Siglec-7 and Siglec-9 expression in primary triple negative and oestrogen receptor positive breast cancer and

Objectives: PD-1/PD-L1 immune checkpoint blockade can be an effective treatment for advanced breast cancer patients. However, patients with oestrogen receptor positive (ER+) tumors often display only low lymphocyte infiltration, while a large part of triple negative (TN) breast tumors does not generate an effective immunotherapy response. Therefore, new treatment strategies have to be developed. Here, we investigate Siglec-7 and Siglec-9 as novel ITIM-bearing inhibitory immune checkpoint receptors similar to PD-1, but expressed on a broader range of immune cells. Methods: We assessed Siglec-7 and Siglec-9 (ligand) expression in TN and ER+ breast cancer tumors and their breast cancer cell line-induced signalling. Results: We report that Siglec-7 and Siglec-9 are highly expressed in TN tumors, and to a low extent in ER+ tumors. Siglec-7 was observed on myeloid cells, T cells, and NK cells and Siglec-9 preferentially on myeloid cells. Expression of sialoglycans, including Siglec-7 and Siglec-9 ligands, was observed in both TN and ER+ breast cancer tissue sections. Expression levels of Siglec-7 and Siglec-9 ligands were higher on Conclusion: These data imply that immunotherapy targeting Siglec receptors may be particularly interesting for TN breast cancer patients not responding to current treatment strategies with tumors displaying high immune cell infiltration.