Unraveling the impact of a glyco-immune checkpoint in bone metastasis

Breast cancer is the most common cancer in women (1). Breast cancer frequently metastasizes to the bones, which significantly impacts patient prognosis and quality of life. This process involves complex interactions between cancer cells and the bone microenvironment, disrupting bone remodeling and leading to osteoclast activation, bone destruction, and the formation of supportive niches for cancer cell proliferation (2). Wang and collaborators have shown an involvement of Siglec-15/sialic acid interaction in the context of bone metastasis associated with breast cancer (3). In their study, the researchers combined analyses of human patient samples with in vitro and in vivo experimentation to elucidate the role of Siglec-15 and interactions with sialoglycan ligands in bone metastasis formation. The authors demonstrate an upregulation of Siglec-15 in the bone metastatic niche in primary human cancer samples and mouse models. They further show that blocking of Siglec-15 inhibits osteoclast formation and Siglec-15 on osteoclasts can inhibit T cell activation. Blockade of Siglec-15 in a syngeneic murine metastasis model also showed a significant reduction of metastasis formation. Importantly, new metastasis formation from existing bone lesions was inhibited by the application of a Siglec-15 blocking antibody. The study by Wang et al. unveils a pivotal role of Siglec-15 in orchestrating the complex interplay between breast cancer cells and the bone microenvironment. The identification of Siglec-15 as a key player in this process not only sheds light on the underlying mechanisms of bone metastasis but also presents a promising avenue for therapeutic interventions (Fig. 1).

Fig. 1.

Siglec-15 influences multiple interactions within the bone metastatic niche. Siglec-15 is expressed on osteoclasts and myeloid cells within the tumor metastatic microenvironment including on tumor-associated macrophages, and interference with Siglec-15 can inhibit osteoclast formation and lead to improved intracellular T cell activation. In addition, the data by Wang and colleagues further demonstrate that metastasis formation coming from bone lesions is strongly diminished when Siglec-15 is blocked with an antibody (the figure was generated with the help of Biorender).

Immunotherapy, in particular with immune checkpoint inhibitors (ICI) has improved the treatment of cancer patients significantly during the past years. ICI block inhibitory receptors on immune cells or their ligands. Blocking antibodies against PD-1/PD-L1, CTLA-4, and recently LAG-3 have been approved for the treatment of cancer (4). If immunotherapy works well, the immune system can clear sometimes even advanced cancer and lead to long-term survival in patients (4). However, only a minority of cancer patients respond to currently available cancer immunotherapies (4). New approaches are therefore needed to improve cancer immunotherapy. The identification of the sialic acid–Siglec axis as an immune-suppressive network in cancer has led to its definition as the first glyco-immune checkpoint (5). Several groups have shown that sialic acid-containing glycans (sialoglycans) are up-regulated in cancer and engagement of sialic acid–binding immunoglobulin-like lectins (Siglecs) on immune cells can be engaged and modulate immune cell activation (6). For example, sialoglycan-mediated engagement of Siglec-7 and Siglec-9 on NK cells could reduce tumor cell killing (7, 8). In addition, some Siglec receptors including Siglec-9 are up-regulated on tumor-infiltrating T cells (9). Inhibition of Siglec-9 on anti-tumor T cells could therefore be an additional strategy to improve cancer immunotherapy. Sialoglycan–Siglec interactions were also shown to influence polarization of tumor-associated macrophages and phagocytosis (10–12).

Siglec-15 has been identified as an inhibitor of T cell activation in an unbiased surface protein screen for factors inhibiting T cell proliferation and activation (13). Furthermore, Siglec-15 has been shown to be up-regulated in several cancer types (14). Siglec receptors are divided depending on their conservation between mammalian species. Conserved human Siglecs include Siglec-1, Siglec-2 (CD22), Siglec-4, and Siglec-15. Rapidly evolving, human CD33-related Siglecs include Siglec-3 (CD33), Siglec-5, Siglec-6, Siglec-7, Siglec-8, Siglec-9, Siglec-10, Siglec-11, Siglec-14, and Siglec-16. Many Siglec receptors have an intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM) or ITIM-like domain, which facilitate the recruitment of SHP1 and SHP2 phosphatases upon activation of the receptors mediating immune inhibition. Some Siglecs have a positively charged amino acid that can lead to the recruitment of DAP10 or DAP12 that contain an immunoreceptor tyrosine-based activation motif mediating cell activation (6). Siglec-15 belongs to the class of conserved Siglec receptors and can recruit DAP12 to transmit activating intracellular signals (15). Siglec-15 has been found on myeloid cells, in particular, macrophages but also osteoclasts (15). On osteoclasts, Siglec-15 has a regulatory role (14, 16). Removal of sialoglycan ligands by sialidase or also the use of a mutant that lacks the essential arginine within the carbohydrate recognition domain for sialic acid binding led to impaired osteoclast development (16). Preclinical data in mouse models have shown that tumor growth could be inhibited by the use of a Siglec-15 blocking antibody (13). Based on these preclinical findings, the humanized antibody NC318 was tested in immunogenic solid cancers and showed some initial promising results in a phase I clinical trial, in particular in patients with non–small cell lung cancer [NSCLC, (14)]. Although monotherapy in a phase II study seemed to have limited efficacy, a phase II trial in combination with the PD-1 blocking pembrolizumab in patients with NSCLC is currently ongoing (NCT04699123).

Wang et al. have shown an involvement of Siglec-15/sialic acid interaction in the context of bone metastasis associated with breast cancer.

The Siglec–sialoglycan glyco-immune checkpoint can be targeted in different ways. A classical approach is the use of Siglec-targeting antibodies that block or engage the function of a certain Siglec receptor as it has been pursued in the case of Siglec-15 (6). The advantage is a high specificity. Another approach is the targeting of sialglycan ligand synthesis with small molecules or an enzymatic removal of sialic acid residues with a sialidase (17, 18). For example, directing a sialidase to HER2 on tumor cells led to a significant activity via inhibitory Siglec receptors on intratumoral myeloid cells including tumor-associated macrophages (10). Currently, a trial testing an approach with a sialidase in patients with solid tumors is ongoing (GLIMMER-01, NCT05259696). The disadvantage of using sialic acid targeting approaches is the lack of specificity. Sialoglycans serve various functions beyond their role as ligands for immuno-modulatory Siglec receptors (19). For example, the stability of surface receptors is dependent on sialic acid residues. Moreover, upon the removal of sialic acid, underlying galactose residues may be exposed, facilitating interactions with galectins (20). Galectins are a class of immune-modulatory lectins that can have a significant influence on cancer progression themselves (20). It remains therefore to be determined what is the better approach, and this might be dependent on the cancer type and stage.

Although Siglec-15 is an activating receptor, its immune-inhibitory effect in cancer has been well documented in preclinical models (14). The exact intracellular signaling, e.g., in osteoclasts or also in intratumoral myeloid cells, requires further investigations. For example, it remains unclear whether sialoglycan ligands need to be on the same cell (in the “cis” position) or are on interacting cells (in the “trans” position). Moreover, the nature of sialoglycan ligands remains to be determined. Some Siglecs can interact with proteins in a sialic acid–independent manner. Further investigations are therefore required.

The identification of Siglec-15 as a key player in breast cancer bone metastasis not only sheds light on the underlying mechanisms but also presents a promising avenue for a more specific therapeutic intervention. The expression of Siglec-15 on osteoclasts, which are important players in this process, supports the potential use of Siglec-15 in patients with bone metastasis in particular breast cancer metastasis. Exploring whether patients with bone metastasis derive greater benefits from Siglec-15 blocking monotherapies compared to those with alternative metastatic patterns presents an intriguing avenue for investigation. Additionally, delving into the potential complementary roles of other agents targeting osteoclasts, such as antibodies targeting RANK ligand, could offer valuable insights for future research. Taken together, the manuscript is a major step forward in the understanding of the role of Siglec receptors in cancer and supports the further investigation of Siglec-15 targeting agents in clinical research.