High mammographic density is associated with an increase in stromal collagen and immune cells within the mammary epithelium

Cecilia W. Huo, Grace Chew, Prue Hill, Dexing Huang, Wendy Ingman, Leigh Hodson, Kristy A. Brown, Astrid Magenau, Amr H. Allam, Ewan McGhee, Paul Timpson, Michael A. Henderson, Erik W. Thompson, Kara Britt

Mammographic density (MD) adjusted for a women's age and body mass index (BMI) is a strong and independent risk factor for breast cancer (BC). While the BC risk attributable to increased MD is significant in the normal population, the biological basis of high MD (HMD) causation and how it raises BC risk remains elusive. We assessed the histological and immunohistochemical differences between matched HMD and low MD (LMD) breast tissues from healthy women to define which cell features may be mediating the increased MD and MD-associated BC risk. Tissue was accrued from 41 women undergoing prophylactic mastectomy due to a high BC risk profile between 2008 and 2013. Tissue slices resected from the mastectomy specimens were X-rayed, then HMD and LMD regions were dissected based on the radiological appearance. The histological composition, aromatase immunoreactivity, hormone receptor status and proliferation status were assessed, as were collagen amount and orientation, epithelial subsets, and immune cell status. HMD tissue had a significantly greater proportion of stroma, collagen and epithelium, and less fat than LMD tissue. Second harmonic generation imaging demonstrated more organised stromal collagen in HMD compared to LMD. There was significantly more aromatase immunoreactivity in both the stromal and glandular regions of HMD than LMD tissues, although no significant differences in levels of oestrogen receptor (ER), progesterone receptor (PR), and Ki-67 expression were detected. The number of macrophages within the epithelium or stroma did not change, however HMD stroma exhibited less CD206+ alternatively activated macrophages. Epithelial cell maturation was not altered in HMD samples, and no evidence of epithelial mesenchymal transition was seen, but there was a significant increase in vimentin+/CD45+ immune cells within the epithelial layer in HMD. We confirmed increased proportions of stroma and epithelium, increased aromatase activity, and no changes in the hormonal receptor or Ki-67 markers status in HMD tissue. The HMD region showed increased collagen deposition and organisation, and decreased alternatively activated macrophages in the stroma. The HMD epithelium may be a site for local inflammation as we observed a significant increase in CD45+/vimentin + immune cells in this area.