BACKGROUND:Sialic acid-binding immunoglobulin-like lectin (Siglec)-6 and Siglec-8 are closely related mast cell (MC) receptors with broad inhibitory activity, but whose functional differences are incompletely understood. METHODS:Proteomic profiling using quantitative mass spectrometry was performed on primary mouse MCs to identify proteins associated with Siglec-6 and Siglec-8. For functional characterization, each receptor was evaluated biochemically and in ex vivo and in vivo inhibition models of IgE and non-IgE-mediated MC activation in Siglec-6- or Siglec-8-expressing transgenic mice. RESULTS:Siglec-6 and Siglec-8 were found in MCs within large complexes, interacting with 66 and 86 proteins, respectively. Strikingly, Siglec-6 and Siglec-8 interacted with a large cluster of proteins involved in IgE and non-IgE-mediated MC activation, including the high affinity IgE receptor, stem cell factor (SCF) receptor KIT/CD117, IL-4 and IL-33 receptors, and intracellular kinases LYN and JAK1. Protein interaction networks revealed Siglec-6 and Siglec-8 had overlapping yet distinct MC functions, with a potentially broader regulatory role for Siglec-6. Indeed, Siglec-6 preferentially interacted with the mature form of KIT at the cell surface, and treatment with an anti-Siglec-6 antibody significantly inhibited SCF-mediated MC activation more in comparison to targeting Siglec-8. CONCLUSION:These data demonstrate a central role for Siglec-6 and Siglec-8 in controlling MC activation through interactions with multiple activating receptors and key signaling molecules. Our findings suggest that Siglec-6 has a role distinct from that of Siglec-8 in regulating MC function and represents a distinct potential therapeutic target in mast cell-driven diseases.

Immunomodulation of mast cell (MC) activity is warranted in allergic and inflammatory diseases where MCs have a central role in pathogenesis. Targeting Siglec-8, an inhibitory receptor on MCs and eosinophils, has shown promising activity in preclinical and clinical studies. While the intracellular pathways that regulate Siglec-8 activity in eosinophils have been well studied, the signaling mechanisms that lead to MC inhibition have not been fully elucidated. Here, we evaluate the intracellular signaling pathways of Siglec-8-mediated inhibition in primary MCs using an anti-Siglec-8 monoclonal antibody (mAb). Phospho-proteomic profiling of FcεRI-activated MCs revealed Siglec-8 mAb-treatment globally inhibited proximal and downstream kinases, leading to attenuated MC activation and degranulation. In fact, Siglec-8 was found to directly interact with FcεRI signaling molecules. Siglec-8 inhibition was dependent on both cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that interact with the SH2 containing protein phosphatase Shp-2 upon Siglec-8 phosphorylation. Taken together, these data support a model in which Siglec-8 regulates proximal FcεRI-induced phosphorylation events through phosphatase recruitment and interaction with FcεRIγ, resulting in global inhibition of MCs upon Siglec-8 mAb engagement.

Mast cells (MC) are key drivers of allergic and inflammatory diseases. Sialic acid-binding immunoglobulin-like lectin (Siglec)-6 is an immunoregulatory receptor found on MCs. While it is recognized that engaging Siglecs with antibodies mediates inhibition across immune cells, the mechanisms that govern this agonism are not understood. Here we generated Siglec-6 mAb clones (AK01 to AK18) to better understand Siglec-6-mediated agonism. Siglec-6 mAbs displayed epitope-dependent receptor internalization and inhibitory activity. We identified a Siglec-6 mAb (AK04) that required Fc-mediated interaction for receptor internalization and induced inhibition and antibody-dependent cellular phagocytosis against MCs. AK04-mediated MC inhibition required Siglec-6 immunoreceptor tyrosine-based inhibitory motif (ITIM) and ITIM-like domains and was associated with receptor cluster formation containing inhibitory phosphatases. Treatment of humanized mice with AK04 inhibited systemic anaphylaxis with a single dose and reduced MCs with chronic dosing. Our findings suggest Siglec-6 activity is epitope dependent and highlight an agonistic Siglec-6 mAb as a potential therapeutic approach in allergic disease.

Background Myeloid cells are the most abundant immune cells within the tumor microenvironment (TME) where they play important roles regulating anti-tumor immunity. Targeting myeloid-specific inhibitory receptors to modulate the TME represents an attractive strategy to improve the therapeutic outcome of current cancer immune therapies. Siglec-10 is an inhibitory receptor expressed on tumor-associated macrophages (TAMs) and dendritic cells that regulates immune cell activation via immunoreceptor tyrosine-based inhibitory motifs. Recently, Siglec-10 was shown to induce immunosuppression and promote tumor immune escape through interaction with CD24. Similarly, CD52 and vascular adhesion protein-1 have been shown to drive immunosuppression via Siglec-10, indicating that Siglec-10 functions as an inhibitory receptor through multiple ligands. Here, we report that Siglec-10 expression is upregulated in human tumors and blockade of Siglec-10 with a monoclonal antibody (mAb) enhances proinflammatory responses and delays tumor growth in vivo by modulating myeloid cell function. Methods Siglec-10 expression was evaluated in human tumors by flow cytometry and RNA-sequencing. Anti-human Siglec-10 mAbs were generated using hybridoma technology and recombinantly produced on mouse IgG1 backbones. Internalization and function of Siglec-10 mAbs were evaluated in vitro by flow cytometry and cytokine production using isolated human CD16+ monocytes. To assess the in vivo activity of a Siglec-10 mAb, transgenic mice expressing Siglec-10 were generated and challenged with poly I:C. Anti-tumor activity of a Siglec-10 mAb was determined using the MC38 syngeneic mouse model. Results RNA-sequencing data revealed high expression of Siglec-10 across multiple tumors compared to normal tissues, including glioblastoma, colorectal carcinoma, and kidney renal clear cell carcinoma. Specifically, Siglec-10 was found to be highly and selectively expressed on intratumor dendritic cells and macrophages. Siglec-10 mAb treatment blocked ligand binding, induced complete receptor internalization, and significantly enhanced TLR-mediated human monocyte activation in vitro. Similarly, in vivo administration of a Siglec-10 mAb induced receptor internalization and enhanced TLR-mediated inflammation as evidenced by increased levels of cytokines, such as TNF and IL-12p40 (figure 1). To assess the potential of a Siglec-10 mAb as an anti-cancer immunotherapy, we established a colon carcinoma model in Siglec-10 transgenic mice. Siglec-10 mAb blockade polarized myeloid cells towards an inflammatory phenotype, enhanced adaptive immune cell activation and promoted anti-tumor activity. Conclusions Siglec-10 is highly expressed on tumor-associated myeloid cells and antibody blockade promotes anti-tumor immunity through activation of TAMs and dendritic cells. Our findings highlight Siglec-10 as a promising myeloid cell target for enhancing anti-tumor immunity in solid tumors. Ethics Approval All legal and ethical requirements have been met with regards to the humane treatment of animals described in this study.

Mast cells are tissue-resident cells that contribute to allergic diseases, among others, due to excessive or inappropriate cellular activation and degranulation. Therapeutic approaches to modulate mast cell activation are urgently needed. Siglec-6 is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-bearing receptor selectively expressed by mast cells, making it a promising target for therapeutic intervention. However, the effects of its engagement on mast cells are poorly defined. Siglec-6 expression and endocytosis on primary human mast cells and mast cell lines were assessed by flow cytometry. SIGLEC6 mRNA expression was examined by single-cell RNAseq in esophageal tissue biopsy samples. The ability of Siglec-6 engagement or co-engagement to prevent primary mast cell activation was determined based on assessments of mediator and cytokine secretion and degranulation markers. Siglec-6 was highly expressed by all mast cells examined, and the SIGLEC6 transcript was restricted to mast cells in esophageal biopsy samples. Siglec-6 endocytosis occurred with delayed kinetics relative to the related receptor Siglec-8. Co-crosslinking of Siglec-6 with FcεRIα enhanced the inhibition of mast cell activation and diminished downstream ERK1/2 and p38 phosphorylation. The selective, stable expression and potent inhibitory capacity of Siglec-6 on human mast cells are favorable for its use as a therapeutic target in mast cell-driven diseases.

Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 infection represents a global health crisis. Immune cell activation via pattern recognition receptors has been implicated as a driver of the hyperinflammatory response seen in COVID-19. However, our understanding of the specific immune responses to SARS-CoV-2 remains limited. Mast cells (MCs) and eosinophils are innate immune cells that play pathogenic roles in many inflammatory responses. Here we report MC-derived proteases and eosinophil-associated mediators are elevated in COVID-19 patient sera and lung tissues. Stimulation of viral-sensing toll-like receptors in vitro and administration of synthetic viral RNA in vivo induced features of hyperinflammation, including cytokine elevation, immune cell airway infiltration, and MC-protease production-effects suppressed by an anti-Siglec-8 monoclonal antibody which selectively inhibits MCs and depletes eosinophils. Similarly, anti-Siglec-8 treatment reduced disease severity and airway inflammation in a respiratory viral infection model. These results suggest that MC and eosinophil activation are associated with COVID-19 inflammation and anti-Siglec-8 antibodies are a potential therapeutic approach for attenuating excessive inflammation during viral infections.

In addition to their well characterized role in mediating IgE-dependent allergic diseases, aberrant accumulation and activation of mast cells (MCs) is associated with many non-allergic inflammatory diseases, whereby their activation is likely triggered by non-IgE stimuli (e.g., IL-33). Siglec-8 is an inhibitory receptor expressed on MCs and eosinophils that has been shown to inhibit IgE-mediated MC responses and reduce allergic inflammation upon ligation with a monoclonal antibody (mAb). Herein, we evaluated the effects of an anti-Siglec-8 mAb (anti-S8) in non-allergic disease models of experimental cigarette-smoke-induced chronic obstructive pulmonary disease and bleomycin-induced lung injury in Siglec-8 transgenic mice. Therapeutic treatment with anti-S8 inhibited MC activation and reduced recruitment of immune cells, airway inflammation, and lung fibrosis. Similarly, using a model of MC-dependent, IL-33-induced inflammation, anti-S8 treatment suppressed neutrophil influx, and cytokine production through MC inhibition. Transcriptomic profiling of MCs further demonstrated anti-S8-mediated downregulation of MC signaling pathways induced by IL-33, including TNF signaling via NF-κB. Collectively, these findings demonstrate that ligating Siglec-8 with an antibody reduces non-allergic inflammation and inhibits IgE-independent MC activation, supporting the evaluation of an anti-Siglec-8 mAb as a therapeutic approach in both allergic and non-allergic inflammatory diseases in which MCs play a role.

Ectoenzyme CD38 is increased on lymphocytes in response to an antigenic challenge and it is hypothesized that targeting these activated lymphocytes could ameliorate pathologic activities in autoimmune diseases. The cynomolgus monkey is an appropriate model for assessing potential effects of targeting CD38 in humans because these species exhibit similar expression profiles. TAK-079 is a human monoclonal antibody (IgG1 λ ) that binds to CD38 and lyses bound cells by complement-dependent cytotoxicity and antibody-dependent cell-mediated cytotoxicity. TAK-079 binds to monkey CD38 with an affinity at EC50 4.5 nM, and the potential activity of TAK-079 was investigated in a monkey collagen-induced arthritis model of autoimmune disease. Prophylactic administration of TAK-079 (3 mg/kg i.v. weekly) was well tolerated and prevented arthritis development compared with vehicle-treated control animals, which exhibited progressive disease with radiographic damage and worsening clinical scores over the study course. Therapeutic treatment of arthritic monkeys with TAK-079 (3 mg/kg i.v. weekly) was also well tolerated and reduced disease progression and symptoms. Arthritis scores and joint swelling were significantly lower than the vehicle control, accompanied by decreases in blood levels of C-reactive protein, alkaline phosphatase, and natural killer, B, and T cells. Histopathology, morphometry, and radiology revealed significantly less joint damage in animals exposed prophylactically to TAK-079 treatment compared with vehicle-treated animals and significantly less damage in animals treated therapeutically with TAK-079 or dexamethasone (0.1 mg/kg oral gavage daily), illustrating potential disease-modifying activity. In conclusion, these data indicate that depletion of CD38-expressing cells could be a therapeutic mechanism for treating autoimmune diseases. SIGNIFICANCE STATEMENT: This study demonstrates that targeting CD38-expressing leukocytes with a cytolytic antibody can ameliorate autoimmune disease in cynomolgus monkeys. The study gives a unique perspective into this therapeutic strategy because the three other anti-CD38 cytolytic antibodies in clinical development (daratumumab, isatuximab, and MOR202) cannot be tested in similar models because they do not crossreact with CD38 expressed by new world primates.

Abstract TAK-079 is a fully human, non-agonistic, IgG1 cell-depleting monoclonal antibody that binds human CD38 with high affinity (KD 1.2 nM, KinExA) and cross reacts with cynomolgus monkey CD38. CD38 is a cADPR hydrolase expressed on human plasmablasts, plasma cells, NK cells and activated T and B cells, but not on mature platelets or red blood cells, based on TAK-079 binding. In patients with rheumatoid arthritis (RA) and systemic lupus erythematousus (SLE), plasma cells, as well as activated B and T cells may be important contributors to disease. Unlike other B cell-selective therapies which target CD20 and do not directly deplete plasmablasts, which are CD20low/negative, CD38 is expressed at high levels on plasmablasts and plasma cells making these cells a direct target of TAK-079. In vitro studies with human blood cells and cell lines showed that binding of TAK-079 to CD38 did not result in PBMC cytokine activation demonstrating that TAK-079 is not an agonist. Rather, it mediated depletion of human B lineage cell lines by ADCC and CDC and in most cases cell lines with increased CD38 expression were more susceptible to cell lysis. This is consistent with findings in healthy cynomolgus monkeys where the efficiency of depletion correlated with level of CD38 expression and TAK-079 dose level. NK cells, which express high levels of CD38, were depleted to a greater extent than CD20+ B cells and CD3+ T cells, which express less CD38. In vivo, TAK-079 potently suppressed the human B cell recall responses to antigen in a mouse adoptive transfer model. Together these data support the further investigation of TAK-079 in autoimmune diseases.

Interleukin-1 receptor–associated kinase 4 (IRAK4) plays a critical role in innate immune signaling by Toll-like receptors (TLRs), and loss of IRAK4 activity in mice and humans increases susceptibility to bacterial infections and causes defects in TLR and IL1 ligand sensing. However, the mechanism by which IRAK4 activity regulates the production of downstream inflammatory cytokines is unclear. Using transcriptomic and biochemical analyses of human monocytes treated with a highly potent and selective inhibitor of IRAK4, we show that IRAK4 kinase activity controls the activation of interferon regulatory factor 5 (IRF5), a transcription factor implicated in the pathogenesis of multiple autoimmune diseases. Following TLR7/8 stimulation by its agonist R848, chemical inhibition of IRAK4 abolished IRF5 translocation to the nucleus and thus prevented IRF5 binding to and activation of the promoters of inflammatory cytokines in human monocytes. We also found that IKKβ, an upstream IRF5 activator, is phosphorylated in response to the agonist-induced TLR signaling. Of note, IRAK4 inhibition blocked IKKβ phosphorylation but did not block the nuclear translocation of NFκB, which was surprising, given the canonical role of IKKβ in phosphorylating IκB to allow NFκB activation. Moreover, pharmacological inhibition of either IKKβ or the serine/threonine protein kinase TAK1 in monocytes blocked TLR-induced cytokine production and IRF5 translocation to the nucleus, but not nuclear translocation of NFκB. Taken together, our data suggest a mechanism by which IRAK4 activity regulates TAK1 and IKKβ activation, leading to the nuclear translocation of IRF5 and induction of inflammatory cytokines in human monocytes.

TAK-079 is a fully human, cytolytic IgG1 mAb against human CD38. We tested the ability of TAK-079 to treat arthritis using a monkey CIA model with features of symmetrical small joint polyarthritis resembling human RA. Monkeys were injected with bovine collagen type II in complete Freund’s adjuvant on Day 0 and Day 21 to induce arthritis. 1 monkey was used as a disease-free, age-matched control (naïve). In monkeys with CIA, the prophylactic group (n=8; 3 mg/kg, QW, iv) started treatment on Day 7 and continued for 8 weeks. The therapeutic treatment groups were randomized, after progression to overt disease, to 1 of 3 treatments: Vehicle (n=5; QW, iv), TAK-079 (n=7; 3 mg/kg, QW, iv), or dexamethasone (Dex; n=4; 0.1 mg/kg QD, po) and treated for 5 weeks. Prophylactic treatment with TAK-079 inhibited arthritis induction and therapeutic treatment reduced arthritic disease to a similar extent as Dex. TAK-079 treatment reduced CD38+ cells in the blood and spleen vs. naïve animal. After the first dose, blood monocytes, B and T cells were each reduced ≤ 55%, but reduction of monocyte and T cell counts were transient despite continued dosing. NK cells showed almost complete depletion. Neutrophil and platelet counts, elevated due to active arthritis, appeared normalized by TAK-079 vs. the naïve animal. TAK-079 treatment reduced systemic and local inflammation. Treated animals suffered less weight loss, and had reduced CRP and ALP levels and increased albumin levels. TAK-079 treatment reduced cartilage degradation and bone resorption in the IP and MCP joints based on radiology findings and confirmed by qualitative histological grading. TAK-079 was effective in treating CIA, supporting its continued investigation for the treatment of autoimmune disease.

The Wnt coreceptor LRP6 is required for canonical Wnt signaling. To understand the molecular regulation of LRP6 function, we generated a series of monoclonal antibodies against the extra cellular domain (ECD) of LRP6 and selected a high-affinity mAb (mAb135) that recognizes cell surface expression of endogenous LRP6. mAb135 enhanced Wnt dependent TCF reporter activation and antagonized DKK1 dependent inhibition of Wnt3A signaling, suggesting a role in modulation of LRP6 function. Detailed analysis of LRP6 domain mutants identified Ser 243 in the first propeller domain of LRP6 as a critical residue for mAb135 binding, implicating this domain in regulating the sensitivity of LRP6 to DKK1. In agreement with this notion, mAb135 directly disrupted the interaction of DKK1 with recombinant ECD LRP6 and a truncated form of the LRP6 ECD containing only repeats 1 and 2. Finally, we found that mAb135 completely protected LRP6 from DKK1 dependent internalization. Together, these results identify the first propeller domain as a novel regulatory domain for DKK1 binding to LRP6 and show that mAb against the first propeller domain of LRP6 can be used to modulate this interaction.

IREM-1 is an inhibitory cell surface receptor with an unknown function and is expressed on myeloid cell lineages, including cell lines derived from acute myeloid leukemia (AML) patients. We have generated a series of monoclonal antibodies (mAbs) against the extracellular domain of IREM-1 and further assessed its expression in normal and AML cells. IREM-1 was restricted to cells from myeloid origin and extensive expression analysis in primary cells obtained from AML patients showed IREM-1 expression in leukemic blasts of 72% (39/54) of samples. We therefore searched for specific IREM-1 mAbs with activity in functional complement-dependent cytotoxicity (CDC) and antibody-dependent cellular cytotoxicity (ADCC). Lead mAbs against IREM-1 showed specific cytotoxic activity against a variety of AML-derived cell lines and freshly isolated blasts from AML patients. Internalization of mAbs upon IREM-1 binding was also shown. In vivo anticancer activity of lead mAbs was observed in an established HL-60 xenograft model with a tumor growth delay of up to 40% and in a model using primary human AML cells, where treatment with anti-IREM-1 mAb resulted in a significant reduction of engrafted human cells. These results demonstrate IREM-1 as a potential novel target for immunotherapy of AML.

Background C-type lectin-like molecule-1 is a transmembrane receptor expressed on myeloid cells, acute myeloid leukemia blasts and leukemic stem cells. To validate the potential of this receptor as a therapeutic target in acute myeloid leukemia, we generated a series of monoclonal antibodies against the extracellular domain of C-type lectin-like molecule-1 and used them to extend the expression profile analysis of acute myeloid leukemia cells and to select cytotoxic monoclonal antibodies against acute myeloid leukemia cells in preclinical models.Design and Methods C-type lectin-like molecule-1 expression was analyzed in acute myeloid leukemia cell lines, and in myeloid derived cells from patients with acute myeloid leukemia and healthy donors. Anti-C-type lectin-like molecule-1 antibody-mediated in vitro cytotoxic activity against acute myeloid leukemia blasts/cell lines and in vivo anti-cancer activity in a mouse xenograft model were assessed. Internalization of C-type lectin-like molecule-1 monoclonal antibodies upon receptor ligation was also investigated.Results C-type lectin-like molecule-1 was expressed in 86.5% (45/52) of cases of acute myeloid leukemia, in 54.5% (12/22) of acute myeloid leukemia CD34+/CD38− stem cells, but not in acute lymphoblastic leukemia blasts (n=5). Selected anti-C-type lectin-like molecule-1 monoclonal antibodies mediated dose-dependent complement-dependent cytotoxicity and antibody-dependent cellular cytotoxicity specifically against acute myeloid leukemia-derived cell lines. Exogenous expression of the transmembrane receptor in HEK293 cells rendered the cells susceptible to antibody-mediated killing by monoclonal antibodies to the receptor. Furthermore, these monoclonal antibodies demonstrated strong complement-dependent cytotoxicity against freshly isolated acute myeloid leukemia blasts (15/16 cases; 94%). The monoclonal antibodies were efficiently internalized upon binding to C-type lectin-like molecule-1 in HL-60 cells. Moreover, a lead chimeric C-type lectin-like molecule-1 monoclonal antibody reduced the tumor size in xenograft mice implanted with HL-60 cells.Conclusions Our results demonstrate that targeting C-type lectin-like molecule-1 with specific cytotoxic monoclonal antibodies is an attractive approach which could lead to novel therapies for acute myeloid leukemia.

NTB-A is a CD2-related cell surface protein expressed on lymphoid cells including B-lymphocytes from Chronic Lymphocytic Leukemia (CLL) and lymphoma patients. We have generated a series of mAbs against NTB-A and assessed their therapeutic potential in preclinical models. Selected mAbs to NTB-A were further tested in functional Complement Dependent Cytotoxicity (CDC) and Antibody Dependent Cellular Cytotoxicty (ADCC) assays in cell lines and B lymphocytes freshly isolated from CLL and lymphoma patients. Potent cytotoxic activity was demonstrated against B cells isolated from CLL patients and B lymphoma cell lines. Chimeric anti-NTB-A mAbs demonstrated anti-tumor activity equal to rituximab against CA46 human lymphoma xenografts in nude mice at a low dose. In a chimpanzee safety study, a single dose of lead anti-NTB-A mAb 994.1 resulted in near-complete depletion of peripheral lymphocytes while having a minimal effect on T cell activation. Taken together, these results demonstrate NTB-A as a promising target with an acceptable safety profile for immunotherapy of leukemia and lymphomas.