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The Journal of Immunology, 2006, 176: 426-440.
Copyright © 2006 by The American Association of Immunologists

Internalizing Antibodies to the C-Type Lectins, L-SIGN and DC-SIGN, Inhibit Viral Glycoprotein Binding and Deliver Antigen to Human Dendritic Cells for the Induction of T Cell Responses

Naveen Dakappagari*, Toshiaki Maruyama*, Mark Renshaw*, Paul Tacken{dagger}, Carl Figdor{dagger}, Ruurd Torensma{dagger}, Martha A. Wild*, Dayang Wu{ddagger}, Katherine Bowdish* and Anke Kretz-Rommel1,*

* Alexion Antibody Technologies, San Diego, CA 92121; {dagger} Department of Tumor Immunology, Nijmegen Centre for Molecular Life Sciences, University Medical Centre Nijmegen, Nijmegen, The Netherlands; and {ddagger} Alexion Pharmaceuticals, Cheshire, CT 06410


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
The C-type lectin L-SIGN is expressed on liver and lymph node endothelial cells, where it serves as a receptor for a variety of carbohydrate ligands, including ICAM-3, Ebola, and HIV. To consider targeting liver/lymph node-specific ICAM-3-grabbing nonintegrin (L-SIGN) for therapeutic purposes in autoimmunity and infectious disease, we isolated and characterized Fabs that bind strongly to L-SIGN, but to a lesser degree or not at all to dendritic cell-specific ICAM-grabbing nonintegrin (DC-SIGN). Six Fabs with distinct relative affinities and epitope specificities were characterized. The Fabs and those selected for conversion to IgG were tested for their ability to block ligand (HIV gp120, Ebola gp, and ICAM-3) binding. Receptor internalization upon Fab binding was evaluated on primary human liver sinusoidal endothelial cells by flow cytometry and confirmed by confocal microscopy. Although all six Fabs internalized, three Fabs that showed the most complete blocking of HIVgp120 and ICAM-3 binding to L-SIGN also internalized most efficiently. Differences among the Fab panel in the ability to efficiently block Ebola gp compared with HIVgp120 suggested distinct binding sites. As a first step to consider the potential of these Abs for Ab-mediated Ag delivery, we evaluated specific peptide delivery to human dendritic cells. A durable human T cell response was induced when a tetanus toxide epitope embedded into a L-SIGN/DC-SIGN-cross-reactive Ab was targeted to dendritic cells. We believe that the isolated Abs may be useful for selective delivery of Ags to DC-SIGN- or L-SIGN-bearing APCs for the modulation of immune responses and for blocking viral infections.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Disclosures
 References
 
Dendritic cell (DC)2-specific ICAM-grabbing nonintegrin (DC-SIGN) (CD209) and liver/lymph node-specific ICAM-3-grabbing nonintegrin (L-SIGN) (CD299/DC-SIGNR) are closely related genes that map to chromosome 19p13.3. Both genes encode a member of the C-type lectin family of type II transmembrane proteins. The two receptors are 77% identical at the amino acid level, have similar three-dimensional structures, and share similar ligands (1). Despite their close evolutionary similarities at the gene and protein level, they are expressed in different tissues. Although DC-SIGN is expressed on DCs and macrophages (2, 3), L-SIGN is found in the endothelial cells of liver, lymph nodes, and placenta, and is absent on DCs and macrophages (4, 5). These receptors are composed of a C-terminal carbohydrate recognition domain (CRD) that is supported by a neck region made up of multiple highly conserved 23-aa repeats and a short cytoplasmic tail at the N terminus (1, 6). Both receptors have been shown to interact with ICAM-3 (2, 4), a molecule expressed on T cells. This interaction is thought to play a role in stabilizing the DC-T cell contact zone. Although L-SIGN and DC-SIGN interact with a number of similar ligands, the outcome of this interaction could potentially be different based on the unique tissue distribution of each receptor.

A number of viruses have been shown to bind to the CRDs of DC-SIGN and L-SIGN, but in a manner that is distinct from ICAM-3 (7, 8, 9). L-SIGN is able to bind and permit entry into the cell of HIV (4, 5), hepatitis C virus (HCV) (10), Ebola virus (11), severe acute respiratory syndrome (SARS) virus (12), CMV (13), and Sindbis virus (14). Current data support the idea that the interaction allows HIV and HCV to be transmitted in trans to other cells, e.g., T cells and hepatocytes (4, 5, 15), whereas it is thought to result in direct infection by other viruses, e.g., Ebola and SARS (12, 16). Recently, L-SIGN has also been shown to serve as a receptor for bacterial pathogens, e.g., Mycobacterium tuberculosis (17) and parasites such as Schistosoma mansoni (18). Because liver sinusoidal endothelial cells (LSECs) are involved in tolerance induction, the use of L-SIGN receptor as a point of entry may explain the difficulty that the immune system has in eradicating these pathogens. Abs to L-SIGN that selectively inhibit pathogen attachment may be prophylactically or therapeutically useful in infectious disease by preventing pathogen entry or adhesion.

The liver is thought to play a central role in immunological tolerance. LSECs, which express L-SIGN and a number of immune cell recognition molecules, have been reported to actively capture potentially harmful antigenic proteins from the circulation and present the processed peptides efficiently to the trafficking leukocytes (19). The presentation of Ags by LSECs to both CD4+ and CD8+ T lymphocytes was found to result in immunological tolerance even in the presence of inflammatory mediators, e.g., IL-12 and IFN-{gamma} (20, 21). Specific targeting of Ag to tolerance-inducing cells by linking an autoantigen to an L-SIGN Ab might result in the induction of T cell tolerance to the presented Ag. In contrast, specific targeting of Ag to DC-SIGN on DCs is expected to raise a stimulatory immune response.

Using phage display and screening technologies, we have successfully isolated a panel of L-SIGN-reactive Fabs with distinct relative affinities and epitope specificities. The ability of Fabs to block HIV binding correlated with their ability to block ICAM-3 binding, while blocking of Ebola binding required distinct Ab properties. We also demonstrated varying degrees of L-SIGN Ab internalization upon receptor binding. The ability of L-SIGN Abs that were cross-reactive with DC-SIGN and internalized by APCs was exploited further to specifically deliver Ag. We demonstrate induction of a robust human T cell response by targeted delivery of an Ag to autologous DCs using a L-SIGN/DC-SIGN-cross-reactive Ab embedded with a tetanus toxoid (TT) epitope.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Disclosures
 References
 
Cell lines and reagents

Stable K562 cell lines expressing DC-SIGN (8) and L-SIGN (18) have been described previously. Fresh human liver nonparenchymal cells were purchased from CellzDirect. These cells are supplied after removal of hepatocytes from total liver cells. Before their use in assays, RBC were lysed and any remaining dead cells were further depleted using a dead cell removal kit (Miltenyi Biotec; catalog 130-090-101), per the manufacturer’s instructions. mAbs mAb162 (reactive only with L-SIGN), mAb1621, and mAb16211 (cross-reactive with DC-SIGN and L-SIGN) were purchased from R&D Systems. An allophycocyanin conjugate of mAb, mAb162 was prepared using a Zenon mouse IgG2b labeling kit (Molecular Probes; catalog Z25251), according to the manufacturer’s instructions. All other Abs used in flow cytometry were purchased from eBioscience. Cytokines, IL-4, and GM-CSF were purchased from StemCell Technologies. Whole TT protein was purchased from Calbiochem. The universal HLA-DR-binding TT epitope, 632DR (aa 632–651, IDKISDVSTIVPYIGPALNI), was chemically synthesized by SynPep. The peptide was purified by reverse-phase chromatography to a single peak, and its identity was confirmed by mass spectrometry.

Construction of human L-SIGN-Fc expression plasmid

Human L-SIGN-Fc was generated by overlap PCR, fusing two PCR fragments derived from cDNA coding for human L-SIGN and the Fc portion of human Ig hG2G4 (22). Flanking primers P1 EcoRV 5'-CAG ATG TGA TAT CTC CAA GGT CCC CAG CTC CCT AAG-3' and P2 XhoI 5'-TGG GCT CGA GTT CGT CTC TGA AGC AGG CTG CG-3' were used to amplify the extracellular domain of human L-SIGN from a human spleen cDNA library. The EcoRV site in the P1 primer allows fusion with the leader sequence. The XhoI site in the P2 primer was used to fuse the fragment with the hG2G4 Fc region. The primers P3 (forward), XhoI 5'-AGA CGA ACT CGA GCG CAA ATG TTG TGT CGA GT-3' and P4 (reverse), stop codon NgoMIV 5'-TGC CGG CCC TGG CAC TCA TTT ACC CAG AGA CAG GGA GAG GCT-3' were used to amplify the hG2G4 Fc region from Glu99 of the hinge domain to the C terminus by using a plasmid containing the hG2G4 C region. The PCR-amplified human L-SIGN and human hG2G4 Fc region fragments were cloned into vector pCR2.1. The resulting plasmid pCR2.1hL-SIGN was digested with EcoRV/XhoI, and the plasmid pCR2.1 hG2G4 was digested with XhoI and NgoMIV. The resulting L-SIGN and hG2G4 Fc fragments were ligated into a modified Apex3P plasmid vector (23). The vector encodes a promoter with Kozak sequence and ATG codon for the initiating methionine.

Expression and purification of recombinant L-SIGN-Fc and DC-SIGN-Fc fusion proteins

The 293 EBNA human embryonic kidney cells were transfected with Apex3P-hL-SIGNhG2G4Fc using Effectene (Qiagen) and were grown in DMEM (Cellgro 10-013-CV) with 10% heat-inactivated FBS, 2 mM L-glutamine, 100 IU/ml penicillin, 100 µg/ml streptomycin, 250 µg/ml G418 sulfate, and 1 µg/ml puromycin at 37°C and 5% CO2. T-175 flasks at 90–95% confluence were washed with HBSS or Dulbecco’s PBS (DPBS) to remove serum proteins, and 30 ml of IS Pro serum-free medium (Irvine Scientific; catalog 91103) supplemented with L-glutamine and penicillin/streptomycin was added to each flask. The supernatant was concentrated and purified by protein A column chromatography. Cloning and expression of DC-SIGN-Fc fusion protein were described previously (8). Briefly, DC-SIGN-Fc consists of the extracellular portion of DC-SIGN (amino acid residues 64–404) fused at the C terminus to a human IgG1-Fc fragment and expressed in the Sig-pIgG1-Fc vector. DC-SIGN-Fc was produced in Chinese hamster ovary K1 cells by cotransfection of DC-SIGN-Sig-pIgG1 Fc (20 µg) and pEE14 vector (5 µg).

Immunization with human L-SIGN

Four BALB/c mice were immunized twice at a 3-wk interval by i.p. administration of 10 µg of recombinant L-SIGN-Fc protein and 100 µl of the adjuvant ImmunEasy (Qiagen) in a total volume of 200 µl. After 3 wk, two of the mice received a third immunization similar to the first two rounds, and two were boosted with 5 x 106 K562/L-SIGN cells. Sera of all mice tested positive when analyzed for binding to K562/L-SIGN cells by flow cytometry. Mice were sacrificed and spleens were frozen immediately in liquid nitrogen.

Phage display library construction

Total RNA was isolated from L-SIGN-immunized mouse spleen samples using TRI Reagent (Molecular Research Center), according to the manufacturer’s protocol. mRNA was purified using Oligotex (Qiagen), according to the manufacturer’s protocol. First strand cDNA was synthesized using SuperScript First-Strand Synthesis System for RT-PCR (Invitrogen Life Technologies), according to the manufacturer’s protocol. Second strand cDNA synthesis and single primer PCR were performed, as previously described (24), with modifications for mouse primers and oligonucleotides. Amplified products were purified using PCR purification columns (Qiagen), digested with appropriate restriction endonucleases, and cloned into an IgG1 {kappa} Fab expression vector. Library size was 1.5 x 109.

Phage display library panning

Two microtiter plates were coated with 100 µl of anti-human IgG Fc-specific Ab (Pierce) at 20 µg/ml in PBS at 4°C overnight. The first plate was washed five times with PBS and blocked with 1% BSA/PBS at 37°C for 1 h. The wells were washed five times with PBS and incubated with 100 µl of rDC-SIGN-Fc (10 µg/ml in PBS) at 37°C for 1 h. The second plate was washed with PBS and blocked with 1% BSA/PBS at 37°C for 1 h. The wells were washed five times with PBS and incubated with 100 µl of recombinant L-SIGN-Fc (10 µg/ml in PBS) at 37°C for 1 h. The first plate was washed and the wells were incubated with 100 µl of library phage for 1 h at 37°C. The second plate was washed and library phage were transferred from the first plate to the second plate and incubated at 37°C for 1.5 h. The wells were washed with PBS with increasing stringency for each round of panning (3, 5, 10, and 10 times), each with a 5-min incubation and vigorous pipetting. The remaining phage were eluted and titrated on LB plates containing carbenicillin and glucose. Eluted phage were propagated in ER2738 cells overnight in the presence of antibiotics, 1 mM isopropyl {beta}-D-thiogalactoside, and helper phage for the next round of panning.

Phage ELISA

Ninety-five single colonies from titration plates from panning rounds 2, 3, and 4 were grown in 1 ml of super broth medium with carbenicillin. Fab phage production was induced with 1 mM {beta}-D-thiogalactoside and helper phage overnight at 30°C. The culture was spun down, and supernatants containing Fab phage were screened by ELISA. Microtiter plates were coated and incubated at 4°C overnight with either anti-human IgG Fc (Pierce) at 8 µg/ml in PBS to determine Ag binding or anti-mouse IgG F(ab')2 (Pierce) at 4 µg/ml in PBS to monitor Fab expression. The plates were washed three times with PBS and blocked with 100 µl of 1% BSA/PBS at 37°C for 1 h. The IgG Fc-treated plates were washed three times with PBS and incubated with 50 µl of L-SIGN-Fc or DC-SIGN-Fc (5 µg/ml in PBS) at 37°C for 2 h before the next step. The plates were washed three times with PBS and incubated with the culture supernatant containing Fab phage at 37°C for 2 h. The plates were washed three times with PBS, and the bound Fab phage were detected with alkaline phosphatase-conjugated anti-mouse IgG F(ab')2 Ab (Pierce) (1:500 in 1% BSA/PBS) at 37°C for 1 h. The plates were washed three times with PBS, and the wells were developed with alkaline phosphatase substrate, p-nitrophenyl phosphate (Sigma-Aldrich).

Screening of Fab phage on cells

Ninety-five single colonies from titration plates (panning rounds 2, 3, and 4) were grown in 1 ml of super broth medium induced for Fab phage production, as described earlier. The cultures were spun down, and 50 µl of supernatant containing Fab phage was incubated with 0.5 x 106 cells (K562 or K562/L-SIGN) in FACS buffer (DPBS with 1% BSA, 0.1% azide) for 1 h at 4°C, washed with FACS buffer, incubated with PE-conjugated goat anti-mouse IgG (Jackson ImmunoResearch Laboratories) at a 1/50 dilution in FACS buffer at 4°C for 30 min, washed and resuspended in 1% formaldehyde, and analyzed on a BD FACSCalibur (BD Biosciences).

DNA sequence analysis

All Fabs showing specific binding to L-SIGN were sent for DNA sequence analysis at Retrogen. The amino acid sequences were deduced and aligned by DNAstar software.

Western blotting

One million K562/L-SIGN cells were lysed in 50 µl of lysis buffer (150 mM NaCl, 25 mM Tris-HCl (pH 7.4), 2 mM EDTA, 1 mM sodium orthovanadate, 10 mM sodium fluoride, 1% Triton X-100, 0.5 mM PMSF, 10 µg/ml aprotinin, and 10 µg/ml leupeptin). Lysis was achieved by gentle rotation at 4°C for 20 min. Cell lysates were centrifuged (14,000 x g, 10 min) to remove cell debris and boiled for 5 min in SDS sample buffer containing 1 mM DTT. Protein lysates were resolved on 4–15% SDS-PAGE gradient gels (Bio-Rad 116-1158), transferred to nitrocellulose membranes, and probed individually with L-SIGN-specific Fabs (1 µg/ml). Protein transfer was monitored with prestained molecular mass standards (Bio-Rad 161-0324). Immunoreactive bands were detected using HRP-conjugated goat anti-mouse IgG (Bio-Rad 170-6516) and ECL (Supersignal West Pico kit; Pierce).

Competition ELISA

Microtiter plates were coated with anti-human IgG Fc (Pierce) at 8 µg/ml in PBS and incubated at 4°C overnight. The plates were washed three times with PBS and blocked with 100 µl of 1% BSA/PBS at 37°C for 1 h. After three washes with PBS, plates were incubated with 50 µl of L-SIGN-Fc (5 µg/ml in PBS) at 37°C for 2 h. The plates were washed three times with PBS and incubated with a constant amount (25 nM) of mAb, mAb162, and 2-fold dilutions of L-SIGN Fabs at 37°C for 2 h. The plates were washed three times with PBS, and mAb162 binding was detected using alkaline phosphate-conjugated anti-mouse Fc{gamma}-specific secondary Ab (Jackson ImmunoResearch Laboratories), followed by alkaline phosphatase substrate, p-nitrophenyl phosphate.

Ab internalization

The assay was done, as described previously (25). Briefly, 0.5 x 106 fresh human liver nonparenchymal cells or K562/L-SIGN cells were incubated with L-SIGN Fabs at 20 µg/ml for 30 min at 4°C in DPBS/1% BSA in duplicate. The unbound Fab was washed off; one sample was incubated at 37°C for an additional 2 h to enable internalization and the second sample was kept at 4°C for 2 h in DPBS/1% BSA/0.1% sodium azide as a noninternalizing control. At the end of the incubation period, cells were washed and incubated with PE-conjugated goat anti-mouse IgG for 30 min at 4°C in DPBS/1% BSA/0.1% sodium azide, washed, fixed in 1% paraformaldehyde, and analyzed on a BD FACSCalibur.

Confocal microscopy

A total of 105 K562/L-SIGN and K562/DC-SIGN cells was incubated with 10 µg/ml various Fabs for 90 min at 37°C in RPMI 1640 supplemented with 10% FCS. Cells were then washed with PBS, fixed in PBS/4% paraformaldehyde, washed again, and adhered to poly(L-lysine)-coated coverslips (20 min at room temperature). Cells were incubated with blocking buffer (PBS/3% BSA/10 mM glycine/0.1% saponin) for 1 h at room temperature. Subsequently, cells were washed with blocking buffer and incubated with 10 µg/ml goat anti-mouse IgG Alexa 647 (Molecular Probes) in blocking buffer for 1 h at room temperature. Cells were then washed with blocking buffer, PBS, and finally with 50 mM Tris-HCl. Finally, coverslips were mounted onto glass slides with Mowiol (Calbiochem, Omnilabo International). Fixed slides were imaged with a Bio-Rad MRC 1024 confocal system operating on a Nikon Optiphot microscope and a Nikon 60X Plan-Apochromatic 1.4 oil immersion lens. Pictures were analyzed with Bio-Rad Lasersharp 2000 and Adobe Photoshop 7.0 software.

Fluorescent bead adhesion assay for ligand blocking

Preparation of carboxylate-modified TransFluorSpheres (488/645 nm, 1.0 µm; Molecular Probes) coated with ICAM-3 Fc (R&D Systems), HIVgp120 (strain JRCSF), or Ebola gp (strain Zaire Mayinga) (viral proteins were kindly provided by D. Burton, The Scripps Research Institute, La Jolla, CA) was previously described (26, 27). For adhesion to ligand-coated fluorescent beads, K562/L-SIGN and K562/DC-SIGN cells (5 x 106/ml) were resuspended in Tris-sodium-BSA buffer (20 mM Tris-HCl (pH 8.0), 150 mM NaCl, 1 mM CaCl2, 2 mM MgCl2, and 0.5% BSA). Fifty thousand cells were preincubated with or without L-SIGN Fabs (20 µg/ml) for 10 min at room temperature in a 96-well V-bottom plate. The ligand-coated fluorescent beads (20 beads/cell) were added, and the suspension was incubated for 30 min at 37°C. After washing, the cells were resuspended in Tris-sodium-BSA buffer. The percentage of cells bound to ligand-coated beads was measured by FACSCalibur in FL-3.

Conversion of mouse Fabs to chimeric IgG

Overlap PCR was used to fuse the mouse {kappa} V regions of the E10 and G10 Fabs with a human {kappa} CH1 region. XbaI and NotI restriction sites at either end of the resulting fragments were used to clone these chimeric L chains into a Lonza vector mammalian expression system (28) adapted in-house for use with these sites. The Fab H chains were cloned by generating a PCR fragment containing the mouse V region with a short primer-derived segment containing human {gamma}1 CH1 sequence, including an existing ApaI site. An E10 or G10 XbaI/ApaI H chain fragment was then inserted into the vector bearing the corresponding L chain, such that human {gamma}1 CH1, hinge, CH2, and CH3 encoding regions were appended to the mouse VH chain regions.

Cloning and expression of peptide epitope-embedded Abs

TT epitope 630DR was inserted by overlap PCR into the CH2 domain between glycines 249 and 250 (Kabat numbering; see Ref.29) with two additional arginines upstream and three downstream of the epitope to give a final insertion of 25 total amino acids (RRIDKISDVSTIVPYIGPALNIRRR). The 5' fragment was amplified using the forward primer E10Age5For: 5'-TTC CCC GAA CCG GTG ACG GTG TCG T-3' that annealed to a region spanning a unique AgeI site upstream of the hinge region (DNA encoding aa 148–157 of the CH1 domain), in combination with the backward primer E10insertionRev: 5'-GCC GAT GTA GGG CAC GAT GGT GCT CAC GTC GCT GAT CTT GTC GAT TCT TCT CCC CAG GAG TTC AGG TGC TGA GGA AGA-3' that annealed to 9 bases of the intron and the DNA encoding aa 244–249 of the CH2 domain. The E10insertionRev primer contained a tail that encoded part of the insertion. The 3' fragment was generated using the forward primer E10insertionFor: 5'-GTG AGC ACC ATC GTG CCC TAC ATC GGC CCC GCC CTG AAC ATC AGA AGA AGA GGA CCG TCA GTC TTC CTC TTC CCC CCA-3' that annealed to glycine 250 and downstream amino acids (250–258) and the reverse primer E10EcoRI3Rev 5'-GAT TAT GAT CAA TGA ATT CTG GCC GTC GCA CTC AT-3' that annealed to a region spanning the stop codon and a unique EcoRI site within the vector at the end of the CH3 region. For PCR, the expand high fidelity PCR system (Roche) was used. The two fragments were gel purified and combined for overlap extension PCR, and the PCR product was digested with AgeI and EcoRI and cloned into the similarly digested E10-IgG parental clone. The sequence of the final cloned product was confirmed by DNA sequencing.

For production of the Ab, plasmids were transiently transfected into 293 EBNA cells using Effectene (Qiagen), per manufacturer’s instructions. Briefly, 1.2 x 107 293 EBNA cells were seeded in 150 mM tissue culture dishes in DMEM/10% FBS. The following day, each dish was transfected with 16 µg of the IgG expression plasmid along with 4 µg of pAdVAntage (Promega) and 800 ng of pEGFP-1 (BD Clontech). The medium was changed to serum-free medium (IS PRO; Irvine Scientific) after 24 h. After an additional 24 h, 2.5 ml of 0.5 M HEPES/20% glucose was added. Cells were incubated 4 days, and the Ab in the medium supernatant was purified by protein A chromatography.

Human subjects and vaccination

Four normal, healthy volunteers were selected who had recently received the standard TT vaccine at their primary care physician’s office. Peripheral blood was drawn from these vaccinated donors after informed consent. The peripheral blood was used as a source to obtain both T cells and monocyte-derived DCs. The study protocol was approved by the institutional review board at Alexion Pharmaceuticals.

Monocyte-derived DCs and PBLs

Peripheral blood drawn from healthy TT-vaccinated individuals was separated on Ficoll density gradients to obtain mononuclear cells (PBMCs). PBMCs were allowed to adhere to flasks for 1 h at 37°C. Nonadherent cells (PBLs) were gently removed and washed, and CD3+ T cells (>95% pure) were magnetically isolated by negative selection (Pan T Cell Isolation Kit II; Miltenyi Biotec). The adherent monocytes from the same donor were cultured in the presence of IL-4 (500 U/ml) and GM-CSF (800 U/ml) for 6–8 days to obtain immature DCs (iDCs) (>95% cells positive for CD11c, DC-SIGN, and HLA-DR, and negative for CD83 expression). X-VIVO 15 medium (Cambrex) supplemented with 2% human serum was used for all DC differentiation and T cell proliferation studies.

Ab-targeted delivery of peptide epitopes

iDCs were incubated with Abs E10 and E10-632DR for 1 h at 37°C, washed, and cocultured with purified CD3+ T cells (10,000 iDCs:100,000 T cells) at 37°C in a 96-well plate. After 4 days of coculture, tritiated thymidine (1 µCi/well; Amersham) was added to the cell cultures and thymidine incorporation was measured after 16–18 h on a microplate scintillation counter (PerkinElmer). Proliferation index was determined as (cpm of Ab treatment)/(cpm of medium treatment). For studies testing presentation of TT epitopes over time, similar procedures were performed, except that purified CD3+ T cells were added to the Ab-treated iDCs either immediately or 2 or 4 days later.

Statistical analysis

Differences between groups were analyzed by two-tailed unpaired Student’s t test. Significance was accepted when p < 0.05.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Disclosures
 References
 
Isolation and binding analysis of a panel of phage-derived Fabs recognizing C-type lectins, L-SIGN or DC-SIGN

An IgG1k phage-displayed Fab library derived from H and L chain coding sequences of mice immunized with human L-SIGN was first negatively selected in one round of panning on human DC-SIGN-Fc protein to remove Fabs with preferential binding to DC-SIGN or the Fc portion. The unbound phage were then used for selecting clones reactive with L-SIGN-Fc fusion protein in three rounds of positive selection. Of 95 clones selected from each of the three rounds of panning, 21 clones expressing phage-displayed Fabs showed a 5- to 100-fold higher binding to K562/L-SIGN compared with K562 cells (Fig. 1A). As illustrated in Fig. 1B, 17 of the 21 Fabs showed specific binding in ELISA to recombinantly produced L-SIGN (O.D >1.0), demonstrating the success of the selection procedure, with two Fabs (D10 and E10) showing some level of cross-reactivity to recombinantly produced DC-SIGN. The relative magnitude of Fab binding to L-SIGN-transfected cells compared with Fab reactivity to recombinant protein can differ. For example, clone E10 showed a fairly robust signal to recombinant protein, but only a modest signal by FACS on cells. Recombinant proteins frequently have slightly different conformation and/or glycosylation patterns compared with the native form on the cell surface. Because the clones are derived from mice immunized with recombinant protein and the library was panned on recombinant protein, identification of a number of clones preferentially recognizing the recombinant protein might be expected.



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  		FIGURE 1. Screening of phage-displayed Fabs for L-SIGN reactivity. A, Ninety-six Fab phage clones selected from three rounds of panning on recombinant DC-SIGN (negative selection) and L-SIGN (positive selection) proteins were screened for binding to K562 (negative control) and K562/L-SIGN-transfected cells by flow cytometry using goat anti-mouse IgG PE conjugate for detection. Only clones showing at least a 5-fold higher binding to K562/L-SIGN cells are shown. B, To select Fab phage clones uniquely reactive with L-SIGN, but not DC-SIGN, K562/L-SIGN cell-reactive clones were screened for reactivity with DC-SIGN-Fc and L-SIGN-Fc fusion proteins in a capture ELISA. mAb162 (L-SIGN specific), mAb1621, and mAb16211 (DC-SIGN/L-SIGN cross-reactive) were used as positive controls. Data are representative of two independent experiments.

 
All 21 clones reactive as phage-displayed Fabs with L-SIGN had unique DNA sequences (data not shown). Based on similarities among the H and L chain CDR3 sequences, a representative panel of six clones (C7, D12, E4, E10, G3, G10) was chosen for further characterization (Table I). Following subcloning to remove the phage gene III coat protein, the purified soluble Fabs of all six clones exhibited a 40- to 100-fold higher binding to K562/L-SIGN compared with K562 cells (Fig. 2A). Three purified Fabs, D12, G3, and E10, also reacted with K562/DC-SIGN cells, but at a lower level (Fig. 2A). This may represent some expected differences (due to expression or other differences that affect efficiency of display) in the analysis of Fabs when evaluated in a phage-displayed format compared with soluble Fabs. The expression of both DC-SIGN and L-SIGN receptors was found to be similar on K562 cells when assessed by SIGN-cross-reactive Ab mAb16211 (Fig. 2B).


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  		Table I. Deduced amino acid sequences of loops comprising CDRs of L-SIGN Fabs

 


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  		FIGURE 2. Receptor specificity analysis of soluble L-SIGN Fabs. A, 5 x 105 K562, K562/DC-SIGN, or K562/L-SIGN cells were incubated with purified soluble Fabs (20 µg/ml) representing six unique clones for 1 h, and the extent of their binding was assessed by flow cytometry using goat anti-mouse IgG PE conjugate. Values represent mean (bars, SD) of two independent experiments. B, Histograms showing similar expression of SIGN molecules on K562 cells after staining with SIGN-cross-reactive mAb16211 (20 µg/ml) and detecting with goat anti-mouse IgG PE conjugate.

 
Relative affinities and epitope specificities of L-SIGN Fabs

The six candidate Fabs were characterized further in terms of their relative affinities and epitope specificities. Although all Fabs displayed high affinity for L-SIGN as demonstrated by their reactivity in ELISA (>0.5 OD) at a concentration of 1 nM, Fab E10 exhibited strong binding even at picomolar concentrations (Fig. 3A). To determine epitope specificities, the ability of each of the six Fabs to inhibit the binding of the lone commercially available L-SIGN-specific Ab, mAb162, to L-SIGN-Fc fusion protein was assayed in a competition ELISA. As shown in Fig. 3B, four Fabs (C7, D12, G10, and E4) inhibited the binding of mAb162 in a concentration-dependent manner, while Fabs G3 and E10 did not compete, similar to the negative control Fab. These similarities and differences in epitope binding are also reflected by their sequences: clones D12, C7, E4, and G10 have closely related CDR, while those of clones E10 and G3 are unrelated to the set of four and to each other (see Table I). To further characterize and differentiate the epitopes recognized by the six Fabs, whole cell lysates of K562/L-SIGN cells prepared under denaturing and reducing conditions were separated by SDS-PAGE, and the membranes were probed with individual L-SIGN Fabs. As depicted in Fig. 3C, two Fabs (clone D12 and E10) recognized a protein band of ~42 kDa with good correlation to monomeric L-SIGN (5), while Fab G3 recognized a higher molecular mass protein band that may correspond to an oligomeric form of L-SIGN, possibly resulting from only partial denaturation of the sample. These data imply that the epitopes recognized by Fabs D12 and E10 are likely to be linear, and further, that although they recognize their epitope on L-SIGN-expressing cells, their epitope is inaccessible in the partially denatured oligomer. The epitopes recognized by Fabs C7, E4, and G10 are probably conformational because these epitopes are destroyed after partial or complete denaturation with concomitant loss of reactivity to either band. Because G3 only recognizes the presumed oligomeric form and not a monomeric form, it probably recognizes a conformational epitope formed on oligomerization. Our data suggest that under the denaturing conditions used (95°C), the oligomeric structure does not fall apart; such observations have been made with other proteins, which oligomerize via coiled-coil domains and require a high concentration of guanidinium chloride for separation (30).



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  		FIGURE 3. Binding affinities and epitope specificities of soluble L-SIGN Fabs selected from the phage display library. A, The relative affinities of L-SIGN Fabs were measured by titrating them over L-SIGN-Fc fusion protein in a capture ELISA. At the highest Ab concentration (20 nM) tested, reactivity with BSA (irrelevant Ag) was <0.2 OD for all six Fabs (data not shown). B, Epitope specificities of L-SIGN Fabs were assessed by inhibiting the binding of a constant amount (25 nM) of mAb162 with a 2-fold titration of L-SIGN Fabs on L-SIGN-Fc fusion protein captured on ELISA plates. mAb162 binding was detected using alkaline phosphatase-conjugated anti-mouse Fc-specific secondary Ab (Jackson ImmunoResearch Laboratories). C, Nature of epitopes bound by Fabs was determined by Western blotting. Expected molecular mass of the monomeric L-SIGN is 42.7 kDa. Data are representative of two independent experiments.

 
L-SIGN Fabs block ligand binding to the receptor

Several viruses, e.g., Ebola, SARS, HIV, and HCV, have been shown to use DC-SIGN and L-SIGN receptors for gaining entry into cells. Both ICAM-3 on T cells and envelope glycoproteins on viruses were found to interact with the CRD of the SIGN receptors in a manner unique to each ligand (7, 9). To determine whether we isolated CRD-reactive Fabs capable of blocking ligand binding, a ligand-coated fluorescent bead-blocking assay was performed. Ligand-coated fluorescent beads not only mimic multimeric binding of the ligand to the cell surface receptor, but also allow quantitation of ligand binding by flow cytometry. First, adhesion of fluorescent beads coated with envelope glycoproteins of Ebola and HIV to K562/DC-SIGN and K562/L-SIGN was assessed in the absence of Abs. As illustrated in Fig. 4A, while Ebola envelope glycoprotein bound equally well to both DC-SIGN- and L-SIGN-expressing cells, HIV envelope glycoprotein bound more strongly to DC-SIGN-expressing cells than to L-SIGN-expressing cells. These differences in viral protein binding to the SIGN molecules correlated with the ability of Fabs to block adhesion of the viral proteins. Although all six Fabs could block to some extent HIVgp120 binding to L-SIGN (39–62%), only two Fabs, C7 and E10, showed significant (p < 0.001) blocking of Ebola gp binding to L-SIGN (70 and 71%, respectively; Fig. 4B), despite recognizing different epitopes. Of the three DC-SIGN-cross-reactive Fabs, D12, E10, and G3, only E10 effectively blocked binding of both viral proteins to DC-SIGN (p < 0.0001; see Fig. 4C). As expected, the other three Fabs uniquely reactive with L-SIGN had no blocking effect on ligand binding to DC-SIGN (Fig. 4C). In addition, three Fabs, C7, E10, and G10, which were most efficient at blocking the HIV gp120 viral protein binding to L-SIGN, also prevented the binding of ICAM-3 to K562/L-SIGN cells most efficiently (see Table II).



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  		FIGURE 4. L-SIGN Fabs block ligand binding to the receptor. A, K562/L-SIGN and K562/DC-SIGN cells were incubated with fluorescent beads coated with envelope glycoproteins of HIV and Ebola in the absence of Abs, and the extent of binding was measured by flow cytometry. B, K562/L-SIGN cells were incubated with fluorescent beads coated with envelope glycoproteins of HIV and Ebola in the presence of L-SIGN Fabs. C, Same as in B, except that K562/DC-SIGN cells were used instead of K562/L-SIGN cells. Percentage of binding is determined as 100 times the number of cells bound to protein-coated beads with Fab divided by the number of cells bound to protein-coated beads without Fab. Values represent mean (bars, SD) of four independent experiments. *, Significant values (p < 0.001) compared with samples not treated with Fabs.

 

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  		Table II. Biological activities mediated by L-SIGN Fabs

 
Conversion of Fab to IgG enhances receptor binding and blocking of viral protein adhesion

Based on the receptor-binding and ligand-blocking results, Fab clones E10 and G10 were converted into chimeric IgGs to increase avidity for the receptor and thereby enhance blocking of virus binding. As shown in Fig. 5A, Fab to IgG conversion greatly improved receptor binding of both clones E10 and G10, but also resulted in some cross-reactivity of G10 with DC-SIGN. This is not a general result of Fab conversion to IgG, because the IgG version of C7 was still uniquely reactive with L-SIGN (data not presented). As depicted in Fig. 5, B and C, full IgGs demonstrated enhanced blocking of viral protein binding compared with their Fab counterparts, for example, while full IgG forms of clones E10 and G10 produced >90% blocking of Ebola gp to L-SIGN, their Fab counterparts showed 71 and 22% blocking, respectively (see Fig. 5C).



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  		FIGURE 5. Conversion of Fab to IgG improves receptor binding and blocking of viral protein adhesion. A, K562, K562/DC-SIGN, and K562/L-SIGN cells were incubated with purified Abs, and the extent of their binding was assessed by flow cytometry using PE-conjugated goat anti-mouse (Fab detection) or goat anti-human (IgG detection) secondary Abs. mAb162 (L-SIGN specific) and mAb16211 (DC-SIGN/L-SIGN cross-reactive) were used as positive controls. B, K562/L-SIGN and K562/DC-SIGN cells were incubated with fluorescent beads coated with HIVgp120 in the presence of Abs and assessed by flow cytometry. C, Same as in B, except that Ebola envelope glycoprotein-coated beads were used instead of HIVgp120-coated beads. mAb162 (L-SIGN specific) and mAbAZND1 (DC-SIGN specific) were used as controls for comparison. Percentage of binding is determined as 100 times the number of cells bound to protein-coated beads with Ab divided by the number of cells bound to protein-coated beads without Ab. Values represent mean (bars, SD) of four independent experiments. *, Highly significant values (p < 0.00005) compared with samples not treated with Abs.

 
L-SIGN Fabs undergo internalization upon binding to the receptor on human liver sinusoidal endothelial cells and L-SIGN-transfected K562 cells

Ab internalization is a prerequisite for delivering autoimmune Ags as Ab-linked cargo into L-SIGN-expressing liver sinusoidal endothelial cells (LSECs) (4, 21). The internalizing potential of the Fabs was assessed on freshly isolated human liver nonparenchymal cells containing ~55% cells that expressed L-SIGN (see Fig. 6A). L-SIGN-positive cells were further characterized using a panel of Abs against endothelial cell markers (CD31, CD54, CD106), immune cell recognition molecules (CD40, CD80, CD86, MHC class I and II), and myeloid cell markers (CD4, CD11c) (19, 31). The observed expression profile of these receptors correlated well with previously described studies on LSECs (19), which were characterized by strong expression levels of CD31, CD54, CD206, and MHC class I and moderate to weak expression of the other markers on LSECs (data not presented).



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  		FIGURE 6. L-SIGN Fabs undergo internalization on human liver sinusoidal endothelial cells. A, The percentage of cells expressing L-SIGN in freshly isolated human liver nonparenchymal cell samples was assessed using L-SIGN-specific Ab mAb162 (allophycocyanin conjugate). B, Fresh human liver nonparenchymal cells were incubated with L-SIGN Fabs at 4°C or 37°C. The level of cell surface Ab remaining after 2 h was measured by flow cytometry. Values represent mean (bars, SD) of two independent experiments.

 
Internalization of Fabs after L-SIGN binding was determined by incubating freshly isolated human liver nonparenchymal cells with the six L-SIGN Fabs at 4°C and 37°C, respectively, and the level of Ab remaining on the cell surface after 2 h was determined by flow cytometry. As shown in Fig. 6B, >40% loss of signal was observed for three Fabs (C7, E10, and G10) and the L-SIGN-specific mAb162 at 37°C compared with 4°C. Three other Fabs (clone D12, E4, and G3) showed a slightly more modest (30–38%) loss of signal. Similar studies comparing the signal remaining on K562/L-SIGN cells further confirmed the observations made on LSECs (see Table II). In addition, confocal microscopy was used as an alternate method to further confirm Ab internalization. As illustrated in Fig. 7, all six L-SIGN Fabs were found inside K562/L-SIGN cells following 90-min incubation at 37°C. Furthermore, while the L-SIGN/DC-SIGN-cross-reactive Ab E10 was internalized by both DC-SIGN and L-SIGN transfectants, the L-SIGN-specific clone E4 was internalized only by L-SIGN transfectants (see bottommost panels designated E4/DC-SIGN and E10/DC-SIGN). The microscopy images are illustrative of a specific internalization process as highlighted by the clustered vesicular staining under the cytoplasmic membrane. Interestingly, these studies show that while all of the L-SIGN Fabs are able to internalize to some extent, the degree of internalization correlated with the capacity of the Abs to block ICAM-3 and HIV binding, but not with their relative affinities.



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  		FIGURE 7. Confirmation of Ab internalization on transfected cells by confocal microscopy. K562/L-SIGN cells were incubated for 90 min with L-SIGN Fabs and imaged with a confocal system using goat anti-mouse IgG Alexa 647 as detection reagent. AZND2 is a SIGN-cross-reactive Ab used as a positive control. As additional specificity controls, two Fab clones, E4 (L-SIGN specific) and E10 (L-SIGN and DC-SIGN cross-reactive), were also incubated with K562 cells expressing DC-SIGN (see lowermost panels on the right). Data are representative of two independent experiments.

 
Ab-targeted delivery of a peptide Ag induces a sustained human T cell response

To consider the use of these Abs in Ab-mediated Ag delivery, we embedded a universal Th epitope, 632DR (32, 33), from TT Ag into the full IgG clone of E10 that cross-reacts with both DC-SIGN and L-SIGN receptors. Peptide 632DR (aa 632–651) was genetically engineered into clone E10 by directed insertion at the junction between hinge and CH2 domain with flanking 20S proteasomal cleavage sites (Fig. 8A) known to facilitate the intracellular release of the inserted epitope (34, 35). The resulting peptide-inserted Ab (designated E10-632DR) expressed as well as the native Ab, E10 (Fig. 8B), and was affinity purified to remove the partially formed Ab seen in crude preparations (Fig. 8, compare C with B). The relative affinity of the peptide-inserted Ab to the DC-SIGN and L-SIGN receptor was found to be similar to the native Ab (see Fig. 8, D and E).



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  		FIGURE 8. Expression of peptide-embedded Abs. A, Schematic representation of E10-632DR H chain showing the location of the embedded TT peptide, 632DR (underlined sequence), with flanking 20S proteasome cleavable linkers (bolded arginines). Lowercase letters represent adjoining Ab sequence. Letter "S" underneath the Ab sequence indicates predicted 20S proteasomal cleavage sites (<www.cbs.dtu.dk/services/NetChop>). B, Nonreducing SDS-PAGE gradient gel (4–15%) showing expression of peptide-embedded (E10-632DR) and native (E10) Abs. Equal amounts of Ab transfection supernatants were detected using anti-L chain-HRP conjugate. Notice the ~2-kDa size difference between peptide-embedded Ab and native Ab at the 203-kDa marker. C, Nonreducing SDS-PAGE gradient gel (4–15%) stained with Coomassie blue showing elimination of partially formed Abs by affinity purification (compare with Fig. 8B). D and E, Relative binding of affinity-purified peptide-embedded Ab and native Ab to DC-SIGN and L-SIGN receptors on K562 cells.

 
As a first step to evaluate targeted delivery, processing, and presentation of the inserted epitope, we took advantage of the cross-reactivity of E10 to DC-SIGN as well as the relative ease of obtaining DCs and autologous T cells from human peripheral blood. iDCs were treated with the native and peptide-embedded Abs for 1 h, washed, and cocultured with autologous T cells from TT-vaccinated donors who had been prescreened for a proliferative T cell response to the whole TT protein (data not presented). As shown in Fig. 9, targeting with E10-632DR elicited a significant (p < 0.005 vs native Ab, E10) T cell proliferative response in donor 13 similar to the free 632DR peptide. However, no proliferative responses were induced in the other three donors by targeting with E10-632DR commensurate with lack of responses to the free peptide in these donors. Furthermore, when donor 13 iDCs were treated with different doses of the Ab, T cell responses directly correlated with the Ab dose, and a significant level (p < 0.00005 vs native Ab) of T cell proliferation was observed even at picomolar concentrations of the targeting Ab (see Fig. 10A). In addition, the proliferative responses induced by E10-632DR could be blocked with excess native Ab E10 (p = 0.00008 vs E10-632DR), but not a control Ab ALXN4100 (see Fig. 10A). Moreover, both the native Ab and peptide-embedded Ab showed equivalent binding to donor 13 iDCs used in the T cell activation assays (see Fig. 10B).



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  		FIGURE 9. Targeted delivery of TT peptide to human DCs activates T cell responses in vaccinated donors. iDCs (>95% DC-SIGN positive) were incubated with Abs (10 µg/ml, 66 nm) and peptide (1 µg/ml, 500 nm) for 1 h at 37°C, washed, cocultured with 100,000 autologous T cells (>95% CD3 positive), and incubated for 5 days. TT protein (100 ng/ml) was added as a positive control without washing. Proliferation was assessed by [3H]thymidine incorporation. *, Highly significant values; p < 0.005 when compared with medium or native Ab, E10. Values represent mean (SD) of six replicate wells.

 


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  		FIGURE 10. Dose-dependent responses to the targeting Ab and blocking of T cell activation by competition with native Ab. A, Ten thousand iDCs from donor 13 were incubated with Abs and free peptide for 1 h at 37°C, washed, and cocultured with 100,000 autologous T cells for 5 days. Proliferation was assessed by [3H]thymidine incorporation. *, Highly significant values; p < 0.00005 when compared with medium or native Ab, E10. Values represent mean (SD) of eight well replicates. This experiment was repeated three times with similar outcomes. B, Dose-dependent binding of peptide-embedded and native Abs to donor 13 iDCs used in the T cell proliferation assays.

 
Delivery of Ags linked to Abs is expected to require internalization and proteasomal cleavage of the Ab to release the linked Ag for presentation to T cells. This type of processing could potentially prolong presentation of Ag to T cells, resulting in a long-lasting immune response. To test this notion, iDCs from donor 13 were treated with E10-632DR or free 632DR peptide for 1 h, washed, and cocultured with autologous T cells either immediately or 2 and 4 days after Ag pulsing. As shown in Fig. 11, only peptide-embedded Ab produced a significant T cell response even 4 days after Ag pulsing, demonstrating that delivering Ag via Abs can result in a sustained immune response, which was not achievable with free peptide. Based on this result, it is conceivable that in vivo targeting with the peptide-embedded Ab would produce a more heightened immune response compared with a free peptide vaccine.



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  		FIGURE 11. Ab targeting produces a sustained immune response. Method similar to Fig. 9, except that donor 13 T cells were added to autologous iDCs either immediately after washing off the Ab or 2 or 4 days later. *, p < 0.05 vs medium. Values represent mean of eight well replicates.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Disclosures
 References
 
L-SIGN is a recently discovered C-type lectin expressed in lymph node and liver sinusoidal endothelial cells and placenta (4, 6). Although the expression of L-SIGN on cells implicated in tolerance induction vs the expression of DC-SIGN on cells potentially capable of raising a stimulatory immune response is very striking, neither functional consequences of the interaction of ICAM-3 on T cells with L-SIGN nor functional consequences of pathogen binding to the receptor are well defined (4, 5), with limited knowledge of the correlation between the two known roles (9). Identification of a panel of Abs that bind to different epitopes on the receptor may assist in elucidation of domains of the receptor involved in the various biologic processes. Abs that specifically recognize the ligand binding domain of L-SIGN may be useful for exploring the biological consequences of receptor activation. Furthermore, it may be feasible to use these same Abs therapeutically for the modulation of immune responses. Abs that block pathogen binding may find immediate therapeutic use in preventing transmission of disease in the host, or in the development of novel therapeutics based on epitope specificity.

Isolating Abs that react selectively with L-SIGN, but not DC-SIGN, is challenging due to the similarities between the two proteins. L-SIGN bears an overall amino acid sequence identity of 77% to DC-SIGN, with even greater identity (88%) in the extracellular domain, the target for Ab binding for therapeutic purposes. Furthermore, the 40 unique amino acids in the 330 aa extracellular domain of L-SIGN are not clustered in one region of the protein (1, 4). The ability to sequentially pan phage-displayed Fab libraries on DC-SIGN (negative selection) and L-SIGN (positive selection) provided us with a powerful tool to successfully identify a panel of high affinity Fab clones that are either uniquely reactive or preferentially reactive with the L-SIGN receptor. Competition ELISAs and Western blot studies revealed a number of interesting features in these Fabs. Fabs with at least three different specificities were identified in competition experiments. Although four of the Fabs competed with binding of L-SIGN-specific mAb162, they did so with differing kinetics, implying that either epitopes bound by these four Fabs are overlapping, but not identical, or their relative affinities are different, or that competition was the result of steric hindrance. Further characterization of the panel demonstrated a variety of distinctions among this subset of Fabs. Western blot studies revealed at least three classes of epitopes recognized by the Fab panel. Although Fabs C7, E4, and G10 recognize conformational epitopes absent in denatured Western blots, Fabs D12 and E10 bound a linear epitope accessible in the monomeric receptor. In contrast, Fab G3 bound L-SIGN-transfected cells as well as or better than the other Fabs in the panel, and appeared to bind an epitope present only in the presumed oligomeric form of the receptor. This is an interesting finding, as crystal structure studies of the extracellular domain demonstrated that receptor oligomerization is required for the recognition of complex carbohydrate ligands by the SIGN molecules (36, 37). As a result of the differences in the epitopes recognized by the Fabs, the biological activities induced or prevented by them could potentially be diverse.

To be able to attribute specific biological functions to different domains of the receptor, we chose to explore two different properties: blocking of ligand binding and Ab internalization. Binding of the Abs to the CRD of the receptor is a prerequisite for their use in modulating the immune response and preventing viral transmission. Interestingly, while Fabs C7 and E10 consistently and effectively blocked the binding of ICAM-3 and both of the viral proteins, Fab G10 was able to block binding of ICAM-3 and HIVgp120, but not the binding of Ebola gp (summarized in Table II). The reason for this differential blocking by Fab G10 may lie in the fact that HIVgp120 binds less strongly to L-SIGN compared with Ebola gp (see Fig. 4A). It is also possible that while Fabs C7 and E10 have their epitopes located directly in the CRD of the receptor, the epitope for G10 may be located outside the CRD, possibly in the neck domain that is important for receptor oligomerization (36, 38). Conversion of Fab G10 to the IgG form of the Ab could allow enhanced blocking possibly through steric interference of the viral binding site, or by steric interference with receptor oligomerization. The latter mechanism is supported by the enhanced blocking of both the viral proteins observed with the IgG form of G10, whereas the first mechanism is supported by the fact that three of the six Fabs in the panel (E4, G10, G3) show a reduced ability to block Ebola gp binding as Fabs. In either case, these results suggest that the Ebola gp binding site is different from the potentially shared binding site of ICAM-3 and HIVgp120. Taken together, these observations suggest that the blocking effects produced by clones C7 and E10 are due to direct competition for the ligand binding site, and clone G10 may block ligand binding through some mechanism of steric interference. For potential use in blocking viral infection, particularly in the event of Ebola infection, a humanized IgG form of clone E10 might be most useful. Additionally, the epitopes defined by these Abs may help in the design of selective small molecule or protein inhibitors.

Conflicting evidence has been presented regarding the internalizing potential of L-SIGN. Based on biochemical studies that L-SIGN does not release 125I-labeled high mannose sugar ligand at endosomal pH, while DC-SIGN does, Guo et al. (39) concluded that L-SIGN is not capable of mediating direct internalization. In contrast, studies using either pseudotype viruses of Ebola (16) or infectious strains of SARS (12) found that virus capture and internalization do occur by L-SIGN-positive cells in cis; this finding was demonstrated by the measurement of viral RNAs isolated from receptor-positive cells. More recently, Ludwig et al. (40) provided further support for L-SIGN-mediated internalization of HCV by monitoring its intracellular localization by confocal microscopy. Our studies demonstrating internalization of L-SIGN Abs on freshly isolated human LSECs by FACS analysis and on L-SIGN-transfected cells by confocal microscopy strongly support the latter virus internalization studies. The conservation of the di-leucine motif and the triacidic cluster in the cytoplasmic tail of both DC-SIGN and L-SIGN, which are known to be required for receptor internalization, lends additional support for the internalizing potential of L-SIGN (1, 4, 41). As LSECs are well designed to take up a wide variety of harmful Ags from circulation by receptor-mediated endocytosis (19), it is likely that L-SIGN may assist in the process of Ag internalization by LSECs.

Ab internalization is a prerequisite for the use of L-SIGN Ab to specifically deliver Ag to LSECs. LSECs appear to be directly involved in tolerance induction through active uptake of many Ags from blood circulation, followed by processing and presentation of the antigenic peptides efficiently to trafficking T cells (19, 42). CD8+ T cells, when exposed to an Ag presented by LSECs, stop producing IL-2 and IFN-{gamma} to become tolerogenic. Furthermore, addition of inflammatory stimuli, e.g., TNF-{alpha}{beta}, IFN-{gamma}, or IL-12, does not rescue the tolerogenic phenotype induced by LSECs (21). Likewise, CD4+ T cells stimulated by Ag-presenting LSECs show a regulatory phenotype characterized by the expression of IL-10 and IL-4 (20). If autoimmune Ags (e.g., proinsulin or myelin basic protein) could be delivered selectively to LSECs via L-SIGN-specific Abs, it might be possible to induce tolerance and thereby block the destructive functions of autoreactive T cells and might be of value in the treatment of autoimmune diseases such as type I diabetes or multiple sclerosis. Alternatively, specific targeting of Ag to DC-SIGN, expected to raise a stimulatory response, could be exploited therapeutically for the treatment of cancer. The ease of obtaining DCs and autologous T cells from human peripheral blood allowed us to first explore the feasibility of delivering Ag using L-SIGN/DC-SIGN Abs to DCs for the induction of Ag-specific T cell responses.

A significant obstacle in testing the targeted delivery approach is the production of Ag-linked Abs. Conventionally, this is achieved by chemical conjugation of Ag to the Ab; however, this method suffers from many problems, particularly in the context of large-scale therapeutic application in humans. It is difficult to control the number and the point of Ag attachment to the Ab and to ensure batch-to-batch production consistency. Ag has also been fused genetically to the C terminus of Fabs and IgGs (43, 44); however, such constructs resulted in significant to complete loss of expression of our peptide-fused Abs. Therefore, we developed a novel approach to embed peptide Ags into the C domain of the Ab by identifying hydrophobic regions for insertion, as most T cell epitopes are hydrophobic, and flexible regions, such as the hinge region, which more easily accommodate inserted peptides. We also relied on the presence of both endogenous and artificially introduced 20S proteasomal cleavage sites (45, 46) shown to be essential for the proper intracellular release of the MHC-binding peptides (34, 35). This novel genetic approach allowed us to successfully produce Ag-embedded Abs in amounts similar to the native Abs without losing the binding affinities for the cognate receptors. Most importantly, the peptide-embedded Abs were taken up specifically by human DCs and the embedded peptide epitope was subsequently processed and presented to autologous T cells for their robust activation. Importantly, the presentation of the embedded peptide was sufficient to activate significant T cell responses observed for up to 4 days after treatment with the targeting Ab. Tacken et al. (47) very recently validated DC-SIGN as a suitable candidate for targeted delivery of Ag to DCs by chemically conjugating keyhole limpet hemocyanin to a humanized anti-DC-SIGN Ab, resulting in a proliferative response of cells from a keyhole limpet hemocyanin-vaccinated donor. Our approach further expands on these studies, allowing for a more refined delivery of specific peptides and precise cleavage for presentation to the T cells, and is to our knowledge the first report of an Ab-targeted single epitope delivery from an infectious agent to human DCs that resulted in a productive T cell response. In further studies, we will address whether targeted delivery of Ag to L-SIGN will induce T cell tolerance to the presented Ags.

In conclusion, we report in this study the identification of human L-SIGN-specific Abs that appear to mediate therapeutically relevant properties, and represent a first step in evaluation of L-SIGN as a clinically important target. These Abs may find utility in exploring the biological function of the receptor, delivering Ags to target organs, modulating immune responses, and preventing the transmission of infectious agents.


   Acknowledgments
 
We thank Linh Tran, Mary Jean Nolan, Peter Calveley, and Bing Lin of Alexion Antibody Technologies for their support with phage panning, Fab to IgG conversion, and Ab purification services.


   Disclosures
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
References
 
N. Dakappagari, T. Maruyama, M. Renshaw, M. A. Wild, D. Wu, K. Bowdish, and A. Kretz-Rommel are employees of Alexion Antibody Technologies, Incorporated, which plans to develop and potentially commercialize the L-SIGN antibodies C7 E10 and E4.


   Footnotes
 
The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

1 Address correspondence and reprint requests to Dr. Anke Kretz-Rommel, Alexion Antibody Technologies, 3985 Sorrento Valley Boulevard, Suite A, San Diego, CA 92121. E-mail address: kretz-rommel{at}alxnsd.com Back

2 Abbreviations used in this paper: DC, dendritic cell; CRD, carbohydrate recognition domain; DC-SIGN, DC-specific ICAM-grabbing nonintegrin; DPBS, Dulbecco’s PBS; HCV, hepatitis C virus; iDC, immature DC; L-SIGN, liver/lymph node-specific ICAM-3-grabbing nonintegrin; LSEC, liver sinusoidal endothelial cell; SARS, severe actue respiratory syndrome; TT, tetanus toxoid. Back

Received for publication April 6, 2005. Accepted for publication October 10, 2005.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Disclosures
 References
 

  1. Soilleux, E. J., R. Barten, J. Trowsdale. 2000. DC-SIGN; a related gene, DC-SIGNR; and CD23 form a cluster on 19p13. J. Immunol. 165: 2937-2942. [Abstract/Free Full Text]
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  4. Bashirova, A. A., T. B. Geijtenbeek, G. C. van Duijnhoven, S. J. van Vliet, J. B. Eilering, M. P. Martin, L. Wu, T. D. Martin, N. Viebig, P. A. Knolle, et al 2001. A dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN)-related protein is highly expressed on human liver sinusoidal endothelial cells and promotes HIV-1 infection. J. Exp. Med. 193: 671-678. [Abstract/Free Full Text]
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