Heterogeneity and immune microenvironment of early invasive estrogen receptor-positive breast cancer reveal an immune-rich subset

Approximately 80% of all breast cancer cases are estrogen receptor-positive (ER+). This subtype is known to have distant recurrences in a subset of patients after adjuvant endocrine therapy, partially due to the heterogeneity of the disease. ER+ breast cancer has been generally classified as an immune-cold disease. Thus, to better inform treatment decisions and to consider the prospect of immunotherapy, the immune microenvironment needs to be thoroughly characterized. In this study, the proteome and transcriptome of the tumour and tumour microenvironment (TME) were characterized using the GeoMx Digital Spatial Profiler (DSP) and NanoString's BC360 gene expression panel. Spatially resolved tumour and TME across a patient's lumpectomy demonstrated substantial heterogeneity in the expression of commonly targeted immune and tumour proteins. Results from this study demonstrated heterogeneity across the tumour and TME in ER+ breast cancer, which may be reflective of a variable immune response.