Human IgG Fc-engineering for enhanced plasma half-life, mucosal distribution and killing of cancer cells and bacteria

Monoclonal IgG antibodies constitute the fastest growing class of therapeutics. Thus, there is an intense interest to design more potent antibody formats, where long plasma half-life is a commercially competitive differentiator affecting dosing, frequency of administration and thereby potentially patient compliance. Here, we report on an Fc-engineered variant with three amino acid substitutions Q311R/M428E/N434W (REW), that enhances plasma half-life and mucosal distribution, as well as allows for needle-free delivery across respiratory epithelial barriers in human FcRn transgenic mice. In addition, the Fc-engineered variant improves on-target complement-mediated killing of cancer cells as well as both gram-positive and gram-negative bacteria. Hence, this versatile Fc technology should be broadly applicable in antibody design aiming for long-acting prophylactic or therapeutic interventions.

A pan-SARS-CoV-2-specific soluble angiotensin-converting enzyme 2-albumin fusion engineered for enhanced plasma half-life and needle-free mucosal delivery

Immunocompromised patients often fail to raise protective vaccine-induced immunity against the global emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants. Although monoclonal antibodies have been authorized for clinical use, most have lost their ability to potently neutralize the evolving Omicron subvariants. Thus, there is an urgent need for treatment strategies that can provide protection against these and emerging SARS-CoV-2 variants to prevent the development of severe coronavirus disease 2019. Here, we report on the design and characterization of a long-acting viral entry-blocking angiotensin-converting enzyme 2 (ACE2) dimeric fusion molecule. Specifically, a soluble truncated human dimeric ACE2 variant, engineered for improved binding to the receptor-binding domain of SARS-CoV-2, was fused with human albumin tailored for favorable engagement of the neonatal fragment crystallizable receptor (FcRn), which resulted in enhanced plasma half-life and allowed for needle-free transmucosal delivery upon nasal administration in human FcRn-expressing transgenic mice. Importantly, the dimeric ACE2-fused albumin demonstrated potent neutralization of SARS-CoV-2 immune escape variants.

Contribution of the ex vivo placental perfusion model in understanding transplacental immunoglobulin G transfer

INTRODUCTION

The acquisition of humoral immunity in utero is essential for the fetus. The crucial protein, which is responsible for this part of immunity, is immunoglobulin-G (IgG). Immune functions of IgGs are mediated via the interaction of the crystallizable fragment (Fc) region of IgG with specific Fc γ receptors (FcγRs). However, an atypical FcγR, the neonatal Fc receptor (FcRn), is a key regulator of IgG transfer across the human placenta. During the last four decades ex vivo placental perfusion studies have contributed significantly to the study of mechanisms of IgG transfer across the multicellular placental barrier.

METHOD

A PubMed search was conducted by using specific keywords: placenta, perfusion and IgG to review manuscripts using human placental perfusion to study the transplacental transfer of IgG. Relevant studies found in reference lists of these manuscripts were also added to the review, and references were included that supported or gave nuance to the discussion of the mechanisms of IgG kinetics in the placenta.

RESULTS AND DISCUSSION

We found twenty publications on the study of transplacental transfer of IgG using human ex vivo placental perfusion, by research groups with partly different settings. This review summarizes knowledge about placental IgG transfer, with a strong focus on the contributions from ex vivo placental perfusion studies.

Potent TRIM21 and complement-dependent intracellular antiviral immunity requires the IgG3 hinge

Humans have four IgG antibody subclasses that selectively or differentially engage immune effector molecules to protect against infections. Although IgG1 has been studied in detail and is the subclass of most approved antibody therapeutics, increasing evidence indicates that IgG3 is associated with enhanced protection against pathogens. Here, we report that IgG3 has superior capacity to mediate intracellular antiviral immunity compared with the other subclasses due to its uniquely extended and flexible hinge region, which facilitates improved recruitment of the cytosolic Fc receptor TRIM21, independently of Fc binding affinity. TRIM21 may also synergize with complement C1/C4-mediated lysosomal degradation via capsid inactivation. We demonstrate that this process is potentiated by IgG3 in a hinge-dependent manner. Our findings reveal differences in how the four IgG subclasses mediate intracellular immunity, knowledge that may guide IgG subclass selection and engineering of antiviral antibodies for prophylaxis and therapy.

Affinity maturation of TCR-like antibodies using phage display guided by structural modeling

TCR-like antibodies represent a unique type of engineered antibodies with specificity toward pHLA, a ligand normally restricted to the sensitive recognition by T cells. Here, we report a phage display-based sequential development path of such antibodies. The strategy goes from initial lead identification through in silico informed CDR engineering in combination with framework engineering for affinity and thermostability optimization, respectively. The strategy allowed the identification of HLA-DQ2.5 gluten peptide-specific TCR-like antibodies with low picomolar affinity. Our method outlines an efficient and general method for development of this promising class of antibodies, which should facilitate their utility including translation to human therapy.

Antibody variable sequences have a pronounced effect on cellular transport and plasma half-life

Monoclonal IgG antibodies are the fastest growing class of biologics, but large differences exist in their plasma half-life in humans. Thus, to design IgG antibodies with favorable pharmacokinetics, it is crucial to identify the determinants of such differences. Here, we demonstrate that the variable region sequences of IgG antibodies greatly affect cellular uptake and subsequent recycling and rescue from intracellular degradation by endothelial cells. When the variable sequences are masked by the cognate antigen, it influences both their transport behavior and binding to the neonatal Fc receptor (FcRn), a key regulator of IgG plasma half-life. Furthermore, we show how charge patch differences in the variable domains modulate both binding and transport properties and that a short plasma half-life, due to unfavorable charge patches, may partly be overcome by Fc-engineering for improved FcRn binding.

•

IgG variable region sequences greatly affect cellular uptake and recycling

•

Variable region charge patches affect FcRn binding and transport

•

The presence of cognate antigen modulates cellular transport and FcRn binding

•

Fc-engineering for improved FcRn binding can overcome unfavorable charge patches

A high-affinity human TCR-like antibody detects celiac disease gluten peptide-MHC complexes and inhibits T cell activation

Antibodies specific for peptides bound to human leukocyte antigen (HLA) molecules are valuable tools for studies of antigen presentation and may have therapeutic potential. Here, we generated human T cell receptor (TCR)-like antibodies toward the immunodominant signature gluten epitope DQ2.5-glia-α2 in celiac disease (CeD). Phage display selection combined with secondary targeted engineering was used to obtain highly specific antibodies with picomolar affinity. The crystal structure of a Fab fragment of the lead antibody 3.C11 in complex with HLA-DQ2.5:DQ2.5-glia-α2 revealed a binding geometry and interaction mode highly similar to prototypic TCRs specific for the same complex. Assessment of CeD biopsy material confirmed disease specificity and reinforced the notion that abundant plasma cells present antigen in the inflamed CeD gut. Furthermore, 3.C11 specifically inhibited activation and proliferation of gluten-specific CD4⁺ T cells in vitro and in HLA-DQ2.5 humanized mice, suggesting a potential for targeted intervention without compromising systemic immunity.

An engineered human albumin enhances half-life and transmucosal delivery when fused to protein-based biologics

Needle-free uptake across mucosal barriers is a preferred route for delivery of biologics, but the efficiency of unassisted transmucosal transport is poor. To make administration and therapy efficient and convenient, strategies for the delivery of biologics must enhance both transcellular delivery and plasma half-life. We found that human albumin was transcytosed efficiently across polarized human epithelial cells by a mechanism that depends on the neonatal Fc receptor (FcRn). FcRn also transported immunoglobulin G, but twofold less than albumin. We therefore designed a human albumin variant, E505Q/T527M/K573P (QMP), with improved FcRn binding, resulting in enhanced transcellular transport upon intranasal delivery and extended plasma half-life of albumin in transgenic mice expressing human FcRn. When QMP was fused to recombinant activated coagulation factor VII, the half-life of the fusion molecule increased 3.6-fold compared with the wild-type human albumin fusion, without compromising the therapeutic properties of activated factor VII. Our findings highlight QMP as a suitable carrier of protein-based biologics that may enhance plasma half-life and delivery across mucosal barriers.

Extended plasma half-life of albumin-binding domain fused human IgA upon pH-dependent albumin engagement of human FcRn in vitro and in vivo

Albumin has a serum half-life of 3 weeks in humans. This feature can be used to improve the pharmacokinetics of shorter-lived biologics. For instance, an albumin-binding domain (ABD) can be used to recruit albumin. A prerequisite for such design is that the ABD-albumin interaction does not interfere with pH-dependent binding of albumin to the human neonatal Fc receptor (FcRn), as FcRn acts as the principal regulator of the half-life of albumin. Thus, there is a need to know how ABDs act in the context of fusion partners and human FcRn. Here, we studied the binding and transport properties of human immunoglobulin A1 (IgA1), fused to a Streptococcus protein G-derived engineered ABD, in in vitro and in vivo systems harboring human FcRn. IgA has great potential as a therapeutic protein, but its short half-life is a major drawback. We demonstrate that ABD-fused IgA1 binds human FcRn pH-dependently and is rescued from cellular degradation in a receptor-specific manner in the presence of albumin. This occurs when ABD is fused to either the light or the heavy chain. In human FcRn transgenic mice, IgA1-ABD in complex with human albumin, gave 4-6-fold extended half-life compared to unmodified IgA1, where the light chain fusion showed the longest half-life. When the heavy chain-fused protein was pre-incubated with an engineered human albumin with improved FcRn binding, cellular rescue and half-life was further enhanced. Our study reveals how an ABD, which does not interfere with albumin binding to human FcRn, may be used to extend the half-life of IgA.

Abbreviations: ABD - Albumin binding domain, ADA – anti-drug-antibodies, ADCC - Antibody-dependent cellular cytotoxicity, ELISA - Enzyme-linked Immunosorbent assay, FcαRI - Fcα receptor, FcγR - Fcγ receptor, FcRn - The neonatal Fc receptor, GST - Glutathione S-transferase, HC - Heavy chain, HERA - Human endothelial cell-based recycling assay, Her2 - Human epidermal growth factor 2, HMEC - Human microvascular endothelial cells, IgG - Immunoglobulin G, IgA - Immunoglobulin A, LC - Light chain, QMP - E505Q/T527M/K573P, WT - Wild type

FcRn is a CD32a coreceptor that determines susceptibility to IgG immune complex-driven autoimmunity

IgG immune complexes (ICs) promote autoimmunity through binding fragment crystallizable (Fc) γ-receptors (FcγRs). Of these, the highly prevalent FcγRIIa (CD32a) histidine (H)-131 variant (CD32a^H) is strongly linked to human autoimmune diseases through unclear mechanisms. We show that, relative to the CD32a arginine (R)-131 (CD32a^R) variant, CD32a^H more avidly bound human (h) IgG1 IC and formed a ternary complex with the neonatal Fc receptor (FcRn) under acidic conditions. In primary human and mouse cells, both CD32a variants required FcRn to induce innate and adaptive immune responses to hIgG1 ICs, which were augmented in the setting of CD32a^H. Conversely, FcRn induced responses to IgG IC independently of classical FcγR, but optimal responses required FcRn and FcγR. Finally, FcRn blockade decreased inflammation in a rheumatoid arthritis model without reducing circulating autoantibody levels, providing support for FcRn’s direct role in IgG IC-associated inflammation. Thus, CD32a and FcRn coregulate IgG IC-mediated immunity in a manner favoring the CD32a^H variant, providing a novel mechanism for its disease association.

Antibody-mediated delivery of T-cell epitopes to antigen-presenting cells induce strong CD4 and CD8 T-cell responses

Targeted delivery of antigen to antigen-presenting cells (APCs) enhances antigen presentation and thus, is a potent strategy for making more efficacious vaccines. This can be achieved by use of antibodies with specificity for endocytic surface molecules expressed on the APC. We aimed to compare two different antibody-antigen fusion modes in their ability to induce T-cell responses; first, exchange of immunoglobulin (Ig) constant domain loops with a T-cell epitope (Troybody), and second, fusion of T-cell epitope or whole antigen to the antibody C-terminus. Although both strategies are well-established, they have not previously been compared using the same system. We found that both antibody-antigen fusion modes led to presentation of the T-cell epitope. The strength of the T-cell responses varied, however, with the most efficient Troybody inducing CD4 T-cell proliferation and cytokine secretion at 10-100-fold lower concentration than the antibodies carrying antigen fused to the C-terminus, both in vitro and after intravenous injection in mice. Furthermore, we exchanged this loop with an MHCI-restricted T-cell epitope, and the resulting antibody enabled efficient cross-presentation to CD8 T cells in vivo. Targeting of antigen to APCs by use of such antibody-antigen fusions is thus an attractive vaccination strategy for increased activation of both CD4 and CD8 peptide-specific T cells.

The neonatal Fc receptor in mucosal immune regulation

The neonatal Fc receptor (FcRn) was first recognized for its role in transfer of maternal IgG to the foetus or newborn, providing passive immunity early in life. However, it has become clear that the receptor is versatile, widely expressed and plays an indispensable role in both immunological and non-immunological processes throughout life. The receptor rescues immunoglobulin G (IgG) and albumin from intracellular degradation and shuttles the ligands across polarized cell barriers, in all cases via a pH-dependent binding-and-release mechanism. These processes secure distribution and high levels of both IgG and albumin throughout the body. At mucosal sites, FcRn transports IgG across polarized epithelial cells where it retrieves IgG in complex with luminal antigens that is delivered to tissue-localized immune cells. In dendritic cells (DCs), FcRn orchestrates processing of IgG-opsonized immune complexes (ICs) in concert with classical Fcγ receptors, which results in antigen presentation and cross-presentation of antigenic peptides on MHC class II and I to CD4+ and CD8+ T cells, respectively. Hence, FcRn regulates transport of the ligands within and across different types of cells, but also processing of IgG-ICs by immune cells. As such, the receptor is involved in immune surveillance and protection against infections. In this brief review, we highlight how FcRn expressed by hematopoietic and non-hematopoietic cells contributes to immune regulation at mucosal barriers-biology that can be utilized in development of biologics and subunit vaccines for non-invasive delivery.

A TRAV26-1-encoded recognition motif focuses the biased T cell response in celiac disease

The semi-public T-cell response towards the gluten epitope DQ2.5-glia-α2 uses a prototypic TCR encoded by the germline segments TRAV26-1 and TRBV7-2. Through mutagenesis experiments, we show that a TRAV26-1encoded recognition motif contacts the MHC β-chain and the TCR CDR3β loop underpinning this conserved T-cell response restricted to the prototypic TCRs.

TRIM21-From Intracellular Immunity to Therapy

Tripartite motif containing-21 (TRIM21) is a cytosolic ubiquitin ligase and antibody receptor that provides a last line of defense against invading viruses. It does so by acting as a sensor that intercepts antibody-coated viruses that have evaded extracellular neutralization and breached the cell membrane. Upon engagement of the Fc of antibodies bound to viruses, TRIM21 triggers a coordinated effector and signaling response that prevents viral replication while at the same time inducing an anti-viral cellular state. This dual effector function is tightly regulated by auto-ubiquitination and phosphorylation. Therapeutically, TRIM21 has been shown to be detrimental in adenovirus based gene therapy, while it may be favorably utilized to prevent tau aggregation in neurodegenerative disorders. In addition, TRIM21 may synergize with the complement system to block viral replication as well as transgene expression. TRIM21 can also be utilized as a research tool to deplete specific proteins in cells and zebrafish embryos. Here, we review our current biological understanding of TRIM21 in light of its versatile functions.

The Neonatal Fc Receptor (FcRn): A Misnomer?

Antibodies are essential components of an adaptive immune response. Immunoglobulin G (IgG) is the most common type of antibody found in circulation and extracellular fluids. Although IgG alone can directly protect the body from infection through the activities of its antigen binding region, the majority of IgG immune functions are mediated via proteins and receptors expressed by specialized cell subsets that bind to the fragment crystallizable (Fc) region of IgG. Fc gamma (γ) receptors (FcγR) belong to a broad family of proteins that presently include classical membrane-bound surface receptors as well as atypical intracellular receptors and cytoplasmic glycoproteins. Among the atypical FcγRs, the neonatal Fc receptor (FcRn) has increasingly gained notoriety given its intimate influence on IgG biology and its ability to also bind to albumin. FcRn functions as a recycling or transcytosis receptor that is responsible for maintaining IgG and albumin in the circulation, and bidirectionally transporting these two ligands across polarized cellular barriers. More recently, it has been appreciated that FcRn acts as an immune receptor by interacting with and facilitating antigen presentation of peptides derived from IgG immune complexes (IC). Here we review FcRn biology and focus on newer advances including how emerging FcRn-targeted therapies may affect the immune responses to IgG and IgG IC.

Publisher Correction: Binding to nanopatterned antigens is dominated by the spatial tolerance of antibodies

In the Supplementary Information file originally published with this Article, the Supplementary references 48-62 were missing; the amended file has now been uploaded.

Plasma Cells Are the Most Abundant Gluten Peptide MHC-expressing Cells in Inflamed Intestinal Tissues From Patients With Celiac Disease

BACKGROUND & AIMS

Development of celiac disease is believed to involve the transglutaminase-dependent response of CD4⁺ T cells toward deamidated gluten peptides in the intestinal mucosa of individuals with specific HLA-DQ haplotypes. We investigated the antigen presentation process during this mucosal immune response.

METHODS

We generated monoclonal antibodies (mAbs) specific for the peptide-MHC (pMHC) complex of HLA-DQ2.5 and the immunodominant gluten epitope DQ2.5-glia-α1a using phage display. We used these mAbs to assess gluten peptide presentation and phenotypes of presenting cells by flow cytometry and enzyme-linked immune absorbent spot (ELISPOT) in freshly prepared single-cell suspensions from intestinal biopsies from 40 patients with celiac disease (35 untreated and 5 on a gluten-free diet) as well as 18 subjects with confirmed noninflamed gut mucosa (controls, 12 presumed healthy, 5 undergoing pancreatoduodenectomy, and 1 with potential celiac disease).

RESULTS

Using the mAbs, we detected MHC complexes on cells from intestinal biopsies from patients with celiac disease who consume gluten, but not from patients on gluten-free diets. We found B cells and plasma cells to be the most abundant cells that present DQ2.5-glia-α1a in the inflamed mucosa. We identified a subset of plasma cells that expresses B-cell receptors (BCR) specific for gluten peptides or the autoantigen transglutaminase 2 (TG2). Expression of MHC class II (MHCII) was not restricted to these specific plasma cells in patients with celiac disease but was observed in an average 30% of gut plasma cells from patients and controls.

CONCLUSIONS

A population of plasma cells from intestinal biopsies of patients with celiac disease express MHCII; this is the most abundant cell type presenting the immunodominant gluten peptide DQ2.5-glia-α1a in the tissues from these patients. These results indicate that plasma cells in the gut can function as antigen-presenting cells and might promote and maintain intestinal inflammation in patients with celiac disease or other inflammatory disorders.

Complement C4 Prevents Viral Infection through Capsid Inactivation

The complement system is vital for anti-microbial defense. In the classical pathway, pathogen-bound antibody recruits the C1 complex (C1qC1r₂C1s₂) that initiates a cleavage cascade involving C2, C3, C4, and C5 and triggering microbial clearance. We demonstrate a C4-dependent antiviral mechanism that is independent of downstream complement components. C4 inhibits human adenovirus infection by directly inactivating the virus capsid. Rapid C4 activation and capsid deposition of cleaved C4b are catalyzed by antibodies via the classical pathway. Capsid-deposited C4b neutralizes infection independent of C2 and C3 but requires C1q antibody engagement. C4b inhibits capsid disassembly, preventing endosomal escape and cytosolic access. C4-deficient mice exhibit heightened viral burdens. Additionally, complement synergizes with the Fc receptor TRIM21 to block transduction by an adenovirus gene therapy vector but is partially restored by Fab virus shielding. These results suggest that the complement system could be altered to prevent virus infection and enhance virus gene therapy efficacy.

•

Complement components C1 and C4 mediate potent neutralization of adenovirus

•

Deposition of C4b on the viral capsid inactivates capsid disassembly

•

C4 exerts direct antiviral functions independent from its role as a C3-convertase

•

C4 antiviral functions synergize with TRIM21-mediated virus neutralization

Binding to nanopatterned antigens is dominated by the spatial tolerance of antibodies

Although repetitive patterns of antigens are crucial for certain immune responses, an understanding of how antibodies bind and dynamically interact with various spatial arrangements of molecules is lacking. Hence, we introduced a new method in which molecularly precise nanoscale patterns of antigens are displayed using DNA origami and immobilized in a surface plasmon resonance set-up. Using antibodies with identical antigen-binding domains, we found that all the subclasses and isotypes studied bind bivalently to two antigens separated at distances that range from 3 to 17 nm. The binding affinities of these antibodies change with the antigen distances, with a distinct preference for antigens separated by approximately 16 nm, and considerable differences in spatial tolerance exist between IgM and IgG and between low- and high-affinity antibodies.

Soluble T-cell receptor design influences functional yield in an E. coli chaperone-assisted expression system

There is a quest for production of soluble protein of high quality for the study of T-cell receptors (TCRs), but expression often results in low yields of functional molecules. In this study, we used an E. coli chaperone-assisted periplasmic production system and compared expression of 4 different soluble TCR formats: single-chain TCR (scTCR), two different disulfide-linked TCR (dsTCR) formats, and chimeric Fab (cFab). A stabilized version of scTCR was also included. Additionally, we evaluated the influence of host (XL1-Blue or RosettaBlue^TM) and the effect of IPTG induction on expression profiles. A celiac disease patient-derived TCR with specificity for gluten was used, and we achieved detectable expression for all formats and variants. We found that expression in RosettaBlue^TM without IPTG induction resulted in the highest periplasmic yields. Moreover, after large-scale expression and protein purification, only the scTCR format was obtained in high yields. Importantly, stability engineering of the scTCR was a prerequisite for obtaining reliable biophysical characterization of the TCR-pMHC interaction. The scTCR format is readily compatible with high-throughput screening approaches that may enable both development of reagents allowing for defined peptide MHC (pMHC) characterization and discovery of potential novel therapeutic leads.

A human endothelial cell-based recycling assay for screening of FcRn targeted molecules

Albumin and IgG have remarkably long serum half-lives due to pH-dependent FcRn-mediated cellular recycling that rescues both ligands from intracellular degradation. Furthermore, increase in half-lives of IgG and albumin-based therapeutics has the potential to improve their efficacies, but there is a great need for robust methods for screening of relative FcRn-dependent recycling ability. Here, we report on a novel human endothelial cell-based recycling assay (HERA) that can be used for such pre-clinical screening. In HERA, rescue from degradation depends on FcRn, and engineered ligands are recycled in a manner that correlates with their half-lives in human FcRn transgenic mice. Thus, HERA is a novel cellular assay that can be used to predict how FcRn-binding proteins are rescued from intracellular degradation.

A TCRα framework-centered codon shapes a biased T cell repertoire through direct MHC and CDR3β interactions

Selection of biased T cell receptor (TCR) repertoires across individuals is seen in both infectious diseases and autoimmunity, but the underlying molecular basis leading to these shared repertoires remains unclear. Celiac disease (CD) occurs primarily in HLA-DQ2.5+ individuals and is characterized by a CD4+ T cell response against gluten epitopes dominated by DQ2.5-glia-α1a and DQ2.5-glia-α2. The DQ2.5-glia-α2 response recruits a highly biased TCR repertoire composed of TRAV26-1 paired with TRBV7-2 harboring a semipublic CDR3β loop. We aimed to unravel the molecular basis for this signature. By variable gene segment exchange, directed mutagenesis, and cellular T cell activation studies, we found that TRBV7-3 can substitute for TRBV7-2, as both can contain the canonical CDR3β loop. Furthermore, we identified a pivotal germline-encoded MHC recognition motif centered on framework residue Y40 in TRAV26-1 engaging both DQB1*02 and the canonical CDR3β. This allowed prediction of expanded DQ2.5-glia-α2-reactive TCR repertoires, which were confirmed by single-cell sorting and TCR sequencing from CD patient samples. Our data refine our understanding of how HLA-dependent biased TCR repertoires are selected in the periphery due to germline-encoded residues.

Human Secretory IgM Antibodies Activate Human Complement and Offer Protection at Mucosal Surface

IgM molecules circulate in serum as large polymers, mainly pentamers, which can be transported by the poly-Ig receptor (pIgR) across epithelial cells to mucosal surfaces and released as secretory IgM (SIgM). The mucosal SIgM molecules have non-covalently attached secretory component (SC), which is the extracellular part of pIgR which is cleaved from the epithelial cell membrane. Serum IgM antibodies do not contain SC and have previously been shown to make a conformational change from 'a star' to a 'staple' conformation upon reaction with antigens on a cell surface, enabling them to activate complement. However, it is not clear whether SIgM similarly can induce complement activation. To clarify this issue, we constructed recombinant chimeric (mouse/human) IgM antibodies against hapten 5-iodo-4-hydroxy-3-nitro-phenacetyl (NIP) and in addition studied polyclonal IgM formed after immunization with a meningococcal group B vaccine. The monoclonal and polyclonal IgM molecules were purified by affinity chromatography on a column containing human SC in order to isolate joining-chain (J-chain) containing IgM, followed by addition of excess amounts of soluble SC to create SIgM (IgM J+ SC+). These SIgM preparations were tested for complement activation ability and shown to be nearly as active as the parental IgM J+ molecules. Thus, SIgM may offer protection against pathogens at mucosal surface by complement-mediated cell lysis or by phagocytosis mediated by complement receptors present on effector cells on mucosa.

Multivalent pIX phage display selects for distinct and improved antibody properties

Phage display screening readily allows for the identification of a multitude of antibody specificities, but to identify optimal lead candidates remains a challenge. Here, we direct the antibody-capsid fusion away from the signal sequence-dependent secretory SEC pathway in E. coli by utilizing the intrinsic signal sequence-independent property of pIX to obtain virion integration. This approach was combined with the use of an engineered helper phage known to improve antibody pIX display and retrieval. By direct comparison with pIII display, we demonstrate that antibody display using this pIX system translates into substantially improved retrieval of desired specificities with favorable biophysical properties in de novo selection. We show that the effect was due to less E. coli host toxicity during phage propagation conferred by the lack of a signal sequence. This pIX combinatorial display platform provides a generic alternative route for obtaining good binders with high stability and may thus find broad applicability.

TRIM21: a cytosolic Fc receptor with broad antibody isotype specificity

Antibodies are key molecules in the fight against infections. Although previously thought to mediate protection solely in the extracellular environment, recent research has revealed that antibody-mediated protection extends to the cytosolic compartment of cells. This postentry viral defense mechanism requires binding of the antibody to a cytosolic Fc receptor named tripartite motif containing 21 (TRIM21). In contrast to other Fc receptors, TRIM21 shows remarkably broad isotype specificity as it does not only bind IgG but also IgM and IgA. When viral pathogens coated with these antibody isotypes enter the cytosol, TRIM21 is rapidly recruited and efficient neutralization occurs before the virus has had the time to replicate. In addition, inflammatory signaling is induced. As such, TRIM21 acts as a cytosolic sensor that engages antibodies that have failed to protect against infection in the extracellular environment. Here, we summarize our current understanding of how TRIM21 orchestrates humoral immunity in the cytosolic environment.

TRIM21 Immune Signaling Is More Sensitive to Antibody Affinity Than Its Neutralization Activity

Ab-coated viruses can be detected in the cytosol by the FcR tripartite motif-containing 21 (TRIM21), which rapidly recruits the proteasomal machinery and triggers induction of immune signaling. As such, TRIM21 plays a key role in intracellular protection by targeting invading viruses for destruction and alerting the immune system. A hallmark of immunity is elicitation of a balanced response that is proportionate to the threat, to avoid unnecessary inflammation. In this article, we show how Ab affinity modulates TRIM21 immune function. We constructed a humanized monoclonal IgG1 against human adenovirus type 5 (AdV5) and a panel of Fc-engineered variants with a wide range of affinities for TRIM21. We found that IgG1-coated viral particles were neutralized via TRIM21, even when affinity was reduced by as much as 100-fold. In contrast, induction of NF-κB signaling was more sensitive to reduced affinity between TRIM21 and the Ab variants. Thus, TRIM21 mediates neutralization under suboptimal conditions, whereas induction of immune signaling is balanced according to the functional affinity for the incoming immune stimuli. Our findings have implications for engineering of antiviral IgG therapeutics with tailored effector functions.

Enhanced FcRn-dependent transepithelial delivery of IgG by Fc-engineering and polymerization

Monoclonal IgG antibodies (Abs) are used extensively in the clinic to treat cancer and autoimmune diseases. In addition, therapeutic proteins are genetically fused to the constant Fc part of IgG. In both cases, the Fc secures a long serum half-life and favourable pharmacokinetics due to its pH-dependent interaction with the neonatal Fc receptor (FcRn). FcRn also mediates transport of intact IgG across polarized epithelial barriers, a pathway that is attractive for delivery of Fc-containing therapeutics. So far, no study has thoroughly compared side-by-side how IgG and different Fc-fusion formats are transported across human polarizing epithelial cells. Here, we used an in vitro cellular transport assay based on the human polarizing epithelial cell line (T84) in which both IgG1 and Fc-fusions were transported in an FcRn-dependent manner. Furthermore, we found that the efficacy of transport was dependent on the format. We demonstrate that transepithelial delivery could be enhanced by Fc-engineering for improved FcRn binding as well as by Fc-polymerization. In both cases, transport was driven by pH-dependent binding kinetics and the pH at the luminal side. Hence, efficient transcellular delivery of IgG-based drugs across human epithelial cells requires optimal pH-dependent FcRn binding that can be manipulated by avidity and Fc-engineering, factors that should inspire the design of future therapeutics targeted for transmucosal delivery.

Developing the IVIG biomimetic, hexa-Fc, for drug and vaccine applications

The remarkable clinical success of Fc-fusion proteins has driven intense investigation for even more potent replacements. Using quality-by-design (QbD) approaches, we generated hexameric-Fc (hexa-Fc), a ~20 nm oligomeric Fc-based scaffold that we here show binds low-affinity inhibitory receptors (FcRL5, FcγRIIb, and DC-SIGN) with high avidity and specificity, whilst eliminating significant clinical limitations of monomeric Fc-fusions for vaccine and/or cancer therapies, in particular their poor ability to activate complement. Mass spectroscopy of hexa-Fc reveals high-mannose, low-sialic acid content, suggesting that interactions with these receptors are influenced by the mannose-containing Fc. Molecular dynamics (MD) simulations provides insight into the mechanisms of hexa-Fc interaction with these receptors and reveals an unexpected orientation of high-mannose glycans on the human Fc that provides greater accessibility to potential binding partners. Finally, we show that this biosynthetic nanoparticle can be engineered to enhance interactions with the human neonatal Fc receptor (FcRn) without loss of the oligomeric structure, a crucial modification for these molecules in therapy and/or vaccine strategies where a long plasma half-life is critical.

Fc Engineering of Human IgG1 for Altered Binding to the Neonatal Fc Receptor Affects Fc Effector Functions

Engineering of the constant Fc part of monoclonal human IgG1 (hIgG1) Abs is an approach to improve effector functions and clinical efficacy of next-generation IgG1-based therapeutics. A main focus in such development is tailoring of in vivo half-life and transport properties by engineering the pH-dependent interaction between IgG and the neonatal Fc receptor (FcRn), as FcRn is the main homeostatic regulator of hIgG1 half-life. However, whether such engineering affects binding to other Fc-binding molecules, such as the classical FcγRs and complement factor C1q, has not been studied in detail. These effector molecules bind to IgG1 in the lower hinge-CH2 region, structurally distant from the binding site for FcRn at the CH2-CH3 elbow region. However, alterations of the structural composition of the Fc may have long-distance effects. Indeed, in this study we show that Fc engineering of hIgG1 for altered binding to FcRn also influences binding to both the classical FcγRs and complement factor C1q, which ultimately results in alterations of cellular mechanisms such as Ab-dependent cell-mediated cytotoxicity, Ab-dependent cellular phagocytosis, and Ab-dependent complement-mediated cell lysis. Thus, engineering of the FcRn-IgG1 interaction may greatly influence effector functions, which has implications for the therapeutic efficacy and use of Fc-engineered hIgG1 variants.

Phage Display Engineered T Cell Receptors as Tools for the Study of Tumor Peptide-MHC Interactions

Cancer immunotherapy has finally come of age, demonstrated by recent progress in strategies that engage the endogenous adaptive immune response in tumor killing. Occasionally, significant and durable tumor regression has been achieved. A giant leap forward was the demonstration that the pre-existing polyclonal T cell repertoire could be re-directed by use of cloned T cell receptors (TCRs), to obtain a defined tumor-specific pool of T cells. However, the procedure must be performed with caution to avoid deleterious cross-reactivity. Here, the use of engineered soluble TCRs may represent a safer, yet powerful, alternative. There is also a need for deeper understanding of the processes that underlie antigen presentation in disease and homeostasis, how tumor-specific peptides are generated, and how epitope spreading evolves during tumor development. Due to its plasticity, the pivotal interaction where a TCR engages a peptide/MHC (pMHC) also requires closer attention. For this purpose, phage display as a tool to evolve cloned TCRs represents an attractive avenue to generate suitable reagents allowing the study of defined pMHC presentation, TCR engagement, as well as for the discovery of novel therapeutic leads. Here, we highlight important aspects of the current status in this field.

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

Interaction with both domain I and III of albumin is required for optimal pH-dependent binding to the neonatal Fc receptor (FcRn)

Albumin is an abundant blood protein that acts as a transporter of a plethora of small molecules like fatty acids, hormones, toxins, and drugs. In addition, it has an unusual long serum half-life in humans of nearly 3 weeks, which is attributed to its interaction with the neonatal Fc receptor (FcRn). FcRn protects albumin from intracellular degradation via a pH-dependent cellular recycling mechanism. To understand how FcRn impacts the role of albumin as a distributor, it is of importance to unravel the structural mechanism that determines pH-dependent binding. Here, we show that although the C-terminal domain III (DIII) of human serum albumin (HSA) contains the principal binding site, the N-terminal domain I (DI) is important for optimal FcRn binding. Specifically, structural inspection of human FcRn (hFcRn) in complex with HSA revealed that two exposed loops of DI were in proximity with the receptor. To investigate to what extent these contacts affected hFcRn binding, we targeted selected amino acid residues of the loops by mutagenesis. Screening by in vitro interaction assays revealed that several of the engineered HSA variants showed decreased binding to hFcRn, which was also the case for two missense variants with mutations within these loops. In addition, four of the variants showed improved binding. Our findings demonstrate that both DI and DIII are required for optimal binding to FcRn, which has implications for our understanding of the FcRn-albumin relationship and how albumin acts as a distributor. Such knowledge may inspire development of novel HSA-based diagnostics and therapeutics.

Dissection of the neonatal Fc receptor (FcRn)-albumin interface using mutagenesis and anti-FcRn albumin-blocking antibodies

Albumin is the most abundant protein in blood and plays a pivotal role as a multitransporter of a wide range of molecules such as fatty acids, metabolites, hormones, and toxins. In addition, it binds a variety of drugs. Its role as distributor is supported by its extraordinary serum half-life of 3 weeks. This is related to its size and binding to the cellular receptor FcRn, which rescues albumin from intracellular degradation. Furthermore, the long half-life has fostered a great and increasing interest in utilization of albumin as a carrier of protein therapeutics and chemical drugs. However, to fully understand how FcRn acts as a regulator of albumin homeostasis and to take advantage of the FcRn-albumin interaction in drug design, the interaction interface needs to be dissected. Here, we used a panel of monoclonal antibodies directed towards human FcRn in combination with site-directed mutagenesis and structural modeling to unmask the binding sites for albumin blocking antibodies and albumin on the receptor, which revealed that the interaction is not only strictly pH-dependent, but predominantly hydrophobic in nature. Specifically, we provide mechanistic evidence for a crucial role of a cluster of conserved tryptophan residues that expose a pH-sensitive loop of FcRn, and identify structural differences in proximity to these hot spot residues that explain divergent cross-species binding properties of FcRn. Our findings expand our knowledge of how FcRn is controlling albumin homeostasis at a molecular level, which will guide design and engineering of novel albumin variants with altered transport properties.

The arabidopsis histone methyltransferase SUVR4 binds ubiquitin via a domain with a four-helix bundle structure

In eukaryotes, different chromatin states facilitate or repress gene expression and restrict the activity of transposable elements. Post-translational modifications (PTMs) of amino acid residues on the N-terminal tails of histones are suggested to define such states. The histone lysine methyltransferase (HKMTase) SU(VAR)3-9 RELATED4 (SUVR4) of Arabidopsis thaliana functions as a repressor of transposon activity. Binding of ubiquitin by the WIYLD domain facilitates the addition of two methyl groups to monomethylated lysine 9 of histone H3. By using nuclear magnetic resonance (NMR) spectroscopy, we identified SUVR4 WIYLD (S4WIYLD) as a domain with a four-helix bundle structure, in contrast to three-helix bundles of other ubiquitin binding domains. NMR titration analyses showed that residues of helix α1 (Q38, L39, and D40) and helix α4 (N68, T70, A71, V73, D74, I76, S78, and E82) of S4WIYLD and residues between the first and second β-strands (T9 and G10) and on β-strands 3 (R42, G47, K48, and Q49) and 4 (H68, R72, and L73) undergo significant chemical shift changes when the two proteins interact. A model of the complex, generated using HADDOCK, suggests that the N-terminal and C-terminal parts of S4WIYLD constitute a surface that interacts with charged residues close to the hydrophobic patch of ubiquitin. The WIYLD domains of the closely related SUVR1 and SUVR2 Arabidopsis proteins also bind ubiquitin, indicating that this is a general feature of this domain. The question of whether SUVR proteins act as both readers of monoubiquitinated H2B and writers of histone PTMs is discussed.

Extending serum half-life of albumin by engineering neonatal Fc receptor (FcRn) binding

A major challenge for the therapeutic use of many peptides and proteins is their short circulatory half-life. Albumin has an extended serum half-life of 3 weeks because of its size and FcRn-mediated recycling that prevents intracellular degradation, properties shared with IgG antibodies. Engineering the strictly pH-dependent IgG-FcRn interaction is known to extend IgG half-life. However, this principle has not been extensively explored for albumin. We have engineered human albumin by introducing single point mutations in the C-terminal end that generated a panel of variants with greatly improved affinities for FcRn. One variant (K573P) with 12-fold improved affinity showed extended serum half-life in normal mice, mice transgenic for human FcRn, and cynomolgus monkeys. Importantly, favorable binding to FcRn was maintained when a single-chain fragment variable antibody was genetically fused to either the N- or the C-terminal end. The engineered albumin variants may be attractive for improving the serum half-life of biopharmaceuticals.

Chimeric anti-CD14 IGG2/4 Hybrid antibodies for therapeutic intervention in pig and human models of inflammation

CD14 is a key recognition molecule of innate immune responses, interacting with several TLRs. TLR signaling cross-talks extensively with the complement system, and combined CD14 and complement inhibition has been proved effective in attenuating inflammatory responses. Pig models of human diseases have emerged as valuable tools to study therapeutic intervention, but suitable neutralizing Abs are rare. Undesired Fc-mediated functions, such as platelet activation and IL-8 release induced by the porcine CD14-specific clone Mil2, limit further studies. Therefore, an inert human IgG2/IgG4 hybrid C region was chosen for an rMil2. As revealed in ex vivo and in vivo pig experiments, rMil2 inhibited the CD14-mediated proinflammatory cytokine response similar to the original clone, but lacked the undesired Fc-effects, and inflammation was attenuated further by simultaneous complement inhibition. Moreover, rMil2 bound porcine FcRn, a regulator of t1/2 and biodistribution. Thus, rMil2, particularly combined with complement inhibitors, should be well suited for in vivo studies using porcine models of diseases, such as sepsis and ischemia-reperfusion injury. Similarly, the recombinant anti-human CD14 IgG2/4 Ab, r18D11, was generated with greatly reduced Fc-mediated effects and preserved inhibitory function ex vivo. Such Abs might be drug candidates for the treatment of innate immunity-mediated human diseases.

A public T cell receptor recognized by a monoclonal antibody specific for the D-J junction of the β-chain

It is becoming increasingly clear that T cell responses against many antigens are dominated by public α/β T cell receptors (TCRs) with restricted heterogeneity. Because expression of public TCRs may be related to resistance, or predisposition to diseases, it is relevant to measure their frequencies. Although staining with tetrameric peptide/major histocompatibility complex (pMHC) molecules gives information about specificity, it does not give information about the TCR composition of the individual T cells that stain. Moreover, next-generation sequencing of TCR does not yield information on pairing of α- and β-chains in single T cells. In an effort to overcome these limitations, we have here investigated the possibility of raising a monoclonal antibody (moAb) that recognizes a public TCR. As a model system, we have used T cells responding to the 91-101 CDR3 peptide of an Ig L-chain (λ2³¹⁵), presented by the MHC class II molecule I-E(d). The CD4⁺ T cell responses against this pMHC are dominated by a receptor composed of Vα3Jα1;Vβ6DβJβ1.1. Even the V(D)J junctions are to a large extent shared between T cell clones derived from different BALB/c mice. We here describe a murine moAb (AB10) of B10.D2 origin that recognizes this public TCR, while binding to peripheral T cells is negligible. Binding of the moAb is abrogated by introduction of two Gly residues in the D-J junction of the CDR3 of the β-chain. A model for the public TCR determinant is presented.

Single-chain variable fragment albumin fusions bind the neonatal Fc receptor (FcRn) in a species-dependent manner: implications for in vivo half-life evaluation of albumin fusion therapeutics

Albumin has a serum half-life of 3 weeks in humans. This has been utilized to extend the serum persistence of biopharmaceuticals that are fused to albumin. In light of the fact that the neonatal Fc receptor (FcRn) is a key regulator of albumin homeostasis, it is crucial to address how fusion of therapeutics to albumin impacts binding to FcRn. Here, we report on a detailed molecular investigation on how genetic fusion of a short peptide or an single-chain variable fragment (scFv) fragment to human serum albumin (HSA) influences pH-dependent binding to FcRn from mouse, rat, monkey, and human. We have found that fusion to the N- or C-terminal end of HSA only slightly reduces receptor binding, where the most noticeable effect is seen after fusion to the C-terminal end. Furthermore, in contrast to the observed strong binding to human and monkey FcRn, HSA and all HSA fusions bound very poorly to mouse and rat versions of the receptor. Thus, we demonstrate that conventional rodents are limited as preclinical models for analysis of serum half-life of HSA-based biopharmaceuticals. This finding is explained by cross-species differences mainly found within domain III (DIII) of albumin. Our data demonstrate that although fusion, particularly to the C-terminal end, may slightly reduce the affinity for FcRn, HSA is versatile as a carrier of biopharmaceuticals.

Effective phagocytosis of low Her2 tumor cell lines with engineered, aglycosylated IgG displaying high FcγRIIa affinity and selectivity

Glycans anchored to residue N297 of the antibody IgG Fc domain are critical in mediating binding toward FcγRs to direct both adaptive and innate immune responses. However, using a full length bacterial IgG display system, we have isolated aglycosylated Fc domains with mutations that confer up to a 160-fold increase in the affinity toward the low affinity FcγRIIa-R131 allele as well as high selectivity against binding to the remarkably homologous human inhibitory receptor, FcγRIIb. The mutant Fc domain (AglycoT-Fc1004) contained a total of 5 amino acid substitutions that conferred an activating to inhibitory ratio of 25 (A/I ratio; FcyRIIa-R131:FcγRIIb). Incorporation of this engineered Fc into trastuzumab, an anti-Her2 antibody, resulted in a 75% increase in tumor cell phagocytosis by macrophages compared to that of the parental glycosylated trastuzumab with both medium and low Her2-expressing cancer cells. A mathematical model has been developed to help explain how receptor affinity and the A/I ratio relate to improved antibody dependent cell-mediated phagocytosis. Our model provides guidelines for the future engineering of Fc domains with enhanced effector function.

Chaperone-assisted thermostability engineering of a soluble T cell receptor using phage display

We here report a novel phage display selection strategy enabling fast and easy selection of thermostabilized proteins. The approach is illustrated with stabilization of an aggregation-prone soluble single chain T cell receptor (scTCR) characteristic of the murine MOPC315 myeloma model. Random mutation scTCR phage libraries were prepared in E. coli over-expressing the periplasmic chaperone FkpA, and such over-expression during library preparation proved crucial for successful downstream selection. The thermostabilized scTCR^mut variants selected were produced in high yields and isolated as monomers. Thus, the purified scTCRs could be studied with regard to specificity and equilibrium binding kinetics to pMHC using surface plasmon resonance (SPR). The results demonstrate a difference in affinity for pMHCs that display germ line or tumor-specific peptides which explains the tumor-specific reactivity of the TCR. This FkpA-assisted thermostabilization strategy extends the utility of recombinant TCRs and furthermore, may be of general use for efficient evolution of proteins.

Selection of nanobodies that target human neonatal Fc receptor

FcRn is a key player in several immunological and non-immunological processes, as it mediates maternal-fetal transfer of IgG, regulates the serum persistence of IgG and albumin, and transports both ligands between different cellular compartments. In addition, FcRn enhances antigen presentation. Thus, there is an intense interest in studies of how FcRn binds and transports its cargo within and across several types of cells, and FcRn detection reagents are in high demand. Here we report on phage display-selected Nanobodies that target human FcRn. The Nanobodies were obtained from a variable-domain repertoire library isolated from a llama immunized with recombinant human FcRn. One candidate, Nb218-H4, was shown to bind FcRn with high affinity at both acidic and neutral pH, without competing ligand binding and interfering with FcRn functions, such as transcytosis of IgG. Thus, Nb218-H4 can be used as a detection probe and as a tracker for visualization of FcRn-mediated cellular transport.

A series of anti-CEA/anti-DOTA bispecific antibody formats evaluated for pre-targeting: comparison of tumor uptake and blood clearance

A series of anti-tumor/anti-chelate bispecific antibody formats were developed for pre-targeted radioimmunotherapy. Based on the anti-carcinoembryonic antigen humanized hT84.66-M5A monoclonal antibody and the anti-DOTA C8.2.5 scFv antibody fragment, this cognate series of bispecific antibodies were radioiodinated to determine their tumor targeting, biodistribution and pharmacokinetic properties in a mouse xenograft tumor model. The in vivo biodistribution studies showed that all the bispecific antibodies exhibited specific high tumor uptake but the tumor targeting was approximately one-half of the parental anti-CEA mAb due to faster blood clearance. Serum stability and FcRn studies showed no apparent reason for the faster blood clearance. A dual radiolabel biodistribution study revealed that the (111)In-DOTA bispecific antibody had increased liver and spleen uptake, not seen for the (125)I-version due to metabolism and release of the radioiodine from the cells. These data suggest increased clearance of the antibody fusion formats by the mononuclear phagocyte system. Importantly, a pre-targeted study showed specific tumor uptake of (177)Lu-DOTA and a tumor : blood ratio of 199 : 1. This pre-targeted radiotherapeutic and substantial reduction in the radioactive exposure to the bone marrow should enhance the therapeutic potential of RIT.

Targeted DNA vaccines for enhanced induction of idiotype-specific B and T cells

Background: Idiotypes (Id) are antigenic determinants localized in variable (V) regions of Ig. Id-specific T and B cells (antibodies) play a role in immunotherapy of Id⁺ tumors. However, vaccine strategies that enhance Id-specific responses are needed. Methods: Id⁺ single-chain fragment variable (scFv) from multiple myelomas and B cell lymphomas were prepared in a fusion format that bivalently target surface molecules on antigen-presenting cells (APC). APC-specific targeting units were either scFv from APC-specific mAb (anti-MHC II, anti-CD40) or chemokines (MIP-1α, RANTES). Homodimeric Id-vaccines were injected intramuscularly or intradermally as plasmids in mice, combined with electroporation. Results: (i) Transfected cells secreted plasmid-encoded Id⁺ fusion proteins to extracellular fluid followed by binding of vaccine molecules to APC. (ii) Targeted vaccine molecules increased Id-specific B and T cell responses. (iii) Bivalency and xenogeneic sequences both contributed to enhanced responses. (iv) Targeted Id DNA vaccines induced tumor resistance against challenges with Id⁺ tumors. (v) Human MIP-1α targeting units enhanced Id-specific responses in mice, due to a cross reaction with murine chemokine receptors. Thus, targeted vaccines designed for humans can be quality tested in mice. (vi) Human Id⁺ scFv from four multiple myeloma patients were inserted into the vaccine format and were successfully tested in mice. (vii) Human MIP-1α vaccine proteins enhanced human T cell responses in vitro. (viii) A hypothetical model for how the APC-targeted vaccine molecules enhance Id-specific T and B cells is presented. Conclusion: Targeted DNA Id-vaccines show promising results in preclinical studies, paving the way for testing in patients.

Next generation phage display by use of pVII and pIX as display scaffolds

Phage display technology has evolved to become an extremely versatile and powerful platform for protein engineering. The robustness of the phage particle, its ease of handling and its ability to tolerate a range of different capsid fusions are key features that explain the dominance of phage display in combinatorial engineering. Implementation of new technology is likely to ensure the continuation of its success, but has also revealed important short comings inherent to current phage display systems. This is in particular related to the biology of the two most popular display capsids, namely pIII and pVIII. Recent findings using two alternative capsids, pVII and pIX, located to the phage tip opposite that of pIII, suggest how they may be exploited to alleviate or circumvent many of these short comings. This review addresses important aspects of the current phage display standard and then discusses the use of pVII and pIX. These may both complement current systems and be used as alternative scaffolds for display and selection to further improve phage display as the ultimate combinatorial engineering platform.

Anti-carcinoembryonic antigen single-chain variable fragment antibody variants bind mouse and human neonatal Fc receptor with different affinities that reveal distinct cross-species differences in serum half-life

Serum half-life of IgG is controlled by the neonatal Fc receptor (FcRn) that interacts with the IgG Fc region and may be increased or decreased as a function of altered FcRn binding. Preclinical evaluations of modified IgGs are frequently carried out in mice, but such IgGs may bind differently to mouse and human FcRn (mFcRn and hFcRn). Here, we report a detailed characterization of a matched set of mouse-human chimeric T84.66 scFv-Fc variants with specificity for the tumor carcinoembryonic antigen and mutations in the FcRn-binding site. Binding to soluble mFcRn and hFcRn was measured using in vitro assays, and the results were compared with blood clearance in vivo in normal (mFcRn bearing) and hFcRn transgenic mice. All variants bound better to mFcRn than to hFcRn. The loss of affinity varied among the mutants, however, and also the hierarchy of binding differed depending on the receptor. The mutations had no major impact on binding to the classical Fcγ receptors. Importantly, the trend of blood clearance in both strains of mice correlated with the hierarchy of binding obtained using soluble FcRn. Consequently, in vitro interaction analysis of engineered IgGs regarding their cross-species FcRn binding ability provides information for prediction of in vivo pharmacokinetics.

DeltaPhage--a novel helper phage for high-valence pIX phagemid display

Phage display has been instrumental in discovery of novel binding peptides and folded domains for the past two decades. We recently reported a novel pIX phagemid display system that is characterized by a strong preference for phagemid packaging combined with low display levels, two key features that support highly efficient affinity selection. However, high diversity in selected repertoires are intimately coupled to high display levels during initial selection rounds. To incorporate this additional feature into the pIX display system, we have developed a novel helper phage termed DeltaPhage that allows for high-valence display on pIX. This was obtained by inserting two amber mutations close to the pIX start codon, but after the pVII translational stop, conditionally inactivating the helper phage encoded pIX. Until now, the general notion has been that display on pIX is dependent on wild-type complementation, making high-valence display unachievable. However, we found that DeltaPhage does facilitate high-valence pIX display when used with a non-suppressor host. Here, we report a side-by-side comparison with pIII display, and we find that this novel helper phage complements existing pIX phagemid display systems to allow both low and high-valence display, making pIX display a complete and efficient alternative to existing pIII phagemid display systems.

A practical approach to T-cell receptor cloning and expression

Although cloning and expression of T-cell Receptors (TcRs) has been performed for almost two decades, these procedures are still challenging. For example, the use of T-cell clones that have undergone limited expansion as starting material to limit the loss of interesting TcRs, must be weighed against the introduction of mutations by excess PCR cycles. The recent interest in using specific TcRs for cancer immunotherapy has, however, increased the demand for practical and robust methods to rapidly clone and express TcRs. Two main technologies for TcR cloning have emerged; the use of a set of primers specifically annealing to all known TcR variable domains, and 5′-RACE amplification. We here present an improved 5′-RACE protocol that represents a fast and reliable way to identify a TcR from 10⁵ cells only, making TcR cloning feasible without a priori knowledge of the variable domain sequence. We further present a detailed procedure for the subcloning of TcRα and β chains into an expression system. We show that a recombination-based cloning protocol facilitates simple and rapid transfer of the TcR transgene into different expression systems. The presented comprehensive method can be performed in any laboratory with standard equipment and with a limited amount of starting material. We finally exemplify the straightforwardness and reliability of our procedure by cloning and expressing several MART-1-specific TcRs and demonstrating their functionality.