Vigdis Lauvrak

A region from the medium chain adaptor subunit (mu) recognizes leucine- and tyrosine-based sorting signals.

Tyrosine-based sorting signals in the cytosolic tails of membrane proteins have been found to bind directly to the medium chain subunit (μ) of the adaptor complexes AP-1 and AP-2. For the leucine-based signals, an interaction with AP-1 and AP-2 has been reported, but no specific interacting subunit has been demonstrated. After searching for molecules interacting with the leucine-based sorting signals within the cytosolic tail of the major histocompatibility complex class II-associated invariant chain using a phage display approach, we identified phage clones with homology to a conserved region of the AP-1 and AP-2 μ chains. To investigate the relevance of these findings, we have expressed regions of mouse μ1 and μ2 chains on phage gene product III and investigated the binding to tail sequences from various transmembrane proteins with known endosomal targeting signals. Enzyme-linked immunosorbent binding assays showed that these phages specifically recognized peptides containing functional leucine- and tyrosine-based sorting signals, suggesting that these regions of the μ1 and μ2 chains interact with both types of sorting signals.

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.

Identification and Characterisation of C1q-Binding Phage Displayed Peptides

Five phage displayed peptide libraries were screened for binders to C1q, the recognition subunit of the classical complement pathway. Two rounds of panning resulted in the isolation and characterisation of several different phage displayed C1q-binding peptides from all five libraries. Two groups of the characterised peptides show sequence similarity with part of the metal ion dependent adhesion site (MIDAS) of integrin A-domains, and the site 187LRNPCPNKEKECQPPF of CD18 (integrin beta2), respectively. These results support binding of complement receptor 3 (CR3, CD11b/CD18, Mac1) to C1q and further suggest C1q binding sites in CR3. We also discuss sequence matches between the characterised peptides and proteins known to interact with C1q, as well as other proteins listed in the SwissProt databank. These findings are of interest for the study of the complement system and may lead to the development of peptides, fusion products or peptido-mimetics with C1q modulating potential.

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 of phage displayed peptides from a random 10-mer library recognising a peptide target

BACKGROUND Peptide display libraries are powerful tools in the search for detailed information about protein-protein interactions. Usual targets for isolation of phage displayed peptide ligands include antibodies, various receptors, other full size proteins or larger fragments thereof. Smaller protein fragments such as synthetic peptides have not been reported as targets for screening of peptide display libraries. OBJECTIVES To investigate whether a protein target used for screening of a peptide display library could be scaled down to peptide size. As the peptide target we wanted to use a sequence derived from the cytosolic tail of MHC class II associated invariant chain containing a leucine class endosomal sorting signal, known to be recognised as an autonomous functional unit during targeting of class II complexes to antigen processing compartments. STUDY DESIGN A screening procedure where a synthetic 15-mer invariant chain peptide was coupled to a methacrylate matrix of high binding capacity was developed, and three rounds of selection were performed from a random 10-mer fUSE5 display library. RESULTS The peptide display library was successfully enriched for phage clones with affinity for the invariant chain peptide. Furthermore, the binding phage clones were able to distinguish between a functional and a mutated form of the target. These clones therefore displayed possible peptide mimetics of signal recognition sites in the cellular sorting machinery. CONCLUSION The size of a protein target may be scaled down to peptide size and be recognised by a 10-mer peptide displayed on filamentous phage. This approach may particularly be useful when the peptide target contains a functional unit for recognition.

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.

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.