A broadly-neutralizing antibody against Ebolavirus glycoprotein that potentiates the breadth and neutralization potency of other antibodies.

Ebolavirus disease (EVD) is caused by multiple species of Ebolavirus. Monoclonal antibodies (mAbs) against the virus glycoprotein (GP) are the only class of therapeutic approved for treatment of EVD caused by Zaire ebolavirus (EBOV). Therefore, mAbs targeting multiple Ebolavirus species may represent the next generation of EVD therapeutics. Broadly reactive anti-GP mAbs were produced; among these, mAbs 11886 and 11883 were broadly neutralizing in vitro. A 3.0 Å cryo-electron microscopy structure of EBOV GP bound to both mAbs shows that 11886 binds a novel epitope bridging the glycan cap (GC), 310 pocket and GP2 N-terminus, whereas 11883 binds the receptor binding region (RBR) and GC. In vitro, 11886 synergized with a range of mAbs with epitope specificities spanning the RBR/GC, including 11883. Notably, 11886 increased the breadth of neutralization by partner mAbs against different Ebolavirus species. These data provide a strategic route to design improved mAb-based next-generation EVD therapeutics.

Generation of a single-cell B cell atlas of antibody repertoires and transcriptomes to identify signatures associated with antigen specificity.

Although new genomics-based pipelines have potential to augment antibody discovery, these methods remain in their infancy due to an incomplete understanding of the selection process that governs B cell clonal selection, expansion, and antigen specificity. Furthermore, it remains unknown how factors such as aging and reduction of tolerance influence B cell selection. Here we perform single-cell sequencing of antibody repertoires and transcriptomes of murine B cells following immunizations with a model therapeutic antigen target. We determine the relationship between antibody repertoires, gene expression signatures, and antigen specificity across 100,000 B cells. Recombinant expression and characterization of 227 monoclonal antibodies revealed the existence of clonally expanded and class-switched antigen-specific B cells that were more frequent in young mice. Although integrating multiple repertoire features such as germline gene usage and transcriptional signatures failed to distinguish antigen-specific from nonspecific B cells, other features such as immunoglobulin G (IgG) subtype and sequence composition correlated with antigen specificity.

CD1a promotes systemic manifestations of skin inflammation.

Inflammatory skin conditions are increasingly recognised as being associated with systemic inflammation. The mechanisms connecting the cutaneous and systemic disease are not well understood. CD1a is a virtually monomorphic major histocompatibility complex (MHC) class I-like molecule, highly expressed by skin and mucosal Langerhans cells, and presents lipid antigens to T-cells. Here we show an important role for CD1a in linking cutaneous and systemic inflammation in two experimental disease models. In human CD1a transgenic mice, the toll-like receptor (TLR)7 agonist imiquimod induces more pronounced splenomegaly, expansion of the peripheral blood and spleen T cell compartments, and enhanced neutrophil and eosinophil responses compared to the wild-type, accompanied by elevated skin and plasma cytokine levels, including IL-23, IL-1α, IL-1ß, MCP-1 and IL-17A. Similar systemic escalation is shown in MC903-induced skin inflammation. The exacerbated inflammation could be counter-acted by CD1a-blocking antibodies, developed and screened in our laboratories. The beneficial effect is epitope dependent, and we further characterise the five best-performing antibodies for their capacity to modulate CD1a-expressing cells and ameliorate CD1a-dependent systemic inflammatory responses. In summary, we show that a therapeutically targetable CD1a-dependent pathway may play a role in the systemic spread of cutaneous inflammation.

Phenotypic determinism and stochasticity in antibody repertoires of clonally expanded plasma cells.

The capacity of humoral B cell-mediated immunity to effectively respond to and protect against pathogenic infections is largely driven by the presence of a diverse repertoire of polyclonal antibodies in the serum, which are produced by plasma cells (PCs). Recent studies have started to reveal the balance between deterministic mechanisms and stochasticity of antibody repertoires on a genotypic level (i.e., clonal diversity, somatic hypermutation, and germline gene usage). However, it remains unclear if clonal selection and expansion of PCs follow any deterministic rules or are stochastic with regards to phenotypic antibody properties (i.e., antigen-binding, affinity, and epitope specificity). Here, we report on the in-depth genotypic and phenotypic characterization of clonally expanded PC antibody repertoires following protein immunization. We find that clonal expansion drives antigen specificity of the most expanded clones (top ∼10), whereas among the rest of the clonal repertoire antigen specificity is stochastic. Furthermore, we report both on a polyclonal repertoire and clonal lineage level that antibody-antigen binding affinity does not correlate with clonal expansion or somatic hypermutation. Last, we provide evidence for convergence toward targeting dominant epitopes despite clonal sequence diversity among the most expanded clones. Our results highlight the extent to which clonal expansion can be ascribed to antigen binding, affinity, and epitope specificity, and they have implications for the assessment of effective vaccines.

A conformation-selective monoclonal antibody against a small molecule-stabilised signalling-deficient form of TNF.

We have recently described the development of a series of small-molecule inhibitors of human tumour necrosis factor (TNF) that stabilise an open, asymmetric, signalling-deficient form of the soluble TNF trimer. Here, we describe the generation, characterisation, and utility of a monoclonal antibody that selectively binds with high affinity to the asymmetric TNF trimer-small molecule complex. The antibody helps to define the molecular dynamics of the apo TNF trimer, reveals the mode of action and specificity of the small molecule inhibitors, acts as a chaperone in solving the human TNF-TNFR1 complex crystal structure, and facilitates the measurement of small molecule target occupancy in complex biological samples. We believe this work defines a role for monoclonal antibodies as tools to facilitate the discovery and development of small-molecule inhibitors of protein-protein interactions.

Distinctive binding properties of human monoclonal LGI1 autoantibodies determine pathogenic mechanisms.

Autoantibodies against leucine-rich glioma inactivated 1 (LGI1) are found in patients with limbic encephalitis and focal seizures. Here, we generate patient-derived monoclonal antibodies (mAbs) against LGI1. We explore their sequences and binding characteristics, plus their pathogenic potential using transfected HEK293T cells, rodent neuronal preparations, and behavioural and electrophysiological assessments in vivo after mAb injections into the rodent hippocampus. In live cell-based assays, LGI1 epitope recognition was examined with patient sera (n = 31), CSFs (n = 11), longitudinal serum samples (n = 15), and using mAbs (n = 14) generated from peripheral B cells of two patients. All sera and 9/11 CSFs bound both the leucine-rich repeat (LRR) and the epitempin repeat (EPTP) domains of LGI1, with stable ratios of LRR:EPTP antibody levels over time. By contrast, the mAbs derived from both patients recognized either the LRR or EPTP domain. mAbs against both domain specificities showed varied binding strengths, and marked genetic heterogeneity, with high mutation frequencies. LRR-specific mAbs recognized LGI1 docked to its interaction partners, ADAM22 and ADAM23, bound to rodent brain sections, and induced internalization of the LGI1-ADAM22/23 complex in both HEK293T cells and live hippocampal neurons. By contrast, few EPTP-specific mAbs bound to rodent brain sections or ADAM22/23-docked LGI1, but all inhibited the docking of LGI1 to ADAM22/23. After intrahippocampal injection, and by contrast to the LRR-directed mAbs, the EPTP-directed mAbs showed far less avid binding to brain tissue and were consistently detected in the serum. Post-injection, both domain-specific mAbs abrogated long-term potentiation induction, and LRR-directed antibodies with higher binding strengths induced memory impairment. Taken together, two largely dichotomous populations of LGI1 mAbs with distinct domain binding characteristics exist in the affinity matured peripheral autoantigen-specific memory pools of individuals, both of which have pathogenic potential. In human autoantibody-mediated diseases, the detailed characterization of patient mAbs provides a valuable method to dissect the molecular mechanisms within polyclonal populations.

Cyto-Mine: An Integrated, Picodroplet System for High-Throughput Single-Cell Analysis, Sorting, Dispensing, and Monoclonality Assurance.

The primary goal of bioprocess cell line development is to obtain high product yields from robustly growing and well-defined clonal cell lines in timelines measured in weeks rather than months. Likewise, high-throughput screening of B cells and hybridomas is required for most cell line engineering workflows. A substantial bottleneck in these processes is detecting and isolating rare clonal cells with the required characteristics. Traditionally, this was achieved by the resource-intensive method of limiting dilution cloning, and more recently aided by semiautomated technologies such as cell sorting (e.g., fluorescence-activated cell sorting) and colony picking. In this paper we report on our novel Cyto-Mine Single Cell Analysis and Monoclonality Assurance System, which overcomes the limitations of current technologies by screening hundreds of thousands of individual cells for secreted target proteins, and then isolating and dispensing the highest producers into microtiter plate wells (MTP). The Cyto-Mine system performs this workflow using a fully integrated, microfluidic Cyto-Cartridge. Critically, all reagents and Cyto-Cartridges used are animal component-free (ACF) and sterile, thus allowing fast, robust, and safe isolation of desired cells.

Rapid isolation of antigen-specific B-cells using droplet microfluidics.

Monoclonal antibodies are powerful tools for scientific research and are the basis of numerous therapeutics. However, traditional approaches to generate monoclonal antibodies against a desired target, such as hybridoma-based techniques and display library methods, are laborious and suffer from fusion inefficiency and display bias, respectively. Here we present a platform, featuring droplet microfluidics and a bead-based binding assay, to rapidly identify and verify antigen-binding antibody sequences from primary cells. We used a defined mixture of hybridoma cells to characterize the system, sorting droplets at up to 100 Hz and isolating desired hybridoma cells, comprising 0.1% of the input, with a false positive rate of less than 1%. We then applied the system to once-frozen primary B-cells to isolate rare cells secreting target-binding antibody. We performed RT-PCR on individual sorted cells to recover the correctly paired heavy- and light-chain antibody sequences, and we used rapid cell-free protein synthesis to generate single-chain variable fragment-format (scFv) antibodies from fourteen of the sorted cells. Twelve of these showed antigen-specific binding by ELISA. Our platform facilitates screening animal B-cell repertoires within days at low cost, increasing both rate and range of discovering antigen-specific antibodies from living organisms. Further, these techniques can be adapted to isolate cells based on virtually any secreted product.

Therapeutic Monoclonal Antibodies for Ebola Virus Infection Derived from Vaccinated Humans.

We describe therapeutic monoclonal antibodies isolated from human volunteers vaccinated with recombinant adenovirus expressing Ebola virus glycoprotein (EBOV GP) and boosted with modified vaccinia virus Ankara. Among 82 antibodies isolated from peripheral blood B cells, almost half neutralized GP pseudotyped influenza virus. The antibody response was diverse in gene usage and epitope recognition. Although close to germline in sequence, neutralizing antibodies with binding affinities in the nano- to pico-molar range, similar to "affinity matured" antibodies from convalescent donors, were found. They recognized the mucin-like domain, glycan cap, receptor binding region, and the base of the glycoprotein. A cross-reactive cocktail of four antibodies, targeting the latter three non-overlapping epitopes, given on day 3 of EBOV infection, completely protected guinea pigs. This study highlights the value of experimental vaccine trials as a rich source of therapeutic human monoclonal antibodies.

Recognition of Amino Acid Motifs, Rather Than Specific Proteins, by Human Plasma Cell-Derived Monoclonal Antibodies to Posttranslationally Modified Proteins in Rheumatoid Arthritis.

OBJECTIVE: Antibodies against posttranslationally modified proteins are a hallmark of rheumatoid arthritis (RA), but the emergence and pathogenicity of these autoantibodies are still incompletely understood. The aim of this study was to analyze the antigen specificities and mutation patterns of monoclonal antibodies (mAb) derived from RA synovial plasma cells and address the question of antigen cross-reactivity. METHODS: IgG-secreting cells were isolated from RA synovial fluid, and the variable regions of the immunoglobulins were sequenced (n = 182) and expressed in full-length mAb (n = 93) and also as germline-reverted versions. The patterns of reactivity with 53,019 citrullinated peptides and 49,211 carbamylated peptides and the potential of the mAb to promote osteoclastogenesis were investigated. RESULTS: Four unrelated anti-citrullinated protein autoantibodies (ACPAs), of which one was clonally expanded, were identified and found to be highly somatically mutated in the synovial fluid of a patient with RA. The ACPAs recognized >3,000 unique peptides modified by either citrullination or carbamylation. This highly multireactive autoantibody feature was replicated for Ig sequences derived from B cells from the peripheral blood of other RA patients. The plasma cell-derived mAb were found to target distinct amino acid motifs and partially overlapping protein targets. They also conveyed different effector functions as revealed in an osteoclast activation assay. CONCLUSION: These findings suggest that the high level of cross-reactivity among RA autoreactive B cells is the result of different antigen encounters, possibly at different sites and at different time points. This is consistent with the notion that RA is initiated in one context, such as in the mucosal organs, and thereafter targets other sites, such as the joints.

Generation of two high affinity anti-mouse FcRn antibodies: Inhibition of IgG recycling in wild type mice and effect in a mouse model of immune thrombocytopenia.

Primary immune thrombocytopenia (ITP) is an autoimmune disease characterized by pathogenic immunoglobulin G (IgG) autoantibodies that bind to platelets, causing their phagocytic removal and leading to reductions in platelet number. The neonatal Fc receptor (FcRn) selectively salvages and recycles IgG, including pathogenic IgG, thereby extending the half-life of IgG in plasma. Two anti-mouse FcRn monoclonal antibodies (mAb) (4470 and 4464) were generated to evaluate the effect of inhibiting IgG recycling. Statistically significant reductions in plasma IgG concentration were observed upon administration of 4470 (10, 30 and 100 mg/kg) in wild-type mice. In a passive mouse model of ITP, 4464 alleviated the reduction in platelet number and/or preserved newly produced platelets when dosed prophylactically as well as in a therapeutic dosing regimen once platelet numbers had already been reduced. These results support the investigation of anti-FcRn therapy as a potential treatment for ITP.

Epitope determines efficacy of therapeutic anti-Tau antibodies in a functional assay with human Alzheimer Tau.

In Alzheimer's disease (AD) and other tauopathies, the cytosolic protein Tau misfolds and forms intracellular aggregates which accumulate within the brain leading to neurodegeneration. Clinical progression is tightly linked to the progressive spread of Tau pathology throughout the brain, and several lines of evidence suggest that Tau aggregates or "seeds" may propagate pathology by spreading from cell to cell in a "prion like" manner. Accordingly, blocking the spread of extracellular seeds with an antibody could be a viable therapeutic approach. However, as the structure of Tau seeds is unknown, it is only possible to rationally design therapeutic Tau antibodies by making a priori assumptions. To avoid this, we developed a robust and quantitative cell based assay and employed an unbiased screening approach to identify the antibody with the highest activity against human Tau seeds. The selected antibody (D), directed to the mid-region of Tau (amino acids 235-250), potently blocked the seeding of human AD Tau and was also fully efficacious against seeds from progressive supranuclear palsy. When we compared this antibody with previously described reference antibodies, we were surprised to find that none of these antibodies showed comparable efficacy against human pathological seeds. Our data highlight the difficulty of predicting antibody accessible epitopes on pathological Tau seeds and question the potential efficacy of some of the Tau antibodies that are currently in clinical development.

Generation and characterization of a high affinity anti-human FcRn antibody, rozanolixizumab, and the effects of different molecular formats on the reduction of plasma IgG concentration.

Rozanolixizumab (UCB7665), a humanized high-affinity anti-human neonatal Fc receptor (FcRn) monoclonal antibody (IgG4P), has been developed to reduce pathogenic IgG in autoimmune and alloimmune diseases. We document the antibody isolation and compare rozanolixizumab with the same variable region expressed in various mono-, bi- and trivalent formats. We report activity data for rozanolixizumab and the different molecular formats in human cells, FcRn-transgenic mice, and cynomolgus monkeys. Rozanolixizumab, considered the most effective molecular format, dose-dependently and selectively reduced plasma IgG concentrations in an FcRn-transgenic mouse model (no effect on albumin). Intravenous (IV) rozanolixizumab dosing in cynomolgus monkeys demonstrated non-linear pharmacokinetics indicative of target-mediated drug disposition; single IV rozanolixizumab doses (30 mg/kg) in cynomolgus monkeys reduced plasma IgG concentration by 69% by Day 7 post-administration. Daily IV administration of rozanolixizumab (initial 30 mg/kg loading dose; 5 mg/kg daily thereafter) reduced plasma IgG concentrations in all cynomolgus monkeys, with low concentrations maintained throughout the treatment period (42 days). In a 13-week toxicology study in cynomolgus monkeys, supra-pharmacological subcutaneous and IV doses of rozanolixizumab (≤ 150 mg/kg every 3 days) were well tolerated, inducing sustained (but reversible) reductions in IgG concentrations by up to 85%, with no adverse events observed. We have demonstrated accelerated natural catabolism of IgG through inhibition of IgG:FcRn interactions in mice and cynomolgus monkeys. Inhibition of FcRn with rozanolixizumab may provide a novel therapeutic approach to reduce pathogenic IgG in human autoimmune disease. Rozanolixizumab is being investigated in patients with immune thrombocytopenia (NCT02718716) and myasthenia gravis (NCT03052751).

Efficacy of an Inhaled IL-13 Antibody Fragment in a Model of Chronic Asthma.

RATIONALE: IL-13 is an important cytokine implicated in the pathogenesis of allergic asthma and is an attractive target for an inhaled therapeutic. OBJECTIVE: To investigate the efficacy of CDP7766, a nebulized inhaled anti-IL-13 monoclonal antibody Fab fragment, in a model of allergic asthma in cynomolgus macaques naturally sensitized to Ascaris suum. METHODS: CDP7766 was nebulized using a vibrating-membrane nebulizer on the basis of eFlow technology. The aerosol generated was analyzed to determine the particle size profile and the biophysical and functional properties of CDP7766. Nebulized CDP7766 (0.1-60 mg/animal, once daily for 5 d) was delivered via the inhaled route. MEASUREMENTS AND MAIN RESULTS: The investigational eFlow nebulizer used in this study generated a respirable aerosol of CDP7766 with no evidence of degradation, loss of potency, aggregation, or formation of particulates. Inhaled CDP7766 was well tolerated in the model (no adverse effects related to local irritation) and significantly inhibited BAL allergen-induced cytokine and chemokine upregulation (60 mg vs. vehicle: eotaxin-3, P < 0.0008; MIP [macrophage inflammatory protein]-1ß, IL-8, IFN-γ, P ≤ 0.01). CDP7766 significantly inhibited the increase in pulmonary resistance stimulated by inhaled allergen, measured 15 minutes and 24 hours after allergen challenge. CONCLUSION: Inhaled CDP7766 potently inhibited the function of IL-13 generated during the airway response to inhaled allergen in cynomolgus macaques, demonstrating the potential of inhaled anti-IL-13 therapeutics for the treatment of allergic asthma.

Generation of Recombinant Monoclonal Antibodies from Immunised Mice and Rabbits via Flow Cytometry and Sorting of Antigen-Specific IgG+ Memory B Cells.

Single B cell screening strategies, which avoid both hybridoma fusion and combinatorial display, have emerged as important technologies for efficiently sampling the natural antibody repertoire of immunized animals and humans. Having access to a range of methods to interrogate different B cell subsets provides an attractive option to ensure large and diverse panels of high quality antibody are produced. The generation of multiple antibodies and having the ability to find rare B cell clones producing IgG with unique and desirable characteristics facilitates the identification of fit-for-purpose molecules that can be developed into therapeutic agents or research reagents. Here, we describe a multi-parameter flow cytometry single-cell sorting technique for the generation of antigen-specific recombinant monoclonal antibodies from single IgG+ memory B cells. Both mouse splenocytes and rabbit PBMC from immunised animals were used as a source of B cells. Reagents staining both B cells and other unwanted cell types enabled efficient identification of class-switched IgG+ memory B cells. Concurrent staining with antigen labelled separately with two spectrally-distinct fluorophores enabled antigen-specific B cells to be identified, i.e. those which bind to both antigen conjugates (double-positive). These cells were then typically sorted at one cell per well using FACS directly into a 96-well plate containing reverse transcriptase reaction mix. Following production of cDNA, PCR was performed to amplify cognate heavy and light chain variable region genes and generate transcriptionally-active PCR (TAP) fragments. These linear expression cassettes were then used directly in a mammalian cell transfection to generate recombinant antibody for further testing. We were able to successfully generate antigen-specific recombinant antibodies from both the rabbit and mouse IgG+ memory B cell subset within one week. This included the generation of an anti-TNFR2 blocking antibody from mice with an affinity of 90 pM.

Development of a therapeutic monoclonal antibody that targets secreted fatty acid-binding protein aP2 to treat type 2 diabetes.

The lipid chaperone aP2/FABP4 has been implicated in the pathology of many immunometabolic diseases, including diabetes in humans, but aP2 has not yet been targeted for therapeutic applications. aP2 is not only an intracellular protein but also an active adipokine that contributes to hyperglycemia by promoting hepatic gluconeogenesis and interfering with peripheral insulin action. Serum aP2 levels are markedly elevated in mouse and human obesity and strongly correlate with metabolic complications. These observations raise the possibility of a new strategy to treat metabolic disease by targeting serum aP2 with a monoclonal antibody (mAb) to aP2. We evaluated mAbs to aP2 and identified one, CA33, that lowered fasting blood glucose, improved systemic glucose metabolism, increased systemic insulin sensitivity, and reduced fat mass and liver steatosis in obese mouse models. We examined the structure of the aP2-CA33 complex and resolved the target epitope by crystallographic studies in comparison to another mAb that lacked efficacy in vivo. In hyperinsulinemic-euglycemic clamp studies, we found that the antidiabetic effect of CA33 was predominantly linked to the regulation of hepatic glucose output and peripheral glucose utilization. The antibody had no effect in aP2-deficient mice, demonstrating its target specificity. We conclude that an aP2 mAb-mediated therapeutic constitutes a feasible approach for the treatment of diabetes.

A fully automated primary screening system for the discovery of therapeutic antibodies directly from B cells.

For a therapeutic antibody to succeed, it must meet a range of potency, stability, and specificity criteria. Many of these characteristics are conferred by the amino acid sequence of the heavy and light chain variable regions and, for this reason, can be screened for during antibody selection. However, it is important to consider that antibodies satisfying all these criteria may be of low frequency in an immunized animal; for this reason, it is essential to have a mechanism that allows for efficient sampling of the immune repertoire. UCB's core antibody discovery platform combines high-throughput B cell culture screening and the identification and isolation of single, antigen-specific IgG-secreting B cells through a proprietary technique called the "fluorescent foci" method. Using state-of-the-art automation to facilitate primary screening, extremely efficient interrogation of the natural antibody repertoire is made possible; more than 1 billion immune B cells can now be screened to provide a useful starting point from which to identify the rare therapeutic antibody. This article will describe the design, construction, and commissioning of a bespoke automated screening platform and two examples of how it was used to screen for antibodies against two targets.

The rapid generation of recombinant functional monoclonal antibodies from individual, antigen-specific bone marrow-derived plasma cells isolated using a novel fluorescence-based method.

Single B cell technologies, which avoid traditional hybridoma fusion and combinatorial display, provide a means to interrogate the naturally-selected antibody repertoire of immunized animals. Many methods enable the sampling of memory B cell subsets, but few allow for the direct interrogation of the plasma cell repertoire, i.e., the subset of B cells responsible for producing immunoglobulin in serum. Here, we describe the use of a robust and simple fluorescence-based technique, called the fluorescent foci method, for the identification and isolation of antigen-specific IgG-secreting cells, such as plasma cells, from heterogeneous bone marrow preparations. Following micromanipulation of single cells, cognate pairs of heavy and light chain variable region genes were recovered by reverse transcription (RT)-polymerase chain reaction (PCR). During the PCR, variable regions were combined with a promoter fragment and a relevant constant region fragment to produce two separate transcriptionally-active PCR (TAP) fragments that were directly co-transfected into a HEK-293F cell line for recombinant antibody expression. The technique was successfully applied to the generation of a diverse panel of high-affinity, functional recombinant antibodies to human tumor necrosis factor (TNF) receptor 2 and TNF derived from the bone marrow of immunized rabbits and rats, respectively. Progression from a bone marrow sample to a panel of functional recombinant antibodies was possible within a 2-week timeframe.

A mixture of functionally oligoclonal humanized monoclonal antibodies that neutralize Clostridium difficile TcdA and TcdB with high levels of in vitro potency shows in vivo protection in a hamster infection model.

Clostridium difficile infections are a major cause of antibiotic-associated diarrhea in hospital and care facility patients. In spite of the availability of effective antibiotic treatments, C. difficile infection (CDI) is still a major cause of patient suffering, death, and substantial health care costs. Clostridium difficile exerts its major pathological effects through the actions of two protein exotoxins, TcdA and TcdB, which bind to and disrupt gut tissue. Antibiotics target the infecting bacteria but not the exotoxins. Administering neutralizing antibodies against TcdA and TcdB to patients receiving antibiotic treatment might modulate the effects of the exotoxins directly. We have developed a mixture of three humanized IgG1 monoclonal antibodies (MAbs) which neutralize TcdA and TcdB to address three clinical needs: reduction of the severity and duration of diarrhea, reduction of death rates, and reduction of the rate of recurrence. The UCB MAb mixture showed higher potency in a variety of in vitro binding and neutralization assays (∼10-fold improvements), higher levels of protection in a hamster model of CDI (82% versus 18% at 28 days), and higher valencies of toxin binding (12 versus 2 for TcdA and 3 versus 2 for TcdB) than other agents in clinical development. Comparisons of the MAb properties also offered some insight into the potential relative importance of TcdA and TcdB in the disease process.