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262 Preclinical characterization of humanized anti-siglec-15 antibodies that rescue T cells from macrophage-mediated immune suppression
  1. Huyen Dinh,
  2. Sam Lam,
  3. Valerie Wall,
  4. Francisco Zapata,
  5. Darbie Whitman,
  6. Ramya Chandrasekaran,
  7. Lauren Loh,
  8. Texia Loh,
  9. Tom Graddis,
  10. Peter Probst and
  11. Myriam Bouchlaka
  1. OncResponse Inc., Seattle, WA, USA


Background Siglec-15 is an immunosuppressive sialic acid-binding Ig-like lectin expressed by myeloid cells, tumor associated macrophages (TAMs), and some human tumors. Interactions between Siglec-15 on TAMs and sialoglycans found on cancer cells contribute to the immunosuppressive tumor microenvironment. Furthermore, Siglec-15 expressed by TAMs inhibits anti-tumor immune responses by engaging unknown immune checkpoint(s) on T cells. Notably, the mutually exclusive expression of Siglec-15 and the checkpoint ligand PD-L1 in the tumor tissue emphasizes Siglec-15 as an attractive target for combination immunotherapy. Anti-Siglec-15 antibody is currently being evaluated in clinical trials for the treatment of cancer.

Methods Anti-Siglec-15 antibodies were cloned from B cells derived from rabbits immunized with human Siglec-15 protein. The antibodies were evaluated for binding to human and cynomolgus Siglec-15 by enzyme-linked immunosorbent assay (ELISA). Top clones were selected based on activity in a panel of functional and phenotypic assays using primary human macrophages and T cells and were subsequently fully humanized. The humanized anti-Siglec-15 IgG1 antibodies were screened for binding to human and cynomolgus Siglec-15 by ELISA, binding to cells expressing Siglec-15, and ability to rescue T cell functional activity (proliferation and IFN-γ) from M2c-mediated immune suppression in vitro. The pharmacokinetics of lead humanized Siglec-15 IgG1 antibodies and their anti-tumor activity were evaluated in humanized mouse models.

Results We have identified a panel of fully humanized anti-Siglec-15 antibodies that bind to recombinant human and cynomolgus Siglec-15 proteins, to Siglec-15-expressing cell lines and immunosuppressive M2c macrophages without appreciable binding to other Siglec family members. Lead antibodies were identified using functional screens modeling Siglec-15-mediated immune suppression by M2c macrophages. These antibodies restored T cell immune responses in two different M2c/CD8 T cell coculture assays. In the first model, lead antibodies rescued the proliferative and IFN-γ responses of anti-CD3-activated human T cells from the inhibitory activity of M2c macrophages. In the second model, these antibodies restored the ability of exhausted CD8 T cells to secrete IFN-γ in the presence of M2c macrophages. Lead antibodies demonstrated a half-life of 6–11 days in humanized FcRn mice, and tumor growth inhibition in humanized NSG-SGM3 mice.

Conclusions We have identified novel humanized anti-Siglec-15 antibodies that restore effector function of activated and exhausted T cells from M2c-mediated immune suppression, with excellent half-life and anti-tumor activity in humanized mouse models. These data provide a strong rationale for further development of these antibodies for anti-cancer immunotherapy.

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