An ‘unwanted journey’: how the immune microenvironment may contribute to breast cancer

By Joyce J.I. Catsman

An ‘unwanted journey’

Sarah* enjoyed her student life like everyone else, until the day she discovered a lump in her breast. At first, she did not dwell on it and continued about her every-day routine. However, it remained in the back of her mind that breast cancer runs in the family. Sarah decided to go to her general practitioner and agreed on some tests, which revealed that she had a BRCA mutation that led to an early stage of abnormal cell growth called ductal carcinoma in situ (DCIS). The option she was offered was surgical removal of her breasts, which stopped her world and started her on what is known as an ‘unwanted journey’¹. Worldwide, 20-25% of diagnosed breast cancers are DCIS, which occurs more often in women than in men. This means that one out of five women with hazardous breast cancer go through such an unwanted journey in their lifetime². This article discusses how researchers are working to develop accurate biomarkers and possible therapies for DCIS, to prevent the ‘unwanted journey’ for many just like Sarah.

DCIS, what is that?

DCIS is a potential breast cancer precursor lesion where unregulated epithelial cells grow inside ductal structures that produce milk upon pregnancy, named terminal duct lobular units (TDLUs) (Figure 1)^3,4,5. Some risk factors for DCIS, include obesity, lifestyle choices such as smoking, and rarely familial mutations such a BRCA mutations^6,7,8. Not every DCIS progresses to an invasive disease⁹. About 80% of all DCIS lesions are asymptomatic, while the remaining 20% are associated with nipple discharge and/or a palpable lump. However, patients are often overtreated due to the lack of biomarkers that can distinguish DCIS lesions that progress to invasive disease from those that remain indolent and do not require treatment. To prevent the development of hazardous breast cancer, women at-risk – who carry a BRCA mutation – typically have their breasts surgically removed^10,11.

New insights into the initiation of the ‘unwanted journey’

As tumors begin to grow, the cancerous cells change the organization of the tissue they reside in, leading to morphological changes that are recognized in cancer research as pre-neoplastic lesions. These tissue changes carry cancerous traits but have not yet metastasized and pose no immediate risk to the patient⁸. In 2021, Messal and his colleagues tracked such morphological aberrations in the TDLUs of surgically removed human breasts¹². These aberrations were found both in breast tissues of BRCA mutation carriers without DCIS or cancer, and in healthy disease-free breasts that did not include DCIS, cancer, or BRCA mutations. This indicates that human breast tissue is already changing in a normal ‘healthy’ setting before it can be classified as DCIS or breast cancer. The researchers identified these aberrations via a new 3D-microscopy technique. They derived this method from a previously developed 3D technique called fast light-microscopic analysis of antibody-stained whole organs (FLASH)^13,14. In FLASH, surgically removed human breast tissue are treated to clear out the lipids, without changing the chemistry and architecture in order to make it transparent. In this way, it is possible to see inside the tissue and get a better look at the morphological changes, as shown in the cover image^12,13. Whilst this workflow allowed the researchers to detect early lesions and local tissue remodeling, the cause and consequences of these aberrations in the TDLUs remain unknown.

Possible connection between the immune microenvironment and the ‘unwanted journey’

Various studies have indicated that immune cells and their factors are associated with DCIS^15,16,17,18. Therefore, we investigated whether the immune microenvironment was involved in the observed aberrations inside the TDLUs of at-risk patients. The immune microenvironment refers to immune cells, cytokines, and other factors. Research has shown that this microenvironment cues tumor progression¹⁵. Based on their interaction with the immune microenvironment, immune cells can be categorized into ‘anti-tumor cells’ that are protective against tumor growth, and ‘pro-tumor cells’ that may stimulate tumor growth¹⁵. In our study, we characterized the immune microenvironment into two disease-free cosmetic breast reductions of ‘healthy’ women, and two cancer-free resections from BRCA mutation carriers who were 30–40-years-old, using immunofluorescence. Immunofluorescence is a 2D-microscopy technique that is used to locate a specific antigen in tissues using an antibody that has been labeled with a fluorescent dye. Via this technique, we looked at specific types of immune cells called T cells. There are two different subtypes of T cells: CD4⁺ T cells that help fight against infections, and CD8⁺ T cells that produce cytotoxins to kill cancer cells and other invaders. Additionally, it is possible to identify CD3⁺ T cells that represent activated T cells. We started by focusing on CD3⁺ T cells to determine their presence in human breast tissue. Literature has shown that CD4⁺ and CD8⁺ T cells belong to the ‘anti-tumor cell’ category, which makes them the ideal cell type to identify in both disease-free breasts and in breast tissues of BRCA mutation carriers^15,16.

Research highlight: potential biomarker or therapy?

Our study profiled the different cell types of the immune system and their distribution in the 3D analyzed human breast. By drawing spatial correlations between the immune profiles and the 3D ductal morphology, we assessed if certain immune cell types were more or less frequent in areas of the ductal trees. Interestingly, the results showed that whilst CD3⁺ T cells occurred in equal numbers throughout the entire human breast, CD4⁺ and CD8⁺ T cells only accumulated in some areas. This indicated activity of the immune system in some TDLUs. However, it remained unclear what the differences in immune cells meant and if they could contribute to a potential biomarker. Further research will be needed to assess whether these immune cells localize to morphologically abnormal ducts, and to investigate their role in the removal of early cancerous cells.

Recognition of the investment of research

Our research concluded that T cell types are differently localized in the epithelium of normal ‘healthy’ human TDLUs and in early lesions. These findings, however, were restricted to a small number of human specimens and therefore further empirical research is needed on this subject. Nevertheless, these results will allow comprehensive follow-up research to find a potential biomarker that distinguishes between DCIS lesions that progress to invasive disease from those that remain indolent and do not require treatment. This could help reduce the overtreatment of DCIS patients. The immune microenvironment likely plays a role in early human ductal transformation and may therefore contribute to breast cancer. This is of interest to women like Sarah since there is hope for future research to prevent their ‘unwanted journeys’.

* Fictional person

If you are going through an ‘unwanted journey’ or know someone with breast cancer and want to talk about it, please contact ‘Borstkankervereniging Nederland’ (030 291 7222) or visit the webpage ‘kanker.nl’.

Author information:

Acknowledgements:

This research was supported by the Netherlands Cancer Institute, division of Molecular Pathology by the group of Prof. Dr. Jelle Wesseling and the group of Prof. Dr. Jacco van Rheenen. This project would never have been possible without the support and guidance of various people at the NKI, with special thanks to my supervisors Prof. Dr. Jelle Wesseling and Dr. Hendrik Messal. 

This article was also supported by the VU:SCI Magazine. I want to thank the editorial team of VU:SCI Magazine, my mentor Madison Carr, and Dr. Hendrik Messal for their feedback and support throughout the writing process of this article.

Cover image source: 3D image of terminal duct lobular units (TDLUs) inside human breast tissue by Dr. Hendrik Messal

Further reading: