Investigating the effects of targeting stromal cell sialylation on the innate immune microenvironment in colorectal cancer

Cancer is a leading cause of mortality worldwide, with colorectal cancer (CRC) standing as a prevalent cause of cancer-related death. Advancements in anti-cancer therapeutics have improved disease outcome for some patients, however the resistant tumour microenvironment (TME) remains a critical contributor to immunosuppression and cancer progression, particularly in stromal-rich CRC tumours. Within the TME, stromal cells such as cancer-associated fibroblasts (CAFs) and mesenchymal stromal cells (MSCs) play pivotal functional roles in modulating tumour growth, enhancing immune evasion and invasion in CRC. Stromal cells have an inherent ability for immunomodulation. It is known that immune cells such as macrophages and T cells are located in the stroma of the TME. There is a need to discover and target novel mechanisms of immunosuppression in stromal-rich CRC to improve immunogenicity in poor response tumours. Sialylation as a hallmark of cancer is a post-translational modification (PTM) in which glycoproteins and glycolipids on the surface of cells are decorated with negatively charged sialic acids, which can propel clusters of cells from tumours, contributing to metastasis. Another key role for sialylation in cancer is tumour immune evasion, through the interaction of sialic acids with immune cell sialic acid-binding immunoglobulin like lectins (Siglecs). Sialylation of cancer cells has been largely explored as a novel therapeutic avenue, however the sialylation pattern and thus sialylation-dependent immunosuppression of stromal cells in the TME of CRC remains elusive, particularly that of innate immune cells. The work of this thesis investigates the potential efficacy of targeting sialylation of stromal cells to overcome immunosuppression in CRC. In primary human in vitro and ex vivo experiments, stromal cell sialylation was compared to CRC epithelium. We observed CRC stromal cell hypersialylation, with stromal cells expressing high levels of overall sialic acids and Siglec-7, -9 and -10 ligands. In immune co-culture experiments, stromal cellspecific induction of Siglec-10 was observed in macrophages and NK cells. Targeting stromal cell sialylation with sialidase led to increased NK cell cytotoxicity, highlighting the immunosuppressive role of stromal cell sialylation in CRC. Using an in vivo mouse model of CRC, stromal cell sialylation was suppressed using two targeting techniques – sialyltransferases inhibitor (SI) and sialidase (Sia), to assess the impact on immune cell infiltration and phenotypes, and the effects on tumour growth. Novel findings revealed sialic acid inhibition of stromal cells enhanced anti-tumour immune responses of macrophages and NK cells, and reduced tumour burden, highlighting the therapeutic potential of targeting stromal cell sialylation in CRC. Novel Siglec-9 ligands, galectin-1 and CD29, were identified in CRC stroma, highlighting these as potential biomarkers for further investigation in stromal-rich CRC. These findings advance our understanding of the complex interplay between stromal cells, sialylation and immune cells in the tumour immune microenvironment of CRC. These findings establish a basis for further exploration of sialylation as a driver of stromal cell-mediated immunosuppression in CRC, offering a promising future for translational research and improving disease outcomes in CRC.

Funder

Research Ireland

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)