T-cell engagers represent one of the most actively pursued modalities in oncology and immunology today, and for good reason. By redirecting cytotoxic T cells against tumor targets without requiring antigen-specific T-cell priming, they bypass one of cancer’s most effective immune evasion mechanisms. Yet despite the approval of blinatumomab in 2014 and a decade of subsequent development, the clinical potential of TCEs remains constrained by a persistent tension: driving sufficient cytotoxicity while avoiding the cytokine release that narrows the therapeutic window.
Solving that tension requires more than incremental optimization of existing formats. It demands a rethinking of TCE architecture: the geometry of the immune synapse, the affinity of each binding arm, and the structural flexibility of the molecule itself.
Why TCE Geometry and Affinity Are the Central Engineering Variables
TCEs function by forming an immune synapse between a T cell and a target cell. The molecule bridges a surface receptor on the T cell, most commonly CD3, and a tumor-associated antigen (TAA) on the cancer cell, bypassing TCR specificity entirely. What this mechanism makes clear is that the structural and biophysical properties of the TCE are not secondary considerations: affinity for CD3, affinity for the TAA, and the physical distance the molecule creates between the two cells are all critical determinants of immune synapse quality and downstream cytotoxic output.
CD3 affinity, in particular, has emerged as a key lever for balancing efficacy and safety. Data from Nona Biosciences’ HBICE® platform demonstrates this directly. In a 2+1 BCMA × CD3 configuration, TCEs carrying CD3 binders of high, medium, and low affinity were tested against human PBMCs co-cultured with NCI-H929 tumor cells. The medium-affinity CD3 binder achieved cytotoxicity equivalent to benchmark TCEs (TABs), while producing significantly lower cytokine release.
Critically, off-target cytokine release did not correlate linearly with CD3 affinity: the medium-affinity binder produced the lowest IL-6 and TNF-α release even in off-target conditions using BCMA-negative HL-60 cells. This finding challenges the assumption that simply reducing CD3 affinity uniformly reduces toxicity, and points instead to a more nuanced affinity-tuning strategy.
The Structural Limitation of Conventional Binder Formats
Almost all CD3 binder modules currently in clinical use are built on conventional heavy chain and light chain architectures. This constrains TCE design in two ways.
First, it limits geometric flexibility: the relative positioning of binding domains, and therefore the distance between T cell and tumor cell, is difficult to modulate within a standard Fab framework.
Second, it introduces manufacturing risk through light chain mispairing, a well-documented challenge in bispecific antibody production.
Heavy chain-only antibodies (HCAbs) address both constraints simultaneously. Single-domain VHH binders are smaller, structurally independent, and can be assembled in configurations that are simply not accessible with conventional Fab modules. Nona Biosciences’ HBICE® platform is built on this principle, using fully human heavy chain-only binders generated through the Harbour Mice® platform to construct TCEs with tunable affinity and expanded geometric options.
The HBICE® Platform: Structural Versatility Across Clinical-Stage Programs
The HBICE® platform supports two primary TCE architectures, 2+1 and 2+2 configurations, each suited to different biological contexts. The 2+1 format, which pairs two TAA-binding domains with one CD3 binder, is particularly relevant for targets with variable or low surface expression, where avidity effects from bivalent TAA engagement can compensate for reduced antigen density. The 2+2 format extends this logic to costimulatory targets, enabling simultaneous engagement of two distinct immune or tumor cell surface molecules.
This structural versatility is not theoretical. Nona Biosciences’ pipeline includes six active HBICE® programs spanning both formats:
|
Program |
Target |
Platform |
Stage |
|
BCMA × CD3 |
2+1 HBICE® |
NMPA Clearance |
|
|
B7H4 × CD3 |
2+1 HBICE® |
FDA IND Clearance |
|
|
B7H4 × 4-1BB |
2+2 HBICE® |
Phase I |
|
|
7024 |
PDL-1 × CD28 |
2+2 HBICE® |
Discovery |
|
7025 |
ROR1 × NKp30 |
2+2 HBICE® |
Discovery |
|
PDL1 × CD40 |
2+2 HBICE® |
IND Enabling |
The breadth of targets from CD3 co-engagement to costimulatory axes like 4-1BB, CD28, and CD40 reflects the platform’s adaptability across oncology mechanisms. The B7H4 × 4-1BB program (7008) reaching Phase I is particularly notable, as 4-1BB agonism requires careful affinity calibration to avoid excessive T-cell activation. Nona’s functional discovery workflow for 4-1BB VH domains specifically evaluates T-cell activation in the presence and absence of CHO cells expressing CD32b, using Utomilumab and Urelumab as reference antibodies to benchmark the fine-tuning of agonist activity.
Enabling Novel Geometries Through Single-Domain Assembly
Beyond the 2+1 and 2+2 formats, VH-based binder modules open a third design dimension: linear binder domain assembly. This configuration allows precise control over the distance between the T cell and the tumor cell within the immune synapse, a parameter that is essentially fixed in conventional bispecific formats. Adjusting this distance has direct implications for immune synapse stability and the efficiency of cytotoxic granule delivery.
Affinity tunability within VH-containing TCEs has been validated experimentally. When human PBMCs were incubated with a TCE containing an SP34-derived Fab CD3 binder versus two TCEs containing different VH CD3 binders, the VH-containing constructs demonstrated a range of cytotoxic activity.
Importantly, only baseline cytotoxicity was observed against TAA-negative tumor cells, confirming that target selectivity is maintained across the affinity spectrum. For a detailed technical overview of these findings, see Nona’s TCE and HBICE® white paper.
Manufacturing Simplification as a Strategic Advantage
The clinical promise of TCEs is inseparable from their manufacturing profile. CAR-T therapies, despite superior efficacy in certain settings, face significant patient access limitations, a direct consequence of complex, individualized manufacturing and the high cost of goods. TCEs, by contrast, are produced through defined, scalable processes with significantly lower COGs and a straightforward supply chain.
The HBICE® platform extends this manufacturing advantage further. By eliminating light chain mispairing, a persistent source of product heterogeneity in conventional bispecific formats, heavy chain-only binder modules reduce the complexity of downstream purification and quality control. This is not a marginal improvement: light chain mispairing can generate off-target species that complicate regulatory submissions and increase batch failure rates. Removing this variable from the manufacturing equation de-risks the entire development path from lead generation through IND filing.
The platform also includes off-the-shelf CD3 binders with cynomolgus cross-reactivity, which accelerates preclinical toxicology studies by enabling direct translation of in vitro findings to non-human primate models without requiring separate binder development for each species.
Building the Next Generation of T-Cell Engagers
The field is moving toward greater structural complexity, multi-specific modalities with three or more arms, myeloid cell engagers alongside T-cell engagers, and combination formats that integrate conditional activation mechanisms. Each of these directions places a premium on the same capabilities that define the HBICE® platform: single-domain binder flexibility, affinity tunability, and manufacturing simplicity.
TCEs optimized through geometry and affinity engineering rather than brute-force potency are better positioned for autoimmune indications as well as oncology, where the requirement for a wide therapeutic window is even more stringent and alternative management options make tolerability non-negotiable.
Nona Biosciences’ HBICE® platform, backed by fully human HCAb discovery through Harbour Mice® and an integrated pipeline from antigen preparation through IND-enabling studies, provides the structural and functional toolkit to engineer TCEs that meet this standard. To explore how the HBICE® platform can support your T-cell engager program from CD3 binder selection through bispecific engineering and preclinical evaluation, connect with the Nona Biosciences discovery team.
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