Abstract 381: Macrophage tracking using multi-modality 3D imaging in xenografts
Authors: Dimant Hemi , McConville Patrick , Orcutt Kelly , Mikule Kieth , Wilcoxen Keith , Lister Deanne , Ahrens Eric T. 
Published: September 2020

A common approach to immunotherapy is to promote the accumulation of specific bone marrow-derived myeloid cells into tumors for anti-tumor activity. While the pro- and anti-tumor effects of differentiated myeloid cells are complex, several novel therapeutic approaches are focused on recruiting macrophages to tumors and transforming their pro-tumor phenotypes of macrophages to anti-tumor phenotypes.

Research efforts to better understand and manipulate this type of macrophage biology is critical for developing and testing therapeutics. These experimental models commonly utilize imaging approaches, be it in vivo using molecular imaging or ex vivo histological approaches, to visualize macrophages in the context of tumors. These different imaging modalities provide different advantages in their ability characterize macrophage distribution, tissue localization and kinetics: In vivo imaging, such as MRI and in vivo fluorescence, gives strong temporal insights and general distribution in target tissues but are limited by sensitivity and resolution. Fluorescence microscopy and histology provide substantially higher resolution and sensitivity regarding TAM localization and cellular morphology but lacks temporal resolution and is only sampling a small portion of target tissue. Cryofluorescence tomography (CFT) is an attractive new modality in which an entire animal or tissue can be imaged ex vivo in 3D with high-resolution and increased sensitivity compared to nuclear in vivo imaging modalities.

To harness the advantages of each imaging modality, we designed a multimodality imaging study enabling the utilization of MRI, FLI and CFT for in vivo and ex vivo visualization of macrophages in tumor bearing animals. In this study, mice bearing xenografts were administered with V-Sense (VS-1000H NIR, Celsense, Inc., Pittsburgh, PA), a perfluorocarbon emulsion containing fluorine-19 (19F) and a NIR fluorophore. When administered IV, V-Sense is preferentially taken up by cells of the recticuloendothlium system, including Kupffer cells and macrophages, especially in inflamed tissues (e.g., tumors) thus enabling imaging using both MRI and fluorescence imaging. Immediately following MRI, animals underwent FLI and were then frozen for CFT.

As expected, 19F signal was evident in mice whose tumors where acute inflammation was present, ) but not detected in the non-inflamed tumors, indicative for macrophage recruitment. However, when imaged using both FLI and CFT, V-Sense signal was detected in tumors of both groups (1 and 2). CFT proved to be the most sensitive, revealing V-Sense signal not only in the tumor and liver but also in several other organs such as lymph nodes and bone marrow that were undetected with MRI or FLI.

The results of this study demonstrated the added value of a multimodality imaging approach using V-Sense, in which low resolution in vivo imaging and high-resolution 3D ex vivo CFT imaging can complement each other, providing rich, multi-resolution layers of immunological information in the same subjects.

Citation Format: Hemi Dimant, Patrick McConville, Kelly Orcutt, Kieth Mikule, Keith Wilcoxen, Deanne Lister, Eric T. Ahrens. Macrophage tracking using multi-modality 3D imaging in xenografts [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 381.