Abstract 1561: Characterization of circulating monocyte infiltration in brain metastasis

Abstract Brain metastases are a devastating condition with extremely poor prognosis. To improve treatment outcomes for patients with brain metastases, the role of the host's immune system in their development needs to be better understood. The contribution of circulating monocytes, in particular, to the progression of brain metastases is not well understood. Histologically and molecularly, it is difficult to differentiate between resident brain macrophages (microglia) and infiltrating circulating monocytes from the systemic circulation. However, advances in genetic engineering have allowed these populations to be distinguished. We hypothesise that infiltration of circulating monocytes into brain metastases increases over time, and correlates with BBB breakdown and increased cell adhesion molecule (CAM) expression. In this study, we have used the Lys-GFP-ki mouse strain in which bone marrow derived monocytes express GFP, but CNS microglia do not. 500 EO771 cells (mouse mammary carcinoma) were microinjected into the left striatum of Lys-GFP-ki mice (n = 28) using stereotaxic guidance and a finely-drawn glass microcapillary. Some animals underwent MRI at 7T on days 7 (n = 7), 14 (n = 4) or 21 (n = 7), and were perfusion-fixed immediately after imaging. T1-weighted 3D data sets were acquired both pre- and post- intravenous injection of 30μL gadodiamide. The remaining animals were perfusion-fixed on days 7 (n = 2), 14 (n = 5) or 21 (n = 3). Immunofluorescent staining was performed on all brains and assessed the expression of F4/80 (macrophage marker), as well as the adhesion molecules VCAM-1, VLA-4, ICAM-1, LFA-1, E- and P-Selectin, PSGL-1, ALCAM and CD34. The percentage of GFP-positive area (indicating recruitment of systemic monocytes) within the tumour increased significantly from day 7 to 14 (p < 0.001) and maintained significance at day 21. In contrast, blood vessel area and number, as determined from CD34 staining, only increased significantly at day 21 (area: p < 0.05, number: p < 0.001). With regards to MRI data, the area of signal enhancement post-gadodiamide injection increased significantly over time and correlated with GFP positive cell infiltration (p < 0.05; y = 0.5913x - 0.2513; r2 = 0.3706). Additionally, the degree of co-localisation increased significantly over time between GFP-positive macrophages and ICAM-1 (p < 0.05), ALCAM (p < 0.05), E- and P-Selectin (p < 0.001, p < 0.05), whilst ALCAM expression also increased significantly on CD34 positive vessels (p < 0.05). In conclusion, it appears that tumour progression in brain metastasis is associated with increasing infiltration of circulating monocytes, which correlates with increasing BBB breakdown. Interestingly, circulating monocyte infiltration appears to occur prior to significant changes in vascular area and density. Analysis of CAM expression, suggests that CAMs, and in particular ICAM-1, may play a role in recruitment of circulating monocytes and in their interactions with cancer cells. Citation Format: Vasiliki Economopoulos, Manuel Sarmiento Soto, James R. Larkin, Danny Allen, Nicola R. Sibson. Characterization of circulating monocyte infiltration in brain metastasis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1561.