Recent in vivo mouse studies have revealed that macrophages can promote metastasis, while certain subsets of monocytes either help or block metastasis. Further studies are warranted to understand the underlying mechanisms, so as to target pharmacologically these immune cells as a means to halt metastasis. Intravital imaging can be performed in mice and has proven extremely useful in uncovering the role of macrophage and monocytes in metastasis, but requires specialized expertise and equipment. Concurrently, microfluidic models have been developed to dissect certain steps of the metastatic spread, because they allow for easier imaging at high resolution and facilitate an improved microenvironment control over more traditional macro-scale in vitro systems.
In this study, a microfluidic model was used to investigate the effects of monocytes and macrophages on cancer cell extravasation. A previously developed microfluidic assay was used to form a 3D microvascular network (MVN) that consists of 3 separate compartments, flanked by media channels connected to reservoirs. In the two lateral compartments, normal human lung fibroblasts were seeded at 5 million/ml, while in the central compartment GFP HUVECs were placed at 4 million/ml all in a 3D fibrin gel. After 4 days, the HUVECs self-assemble into a MVN, which forms lumens that open directly to the media channel. Therefore, cancer cells can be perfused within the MVN via the reservoirs, and their transendothelial migration is tracked by confocal imaging over time for up to 12 hours. In this study, we mix the HUVECs with the macrophages (ratio 40:1) such that the macrophages are found in the fibrin on the outside of the vessels. Alternatively, we perfuse monocytes in the MVN, most of which have extravasated 4 days later (94±2%), thus replicating macrophage differentiation from recruited monocytes. In another study, monocytes were perfused with cancer cells in the MVN to test whether the monocytes could affect cancer cell extravasation intra-luminally, as they would in the blood circulation. Both monocytes and macrophages were derived from healthy donors' blood.
Monocyte-derived macrophages (MDMs) were on average 33.4±4.4 μm (mean±SEM) away from the MVN, of which 40% were found within 10 μm. Interestingly, 46.4±3.1% stained positive for MRC-1. The MDM:MDA-MB-231 ratio was 2±0.5. The presence of MDM around the microvascular networks did not affect cancer cell extravasation rate; 10 hours after the cancer cells were perfused within the MVN, 57±15 vs. 60±6% of MDA-MB-231 had extravasated with or without MDMs, respectively. 11±6% of MDA-MB-231s came in contact with MDMs, for 5.1±0.6 hrs. Contact was mostly established after cancer cells had extravasated (73±19% of cancer cells), and less often while the cancer cell was extravasating (15±9%) or inside the vessels (12±12%). At t = 0, the average shortest distance between a MDM and a cancer cell arrested within the vessels was 98.4±6.3 μm, and was the same for cancer cells that did or did not eventually extravasate. However, the cancer cells that extravasated migrated further away from the μVN surface in presence of MDMs; 4.5 hrs after extravasation, cells were found 16.5±4.9 vs. 5.4±2.1 μm away from the endothelial wall with or without MDMs, respectively (p = 0.044, unpaired student t-test). In a separate study, monocytes were found to significantly decrease cancer cell extravasation. 11 hrs after cancer cells were perfused with monocytes inside the MVN, 19.8±6.1% of MDA-MB-231 had extravasated vs. 53.5±11% when cancer cells were perfused alone (p = 0.041, unpaired t-test). In conclusion, monocytes but not macrophage seem to affect cancer cell extravasation. Further studies will generalize the results to other cancer cell lines, and investigate how monocytes might decrease extravasation, and how this effect could be exacerbated pharmacologically to decrease metastasis.
Citation Format: Alexandra Boussommier-Calleja, Roger D. Kamm. The role of macrophages and monocytes during cancer cell extravasation in 3D vascularized microfluidic models [abstract]. In: Proceedings of the Second CRI-CIMT-EATI-AACR International Cancer Immunotherapy Conference: Translating Science into Survival; 2016 Sept 25-28; New York, NY. Philadelphia (PA): AACR; Cancer Immunol Res 2016;4(11 Suppl):Abstract nr A126.
- ©2016 American Association for Cancer Research.