Monovalent single-chain Fv fragments and bivalent miniantibodies bound to vesicular stomatitis virus protect against lethal infection

Antibody bivalency improves antiviral efficacy by inhibiting virion release independently of Fc gamma receptors

Across the animal kingdom, multivalency discriminates antibodies from all other immunoglobulin superfamily members. The evolutionary forces conserving multivalency above other structural hallmarks of antibodies remain, however, incompletely defined. Here, we engineer monovalent either Fc-competent or -deficient antibody formats to investigate mechanisms of protection of neutralizing antibodies (nAbs) and non-neutralizing antibodies (nnAbs) in virus-infected mice. Antibody bivalency enables the tethering of virions to the infected cell surface, inhibits the release of virions in cell culture, and suppresses viral loads in vivo independently of Fc gamma receptor (FcγR) interactions. In return, monovalent antibody formats either do not inhibit virion release and fail to protect in vivo or their protective efficacy is largely FcγR dependent. Protection in mice correlates with virus-release-inhibiting activity of nAb and nnAb rather than with their neutralizing capacity. These observations provide mechanistic insights into the evolutionary conservation of antibody bivalency and help refining correlates of nnAb protection for vaccine development.

Phage-based vaccines

Bacteriophages, or more colloquially as phages, are viruses that possess the ability to infect and replicate with bacterial cells. They are assembled from two major types of biomolecules, the nucleic acids and the proteins, with the latter forming a capsid and the former being encapsulated. In the eukaryotic hosts, phages are inert particulate antigens and cannot trigger pathogenesis. In recent years, many studies have been explored about using phages as nanomedicine platforms for developing vaccines due to their unique biological characteristics. The whole phage particles can be used for vaccine design in the form of phage-displayed vaccines or phage DNA vaccines. Phage-displayed vaccines are the phages with peptide or protein antigens genetically displayed on their surfaces as well as those with antigens chemically conjugated or biologically bound on their surfaces. The phages can then deliver the immunogenic peptides or proteins to the target cells or tissues. Phage DNA vaccines are the eukaryotic promoter-driven vaccine genes inserted in the phage genomes, which are carried by phages to the target cells to generate antigens. The antigens, either as the immunogenic peptides or proteins displayed on the phages, or as the products expressed from the vaccine genes, can serve as vaccines to elicit immune responses for disease prevention and treatment. Both phage-displayed vaccines and phage DNA vaccines promise a brilliant future for developing vaccines. This review presents the recent advancements in the field of phage-based vaccines and their applications in both the prevention and treatment of various diseases. It also discusses the challenges and perspectives in moving this field forwards.

The conduit system exports locally secreted IgM from lymph nodes

IgM provides early protection against pathogens. How IgM is exported out of lymph nodes remains unknown. Thierry et al. report that B cells utilize a system of paracortical conduits to rapidly export their IgM to the periphery.

Immunoglobulin M (IgM) is the first type of antibody produced during acute infections and thus provides an early line of specific defense against pathogens. Being produced in secondary lymphoid organs, IgM must rapidly be exported to the blood circulation. However, it is currently unknown how such large pentameric molecules are released from lymph nodes (LNs). Here, we show that upon immunization, IgM transiently gains access to the luminal side of the conduit system, a reticular infrastructure enabling fast delivery of tissue-derived soluble substances to the LN parenchyma. Using microinjections of purified IgM, we demonstrate that conduit-associated IgM is delivered by neither the afferent lymph nor the blood, but is locally conveyed by conduits. Exploiting in vivo models, we further demonstrate that conduit-associated IgM is locally and transiently produced by activated, antigen-specific B cells migrating in the T cell zone. Thus, our study reveals that the conduit system is coopted by B cells to rapidly export secreted IgM out of LNs.

Evaluation of a novel type of imaging probe based on a recombinant bivalent mini-antibody construct for detection of CD44v6-expressing squamous cell carcinoma

We have developed the CD44v6-targeting human bivalent antibody fragment AbD19384, an engineered recombinant human bivalent Fab antibody formed via dimerization of dHLX (synthetic double helix loop helix motif) domains, for potential use in antibody-based molecular imaging of squamous cell carcinoma in the head and neck region. This is a unique construct that has, to the best of our knowledge, never been assessed for molecular imaging in vivo before. The objective of the present study was to evaluate for the first time the in vitro and in vivo binding properties of radio-iodinated AbD19384, and to assess its utility as a targeting agent for molecular imaging of CD44v6-expressing tumors. Antigen specificity and binding properties were assessed in vitro. In vivo specificity and biodistribution of 125I-AbD19384 were next evaluated in tumor-bearing mice using a dual-tumor setup. Finally, AbD19384 was labeled with 124I, and its imaging properties were assessed by small animal PET/CT in tumor bearing mice, and compared with 2-deoxy-2-[18F]fluoro-D-glucose (18F-FDG). In vitro studies demonstrated CD44v6-specific binding with slow off-rate for AbD19384. A favorable biodistribution profile was seen in vivo, with tumor-specific uptake. Small animal PET/CT images of 124I-AbD19384 supported the results through clearly visible high CD44v6-expressing tumors and faintly visible low expressing tumors, with superior imaging properties compared to 18F-FDG. Tumor-to-blood ratios increased with time for the conjugate (assessed up to 72 h p.i.), although 48 h p.i. proved best for imaging. Biodistribution and small-animal PET studies demonstrated that the recombinant Fab-dHLX construct AbD19384 is a promising tracer for imaging of CD44v6 antigen expression in vivo, with the future aim to be used for individualized diagnosis and early detection of squamous cell carcinomas in the head and neck region. Furthermore, this proof-of-concept research established the feasibility of using recombinant Fab-dHLX constructs for in vivo imaging of tumor biomarkers.

Longitudinal analysis of the peripheral B cell repertoire reveals unique effects of immunization with a new influenza virus strain

Background

Despite the potential to produce antibodies that can neutralize different virus (heterotypic neutralization), there is no knowledge of why vaccination against influenza induces protection predominantly against the utilized viral strains (homotypic response). Identification of structural patterns of the B cell repertoire associated to heterotypic neutralization may contribute to identify relevant epitopes for a universal vaccine against influenza.

Methods

Blood samples were collected from volunteers immunized with 2008/2009 trivalent inactivated vaccine (TIV), pandemic H1N1 (pdmH1N1) monovalent inactivated vaccine (MIV) and the 2014/2015 TIV. Neutralization was assessed by hemagglutination and microneutralization test. IgG V_H amplicons derived from peripheral blood RNA from pre-immune and 7 days post vaccination were subjected to 454-Roche sequencing. Full reconstruction of the sampled repertoires was done with ImmunediveRsity.

Results

The TIV induced a predominantly homotypic neutralizing serologic response, while the 09 MIV induced a heterotypic neutralizing seroconversion in 17 % of the individuals. Both the 08/09 and the 14/15 TIV were associated with a reduction in clonotypic diversity, whereas 09 MIV was the opposite. Moreover, TIV and MIV induced distinctive patterns of IGHV segment use that are consistent with B cell selection by conserved antigenic determinants shared by the pre-pandemic and the pandemic strains. However, low somatic hypermutation rates in IgG after 09 MIV immunization, but not after 08/09 and 14/15 TIV immunization were observed. Furthermore, no evidence of the original antigenic sin was found in the same individuals after vaccination with the three vaccines.

Conclusions

Immunization with a new influenza virus strain (2009 pdmH1N1) induced unique effects in the peripheral B cell repertoire clonal structure, a stereotyped response involving distinctive IGHV segment use and low somatic hypermutation levels. These parameters were contrastingly different to those observed in response to pre-pandemic and post-pandemic vaccination, and may be the result of clonal selection of common antigenic determinants, as well as germinal center-independent responses that wane as the pandemic strain becomes seasonal. Our findings may contribute in the understanding of the structural and cellular basis required to develop a universal influenza vaccine.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-015-0239-y) contains supplementary material, which is available to authorized users.

Reduction of enterotoxin induced fluid accumulation in ileal loops of neonatal calves with anti-F5 fimbriae recombinant antibody

Neonatal calf colibacillosis caused by enterotoxigenic Escherichia coli (ETEC) is an economically significant problem in most parts of the world. The most common ETEC found in calves express the F5 (K99) fimbriae, which are necessary for the attachment of the bacteria to the ganglioside receptors on enterocytes. It is known that prevention of ETEC F5(+) adhesion to its ganglioside receptors with specific antibodies protects calves from colibacillosis. Previously we have described the development and characterization of a mouse recombinant antibody fragment (moRAb) that prevents F5 fimbrial protein induced agglutination of horse red blood cells (HRBC), which exhibit the same gangloside receptor for F5 fimbriae. Here we demonstrate that this recombinant antibody fragment inhibits in vitro the attachment of ETEC F5(+) bacteria to HRBC as well as isolated calf enterocytes, and in vivo it decreases fluid accumulation in intestinal loops of calves. Thus, correct oral administration of this anti-F5 moRAb may serve as an immunoprophylactic for cost effective control of colibacillosis in calves.

Single-chain fragment variable passive immunotherapies for neurodegenerative diseases

Accumulation of misfolded proteins has been implicated in a variety of neurodegenerative diseases including prion diseases, Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD). In the past decade, single-chain fragment variable (scFv) -based immunotherapies have been developed to target abnormal proteins or various forms of protein aggregates including Aβ, SNCA, Htt, and PrP proteins. The scFvs are produced by fusing the variable regions of the antibody heavy and light chains, creating a much smaller protein with unaltered specificity. Because of its small size and relative ease of production, scFvs are promising diagnostic and therapeutic reagents for protein misfolded diseases. Studies have demonstrated the efficacy and safety of scFvs in preventing amyloid protein aggregation in preclinical models. Herein, we discuss recent developments of these immunotherapeutics. We review efforts of our group and others using scFv in neurodegenerative disease models. We illustrate the advantages of scFvs, including engineering to enhance misfolded conformer specificity and subcellular targeting to optimize therapeutic action.

Viral replicative capacity is the primary determinant of lymphocytic choriomeningitis virus persistence and immunosuppression

The Clone 13 (Cl13) strain of lymphocytic choriomeningitis virus is widely studied as a model of chronic systemic viral infection. Here, we used reverse genetic techniques to identify the molecular basis of Cl13 persistence and immunosuppression, the characteristics differentiating it from the closely related Armstrong strain. We found that a single-point mutation in the Cl13 polymerase was necessary and partially sufficient for viral persistence and immunosuppression. A glycoprotein mutation known to enhance dendritic cell targeting accentuated both characteristics but when introduced alone, failed to alter the phenotype of the Armstrong strain. The decisive polymerase mutation increased intracellular viral RNA load in plasmacytoid dendritic cells, which we identified as a main initial target cell type in vivo, and increased viremia in the early phase of infection. These findings establish the enhanced replicative capacity as the primary determinant of the Cl13 phenotype. Viral persistence and immunosuppression can, thus, represent a direct consequence of excessive viral replication overwhelming the host's antiviral defense.

A single-chain fragment variable recombinant antibody against F5 fimbria of enterotoxigenic Escherichia coli inhibits agglutination of horse red blood cells induced by F5 protein

Bovine colibacillosis caused by enterotoxigenic Escherichia coli (ETEC) is a worldwide problem. Adhesion of ETEC to intestinal cell receptors mediated by the surface protein F5 fimbriae is the initial step in the establishment of colibacillosis. Prevention of ETEC F5(+) adhesion to enterocytes protects newborn calves against collibacillosis. On the enterocytes, the F5 fimbriae bind to a ganglioside that is also found on horse red blood cells. Thus, the presence of F5 fimbriae induces haemagglutination, which is useful as an indicator in a functional assay system. In this study, recombinant anti-F5 scFv antibody fragment produced in E. coli HB2151 reacted with F5 fimbriae in ELISA and Western immunoblot, and prevented haemagglutination induced by the binding of the F5 fimbriae to its natural host receptors on horse red blood cells. Given the ease with which recombinant antibodies can be mass-produced, the presently described scFv may hold promise as a prophylactic agent for colibacillosis.

Paracrine inhibition of prion propagation by anti-PrP single-chain Fv miniantibodies

Prion diseases are characterized by the deposition of PrP(Sc), an abnormal form of the cellular prion protein PrP(C). A growing body of evidence suggests that antibodies to PrP(C) can antagonize deposition of PrP(Sc). However, host tolerance hampers the induction of immune responses to PrP(C), and cross-linking of PrP(C) by bivalent anti-PrP antibodies is neurotoxic. In order to obviate these problems, we explored the antiprion potential of recombinant single-chain antibody (scFv) fragments. scFv fragments derived from monoclonal anti-PrP antibody 6H4, flagged with c-myc and His6 tags, were correctly processed and secreted by mammalian RD-4 rhabdomyosarcoma cells. When cocultured with cells secreting anti-PrP scFv, chronically prion-infected neuroblastoma cells ceased to produce PrP(Sc), even if antibody-producing cells were physically separated from target cells in transwell cultures. Expression of scFv with irrelevant specificity, or of similarly tagged molecules, was not curative. Therefore, eukaryotically expressed scFv exerts a paracrine antiprion activity. The effector functions encoded by immunoglobulin constant domains are unnecessary for this effect. Because of their small size and their monovalent binding, scFv fragments may represent candidates for gene transfer-based immunotherapy of prion diseases.

Immunological factors that affect the in vivo fate of T7 phage in the mouse

Phage display is a powerful method to study organ and tissue specific addresses. As part of our studies on the in vivo panning of tissue-homing peptide libraries, we examined the survival of T7 phage in the blood of C57BL/6J mice to estimate the half-life of T7 phage and the factors responsible for its inactivation. Amplified and purified T7 phage particles with or without random peptide library inserts were injected intravenously into the tail vein of wild-type (C57BL/6J) and immunocompromized (C57BL/6J) female mice. In wild-type mice, both the parent phage as well as phage carrying a peptide library were eliminated quickly from the blood, with only approximately 1% survival of detectable infectious phage after 60 min of injection. In SCID (C57BL/6J-Prkdc(scid)) mice, phage titers were stable over the same period of time with or without peptide library, suggesting a role for either B- or T cells or both in phage inactivation. The presumed role of B cell was indicated by demonstration of stable phage in the B-cell deficient mouse (C57BL/10-Igh-6(tm1Cgn)). In other immunocompromized mice, the phage titers were unstable, similar to that found in wild-type mice. In no case, was there a difference between phage with or without random peptide library. These data indicate that the presence of random C-X7-C peptides on the T7 phage coat protein does not affect the clearance of the phage in murine blood. Most likely, host immune factors play a role in the neutralization of T7 phage in blood by reacting with B-cell dependent immunoglobin.

Virus-neutralizing activity mediated by the Fab fragment of a hemagglutinin-specific antibody is sufficient for the resolution of influenza virus infection in SCID mice

Antibodies (Abs) contribute to the control of influenza virus infection in vivo by reducing progeny virus yield from infected cells (yield reduction [YR]) and by inhibiting progeny virus from spreading the infection to new host cells (virus neutralization [VN]). Previous studies showed that the infection could be resolved in severe combined immunodeficiency (SCID) mice by treatment with hemagglutinin (HA)-specific monoclonal antibodies (MAbs) that exhibit both VN and YR activities but not by MAbs that exhibited only YR activity. To determine whether virus clearance requires both activities, we measured the therapeutic activity of an HA-specific MAb (VN and YR) and its Fab fragment (VN) by intranasal (i.n.) administration to infected SCID mice. Immunoglobulin G (IgG) and Fab cleared the infection with i.n. 50% effective doses (ED(50)s) of 16 and 90 pmol, respectively. To resolve an established infection solely by VN activity, Fab must be present in the respiratory tract at an effective threshold concentration until all infected cells have died and production of virus has ceased. Because IgG and Fab had different half-lives in the respiratory tract (22 and 8 h, respectively) and assuming that both operated mainly or solely by VN, it could be estimated that clearance was achieved 24 h after Ab treatment when both reagents were present in the respiratory tract at approximately 10 pmol. This dose was approximately 200 times larger than the respiratory tract-associated Ab dose resulting from administration of the intraperitoneal ED(50) (270 pmol) of IgG. This indicated that our procedure of i.n. administration of Ab did not make optimal use of the Ab's therapeutic activity.

Recombinant single-chain variable fragment antibodies directed against Clostridium difficile toxin B produced by use of an optimized phage display system

Recombinant antibody cloning and phage display technologies were used to produce single-chain antibodies (scFv) against Clostridium difficile toxin B. The starting material was the mouse B cell hybridoma line 5A8, which generates a monoclonal antibody against the toxin. The integrated cloning, screening, and phage display system of Krebber et al. (J. Immunol. Methods 201:35-55, 1997) allowed us to rapidly obtain toxin B-binding scFv sequences derived from the hybridoma cell line. The best candidate scFv sequences, based on preliminary enzyme-linked immunosorbent assay (ELISA) screening data were then subcloned into the compatible expression vector. Recombinant single-chain antibodies were expressed in Escherichia coli. A 29-kDa band was observed on polyacrylamide gel electrophoresis as predicted. The expressed product was characterized by immunoblotting and detection with an anti-FLAG antibody. The toxin B-binding function of the single-chain antibody was shown by a sandwich ELISA. The antibody was highly specific for toxin B and did not cross-react with material isolated from a toxin B-negative C. difficile strain. The sensitivity of the soluble single-chain antibody is significantly higher than the original monoclonal antibody based on ELISA data and could detect a minimum of 10 ng of toxin B/well. Competitive ELISAs established that the affinity of the 5A8 parent antibody and the best representative (clone 10) of the single-chain antibodies were similar and in the range of 10(-8) M. We propose that recombinant antibody technology is a rapid and effective approach to the development of the next generation of immunodiagnostic reagents.

Minimal structural elements of an inhibitory anti-ATF1/CREB single-chain antibody fragment (scFv41.4)

Antibody variable domains represent potential structural models for the rational design of therapeutic molecules that bind cellular proteins with high affinity and specificity. The Activating Transcription Factor 1 (ATF1)/Cyclic AMP Response Element Binding Protein (CREB) family of transcription factors are particularly relevant targets due to their strong association with melanoma and clear cell sarcoma. Biochemical and structural investigations were performed to optimize a single-chain antibody fragment (scFv), scFv41.4, that disrupts the binding of ATF1/CREB to cyclic-AMP response elements (CRE) in vitro and inhibits transcriptional activation in cells. Molecular modeling and ligand docking simulations suggested that scFv41.4 could function as a disulfide-deficient single domain scFv. Functional studies verified that deletion of the light chain did not result in reduced inhibitory activity. The isolated heavy chain was predicted to assume a relaxed structural conformation that maintained a functional antigen binding pocket. The minimal structural elements necessary for intracellular function were further analyzed by selective deletion of CDR1 and CDR2. V(H)-CDR1 and V(H)-CDR3 were shown to play a key role in antigen binding activity, but V(H)-CDR2 was dispensable. Thus, scFv41.4 represents a unique molecule with potential for use in the design of peptidomimetic derivatives having therapeutic application to human cancer.

A divalent antibody format is required for neutralization of human cytomegalovirus via antigenic domain 2 on glycoprotein B

Glycoprotein B (gB) of human cytomegalovirus (HCMV) is the dominating protein in the envelope of this virus and gives rise to virus-neutralizing antibodies in most infected individuals. We have previously isolated a neutralizing human antibody specific for antigenic domain 2 (AD-2) on gB, a poorly immunogenic epitope, which nevertheless is capable of eliciting potent neutralizing antibodies. In order to define parameters important for the neutralization of HCMV via gB, we have investigated the virus-neutralizing capacity and the kinetics of the interaction with AD-2 of the monomeric and dimeric forms of a single chain variable fragment (scFv) corresponding to this antibody. We demonstrate here that neutralization of HCMV via AD-2 on gB can be mediated by dimeric scFv, while monomeric fragments cannot mediate neutralization of the virus, despite a slow dissociation from the intact glycoprotein. This finding is discussed in the context of possible mechanisms for antibody-mediated virus neutralization.

Genetic and structural determinants of virus neutralizing antibodies

Neutralizing antibodies (Abs) are the principal protective mechanism against disease caused by reinfection with viruses. Ab-mediated neutralization of viruses is a complex process comprising multiple mechanisms. Every structural aspect of Abs is potentially capable of modulating the level of neutralizing activity or the mechanisms of neutralization. The focus of our laboratory is to understand the genetic and structural basis of Ab-mediated neutralization of human viral pathogens. We demonstrated the unexpected finding that virus antigen-binding fragments of Abs (Fabs) mediate potent virus neutralizing effects in vivo. This work has led to a broad investigation of the importance of the genetics, chemistry, and structure of the combining site to the neutralizing activity of antiviral Abs. Ongoing work in our laboratory reveals that effect or functions specified by the Ab isotype such as polymer formation, interactions with complement, interactions with Fc receptors, and the ability to transcytose mucosal epithelia, also modulate the mechanism and level of neutralizing effects mediated by antiviral Abs.

Neutralisation and binding of VHS virus by monovalent antibody fragments

We have previously reported the cloning and characterisation of the heavy and light chain variable domain genes encoding three monoclonal antibodies (Mabs) that bind viral haemorrhagic septicaemia virus (VHSV). Two of these antibodies, 3F1H10 and 3F1A2 both neutralised the virus though 3F1A2 appeared to recognise a broader range of virus isolates. The variable domains of these two antibodies differ by only four residues (Lorenzen et al., 2000a. Fish Shellfish Immunol. 10, 129-142). To further study the mechanism of neutralisation, Fab fragments as well as a series of recombinant bacterial single chain antibody (scAb) fragments were generated from the three anti-VHSV Mabs and their variable domain genes, respectively. Fabs and scAbs derived from the neutralising Mabs were both able to neutralise the VHSV type 1 isolate DK-F1. In addition, a series of scAb fragments were produced using the 3F1H10 variable heavy (VH) chain and variable light (Vkappa) chain domains but containing, either alone or in dual combination, each of the four different residues present in 3F1A2. The dissociation constants of Mabs 3F1H10 and 3F1A2 and their respective Fab and scAb fragments were measured by BIAcore analysis and found to correlate with the capacity of each molecule to neutralise DK-F1. These investigations, together with computer assisted molecular analysis of the theoretical influence of each mutation on antigen binding, led to the identification of a single mutation at position 35a in the VH domain as having the most marked impact on viral neutralisation.

Neutralizing antiviral antibody responses

Neutralizing antibodies are evolutionarily important effectors of immunity against viruses. Their evaluation has revealed a number of basic insights into specificity, rules of reactivity (tolerance), and memory—namely, (1) Specificity of neutralizing antibodies is defined by their capacity to distinguish between virus serotypes; (2) B cell reactivity is determined by antigen structure, concentration, and time of availability in secondary lymphoid organs; and (3) B cell memory is provided by elevated protective antibody titers in serum that are depending on antigen stimulation. These perhaps slightly overstated rules are simple, correlate with in vivo evidence as well as clinical observations, and appear to largely demystify many speculations about antibodies and B cell physiology. The chapter also considers successful vaccines and compares them with those infectious diseases where efficient protective vaccines are lacking, it is striking to note that all successful vaccines induce high levels of neutralizing antibodies (nAbs) that are both necessary and sufficient to protect the host from disease. Successful vaccination against infectious diseases such as tuberculosis, leprosy, or HIV would require induction of additional long-lasting T cell responses to control infection.

Immunoprophylaxis in fish by injection of mouse antibody genes

Antibodies are a crucial part of the body's specific defense against infectious diseases and have considerable potential as therapeutic and prophylactic agents in humans and animals. The development of recombinant single-chain antibodies allows a genetic application strategy for prevention of infectious diseases. To test this in a fish model, a gene construct encoding a neutralizing single-chain antibody to the fish-pathogenic rhabdovirus VHSV (viral hemorrhagic septicemia virus) was administered to rainbow trout by intramuscular injection of plasmid DNA. Circulating recombinant antibodies could later be detected in the fish, and protective immunity to the viral disease was established.

Virus neutralization by germ-line vs. hypermutated antibodies

Mice infected with vesicular stomatitis virus (VSV), a cytopathic virus closely related to rabies virus, mount a virus-neutralizing antibody response protecting against lethal disease. VSVneutralizing monoclonal IgGs isolated from primary immune responses were devoid of somatic mutations, whereas most secondary and all hyperimmune response IgGs tested were hypermutated. A comparative analysis of recombinant single-chain antibody fragments (scFv-Ckappa) revealed that even the germ-line precursor of one hypermutated antibody bound and neutralized VSV. Four somatic amino acid substitutions in V(H) increased by 300-fold the binding strength of monovalent scFv-Ckappa. The multivalent binding avidity of germ-line scFv-Ckappa was increased by more than 10-fold compared with the monovalent binding strength. In contrast, hypermutated scFv-Ckappa did not show such avidity effects. Thus the overall binding difference between the germ-line and the hypermutated VSV-neutralizing antibody was only 10- to 15-fold. This may explain why primary germ-line antibodies and secondary hypermutated antibodies directed against pathogens such as viruses and bacteria expressing repetitive antibody determinants show rather similar binding qualities, whereas monovalently binding hapten-specific antibodies can show "affinity maturation" effects of up to 1000-fold.

Analysis of competition binding between soluble and membrane-bound ligands for cell surface receptors

Binding of the Fc portion of IgG coated on targets to Fcgamma receptors (e.g., CD16) expressed on leukocytes (i.e., 2D binding) is an initiating step for immune responses such as phagocytosis or antibody-dependent cellular cytotoxicity. In vivo, circulating leukocytes are exposed to plasma IgG. The competition from soluble IgG (i.e., 3D binding) may affect the 2D binding. Many cell surface receptors, CD16 included, have soluble counterparts. While their physiological significance is not clear, receptor-based competitive inhibition therapy, in which soluble receptors, ligands, and their analogs are employed to compete with surface-bound receptors and ligands to prevent unwanted adhesion, is widely used to treat various diseases. To provide a quantitative basis for design of these therapeutic approaches, we developed a mathematical model for 2D and 3D competition binding. The model relates cell-surface adhesion (in the presence and absence of dislodging forces) to the concentration of the soluble competitor, the densities of the surface-bound receptors and ligands, as well as the binding affinities of the 2D and 3D interactions. Binding of CD16-expressing cells to an IgG-coated surface in the presence of a soluble competitor (IgG or anti-CD16 antibody) was quantified by a centrifugation assay. The agreement between experiment and theory supports the validity of the model, which could be useful in predicting the efficacy of the competitor.

Secondary rearrangements and hypermutation generate sufficient B cell diversity to mount protective antiviral immunoglobulin responses

Variable (V) region gene replacement was recently implicated in B cell repertoire diversification, but the contribution of this mechanism to antibody responses is still unknown. To investigate the role of V gene replacements in the generation of antigen-specific antibodies, we analyzed antiviral immunoglobulin responses of "quasimonoclonal" (QM) mice. The B cells of QM mice are genetically committed to exclusively express the anti-(4-hydroxy-3-nitrophenyl) acetyl specificity. However, approximately 20% of the peripheral B cells of QM mice undergo secondary rearrangements and thereby potentially acquire new specificities. QM mice infected with vesicular stomatitis virus (VSV), lymphocytic choriomeningitis virus, or poliovirus mounted virus-specific neutralizing antibody responses. In general, kinetics of the antiviral immunoglobulin responses were delayed in QM mice; however, titers similar to control animals were eventually produced that were sufficient to protect against VSV-induced lethal disease. VSV neutralizing single-chain Fv fragments isolated from phage display libraries constructed from QM mice showed VH gene replacements and extensive hypermutation. Thus, our data demonstrate that secondary rearrangements and hypermutation can generate sufficient B cell diversity in QM mice to mount protective antiviral antibody responses, suggesting that these mechanisms might also contribute to the diversification of the B cell repertoire of normal mice.

IgD can largely substitute for loss of IgM function in B cells

The mu and delta heavy chains of IgM and IgD, the first antibody isotypes expressed during bone-marrow B-cell development, are encoded by a common transcription unit. Expression of the mu chain on the surface of late pre-B cells allows their further development to immature B cells. Coexpression of the delta chain and emigration of the immature B cells to the periphery eventually leads to the development of naive mature IgM/IgD double-positive cells. Although IgM is important in driving B-cell development, the contribution of IgD is not clear. Here we investigate the function of IgD. We generated mice deficient in IgM (IgM-/- mice) by deleting the mu region in embryonic stem cells. IgM-/- mice showed normal B-cell development and maturation, with IgD replacing membrane-bound and secretory IgM. Moreover, specific B-cell responses and isotype class switches occurred during immunization or infection. In contrast to mice deficient in B cells, IgM-/- mice survived infection with vesicular stomatitis virus by developing neutralizing immunoglobulins, but they were more susceptible than wild-type controls with delayed specific immunoglobulin responses. These data lead us to conclude that IgD is largely able to substitute for IgM functions.

A recombinant single chain antibody neutralizes coronavirus infectivity but only slightly delays lethal infection of mice

The variable region genes of a murine anti-coronavirus monoclonal antibody (mAb) were joined by assembly polymerase chain reaction and expressed in Escherichia coli in a single chain variable fragment (scFv) configuration. After induction of expression, the expected 32-kDa protein was identified by Western immunoblotting with specific rabbit anti-idiotype antibodies. The scFv fragments were purified from soluble cytoplasmic preparations by affinity chromatography on nickel agarose, which was possible with an N-terminal but not with a C-terminal histidine tag. Purified scFv fragments retained the antigen-binding properties of the parental antibody, could inhibit its binding to viral antigens with apparently higher efficiency than monovalent antigen-binding (Fab) fragments, but neutralized viral infectivity with lower efficiency (about sevenfold at a molar level). To evaluate the usefulness of these smaller and less immunogenic molecules in the treatment of viral diseases, mice were treated with purified recombinant scFv fragments and challenged with a lethal viral dose. A small delay in mortality was observed for the scFv-treated animals. Therefore, even though the scFv could neutralize viral infectivity in vitro, the same quantity of fragments that partially protected mice in the form of Fab only slightly delayed virus-induced lethality when injected as scFv fragments, probably because of a much faster in vivo clearance: the biologic half-life was estimated to be about 6 min. Since a scFv derived from a highly neutralizing and protective mAb is only marginally effective in the passive protection of mice from lethal viral infection, the use of such reagents for viral immunotherapy will require strategies to overcome stability limitations.

Phage-displayed and soluble mouse scFv fragments neutralize rabies virus

A phage-display technology was used to produce a single-chain Fv antibody fragment (scFv) from the 30AA5 hybridoma secreting anti-glycoprotein monoclonal antibody (MAb) that neutralizes rabies virus. ScFv was constructed and then cloned for expression as a protein fusion with the g3p minor coat protein of filamentous phage. The display of antibody fragment on the phage surface allows its selection by affinity using an enzyme-linked immunosorbent assay (ELISA); the selected scFv fragment was produced in a soluble form secreted by E. coli. The DNA fragment was sequenced to define the germline gene family and the amino-acid subgroups of the heavy (VH) and light (VL) chain variable regions. The specificity characteristics and neutralization capacity of phage-displayed and soluble scFv fragments were found to be identical to those of the parental 30AA5 MAb directed against antigenic site II of rabies glycoprotein. Phage-display technology allows the production of new antibody molecule forms able to neutralize the rabies virus specifically. The next step could be to engineer and produce multivalent and multispecific neutralizing antibody fragments. A cocktail of multispecific neutralizing antibodies could contain monovalent, bivalent or tetravalent scFv fragments, for passive immunoglobulin therapy.

The role of antibody concentration and avidity in antiviral protection

Neutralizing antibodies are necessary and sufficient for protection against infection with vesicular stomatitis virus (VSV). The in vitro neutralization capacities and in vivo protective capacities of a panel of immunoglobulin G monoclonal antibodies to the glycoprotein of VSV were evaluated. In vitro, neutralizing activity correlated with avidity and with neutralization rate constant, a measure of on-rate. However, in vivo, protection was independent of immunoglobulin subclass, avidity, neutralization rate constant, and in vitro neutralizing activity; above a minimal avidity threshold, protection depended simply on a minimum serum concentration. These two biologically defined thresholds of antibody specificity offer hope for the development of adoptive therapy with neutralizing antibodies.

Reliable cloning of functional antibody variable domains from hybridomas and spleen cell repertoires employing a reengineered phage display system

A prerequisite for the use of recombinant antibody technologies starting from hybridomas or immune repertoires is the reliable cloning of functional immunoglobulin genes. For this purpose, a standard phage display system was optimized for robustness, vector stability, tight control of scFv-delta geneIII expression, primer usage for PCR amplification of variable region genes, scFv assembly strategy and subsequent directional cloning using a single rare cutting restriction enzyme. This integrated cloning, screening and selection system allowed us to rapidly obtain antigen binding scFvs derived from spleen-cell repertoires of mice immunized with ampicillin as well as from all hybridoma cell lines tested to date. As representative examples, cloning of monoclonal antibodies against a his tag, leucine zippers, the tumor marker EGP-2 and the insecticide DDT is presented. Several hybridomas whose genes could not be cloned in previous experimental setups, but were successfully obtained with the present system, expressed high amounts of aberrant heavy and light chain mRNAs, which were amplified by PCR and greatly exceeded the amount of binding antibody sequences. These contaminating variable region genes were successfully eliminated by employing the optimized phage display system, thus avoiding time consuming sequencing of non-binding scFv genes. To maximize soluble expression of functional scFvs subsequent to cloning, a compatible vector series to simplify modification, detection, multimerization and rapid purification of recombinant antibody fragments was constructed.