Gøril Berntzen

Cross-species Binding Analyses of Mouse and Human Neonatal Fc Receptor Show Dramatic Differences in Immunoglobulin G and Albumin Binding

The neonatal Fc receptor (FcRn) regulates the serum half-life of both IgG and albumin through a pH-dependent mechanism that involves salvage from intracellular degradation. Therapeutics and diagnostics built on IgG, Fc, and albumin fusions are frequently evaluated in rodents regarding biodistribution and pharmacokinetics. Thus, it is important to address cross-species ligand reactivity with FcRn, because in vivo testing of such molecules is done in the presence of competing murine ligands, both in wild type (WT) and human FcRn (hFcRn) transgenic mice. Here, binding studies were performed in vitro using enzyme-linked immunosorbent assay and surface plasmon resonance with recombinant soluble forms of human (shFcRnWT) and mouse (smFcRnWT) receptors. No binding of albumin from either species was observed at physiological pH to either receptor. At acidic pH, a 100-fold difference in binding affinity was observed. Specifically, smFcRnWT bound human serum albumin with a KD of ∼90 μm, whereas shFcRnWT bound mouse serum albumin with a KD of 0.8 μm. shFcRnWT ignored mouse IgG1, and smFcRnWT bound strongly to human IgG1. The latter pair also interacted at physiological pH with calculated affinity in the micromolar range. In all cases, binding of albumin and IgG from either species to both receptors were additive. Cross-species albumin binding differences could partly be explained by non-conserved amino acids found within the α2-domain of the receptor. Such distinct cross-species FcRn binding differences must be taken into consideration when IgG- and albumin-based therapeutics and diagnostics are evaluated in rodents for their pharmacokinetics.

Ligand binding and antigenic properties of a human neonatal Fc receptor with mutation of two unpaired cysteine residues

The neonatal Fc receptor (FcRn) is a major histocompatibility complex class I‐related molecule that regulates the half‐life of IgG and albumin. In addition, FcRn directs the transport of IgG across both mucosal epithelium and placenta and also enhances phagocytosis in neutrophils. This new knowledge gives incentives for the design of IgG and albumin‐based diagnostics and therapeutics. To study FcRn in vitro and to select and characterize FcRn binders, large quantities of soluble human FcRn are needed. In this report, we explored the impact of two free cysteine residues (C48 and C251) of the FcRn heavy chain on the overall structure and function of soluble human FcRn and described an improved bacterial production strategy based on removal of these residues, yielding ∼ 70 mg·L−1 of fermentation of refolded soluble human FcRn. The structural and functional integrity was proved by CD, surface plasmon resonance and MALDI‐TOF peptide mapping analyses. The strategy may generally be translated to the large‐scale production of other major histocompatibility complex class I‐related molecules with nonfunctional unpaired cysteine residues. Furthermore, the anti‐FcRn response in goats immunized with the FcRn heavy chain alone was analyzed following affinity purification on heavy chain‐coupled Sepharose. Importantly, purified antibodies blocked the binding of both ligands to soluble human FcRn and were thus directed to both binding sites. This implies that the FcRn heavy chain, without prior assembly with human β2‐microglobulin, contains the relevant epitopes found in soluble human FcRn, and is therefore sufficient to obtain binders to either ligand‐binding site. This finding will greatly facilitate the selection and characterization of such binders.

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.

Identification of a Polymeric Ig Receptor Binding Phage-displayed Peptide That Exploits Epithelial Transcytosis without Dimeric IgA Competition

The polymeric Ig receptor (pIgR), also called membrane secretory component (SC), mediates epithelial transcytosis of polymeric immunoglobulins (pIgs). J Chain-containing polymeric IgA (pIgA) and pentameric IgM bind pIgR at the basolateral epithelial surface. After transcytosis, the extracellular portion of the pIgR is cleaved at the apical side, either complexed with pIgs as bound SC or unoccupied as free SC. This transport pathway may be exploited to target bioactive molecules to the mucosal surface. To identify small peptide motifs with specific affinity to human pIgR, we used purified free SC and selection from randomized, cysteine-flanked 6- and 9-mer phage-display libraries. One of the selected phages, called C9A, displaying the peptide CVVWMGFQQVC, showed binding both to human free SC and SC complexed with pIgs. However, the pneumococcal surface protein SpsA (Streptococcus pneumoniae secretory IgA-binding protein), which binds human SC at a site distinct from the pIg binding site, competed with the C9A phage for binding to SC. The C9A phage showed greatly increased transport through polarized Madin-Darby canine kidney cells transfected with human pIgR. This transport was not affected by pIgA nor did it inhibit pIgR-mediated pIgA transcytosis. A free peptide of identical amino acid sequence as that displayed by the C9A phage inhibited phage interaction with SC. This implied that the C9A peptide sequence may be exploited for pIgR-mediated epithelial transport without interfering with secretory immunity.

Selection and Characterization of Cyclic Peptides that Bind to a Monoclonal Antibody Against Meningococcal L3,7,9 lipopolysaccharides

There is still no general vaccine for prevention of disease caused by group‐B meningococcal strains. Meningococcal lipopolysaccharides (LPSs) have received attention as potential vaccine candidates, but concerns regarding their safety have been raised. Peptide mimics of LPS epitopes may represent safe alternatives to immunization with LPS. The monoclonal antibody (MoAb) 9‐2‐L3,7,9 [ 1 ] specific for Neisseria meningitidis LPS immunotype L3,7,9 is bactericidal and does not cross‐react with human tissue. To explore the possibility of isolating peptide mimics of the epitope recognized by MoAb 9‐2‐L3,7,9, we have constructed two phage display libraries of six and nine random amino acids flanked by cysteines. Furthermore, we developed a system for the easy exchange of peptide‐encoding sequences from the phage‐display system to a hepatitis B core (HBc) expression system. Cyclic peptides that specifically bound MoAb 9‐2‐L3,7,9 at a site overlapping with the LPS‐binding site were selected from both libraries. Three out of four tested peptides which reacted with MoAb 9‐2‐L3,7,9 were successfully presented as fusions to the immunodominant loop of HBc particles expressed in Escherichia coli. However, both peptide conjugates to keyhole limpet haemocyanin and HBc particle fusions failed to give an anti‐LPS response in mice.

Identification of a High Affinity FcγRIIA-binding Peptide That Distinguishes FcγRIIA from FcγRIIB and Exploits FcγRIIA-mediated Phagocytosis and Degradation

FcγRIIA is a key activating receptor linking immune complex formation with cellular effector functions. FcγRIIA has 93% identity with an inhibitory receptor, FcγRIIB, which negatively regulates FcγRIIA. FcγRIIA is important in the therapeutic action of several monoclonal antibodies. Binding molecules that discriminate FcγRIIA from FcγRIIB may optimize receptor activity and serve as a lead for development of therapeutics with FcγRIIA as a key target. Here we report the use of phage display libraries to select short peptides with distinct FcγRIIA binding properties. An 11-mer peptide (WAWVWLTETAV) was characterized that bound FcγRIIA with a Kd of 500 nm. It mediated cell internalization and degradation of a model antigen. The peptide-binding site on FcγRIIA was shown to involve Phe163 and the IgG binding amino acids Trp90 and Trp113. It is thus overlapping but not identical to that of IgG. Neither activating receptors FcγRI and FcγRIII, nor FcγRIIB, all of which lack Phe163, bound the peptide.

Processing of an Antigenic Sequence from IgG Constant Domains for Presentation by MHC Class II

Targeting of T cell epitopes to APC enhances T cell responses. We used an APC-specific Ab (anti-IgD) and substituted either of 18 loops connecting β strands in human IgG constant H (CH) domains with a characterized T cell peptide epitope. All Ab-epitope fusion molecules were secreted from producing cells except IgG-loop 2(BC)CH1, and comparing levels, a hierarchy appeared with fusions involving CH2≥CH1>CH3. Within each domain, fusion at loop 6(FG) showed best secretion, while low secretion correlated with the substitution of native loops that contain conserved amino acids buried within the folded molecule. Comparing the APC-specific rAb molecules for their ability to induce T cell activation in vitro, the six mutants with epitope in CH2 were the most effective, with loop 4CH2 ranking on top. The CH1 mutants were more resistant to processing, and the loop 6CH1 mutant only induced detectable activation. The efficiency of the CH3 mutants varied, with loop 6CH3 being the least effective and equal to loop 6 CH1. Considering both rAb secretion level and T cell activation efficiency, a total of eight loops may carry T cell epitopes to APC for processing and presentation to T cells, namely, all in CH2 in addition to loop 6 in CH1 and CH3. Comparing loop 4CH2 with loop 6CH1 mutants after injection of Ab in BALB/c mice, the former was by far the most efficient and induced specific T cell activation at concentrations at least 100-fold lower than loop 6CH1.

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.

Identification of a high affinity FcgammaRIIA-binding peptide that distinguishes FcgammaRIIA from FcgammaRIIB and exploits FcgammaRIIA-mediated phagocytosis and degradation.

FcγRIIA is a key activating receptor linking immune complex formation with cellular effector functions. FcγRIIA has 93% identity with an inhibitory receptor, FcγRIIB, which negatively regulates FcγRIIA. FcγRIIA is important in the therapeutic action of several monoclonal antibodies. Binding molecules that discriminate FcγRIIA from FcγRIIB may optimize receptor activity and serve as a lead for development of therapeutics with FcγRIIA as a key target. Here we report the use of phage display libraries to select short peptides with distinct FcγRIIA binding properties. An 11-mer peptide (WAWVWLTETAV) was characterized that bound FcγRIIA with a Kd of 500 nm. It mediated cell internalization and degradation of a model antigen. The peptide-binding site on FcγRIIA was shown to involve Phe163 and the IgG binding amino acids Trp90 and Trp113. It is thus overlapping but not identical to that of IgG. Neither activating receptors FcγRI and FcγRIII, nor FcγRIIB, all of which lack Phe163, bound the peptide.

Identification of a High Affinity Fc gamma RIIA-binding Peptide That Distinguishes Fc gamma RIIA from Fc gamma RIIB and Exploits Fc gamma RIIA-mediated Phagocytosis and Degradation

FcγRIIA is a key activating receptor linking immune complex formation with cellular effector functions. FcγRIIA has 93% identity with an inhibitory receptor, FcγRIIB, which negatively regulates FcγRIIA. FcγRIIA is important in the therapeutic action of several monoclonal antibodies. Binding molecules that discriminate FcγRIIA from FcγRIIB may optimize receptor activity and serve as a lead for development of therapeutics with FcγRIIA as a key target. Here we report the use of phage display libraries to select short peptides with distinct FcγRIIA binding properties. An 11-mer peptide (WAWVWLTETAV) was characterized that bound FcγRIIA with a Kd of 500 nm. It mediated cell internalization and degradation of a model antigen. The peptide-binding site on FcγRIIA was shown to involve Phe163 and the IgG binding amino acids Trp90 and Trp113. It is thus overlapping but not identical to that of IgG. Neither activating receptors FcγRI and FcγRIII, nor FcγRIIB, all of which lack Phe163, bound the peptide.