Investigating the sialoglycan-Siglec axis in the prostate cancer tumour immune microenvironment

Abstract

Although immunotherapy which blocks immune checkpoint inhibitors is a mainstay agent for many cancer types, so far, prostate cancer patients have seen few benefits. One mechanism of immune evasion is the presentation of sialic acid on the surface of cancer cells. Sialyltransferases enzymes ST3GAL1 and ST6GAL1 are responsible for converting the addition of sialic acid to terminal glycans (known as sialoglycans) on the cell surface. The recognition of sialoglycans is mediated by Siglecs, which are expressed on the surface of immune cells and function as immune checkpoints. Despite engagement of the sialoglycan-siglec axis demonstrating inhibition of immune cell-mediated tumour clearance in alternative cancers, little is known about these interactions in prostate cancer. The overall aim of this study was to profile the sialyltransferases ST3GAL1 and ST6GAL1, their associated sialoglycans and Siglecs within the prostate cancer tumour immune microenvironment. The gene expression of sialyltransferases were explored in The Cancer Genome Atlas prostate cancer data set which revealed ST6GAL1 as the most highly expressed sialyltransferase gene, and ST3GAL1 as the most frequently altered. Immunohistochemistry identified ST3GAL1 protein expression to be upregulated in patient prostate cancer tissue compared to normal. Siglec-Fcs were utilised to comprehensively profile sialoglycans in prostate cancer cell lines and patient tissue by immunofluorescence, ELISA, and flow cytometry revealing ST3GAL1, ST6GAL1 and the AR targeting therapy enzalutamide to drive the expression of sialoglycans with the capacity to engage immunosuppressive Siglecs. Immunofluorescence in patient tissue detected the expression of Siglecs on tumour associated macrophages and was utilised for the optimisation of a novel antibody panel for exploring the sialoglycansiglec axis by Imaging Mass Cytometry. Furthermore, murine cancer models were applied to explore the effect of ST6GAL1 and ST3GAL1 silencing on tumour growth and revealed improved anti-cancer immunity which prevented tumour engraftment. In conclusion, the data presented within this study provides rationale for the use of glyco-immune checkpoint targeting therapies in the treatment of prostate cancer.