Abstract
Breast cancer is currently one of the most commonly diagnosed cancers, both in New Zealand and globally. Making up 75% of breast cancer diagnoses is the subtype, oestrogen receptor positive (ER+) breast cancer, characterised by tumoural overexpression of the oestrogen receptor. The development of endocrine therapies, such as letrozole, to prevent oestrogen signaling in ER+ breast cancer has greatly improved patient outcomes. However, a significant proportion of treated patients develop resistance and experience disease progression.
A growing area of interest is the use of immunotherapies, including immune checkpoint inhibitors, to treat ER+ breast cancer. One key immune checkpoint in mammals is the negative co-stimulatory interaction between T cells and antigen presenting cells as a mechanism of controlling autoimmune reactivity. However, solid cancers can hijack this
mechanism by upregulating immune checkpoint ligands as a way to evade immune mediated destruction. Immune checkpoint inhibitors block this interaction to prevent tumour cells
from going undetected by the immune system. With excellent efficacy displayed in some solid tumour types, immune checkpoint inhibitors could be a good avenue for treatment of ER+ breast cancer, yet knowledge of their molecular mechanisms in ER+ breast cancer is
limited.
This study aimed to investigate the molecular actions of anti-PD-1 and anti-PD-L1 immune checkpoint inhibitors in mouse mammary tumours as a model for ER+ breast cancer. Understanding the molecular basis behind treatments is crucial to understand mechanisms
of resistance and to identify patients likely to respond to treatment. These aims were addressed through investigation of transcriptional and cellular changes in mouse mammary tumours treated with anti-PD-1, anti-PD-L1, letrozole, and a combination of letrozole + anti-
PD-1/anti-PD-L1. RNA-seq and immunohistochemistry/immunofluorescence analyses identified that both anti-PD-1 and anti-PD-L1 monotherapies had minimal influence on both transcriptional responses and cellular numbers. However, the combination of letrozole +
anti-PD-1 treatment displayed encouraging synergy through enhanced activation of T cells and decreased mast cell numbers. The interplay between letrozole and anti-PD-1 treatments
has been suggested to occur through the combination of inhibition of the ERβ/Ccl2 pathway and inhibition of PD-1/PD-L1 signaling caused by letrozole and anti-PD-1 treatments respectively. Together with observations from previous studies in other tumour types, this
work suggests that anti-PD-1 may promote the release of Ccl2 from mast cells, which feeds back to T cells to maintain suppression, and letrozole treatment in combination could prevent
the production of Ccl2 through reduced ERβ signaling. Finally, letrozole + anti-PD-L1 treatment did not show these cellular changes, suggesting that the interaction of PD-1 with its alternative ligand PD-L2, also thought to be expressed by mast cells, may maintain T cell suppression.
Anti-PD-1 and anti-PD-L1 treatments alone do not appear to cause a large anti-tumoural response in ER+ breast cancer, but the synergism of letrozole + anti-PD-1 treatments indicates an encouraging avenue of further research to validate the mechanisms proposed. Combining other treatments has shown great promise in reducing tumour resistance and
improving response, so this combination may be of use in ER+ breast cancer patients.