Difficulty: Intermediate to Advanced
Prerequisites: Part 1 of these notes (Synthetic Immunology Overview and Therapeutic Antibodies). Familiarity with antibody structure (Fab, Fc, variable and constant regions), ADCC, CDC, and the basic generations of monoclonal antibodies.
Therapeutic antibodies are effective but come with limitations: large size, poor tissue penetration, high cost, and short half-life for some formulations. This part of the course covers the next wave of engineered proteins designed to overcome those limitations. Chimeric proteins combine receptor-binding domains with stabilising scaffolds (typically Fc regions). Antibody derivatives are smaller molecules built from variable domains that can bispecifically link immune effector cells to targets. Antibody mimetics are entirely synthetic scaffolds with antibody-like binding properties at a fraction of the size and cost. Together, these represent the expanding toolkit of synthetic immunology beyond full-length antibodies.
Chimeric proteins (e.g. etanercept, rilonacept, abatacept) fuse receptor-binding domains to Fc regions for stability. Antibody derivatives (BiTEs, DARTs, TandAbs, nanobodies) are smaller, cheaper, bispecific molecules that recruit immune cells to targets. Antibody mimetics (DARPins, avimers, anticalins) are tiny synthetic scaffolds with high specificity. All trade improved tissue penetration and lower cost for shorter serum half-lives.
Chimeric protein (fusion protein)
A synthetic protein that combines the binding domain of an endogenous receptor with a stabilising component, typically the Fc region of IgG1 or albumin. Designed to capture and neutralise specific cytokines or block receptor-ligand interactions.
Think of it as: taking the "catcher's mitt" from a natural receptor and bolting it onto a long-lasting scaffold so it circulates in the blood and intercepts inflammatory signals.
Etanercept
A fusion protein consisting of TNF receptor 2 (TNFR2) linked to the Fc region of human IgG1. Binds and neutralises TNF. Prescribed for autoimmune diseases including RA. Equally effective as anti-TNF antibodies but with a shorter half-life (requires twice-weekly subcutaneous injection).
Rilonacept
A dimeric fusion protein combining the ligand-binding domains of IL-1 receptor type 1 (IL-1R1) and IL-1R accessory protein (AcP), linked to the Fc region of IgG1. Approved for cryopyrin-associated periodic syndrome (CAPS). Must be administered weekly due to short half-life.
Belatacept and abatacept
Fusion proteins composed of the extracellular domain of CTLA-4 linked to the Fc region of IgG1. They block T cell co-stimulation by competing with CD28 for B7 binding on antigen-presenting cells. Belatacept is used to prevent transplant rejection; abatacept is approved for RA.
In simple terms: these proteins mimic the natural brake on T cells (CTLA-4), using it as a decoy to prevent T cell activation.
Bispecific T cell engager (BiTE)
A single polypeptide chain containing the variable domains of two different antibodies, connected by a flexible linker. One end binds CD3-epsilon on T cells; the other binds a tumour-associated antigen. Forces a cytolytic synapse between the T cell and its target, independent of ADCC.
Think of it as: a molecular handcuff that physically drags a T cell onto a tumour cell.
Dual affinity retargeting (DART) antibody
Similar in function to BiTEs but composed of two polypeptide chains linked by interchain disulphide bridges. The more rigid configuration may explain the DART format's superior performance over BiTEs in some comparative studies.
Tetravalent tandem antibody (TandAb)
Two polypeptide chains that dimerise to form a molecule with four antigen-binding sites (~110 kDa). Developed by Affimed. Can engage both T cells and NK cells with their targets.
Nanobody
An antibody derivative consisting of the variable domain of heavy-chain-only antibodies found naturally in camelids (camels, llamas). Only 12 to 15 kDa. Excellent tissue penetration but short serum half-life. Can be multimerised or fused to albumin-binding domains for extended circulation.
In simple terms: a tiny fragment of a camel antibody that can squeeze into tissues full-size antibodies cannot reach.
Designed ankyrin repeat protein (DARPin)
An antibody mimetic built from 3 to 5 fully synthetic ankyrin repeats flanked by capping motifs. High specificity, low immunogenicity (lacks T cell epitopes), produced in bacteria.
Avimer
An antibody mimetic based on A domains from cell surface receptors. Each A domain has low affinity for its target, but multimerisation via genetic oligomerisation produces high overall avidity. Example: AMG 220, a 12-kDa scaffold binding IL-6 with picomolar affinity.
Anticalin
An antibody mimetic derived from lipocalins, a family of transport proteins with a rigid beta-barrel structure and four randomisable loops. Commercially developed by Pieris.
Combine a receptor's binding domain with a stabilising scaffold (Fc region or albumin)
Enable high affinity and specificity, similar to therapeutic antibodies
Key examples:
Etanercept: TNFR2 + Fc of IgG1. Blocks TNF. Used for RA, IBD. Equally effective as anti-TNF antibodies but shorter half-life (twice-weekly injection vs every 2 to 4 weeks).
Rilonacept: IL-1R1 + IL-1R-AcP + Fc of IgG1. Traps both IL-1-alpha and IL-1-beta. Approved for CAPS (Muckle-Wells syndrome, familial cold autoinflammatory syndrome). Weekly administration.
Belatacept / abatacept: CTLA-4 extracellular domain + Fc of IgG1. Block T cell co-stimulation by outcompeting CD28 for B7 binding (CTLA-4 has higher affinity for B7 than CD28). Belatacept inhibits transplant rejection; abatacept treats RA.
Chimeric activators (experimental): a synthetic protein with two binding domains for distinct cell surface proteins can modulate cell-type specificity. Example: IFN-alpha engineered to activate only cells bearing EGFR.
Advantages over antibodies: smaller size enables better tissue penetration, equally priced and safe, similar efficacy
Disadvantages: shorter serum half-life (days vs weeks)
Generated from the variable regions of immunoglobulins. Cheaper to produce, enhanced tissue penetration, lower immunogenicity, but shorter serum half-life.
BiTE antibodies (Bispecific T cell engagers)
Single polypeptide chain: V_La-V_Ha-linker-V_Lb-V_Hb
One arm binds CD3-epsilon on T cells; the other binds a tumour antigen
Force cytotoxic T cells to kill the target, independent of ADCC
Key example: blinatumomab (55 kDa). Binds CD19 + CD3-epsilon. Forces a cytolytic synapse between T and B cells. T cell activation depends solely on blinatumomab binding (no co-activation signal needed). Achieves serial B cell lysis at effector-to-target ratio of 1:90. Half-life only ~2 hours, so administered as continuous IV infusion. In trials for non-Hodgkin lymphoma, CLL, and ALL.
Other BiTEs in development: AMG 330 (CD33 + CD3-epsilon, for AML), AMG 212 (PSMA + CD3-epsilon, for prostate cancer), MT110 (EpCAM + CD3-epsilon, for solid cancers)
DART antibodies (Dual affinity retargeting)
Two polypeptide chains linked by interchain disulphide bridges: V_La-V_Hb and V_Lb-V_Ha
Also ~55 kDa, bispecific, designed to crosslink CD3-positive T cells and target cells
More rigid configuration than BiTEs, with limited flexibility between antigen-binding moieties
In side-by-side comparison (CD19 x CD3), the DART format consistently outperformed BiTE in: maximal B cell lysis, concentration for half-maximal lysis, and T cell activation markers
Developed by MacroGenics. Examples: MGD006 (CD123 + CD3, for AML), MGD007 (gpA33 + CD3, for colorectal cancer)
TandAb antibodies (Tetravalent tandem antibodies)
Two polypeptide chains that dimerise to form four antigen-binding sites (~110 kDa)
Developed by Affimed
Examples: AFM11 (CD19 + CD3, for non-Hodgkin lymphoma), AFM13 (CD30 + CD16a, recruits NK cells for Hodgkin lymphoma, higher ADCC potency than monoclonal antibodies due to two antigen-binding sites giving higher avidity), AFM21 (EGFRvIII + CD3, for solid tumours)
Affimed has also developed trispecific TandAbs: two identical effector-binding sites plus two different target-binding sites
Nanobodies
Derived from camelid heavy-chain-only antibodies
Comprise the V_HH domain, 12 to 15 kDa
No nanobody approved yet, but several in clinical phases
Advantages: excellent tissue penetration, can be produced in bacteria, modular (can be multimerised)
Disadvantages: short serum half-life, may trigger enhanced immune response (camelid origin)
Key examples: ozoralizumab (3 V_HH: 2 bind TNF + 1 binds albumin, in trials for RA), ALX-0061 (2 V_HH: 1 binds IL-6R + 1 binds albumin, Phase II for RA), ALX-0761 (trivalent, neutralises IL-17A and IL-17F + binds albumin, preclinical for RA)
Synthetic proteins containing antigen-binding sites reminiscent of antibodies. Single polypeptide, derived from existing human scaffold proteins. High specificity at low molecular weight. Can be produced in bacteria (cheaper and faster). ~50 scaffolds discovered, few tested clinically.
DARPins (Designed ankyrin repeat proteins)
3 to 5 synthetic ankyrin repeats flanked by N- and C-terminal capping motifs
Lack T cell epitopes, so low immunogenicity
Commercially developed by Molecular Partners
Example: DARPin bi53_79 blocks IgE binding to Fc-epsilon-RI and strips already-bound IgE from effector cells. Outperformed omalizumab in preventing allergic reactions in vivo.
Avimers (Avidity multimers)
Based on A domains from the extracellular portion of cell surface receptors
Each A domain has low affinity; multimerisation through genetic oligomerisation produces high avidity
Discovered by Avidia (now part of Amgen)
Example: AMG 220, a 12-kDa scaffold of three A domains from the LDL receptor, binds IL-6 with picomolar affinity. Phase I for Crohn's disease.
Anticalins
Derived from lipocalins: transport proteins with a rigid beta-barrel and four randomisable loops
Libraries from various species and cell types, randomised at 16 positions
Commercially developed by Pieris
Examples: PRS-010 (binds CTLA-4) and PRS-060 (binds IL-4R-alpha, the receptor for Th2 cytokine IL-4). Both preclinical.
Feature | Full antibody | Antibody derivative | Antibody mimetic |
|---|---|---|---|
Molecular weight | ~150 kDa | 12 to 110 kDa | 12 to 20 kDa |
Tissue penetration | Low | Good | Good |
Serum half-life | Weeks | Hours to days | Hours |
Immunogenicity | Variable | Low to moderate | Low |
Production cost | High | Moderate | Low (bacterial) |
Specificity | Monospecific or bispecific | Bispecific or multispecific | Monospecific |
ADCC/CDC capability | Yes (via Fc) | Generally no | No |
Etanercept (Enbrel) is one of the most widely prescribed biologics for RA worldwide, demonstrating how chimeric proteins have become mainstream therapy.
Blinatumomab (a BiTE) received FDA approval in 2014 for relapsed/refractory ALL, becoming the first bispecific T cell engager to reach market. It demonstrated that T cells can be harnessed to destroy cancer without classical ADCC.
"Antibody derivatives are just smaller versions of antibodies." They are structurally distinct. Antibody derivatives are built from variable domain fragments connected by linkers, not truncated whole antibodies. This changes their pharmacokinetics, tissue penetration, and effector mechanisms entirely.
"BiTEs and DARTs are interchangeable." They differ in structure (single-chain vs two-chain with disulphide bridges) and rigidity. In head-to-head comparison, the DART format outperformed the BiTE format in B cell lysis and T cell activation.
"Smaller molecules are always better." Smaller derivatives and mimetics gain tissue penetration and lower production cost, but lose serum half-life. Blinatumomab, for instance, has a half-life of only ~2 hours and must be given as a continuous IV infusion.
"Nanobodies are fully human." They are derived from camelid heavy-chain antibodies and may trigger enhanced immune responses compared with human monoclonal antibodies, even after humanisation and sequence optimisation.
Be able to compare BiTEs, DARTs, and TandAbs structurally and functionally.
Know the three main classes of antibody mimetics (DARPins, avimers, anticalins) and their scaffolds.
Understand why chimeric proteins (e.g. etanercept, rilonacept, abatacept) are classified separately from therapeutic antibodies.
The comparison table (antibodies vs derivatives vs mimetics) is high-yield for exam questions about trade-offs between molecular size, half-life, cost, and tissue penetration.
Expect questions on blinatumomab: how it works (bispecific CD19/CD3-epsilon, forces cytolytic synapse), why it needs continuous infusion (2-hour half-life), and what it demonstrates about T cell-mediated killing independent of ADCC.
True or False: Etanercept is a monoclonal antibody that targets TNF. (False. Etanercept is a chimeric fusion protein consisting of TNFR2 linked to the Fc of IgG1.)
Fill in the blank: A BiTE antibody forces a ______ synapse between a T cell and a target cell. (cytolytic)
True or False: DARTs are single-polypeptide-chain molecules, like BiTEs. (False. DARTs consist of two polypeptide chains linked by interchain disulphide bridges.)
Fill in the blank: Nanobodies are derived from heavy-chain-only antibodies found naturally in ______. (camelids, e.g. camels, llamas)
True or False: Antibody mimetics can mediate ADCC via an Fc region. (False. Antibody mimetics lack an Fc region and cannot mediate ADCC or CDC.)
Q: Explain the mechanism by which blinatumomab destroys B cells, and state why no co-stimulatory signal is required.
A: Blinatumomab is a BiTE that bispecifically binds CD19 on B cells and CD3-epsilon on T cells, forcing the formation of a cytolytic synapse. T cell activation is solely dependent on blinatumomab binding. The physical proximity induced by the BiTE is sufficient to trigger T cell activation and serial lysis of B cells via granzyme and perforin release, without any additional co-stimulatory signal.
Q: Compare the structural and functional differences between BiTE and DART antibody formats.
A: BiTEs are single polypeptide chains (V_La-V_Ha-linker-V_Lb-V_Hb), while DARTs consist of two polypeptide chains linked by interchain disulphide bridges (V_La-V_Hb and V_Lb-V_Ha). Both are ~55 kDa and bispecific. The DART format is more rigid due to limited flexibility between antigen-binding moieties, which may explain its superior performance in comparative studies regarding maximal B cell lysis, half-maximal lysis concentration, and T cell activation.
Q: Why are antibody mimetics considered a promising alternative to therapeutic antibodies, and what is their main limitation?
A: Antibody mimetics offer high specificity at low molecular weight, enabling better tissue penetration and cheaper production (in bacteria). They also appear to have low immunogenicity. Their main limitation is a short serum half-life, which necessitates more frequent dosing.
Q: Describe how abatacept interferes with T cell activation, referring to the roles of CTLA-4, CD28, and B7.
A: Abatacept is a fusion protein of the CTLA-4 extracellular domain and the Fc of IgG1. On antigen-presenting cells, B7 can bind either CTLA-4 (leading to T cell anergy) or CD28 (leading to T cell activation). CTLA-4 has higher affinity for B7 than CD28 does. Abatacept acts as a soluble decoy, binding B7 and blocking the CD28-B7 co-stimulatory interaction, thereby preventing T cell activation.
Chimeric proteins connect back to antibody Fc region biology (covered in Part 1) and receptor-ligand interactions from basic cell signalling.
BiTEs and DARTs build on the concept of T cell cytotoxicity (granzyme, perforin, CD3 signalling) from basic immunology.
Antibody mimetics connect to protein engineering and scaffold design, topics relevant to biotech and structural biology courses.
Chimeric proteins, fusion proteins, etanercept, Enbrel, rilonacept, Arcalyst, belatacept, abatacept, CTLA-4-Ig, BiTE antibody, blinatumomab, DART antibody, TandAb, nanobody, camelid antibody, V_HH, DARPin, avimer, anticalin, antibody derivative, antibody mimetic, bispecific antibody, CD3-epsilon, CD19, cytolytic synapse, T cell engager, scaffold protein, ankyrin repeat, lipocalin, tissue penetration, serum half-life, cancer immunotherapy