Ibarlucea-Benitez et al. generated Siglec-E KO and combined Siglec-E KO/human Siglec-7/9 transgenic mice and showed that these receptors are expressed on peripheral and tumor-infiltrating immune cells. Siglec-E KO enhanced antitumor responses only in a lung colonization model, which lacked a strong immune-suppressive milieu. Murine expression of Siglecs E or 7/9 inhibited endogenous and antibody-mediated antitumor responses, which were reduced by increasing tumor levels of Siglec ligands. Siglec-E targeting and immune checkpoint inhibition synergized, and Siglec-7 or -9 blockade reduced tumor burden in Siglec-7/9 transgenic/Siglec-E KO mice.
Contributed by Paula Hochman
ABSTRACT: Given the role of myeloid cells in T cell activation and in the antitumor response, targeting checkpoint molecules expressed on this population represents a promising strategy to augment antitumor immunity. However, myeloid checkpoints that can be effectively used as immunotherapy targets are still lacking. Here, we demonstrate the therapeutic potential of targeting the myeloid receptors Siglec-7 and Siglec-9 in vivo. By using a humanized immunocompetent murine model, we demonstrate that human Siglec-7 and Siglec-9, in addition to the murine homolog Siglec-E, inhibit the endogenous antitumor immune response, as well as the response to tumor-targeting and immune checkpoint inhibiting antibodies in vivo. The impact of these Siglecs on tumor progression is highly dependent on the anatomical distribution of the tumor and, as a consequence, the local tumor microenvironment, as tumors with a more immune-suppressive tumor microenvironment are less sensitive to Siglec perturbation. Finally, to assess the potential of these two receptors as targets for immunotherapy, we developed Fc engineered blocking antibodies to Siglec-7 and Siglec-9 and demonstrate that Siglec-7 and Siglec-9 blockade can significantly reduce tumor burden in vivo, demonstrating the therapeutic potential of targeting these two receptors.