Overactivation of FcγRI by immune complexes (IC) is implicated in various autoimmune disorders and neuropathies. Currently, no effective FcγRI-specific blocking antibodies are available. Here we report preclinical data revealing two anti-FcγRI antibodies, C01 and C04, with high affinity, Fab-mediated binding within the IgG binding site on extracellular domain 2 of FcγRI. Both C01 and C04 block 90% of IgG and IC binding, and displace ~60% of pre-bound ICs without activating FcγRI, thereby minimizing the risk of aggravating inflammation. In the context of autoimmunity, C01 and C04 inhibit RA patient-derived autoantibody-IC binding to monocytes, macrophages and activated neutrophils, meanwhile they also inhibit the binding of opsonized platelets to monocytes from patients with immune thrombocytopenia. In vivo, C01 and C04 reduce IgG-dependent platelet depletion in humanized immunodeficient FcRγ-/- mice. Structural studies confirm that C01 and C04 achieve their blocking effects through Fab-mediated binding to FcγRI. Our data thus suggest that C01 and C04 may offer therapeutic potential for autoimmune disorders.

Antibodies that specifically bind to individual human fragment crystallizable γ receptors (FcγRs) are of interest as research tools in studying immune cell functions, as well as components in bispecific antibodies for immune cell engagement in cancer therapy. Monoclonal antibodies for human low-affinity FcγRs have been successfully generated by hybridoma technology and are widely used in pre-clinical research. However, the generation of monoclonal antibodies by hybridoma technology that specifically bind to the high-affinity receptor FcγRI is challenging. Monomeric mouse IgG2a, IgG2b, and IgG3 bind human FcγRI with high affinity via the Fc part, leading to an Fc-mediated rather than a fragment for antigen binding (Fab)-mediated selection of monoclonal antibodies. Blocking the Fc-binding site of FcγRI with an excess of human IgG or Fc during screening decreases the risk of Fc-mediated interactions but can also block the potential epitopes of new antibody candidates. Therefore, we replaced hybridoma technology with phage display of a single-chain fragment variable (scFv) antibody library that was generated from mice immunized with FcγRI-positive cells and screened it with a cellular panning approach assisted by next-generation sequencing (NGS). Seven new FcγRI-specific antibody sequences were selected with this methodology, which were produced as Fc-silent antibodies showing FcγRI-restricted specificity.

Signal inhibitory receptor on leukocytes-1 (SIRL-1) is an immune inhibitory receptor expressed on human granulocytes and monocytes that dampens antimicrobial functions. We previously showed that sputum neutrophils from infants with severe respiratory syncytial virus (RSV) bronchiolitis have decreased SIRL-1 surface expression compared with blood neutrophils and that SIRL-1 surface expression is rapidly lost from in vitro activated neutrophils. This led us to hypothesize that activated neutrophils lose SIRL-1 by ectodomain shedding. Here, we developed an ELISA and measured the concentration of soluble SIRL-1 (sSIRL-1) in patients with RSV bronchiolitis and hospitalized patients with COVID-19, which are both characterized by neutrophilic inflammation. In line with our hypothesis, sSIRL-1 concentration was increased in sputum compared with plasma of patients with RSV bronchiolitis and in serum of hospitalized patients with COVID-19 compared with control serum. In addition, we show that in vitro activated neutrophils release sSIRL-1 by proteolytic cleavage and that this diminishes the ability to inhibit neutrophilic reactive oxygen species production via SIRL-1. Finally, we found that SIRL-1 shedding is prevented by proteinase 3 inhibition and by extracellular adherence protein from Staphylococcus aureus. Notably, we recently showed that SIRL-1 is activated by PSMα3 from S. aureus, suggesting that S. aureus may counteract SIRL-1 shedding to benefit from preserved inhibitory function of SIRL-1. In conclusion, we report that SIRL-1 is released from activated neutrophils by proteinase 3 cleavage and that endogenous sSIRL-1 protein is present in vivo.

Current combination therapies elicit high response rates in B cell malignancies, often using CD20 antibodies as the backbone of therapy. However, many patients eventually relapse or develop progressive disease. Therefore, novel CD20 antibodies combining multiple effector mechanisms were generated. To study whether neutrophil-mediated destruction of B cell malignancies can be added to the arsenal of effector mechanisms, we chimerized a panel of five previously described murine CD20 antibodies to the human IgG1, IgA1 and IgA2 isotype. Of this panel, we assessed in vitro antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC) and direct cell death induction capacity and studied the efficacy in two different in vivo mouse models. IgA antibodies outperformed IgG1 antibodies in neutrophil-mediated killing in vitro, both against CD20-expressing cell lines and primary patient material. In these assays, we observed loss of CD19 with both IgA and IgG antibodies. Therefore, we established a novel method to improve the assessment of B-cell depletion by CD20 antibodies by including CD24 as a stable cell marker. Subsequently, we demonstrated that only IgA antibodies were able to reduce B cell numbers in this context. Additionally, IgA antibodies showed efficacy in both an intraperitoneal tumor model with EL4 cells expressing huCD20 and in an adoptive transfer model with huCD20-expressing B cells. Taken together, we show that IgA, like IgG, can induce ADCC and CDC, but additionally triggers neutrophils to kill (malignant) B cells. We conclude that antibodies of the IgA isotype offer an attractive repertoire of effector mechanisms for the treatment of CD20-expressing malignancies.

Based on their mechanisms-of-action, CD20 monoclonal antibodies (mAbs) are grouped into Type I [complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC)] and Type II [programmed cell death (PCD) and ADCC] mAbs. We generated 17 new hybridomas producing CD20 mAbs of different isotypes and determined unique heavy and light chain sequence pairs for 13 of them. We studied their epitope binding, binding kinetics and structural properties and investigated their predictive value for effector functions, i.e. PCD, CDC and ADCC. Peptide mapping and CD20 mutant screens revealed that 10 out of these 11 new mAbs have an overlapping epitope with the prototypic Type I mAb rituximab, albeit that distinct amino acids of the CD20 molecule contributed differently. Binding kinetics did not correlate with the striking differences in CDC activity among the mIgG2c mAbs. Interestingly, chimerization of mAb m1 resulted in a mAb displaying both Type I and II characteristics. PCD induction was lost upon introduction of a mutation in the framework of the heavy chain affecting the elbow angle, supporting that structural changes within this region can affect functional activities of CD20 mAbs. Together, these new CD20 mAbs provide further insights in the properties dictating the functional efficacy of CD20 mAbs.

Interactions within the hematopoietic niche in the BM microenvironment are essential for maintenance of the stem cell pool. In addition, this niche is thought to serve as a sanctuary site for malignant progenitors during chemotherapy. Therapy resistance induced by interactions with the BM microenvironment is a major drawback in the treatment of hematologic malignancies and bone-metastasizing solid tumors. To date, studying these interactions was hampered by the lack of adequate in vivo models that simulate the human situation. In the present study, we describe a unique human-mouse hybrid model that allows engraftment and outgrowth of normal and malignant hematopoietic progenitors by implementing a technology for generating a human bone environment. Using luciferase gene marking of patient-derived multiple myeloma cells and bioluminescent imaging, we were able to follow pMM cells outgrowth and to visualize the effect of treatment. Therapeutic interventions in this model resulted in equivalent drug responses as observed in the corresponding patients. This novel human-mouse hybrid model creates unprecedented opportunities to investigate species-specific microenvironmental influences on normal and malignant hematopoietic development, and to develop and personalize cancer treatment strategies.

The development of resistance to chemotherapy is a major obstacle for lasting effective treatment of cancer. Here, we demonstrate that endogenous mesenchymal stem cells (MSCs) become activated during treatment with platinum analogs and secrete factors that protect tumor cells against a range of chemotherapeutics. Through a metabolomics approach, we identified two distinct platinum-induced polyunsaturated fatty acids (PIFAs), 12-oxo-5,8,10-heptadecatrienoic acid (KHT) and hexadeca-4,7,10,13-tetraenoic acid (16:4(n-3)), that in minute quantities induce resistance to a broad spectrum of chemotherapeutic agents. Interestingly, blocking central enzymes involved in the production of these PIFAs (cyclooxygenase-1 and thromboxane synthase) prevents MSC-induced resistance. Our findings show that MSCs are potent mediators of resistance to chemotherapy and reveal targets to enhance chemotherapy efficacy in patients.

Abstract Abstract 3412 Interactions with the hematopoietic niche in the bone marrow (BM) microenvironment are essential for hematopoietic stem cell (HSC) self-renewal. In addition, in hematological malignancies this niche is considered to serve as a sanctuary site for leukemic stem cells during chemotherapy, and to contribute to disease relapse. Although many advances have been made in understanding how the niche regulates HSC self-renewal and confers therapy resistance, most of this knowledge is based on genetic loss- or gain-of-function murine models. Since these models do not recapitulate the human physiology, there is a need for models that more closely resemble the human niche. Here, we describe a unique humanized model, which implements a novel scaffold-based technology for generating a human bone environment in RAG2−/−gc−/−-mice. Inoculation of these mice with normal human CD34+ hematopoietic progenitor cells, isolated from umbilical cord blood, resulted in homing to the human bone environment and the generation of human hematopoietic cells of distinct lineages, but more importantly also the engraftment of CD34+ cells themselves. In a next series of experiments the supportive nature of the humanized niche was further investigated with patient-derived acute myeloid leukemia (pAML) and multiple myeloma (pMM) cells, two hematopoietic malignancies that are highly dependent on the BM microenvironment for survival and growth. Inoculation of the humanized mice with pAML cells, obtained from a poor-risk patient (M1; complex karyotype) or cells from a good risk AML patient (M4; inv(16)) revealed the ability of the reconstructed human bone environment to support outgrowth of the leukemia with the cells having a similar phenotype as those from the patient sample. Interestingly, engraftment of good risk AML samples, including inv(16), has been reported to be very difficult in the NOD/SCID-based AML xenotransplant model. The humanized model that we developed was further substantiated by the ability to support the outgrowth of pMM from 7 out of 7 patients. MM is a hematological malignancy that fails to grow in mouse tissues without extra support, e.g. fetal human bone chips. Moreover, the outgrowth of pMM in our humanized model is accompanied by an increase in osteoclast activity, indicating the presence of bone resorption, one of the most relevant clinical sequelae of MM. In addition, by gene-marking pMM cells with luciferase and using bioluminescent imaging, we were able to follow myeloma outgrowth in time. Treatment of pMM-bearing mice with identical drugs as given to the patients showed that the pMM cells growing in the humanized environment in the mice responded similar as the MM patients. Hence, our model allows, for the first time, to investigate essential interactions within the human BM microenvironment for the development of normal and malignant hematopoiesis and thus for therapy development. Disclosures: de Bruijn: Xpand Biotechnology BV: Employment. Weers:Genmab BV: Employment. Parren:Genmab BV: Employment.

BackgroundIncorporation of the chimeric CD20 monoclonal antibody rituximab in the treatment schedule of patients with non-Hodgkin's lymphoma has significantly improved outcome. Despite this success, about half of the patients do not respond to treatment or suffer from a relapse and additional therapy is required. A low CD20-expression level may in part be responsible for resistance against rituximab. We therefore investigated whether the CD20-expression level related resistance to rituximab could be overcome by a new group of CD20 mAbs (HuMab-7D8 and ofatumumab) targeting a unique membrane-proximal epitope on the CD20 molecule.Design and MethodsBy retroviral transduction of the CD20 gene into CD20-negative cells and clonal selection of transduced cells a system was developed in which the CD20-expression level is the only variable. These CD20 transduced cells were used to study the impact of rituximab and HuMab-7D8 mediated complement-dependent cytotoxicity. To study the in vivo efficacy of these mAbs an in vivo imaging system was generated by retroviral expression of the luciferase gene in the CD20-positive cells.ResultsWe show that HuMab-7D8 efficiently killed CD20(low) cells that are not susceptible to rituximab-induced killing in vitro. In a mouse xenograft model, we observed a comparable increase in survival time between HuMab-7D8 and rituximab-treated mice. Most significantly, however, HuMab-7D8 eradicated all CD20-expressing cells both in the periphery as well as in the bone marrow whereas after rituximab treatment CD20(low) cells survived.ConclusionsCells that are insensitive to in vitro and in vivo killing by rituximab as the result of their low CD20-expression profile may be efficiently killed by an antibody against the membrane-proximal epitope on CD20. Such antibodies should, therefore, be explored to overcome rituximab resistance in the clinic.

Mutations of the genes encoding APC or ß-catenin in colon carcinoma induce the constitutive formation of nuclear ß-catenin/Tcf-4 complexes, resulting in activated transcription of Tcf target genes1,2. To study the physiological role of Tcf-4 (which is encoded by the Tcf7l2 gene), we disrupted Tcf7l2 by homologous recombination. Tcf7l2-/- mice die shortly after birth. A single histopathological abnormality was observed. An apparently normal transition of intestinal endoderm into epithelium occurred at approximately embryonic day (E) 14.5. However, no proliferative compartments were maintained in the prospective crypt regions between the villi. As a consequence, the neonatal epithelium was composed entirely of differentiated, non-dividing villus cells. We conclude that the genetic program controlled by Tcf-4 maintains the crypt stem cells of the small intestine. The constitutive activity of Tcf-4 in APC-deficient human epithelial cells may contribute to their malignant transformation by maintaining stem-cell characteristics.

Tcf/Lef transcription factors mediate signalling from Wingless/Wnt proteins by recruiting Armadillo/β-catenin as a transcriptional co-activator 1 , 2 , 3 , 4 , 5 , 6 , 7 . However, studies of Drosophila, Xenopus and Caenorhabditis elegans have indicated that Tcf factors may also be transcriptional repressors 6 , 8 , 9 , 10 , 11 , 12 , 13 . Here we show that Tcf factors physically interact with members of the Groucho family of transcriptional repressors. In transient transfection assays, the Xenopus Groucho homologue XGrg-4 inhibited activation of transcription of synthetic Tcf reporter genes. In contrast, the naturally truncated Groucho-family member XGrg-5 enhanced transcriptional activation. Injection of XGrg-4 into Xenopus embryos repressed transcription of Siamois and Xnr-3 , endogenous targets of β-catenin–Tcf. Dorsal injection of XGrg-4 had a ventralizing effect on Xenopus embryos. Secondary-axis formation induced by a dominant-positive Armadillo–Tcf fusion protein was inhibited by XGrg-4 and enhanced by XGrg-5. These data indicate that expression of Tcf target genes is regulated by a balance between Armadillo and Groucho.