM2015-LB-236