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Simple Summary Changes in glycosylation, such as incomplete synthesis and higher density of O-glycans on the cell surface, are frequently observed in cancer cells. Several types of truncated O-glycan structures, e.g., T/Tn antigens, are suspected to disrupt molecular interactions between tumor microenvironment and immune cells, for instance, facilitating cancer immune-escape. Therefore, numerous exogenous lectins targeting aberrant O-glycans are interesting tools for cancer diagnosis, prognosis, and therapy. However, the ability of exolectins to detect subtle alterations in the glycome of tumor cells and to interfere in tumor/healthy cell interactions remains largely unknown. The present article reports for the first time that the Helix pomatia (HPA) lectin, a well-known T/Tn-specific lectin, currently used as a tool in cancer diagnostics, kills Tn-positive leukemia cells and Tn-negative lymphoma cells but does not affect healthy lymphocytes. Thus, HPA could be used to discriminate between tumor and healthy cells, and detect subtle alterations in the glycosylation profile. Abstract Morniga G is a T/Tn-specific lectin, inducing cell death in Tn-positive leukemias but not in healthy lymphocytes. Helix pomatia lectin (HPA) is another T/Tn-specific lectin, currently used as tool for cancer diagnostics. The HPA-mediated tumor cell death was evaluated on human leukemia and mouse lymphoma cells, and compared to the effect of Morniga G. Both lectins induced an equivalent percentage of cell death in Tn-positive Jurkat human leukemia. In contrast, EL4 mouse lymphoma resisted Morniga G-mediated cytotoxicity but were killed by HPA at concentrations of 2.5 μg/mL (0.032 nM) and higher. In both malignant cells, HPA-mediated cell death showed features compatible with apoptosis (annexin-externalization, caspase-activation, mitochondrial membrane depolarization, and ROS production). Cytometry analysis indicated that EL4 cells are T/Tn-negative. Because previous results showed a high amount of N-acetylgalactosamine (GalNAc, sugar present in Tn antigen) on EL4 cell surface, this GalNAc could be involved in the formation of truncated O-glycans other than the T/Tn residues. When compared to Morniga G, bioinformatic analysis suggested that HPA benefits from an extended carbohydrate-binding site, better adapted than Morniga G to the accommodation of more complex branched and truncated O-glycans (such as core 2). Finally, HPA killed EL4 cells but not healthy lymphocytes in a mixture of lymphoma cells + lymphocytes, suggesting that HPA selectively triggers tumor cell death.

CD44 variant isoforms are often upregulated in cancer and associated with increased aggressive tumor phenotypes. The CD44v9 is one of the major protein splice variant isoforms expressed in human gastrointestinal cancer cells. Immunodetection of CD44 isoforms like CD44v9 in tumor tissue is almost exclusively performed by using specific monoclonal antibodies. However, the structural variability conferred by both the alternative splicing and CD44 protein glycosylation is disregarded. In the present work, we have evaluated the role of O-glycosylation using glycoengineered gastric cancer models in the detection of CD44v9 by monoclonal antibodies. We demonstrated, using different technical approaches, that the presence of immature O-glycan structures, such as Tn and STn, enhance CD44v9 protein detection. These findings can have significant implications in clinical applications mainly at the detection and targeting of this cancer-related CD44v9 isoform and highlight the utmost importance of considering glycan structures in cancer biomarker detection and in therapy targeting. This work was funded by FEDER funds through the Operational Programme for Competitiveness Factors-COMPETE, grant numbers POCI-01-0145-FEDER-016585; POCI-01-0145-FEDER-007274; OCI-01-0145-FEDER-031028; and national funds through the Foundation for Science and Technology (FCT), grant numbers PTDC/BBB-EBI/0567/2014 (to CAR), UID/BIM/04293/2013, and PTDC/MED-QUI/29780/2017; and the project NORTE-01-0145-FEDER-000029, supported by Norte Portugal Regional Programme (NORTE 2020), under the PORTUGAL 2020 Partnership Agreement, through the European Regional Development Fund (ERDF). F. Pinto received a fellowship from FCT (SFRH/BPD/115730/2016).