The invention provides methods and compositions for treating Siglec-9 mediated cancer in a subject, where the cells of the cancer express sialylated Core-1-MUC1 glycoproteins that engage with Siglec-9 expressed on certain immune cells, for example, monocytes and macrophages of the subject. Prior to treatment, the cancerous cells may evade the immune system of the host by binding Siglec-9 expressed by the immune cells, whereupon binding activates a number of pro-tumorigenic, Siglec-9 mediated activities in or via the immune cells. However, when treated with an inhibitor of Siglec-9 activity, the Siglec-mediated activities can be mitigated and the host immune system can recognize and elicit an immune response against the cancer cells expressing the sialylated Core-1 MUC1 glycoproteins.
