Tumour microenvironment shapes eosinophil plasticity in murine models of breast cancer

Thesis

Eosinophils have emerged as important mediators of successful response to immune checkpoint blockade (ICB) treatment in breast cancer. However, in contrast to colorectal or lung tumours, depletion of eosinophils or induction of eosinophilia in murine models of breast cancer did not alter tumour growth. This indicates that the breast tumour microenvironment (TME) can evade eosinophil cytotoxicity by shaping their phenotypic and functional plasticity.

Using NT193, 4T1, and E0771 murine models of breast cancer, the heterogeneity of eosinophils in the TME was explored by spectral flow cytometry. This analysis identified two novel subsets of tumour-associated eosinophils. While eosinophils express relatively high levels of the Ly6C glycoprotein in healthy tissues, such as bone marrow, blood and mammary fat pad, they gradually lose Ly6C expression in the TME during tumour progression, resulting in Ly6C+ and Ly6C- eosinophil populations. Further experiments revealed that Ly6C+ eosinophils represent a long-lived population that naturally transitions into a Ly6C- state with reduced cytotoxic potential.

To understand the underlying differences between these two eosinophil subsets, both Ly6C+ and Ly6C- populations were analysed by bulk mRNA sequencing. Ly6C+ eosinophils were more responsive to IFNγ and IFNβ, and upregulated pathways associated with cytotoxicity. Indeed, stimulation of both eosinophil subsets with IFNs enhanced their cytotoxic abilities ex vivo. Because of the proposed role of eosinophils in mediating ICB responses and the role of IFN in regulating eosinophil cytotoxicity, the impact of ICB and anti-IFN treatment on eosinophil subset regulation was examined in NT193 tumours. ICB treatment led Ly6C+ eosinophils to a more activated phenotype, while blocking IFN signalling resulted in defective degranulation.

These data provide new evidence that eosinophils are a highly plastic population shaped into a less active state by the progressing breast TME. While IFNs are potent activators of eosinophil cytotoxicity, further identification of factors driving the eosinophil transition into the Ly6C- state might help with preventing the loss of anti-tumorigenic functions and improve immunotherapy outcomes.

Authors: Varyova, Z

• DOI: 10.5287/ora-nokngdavp
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: tumour microenvironment; immunotherapy; breast cancer; eosinophils