NKG2D-bispecific enhances NK and CD8+ T cell antitumor immunity.

BACKGROUND: Cancer immunotherapy approaches that elicit immune cell responses, including T and NK cells, have revolutionized the field of oncology. However, immunosuppressive mechanisms restrain immune cell activation within solid tumors so additional strategies to augment activity are required. METHODS: We identified the co-stimulatory receptor NKG2D as a target based on its expression on a large proportion of CD8+ tumor infiltrating lymphocytes (TILs) from breast cancer patient samples. Human and murine surrogate NKG2D co-stimulatory receptor-bispecifics (CRB) that bind NKG2D on NK and CD8+ T cells as well as HER2 on breast cancer cells (HER2-CRB) were developed as a proof of concept for targeting this signaling axis in vitro and in vivo. RESULTS: HER2-CRB enhanced NK cell activation and cytokine production when co-cultured with HER2 expressing breast cancer cell lines. HER2-CRB when combined with a T cell-dependent-bispecific (TDB) antibody that synthetically activates T cells by crosslinking CD3 to HER2 (HER2-TDB), enhanced T cell cytotoxicity, cytokine production and in vivo antitumor activity. A mouse surrogate HER2-CRB (mHER2-CRB) improved in vivo efficacy of HER2-TDB and augmented NK as well as T cell activation, cytokine production and effector CD8+ T cell differentiation. CONCLUSION: We demonstrate that targeting NKG2D with bispecific antibodies (BsAbs) is an effective approach to augment NK and CD8+ T cell antitumor immune responses. Given the large number of ongoing clinical trials leveraging NK and T cells for cancer immunotherapy, NKG2D-bispecifics have broad combinatorial potential.

Characterizing and Quantitating Therapeutic Tethered Multimeric Antibody Degradation Using Affinity Capture Mass Spectrometry.

There are many pharmacokinetic challenges associated with administering protein therapeutics, including biotransformation via clipping, deamidation, isomerization, oxidation, etc. In the case of engineered multivalent tethered antibody formats, proteolysis or deconjugation at the fusion or conjugation site present further issues. Unlike degradations associated with antibody drug conjugates, such biotransformations of tethered antibody formats usually result in degraded products with large mass differences. These large differences can result in processing or mass spectrometry response bias among the resulting product species that can lead to inaccurate stability quantitation. Herein, we describe an assay strategy for characterizing and quantitating degradations accurately for multivalent antibodies by incorporating response bias corrections. For the multivalent tethered antibody molecules selected, an ∼30-80% difference in response, compared to the cleaved product, was observed. To correct for the response bias, selected tethered multivalent antibodies and an IgG antibody (representing the stable intact and the degraded product species, respectively) were spiked in serum at known ratios for analysis. Following affinity capture, we generated calibration curves (five-parameter logistic fit p < 0.05) by plotting the measured ratios of the MS ion responses against the known spiked-in ratios (CVs < 8% for calibration standards). The qualified calibration curve (accuracy within 8% and 2% for measuring degradations of 5% and 15% product, respectively) was then used, through interpolation, to determine stability profiles for the same multivalent tethered antibody formats from both in vitro serum and pharmacokinetic study samples.

Bispecific T-cell engagers: Towards understanding variables influencing the in vitro potency and tumor selectivity and their modulation to enhance their efficacy and safety.

Bispecific molecules redirecting the cytotoxicity of T-cells are a growing class of therapeutics with numerous molecules being tested in clinical trials. However, it has been a long way since the proof of concept studies in the mid 1980's. In the process we have learnt about the impact of different variables related to the bispecific molecule and the target antigen on the potency of this type of drugs. This work reviews the insights gained and how that knowledge has been used to design more potent bispecific T-cell engagers. The more recent advancement of antibodies with this modality into safety studies in non-human primates and as well as in clinical studies has revealed potential toxicity liabilities for the mode of action. Modifications in existing antibody formats and new experimental molecules designed to mitigate these problems are discussed.

An anti-CD3/anti-CLL-1 bispecific antibody for the treatment of acute myeloid leukemia.

Acute myeloid leukemia (AML) is a major unmet medical need. Most patients have poor long-term survival, and treatment has not significantly changed in 40 years. Recently, bispecific antibodies that redirect the cytotoxic activity of effector T cells by binding to CD3, the signaling component of the T-cell receptor, and a tumor target have shown clinical activity. Notably, blinatumomab is approved to treat relapsed/refractory acute lymphoid leukemia. Here we describe the design, discovery, pharmacologic activity, pharmacokinetics, and safety of a CD3 T cell-dependent bispecific (TDB) full-length human IgG1 therapeutic antibody targeting CLL-1 that could potentially be used in humans to treat AML. CLL-1 is prevalent in AML and, unlike other targets such as CD33 and CD123, is not expressed on hematopoietic stem cells providing potential hematopoietic recovery. We selected a high-affinity monkey cross-reactive anti-CLL-1 arm and tested several anti-CD3 arms that varied in affinity, and determined that the high-affinity CD3 arms were up to 100-fold more potent in vitro. However, in mouse models, the efficacy differences were less pronounced, probably because of prolonged exposure to TDB found with lower-affinity CD3 TDBs. In monkeys, assessment of safety and target cell depletion by the high- and low-affinity TDBs revealed that only the low-affinity CD3/CLL1 TDB was well tolerated and able to deplete target cells. Our data suggest that an appropriately engineered CLL-1 TDB could be effective in the treatment of AML.

Mechanism for neutralizing activity by the anti-CMV gH/gL monoclonal antibody MSL-109.

Cytomegalovirus (CMV) is a widespread opportunistic pathogen that causes birth defects when transmitted transplacentally and severe systemic illness in immunocompromised individuals. MSL-109, a human monoclonal IgG isolated from a CMV seropositive individual, binds to the essential CMV entry glycoprotein H (gH) and prevents infection of cells. Here, we suggest a mechanism for neutralization activity by MSL-109. We define a genetic basis for resistance to MSL-109 and have generated a structural model of gH that reveals the epitope of this neutralizing antibody. Using surface-based, time-resolved FRET, we demonstrate that gH/gL interacts with glycoprotein B (gB). Additionally, we detect homodimers of soluble gH/gL heterodimers and confirm this novel oligomeric assembly on full-length gH/gL expressed on the cell surface. We show that MSL-109 perturbs the dimerization of gH/gL:gH/gL, suggesting that dimerization of gH/gL may be required for infectivity. gH/gL homodimerization may be conserved between alpha- and betaherpesviruses, because both CMV and HSV gH/gL demonstrate self-association in the FRET system. This study provides evidence for a novel mechanism of action for MSL-109 and reveals a previously undescribed aspect of viral entry that may be susceptible to therapeutic intervention.

Characterization of Chain Pairing Variants of Bispecific IgG Expressed in a Single Host Cell by High-Resolution Native and Denaturing Mass Spectrometry.

Bispecific antibodies, including bispecific IgG, show some promise in clinical trials as a means to extend the therapeutic potential of antibodies. Bispecific IgG can be made by separate expression and purification of each parent half antibody followed by in vitro reconstitution. Generating bispecific IgG by coexpression of two different light and heavy chains in a single host cell is potentially more efficient because it obviates the need for two separate cell lines and purification processes. However, this workflow may produce unwanted mispaired IgG species in addition to the desired bispecific IgG. Development and identification of designs that facilitate cognate light chain pairing may benefit from more refined methods to identify and quantify low levels of mispaired IgG. Using an anti-IL-4/IL-13 bispecific IgG, a mass spectrometric characterization method was developed using native or denaturing conditions by direct infusion into an Exactive Plus Extended Mass Range Orbitrap instrument. The high mass resolving power of the instrument allows unambiguous identification and accurate quantification of all light and heavy chain pairing variants in a mixture of bispecific IgG assembled in vivo upon coexpression down to 1% impurity. Preferential pairing of the anti-IL-13 light chain to its cognate heavy chain was observed, which may be leveraged to guide the design of a single-cell solution for streamlined production of bispecific IgG. Additionally, the utility of native mass spectrometry in deconvoluting complex antibody mixtures and in antigen-binding experiments to understand the contribution of doubly light chain mispaired bispecific IgG was demonstrated.

CLEC5a-directed bispecific antibody for effective cellular phagocytosis.

While antibody-dependent cellular phagocytosis mediated by activating Fcγ receptor is a key mechanism underlying many antibody drugs, their full therapeutic activities can be restricted by the inhibitory Fcγ receptor IIB (FcγRIIB). Here, we describe a bispecific antibody approach that harnesses phagocytic receptor CLEC5A (C-type Lectin Domain Containing 5A) to drive Fcγ receptor-independent phagocytosis, potentially circumventing the negative impact of FcγRIIB. First, we established the effectiveness of such an approach by constructing bispecific antibodies that simultaneously target CLEC5A and live B cells. Furthermore, we demonstrated its in vivo application for regulatory T cell depletion and subsequent tumor regression.

Nonclinical Pharmacokinetics and Pharmacodynamics Characterization of Anti-CD79b/CD3 T Cell-Dependent Bispecific Antibody Using a Surrogate Molecule: A Potential Therapeutic Agent for B Cell Malignancies.

The T cell-dependent bispecific (TDB) antibody, anti-CD79b/CD3, targets CD79b and CD3 cell-surface receptors expressed on B cells and T cells, respectively. Since the anti-CD79b arm of this TDB binds only to human CD79b, a surrogate TDB that binds to cynomolgus monkey CD79b (cyCD79b) was used for preclinical characterization. To evaluate the impact of CD3 binding affinity on the TDB pharmacokinetics (PK), we utilized non-tumor-targeting bispecific anti-gD/CD3 antibodies composed of a low/high CD3 affinity arm along with a monospecific anti-gD arm as controls in monkeys and mice. An integrated PKPD model was developed to characterize PK and pharmacodynamics (PD). This study revealed the impact of CD3 binding affinity on anti-cyCD79b/CD3 PK. The surrogate anti-cyCD79b/CD3 TDB was highly effective in killing CD79b-expressing B cells and exhibited nonlinear PK in monkeys, consistent with target-mediated clearance. A dose-dependent decrease in B cell counts in peripheral blood was observed, as expected. Modeling indicated that anti-cyCD79b/CD3 TDB's rapid and target-mediated clearance may be attributed to faster internalization of CD79b, in addition to enhanced CD3 binding. The model yielded unbiased and precise curve fits. These findings highlight the complex interaction between TDBs and their targets and may be applicable to the development of other biotherapeutics.

Novel Anti-LY6G6D/CD3 T-Cell-Dependent Bispecific Antibody for the Treatment of Colorectal Cancer.

New therapeutics and combination regimens have led to marked clinical improvements for the treatment of a subset of colorectal cancer. Immune checkpoint inhibitors have shown clinical efficacy in patients with mismatch-repair-deficient or microsatellite instability-high (MSI-H) metastatic colorectal cancer (mCRC). However, patients with microsatellite-stable (MSS) or low levels of microsatellite instable (MSI-L) colorectal cancer have not benefited from these immune modulators, and the survival outcome remains poor for the majority of patients diagnosed with mCRC. In this article, we describe the discovery of a novel T-cell-dependent bispecific antibody (TDB) targeting tumor-associated antigen LY6G6D, LY6G6D-TDB, for the treatment of colorectal cancer. RNAseq analysis showed that LY6G6D was differentially expressed in colorectal cancer with high prevalence in MSS and MSI-L subsets, whereas LY6G6D expression in normal tissues was limited. IHC confirmed the elevated expression of LY6G6D in primary and metastatic colorectal tumors, whereas minimal or no expression was observed in most normal tissue samples. The optimized LY6G6D-TDB, which targets a membrane-proximal epitope of LY6G6D and binds to CD3 with high affinity, exhibits potent antitumor activity both in vitro and in vivo. In vitro functional assays show that LY6G6D-TDB-mediated T-cell activation and cytotoxicity are conditional and target dependent. In mouse xenograft tumor models, LY6G6D-TDB demonstrates antitumor efficacy as a single agent against established colorectal tumors, and enhanced efficacy can be achieved when LY6G6D-TDB is combined with PD-1 blockade. Our studies provide evidence for the therapeutic potential of LY6G6D-TDB as an effective treatment option for patients with colorectal cancer.

A role for sperm surface protein disulfide isomerase activity in gamete fusion: evidence for the participation of ERp57.

In mammals, sperm-egg interaction is based on molecular events either unique to gametes or also present in somatic cells. In gamete fusion, it is unknown which features are gamete specific and which are shared with other systems. Conformational changes mediated by thiol-disulfide exchange are involved in the activation of some virus membrane fusion proteins. Here we asked whether that mechanism is also operative in sperm-egg fusion. Different inhibitors of protein disulfide isomerase (PDI) activity were able to inhibit sperm-egg fusion in vitro. While pretreatment of oocytes had no effect, pretreatment of sperm reduced their fusion ability. Some members of the PDI family were detected on the sperm head, and use of specific antibodies and substrates suggested that the oxidoreductase ERp57 has a role in gamete fusion. The results support the idea that thiol-disulfide exchange is a mechanism that may act in gamete fusion to produce conformational changes in fusion-active proteins.

Anti-LYPD1/CD3 T-Cell-Dependent Bispecific Antibody for the Treatment of Ovarian Cancer.

Ovarian cancer is a diverse class of tumors with very few effective treatment options and suboptimal response rates in early clinical studies using immunotherapies. Here we describe LY6/PLAUR domain containing 1 (LYPD1) as a novel target for therapeutic antibodies for the treatment of ovarian cancer. LYPD1 is broadly expressed in both primary and metastatic ovarian cancer with ∼70% prevalence in the serous cancer subset. Bispecific antibodies targeting CD3 on T cells and a tumor antigen on cancer cells have demonstrated significant clinical activity in hematologic cancers. We have developed an anti-LYPD1/CD3 T-cell-dependent bispecific antibody (TDB) to redirect T-cell responses to LYPD1 expressing ovarian cancer. Here we characterize the nonclinical pharmacology of anti-LYPD1/CD3 TDB and show induction of a robust polyclonal T-cell activation and target dependent killing of LYPD1 expressing ovarian cancer cells resulting in efficient in vivo antitumor responses in PBMC reconstituted immune-deficient mice and human CD3 transgenic mouse models. Anti-LYPD1/CD3 TDB is generally well tolerated at high-dose levels in mice, a pharmacologically relevant species, and showed no evidence of toxicity or damage to LYPD1 expressing tissues.

Harnessing MerTK agonism for targeted therapeutics.

Phagocytosis plays important roles both in homeostasis and under pathological conditions. Fcγ receptor-mediated phagocytosis has been exploited as an integral mechanism for antibody-based therapies. Unlike Fcγ receptor-mediated phagocytosis, MerTK-mediated phagocytic clearance is immunologically silent. Here, we describe a bispecific antibody approach to harness MerTK for targeted clearance without inducing proinflammatory cytokine release associated with Fcγ receptor engagement. We generated bispecific antibodies targeting live B cells or amyloid beta aggregates to demonstrate the feasibility and versatility of this new approach.

Dissecting the molecular basis of high viscosity of monospecific and bispecific IgG antibodies.

Some antibodies exhibit elevated viscosity at high concentrations, making them poorly suited for therapeutic applications requiring administration by injection such as subcutaneous or ocular delivery. Here we studied an anti-IL-13/IL-17 bispecific IgG4 antibody, which has anomalously high viscosity compared to its parent monospecific antibodies. The viscosity of the bispecific IgG4 in solution was decreased by only ~30% in the presence of NaCl, suggesting electrostatic interactions are insufficient to fully explain the drivers of viscosity. Intriguingly, addition of arginine-HCl reduced the viscosity of the bispecific IgG4 by ~50% to its parent IgG level. These data suggest that beyond electrostatics, additional types of interactions such as cation-π and/or π-π may contribute to high viscosity more significantly than previously understood. Molecular dynamics simulations of antibody fragments in the mixed solution of free arginine and explicit water were conducted to identify hotspots involved in self-interactions. Exposed surface aromatic amino acids displayed an increased number of contacts with arginine. Mutagenesis of the majority of aromatic residues pinpointed by molecular dynamics simulations effectively decreased the solution's viscosity when tested experimentally. This mutational method to reduce the viscosity of a bispecific antibody was extended to a monospecific anti-GCGR IgG1 antibody with elevated viscosity. In all cases, point mutants were readily identified that both reduced viscosity and retained antigen-binding affinity. These studies demonstrate a new approach to mitigate high viscosity of some antibodies by mutagenesis of surface-exposed aromatic residues on complementarity-determining regions that may facilitate some clinical applications.

Target arm affinities determine preclinical efficacy and safety of anti-HER2/CD3 bispecific antibody.

Systemic cytokine release and on-target/off-tumor toxicity to normal tissues are the main adverse effects limiting the clinical utility of T cell-redirecting therapies. This study was designed to determine how binding affinity for CD3 and tumor target HER2 impact the efficacy and nonclinical safety of anti-HER2/CD3 T cell-dependent antibodies (TDBs). Affinity was found to be a major determinant for the overall tolerability. Higher affinity for CD3 associated with rapidly elevated peripheral cytokine concentrations, weight loss in mice, and poor tolerability in cynomolgus monkeys. A TDB with lower CD3 affinity was better tolerated in cynomolgus monkeys compared with a higher CD3-affinity TDB. In contrast to tolerability, T cell binding affinity had only limited impact on in vitro and in vivo antitumor activity. High affinity for HER2 was critical for the tumor-killing activity of anti-HER2/CD3 TDBs, but higher HER2 affinity also associated with a more severe toxicity profile, including cytokine release and damage to HER2-expressing tissues. The tolerability of the anti-HER2/CD3 was improved by implementing a dose-fractionation strategy. Fine-tuning the affinities for both the tumor target and CD3 is likely a valuable strategy for achieving maximal therapeutic index of CD3 bispecific antibodies.

Izumo is part of a multiprotein family whose members form large complexes on mammalian sperm.

Izumo, a sperm membrane protein, is essential for gamete fusion in the mouse. It has an Immunoglobulin (Ig) domain and an N-terminal domain for which neither the functions nor homologous sequences are known. In the present work we identified three novel proteins showing an N-terminal domain with significant homology to the N-terminal domain of Izumo. We named this region "Izumo domain," and the novel proteins "Izumo 2," "Izumo 3," and "Izumo 4," retaining "Izumo 1" for the first described member of the family. Izumo 1-3 are transmembrane proteins expressed specifically in the testis, and Izumo 4 is a soluble protein expressed in the testis and in other tissues. Electrophoresis under mildly denaturing conditions, followed by Western blot analysis, showed that Izumo 1, 3, and 4 formed protein complexes on sperm, Izumo 1 forming several larger complexes and Izumo 3 and 4 forming a single larger complex. Studies using different recombinant Izumo constructs suggested the Izumo domain possesses the ability to form dimers, whereas the transmembrane domain or the cytoplasmic domain or both of Izumo 1 are required for the formation of multimers of higher order. Co-immunoprecipitation studies showed the presence of other sperm proteins associated with Izumo 1, suggesting Izumo 1 forms a multiprotein membrane complex. Our results raise the possibility that Izumo 1 might be involved in organizing or stabilizing a multiprotein complex essential for the function of the membrane fusion machinery.

Participation of cysteine-rich secretory proteins (CRISP) in mammalian sperm-egg interaction.

Mammalian fertilization is a complex multi-step process mediated by different molecules present on both gametes. CRISP1 (cysteine-rich secretory protein 1) is an epididymal protein thought to participate in gamete fusion through its binding to egg-complementary sites. Structure-function studies using recombinant fragments of CRISP1 as well as synthetic peptides reveal that its egg-binding ability resides in a 12 amino acid region corresponding to an evolutionary conserved motif of the CRISP family, named Signature 2 (S2). Further experiments analyzing both the ability of other CRISP proteins to bind to the rat egg and the amino acid sequence of their S2 regions show that the amino acid sequence of the S2 is needed for CRISP1 to interact with the egg. CRISP1 appears to be involved in the first step of sperm binding to the zona pellucida, identifying a novel role for this protein in fertilization. The observation that sperm testicular CRISP2 is also able to bind to the egg surface suggests a role for this protein in gamete fusion. Subsequent experiments confirmed the participation of CRISP2 in this step of fertilization and revealed that CRISP1 and CRISP2 interact with common egg surface binding sites. Together, these results suggest a functional cooperation between CRISP1 and CRISP2 to ensure the success of fertilization. These observations contribute to a better understanding of the molecular mechanisms underlying mammalian fertilization.

Immune repertoire mining for rapid affinity optimization of mouse monoclonal antibodies.

Traditional hybridoma and B cell cloning antibody discovery platforms have inherent limits in immune repertoire sampling depth. One consequence is that monoclonal antibody (mAb) leads often lack the necessary affinity for therapeutic applications, thus requiring labor-intensive and time-consuming affinity in vitro engineering optimization steps. Here, we show that high-affinity variants of mouse-derived mAbs can be rapidly obtained by testing of somatic sequence variants obtained by deep sequencing of antibody variable regions in immune repertories from immunized mice, even with a relatively sparse sampling of sequence variants from large sequence datasets. Affinity improvements can be achieved for mAbs with a wide range of affinities. The optimized antibody variants derived from immune repertoire mining have no detectable in vitro off-target binding and have in vivo clearance comparable to the parental mAbs, essential properties in therapeutic antibody leads. As generation of antibody variants in vitro is replaced by mining of variants generated in vivo, the procedure can be applied to rapidly identify affinity-optimized mAb variants.

Single cell-produced and in vitro-assembled anti-FcRH5/CD3 T-cell dependent bispecific antibodies have similar in vitro and in vivo properties.

Bispecific antibody production using single host cells has been a new advancement in the antibody engineering field. We previously showed comparable in vitro biological activity and in vivo mouse pharmacokinetics (PK) for two novel single cell variants (v10 and v11) and one traditional dual cell in vitro-assembled anti-human epidermal growth factor receptor 2/CD3 T-cell dependent bispecific (TDB) antibodies. Here, we extended our previous work to assess single cell-produced bispecific variants of a novel TDB against FcRH5, a B-cell lineage marker expressed on multiple myeloma (MM) tumor cells. An in vitro-assembled anti- FcRH5/CD3 TDB antibody was previously developed as a potential treatment option for MM. Two bispecific antibody variants (designs v10 and v11) for manufacturing anti-FcRH5/CD3 TDB in single cells were compared to in vitro-assembled TDB in a dual-cell process to understand whether differences in antibody design and production led to any major differences in their in vitro biological activity, in vivo mouse PK, and PK/pharmacodynamics (PD) or immunogenicity in cynomolgus monkeys (cynos). The binding, in vitro potencies, in vitro pharmacological activities and in vivo PK in mice and cynos of these single cell TDBs were comparable to those of the in vitro-assembled TDB. In addition, the single cell and in vitro-assembled TDBs exhibited robust PD activity and comparable immunogenicity in cynos. Overall, these studies demonstrate that single cell-produced and in vitro-assembled anti-FcRH5/CD3 T-cell dependent bispecific antibodies have similar in vitro and in vivo properties, and support further development of single-cell production method for anti-FcRH5/CD3 TDBs and other single-cell bispecifics.

Expanding the Boundaries of Biotherapeutics with Bispecific Antibodies.

Bispecific antibodies have moved from being an academic curiosity with therapeutic promise to reality, with two molecules being currently commercialized (Hemlibra® and Blincyto®) and many more in clinical trials. The success of bispecific antibodies is mainly due to the continuously growing number of mechanisms of actions (MOA) they enable that are not accessible to monoclonal antibodies. One of the earliest MOA of bispecific antibodies and currently the one with the largest number of clinical trials is the redirecting of the cytotoxic activity of T-cells for oncology applications, now extending its use in infective diseases. The use of bispecific antibodies for crossing the blood-brain barrier is another important application because of its potential to advance the therapeutic options for neurological diseases. Another noteworthy application due to its growing trend is enabling a more tissue-specific delivery or activity of antibodies. The different molecular solutions to the initial hurdles that limited the development of bispecific antibodies have led to the current diverse set of bispecific or multispecific antibody formats that can be grouped into three main categories: IgG-like formats, antibody fragment-based formats, or appended IgG formats. The expanded applications of bispecific antibodies come at the price of additional challenges for clinical development. The rising complexity in their structure may increase the risk of immunogenicity and the multiple antigen specificity complicates the selection of relevant species for safety assessment.

Avidity-based binding to HER2 results in selective killing of HER2-overexpressing cells by anti-HER2/CD3.

A primary barrier to the success of T cell-recruiting bispecific antibodies in the treatment of solid tumors is the lack of tumor-specific targets, resulting in on-target off-tumor adverse effects from T cell autoreactivity to target-expressing organs. To overcome this, we developed an anti-HER2/CD3 T cell-dependent bispecific (TDB) antibody that selectively targets HER2-overexpressing tumor cells with high potency, while sparing cells that express low amounts of HER2 found in normal human tissues. Selectivity is based on the avidity of two low-affinity anti-HER2 Fab arms to high target density on HER2-overexpressing cells. The increased selectivity to HER2-overexpressing cells is expected to mitigate the risk of adverse effects and increase the therapeutic index. Results included in this manuscript not only support the clinical development of anti-HER2/CD3 1Fab-immunoglobulin G TDB but also introduce a potentially widely applicable strategy for other T cell-directed therapies. The potential of this discovery has broad applications to further enable consideration of solid tumor targets that were previously limited by on-target, but off-tumor, autoimmunity.