Abstract 4060: In vitro human tumor-associated macrophage model implicates macrophage proliferation as a mechanism for maintaining tumor-associated macrophage populations

Prostate cancer tumor growth and disease progression is highly influenced by non-cancerous host cells within the tumor microenvironment. Tumor-associated macrophages (TAMs) are a critical component of this microenvironment and can comprise up to 50% of prostate cancer tumors. Additionally, high TAM infiltrate correlates with poor prostate cancer patient prognosis. Macrophages have different phenotypes and functions depending on the immune response in which they are involved. The majority of prostate cancer TAMs resemble alternatively-activated M2 macrophages which, in healthy individuals, participate in the Th2 immune response which is primarily involved in wound healing. Macrophages contribute to Th2 responses by promoting tissue remodeling, cell proliferation, angiogenesis, and immune suppression. These functions are taken advantage of by cancer cells to promote tumor growth, immune evasion, and disease progression. Techniques for studying TAMs in vitro are limited and are confounded by the lack of procedure consensus for in vitro macrophage differentiation and polarization. We have established an optimized protocol for mimicking TAMs in vitro using macrophages derived from human monocytes isolated from whole blood. (doi: 10.2144/000114435). These cell populations have been validated as an accurate TAM model by their marker expression and immunomodulatory functions. It has recently been widely accepted that tissue populations of macrophages are maintained by proliferation of tissue-resident macrophages seeded during embryogenesis rather than by recruitment of monocytes from the blood. Additionally, it has been reported that local macrophage proliferation is a prominent method for maintaining macrophage populations in Th2 diseases and TAM populations in breast cancer models. However, how and whether in vitro models recapitulate this macrophage behavior is has been largely ignored and remains unaddressed. To address this question, we investigated the proliferation status of our optimized in vitro TAM model by observing the expression of the proliferation marker Ki67 and EdU incorporation by flow cytometry and immunofluorescence. We found that subsets of these macrophages stain positively for Ki67 and incorporate EdU indicating that they proliferate in culture. These findings have wide implications for cancer as it suggests that the large M2 TAM populations found in prostate cancer tumors may be maintained by macrophage proliferation. This supports a mechanism for the origin and maintenance of TAM populations that is currently ignored by the majority of the cancer field. Further work will investigate the proliferation status of TAMs in vivo using MycCaP prostate cancer tumors in syngeneic mouse models. Citation Format: Amber E. de Groot, Kenneth J. Pienta. In vitro human tumor-associated macrophage model implicates macrophage proliferation as a mechanism for maintaining tumor-associated macrophage populations [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4060.