Establishment of stable mouse/human-human hybrid cell lines producing large amounts of anti-tetanus human monoclonal antibodies with high neutralizing activity

Epitope mapping of neutralising anti-SARS-CoV-2 monoclonal antibodies: Implications for immunotherapy and vaccine design

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is the causative agent of the coronavirus disease 2019 (COVID-19) pandemic. This disease has currently affected more than 346 million people and resulted in more than 5.5 million deaths in many countries. Neutralising monoclonal antibodies (MAbs) against the SARS-CoV-2 virus could serve as prophylactic/therapeutic agents in COVID-19 infection by providing passive protection against the virus in individuals. Until now, no Food and Drug Administration/European Medicines Agency-approved neutralising MAb against SARS-CoV-2 virus exists in the market, though a number of MAbs have been authorised for emergency use. Therefore, there is an urgent need for development of efficient anti-SARS-CoV-2 neutralising MAbs for use in the clinic. Moreover, neutralising anti-SARS-CoV-2 MAbs could be used as beneficial tools for designing epitope-based vaccines against the virus. Given that the target epitope of a MAb is a crucial feature influencing its neutralising potency, target epitopes of neutralising anti-SARS-CoV-2 MAbs already reported in the literature and reactivity of these MAbs with SARS-CoV-2 variants are reviewed herein.

Neutralization of tetanus toxin by a novel chimeric monoclonal antibody

PURPOSE

Tetanus is a life-threatening disease characterized by muscle spasm caused by neurotoxin of Clostridium tetani. Given the current passive immunotherapy of tetanus with human anti-toxin polyclonal antibodies (PAbs) and the limitations of such preparations, neutralizing monoclonal antibodies (MAbs), especially chimeric or human antibodies with reduced immunogenicity might be considered as an alternative source.

METHODS

A mouse-human chimeric MAb, designated c-1F2C2, was generated and its binding specificities to various recombinant fragments of tetanus toxin, generated in E. coli, were determined. In vivo toxin neutralizing activity of c-1F2C2 was evaluated and compared with that of a commercially available human anti-toxin PAb in a mouse model. The possible mechanisms of toxin neutralizing activity of c-1F2C2 were investigated by assessing its inhibitory effects on toxin receptors binding, including GT1b ganglioside receptor and those expressed on PC12 cells.

RESULTS

In vivo neutralizing assay showed that c-1F2C2 was able to protect mice against tetanus toxin with an estimated potency of 7.7 IU/mg comparing with 1.9 IU/mg of the commercial human anti-toxin PAb for 10 MLD toxin and 10 IU/mg versus 1.9 IU/mg of the PAb for 2.5 MLD toxin. c-1F2C2 recognized fragment C of the toxin, which is responsible for binding of the toxin to its receptor on neuronal cells. Accordingly, the chimeric MAb partially prevented the toxin from binding to its receptors on PC12 cells (37% inhibition).

CONCLUSION

The chimeric MAb c-1F2C2 displayed similar structural and functional characteristics compared to its murine counterpart and might be useful for passive immunotherapy of tetanus.

Novel neutralizing human monoclonal antibodies against tetanus neurotoxin

Tetanus is a fatal disease caused by tetanus neurotoxin (TeNT). TeNT is composed of a light chain (Lc) and a heavy chain, the latter of which is classified into two domains, N-terminus Hn and C-terminus Hc. Several TeNT-neutralizing antibodies have been reported, but it remains unclear which TeNT domains are involved in neutralization. To further understand the mechanism of these antibodies, we isolated TeNT-reactive human antibody clones from peripheral blood mononuclear cells. We then analyzed the reactivity of the isolated antibody clones to each protein domain and their inhibition of Hc-ganglioside GT1b binding, which is critical for TeNT toxicity. We also investigated the TeNT-neutralizing ability of isolated antibody clones and showed that an Hn-reactive clone protected strongly against TeNT toxicity in mice. Furthermore, combination treatment of Hn-reactive antibody clones with both Hc-reactive and TeNT mix (the mixture of Hc, Hn, and Lc proteins)–reactive antibody clones enhanced the neutralizing effect. These results indicated that antibody clones targeting Hn effectively neutralized TeNT. In addition, the use of a cocktail composed of Hc-, Hn-, and TeNT mix–reactive antibodies provided enhanced protection compared to the use of each antibody alone.

Structural basis of tetanus toxin neutralization by native human monoclonal antibodies

Four potent native human monoclonal antibodies (mAbs) targeting distinct epitopes on tetanus toxin (TeNT) are isolated with neutralization potency ranging from approximately 17 mg to 6 mg each that are equivalent to 250 IU of human anti-TeNT immunoglobulin. TT0170 binds fragment B, and TT0069 and TT0155 bind fragment AB. mAb TT0067 binds fragment C and blocks the binding of TeNT to gangliosides. The co-crystal structure of TT0067 with fragment C of TeNT at a 2.0-Å resolution demonstrates that mAb TT0067 directly occupies the W pocket of one of the receptor binding sites on TeNT, resulting in blocking the binding of TeNT to ganglioside on the surface of host cells. This study reveals at the atomic level the mechanism of action by the TeNT neutralizing antibody. The key neutralization epitope on the fragment C of TeNT identified in our work provides the critical information for the development of fragment C of TeNT as a better and safer tetanus vaccine.

Contribution of Fc fragment of monoclonal antibodies to tetanus toxin neutralization

BACKGROUND

Monoclonal antibodies (MAbs) against neurotoxin of Clostridium tetani are considered as a novel source of immunoglobulins for passive immunotherapy of tetanus. Toxin neutralization is classically attributed to the Fab and F(ab')2 fragments of antibodies. Herein, we generated Fab and F(ab')2 fragments of three toxin neutralizing mouse MAbs and compared their neutralizing activities to those of their intact molecules.

METHODS

Fab and F (ab')2 fragments of the antibodies were generated by papain and pepsin digestions, respectively, and their toxin neutralizing activities were compared with those of the intact antibodies in an in vivo toxin neutralization assay.

RESULTS

While low doses of the intact MAbs were able to fully protect the mice against tetanus toxin, none of the mice which received Fab or F(ab')2 fragments survived until day 14, even at the highest administered dose. All mice receiving human polyclonal anti-tetanus immunoglobulin or their fragments were fully protected.

CONCLUSION

Reduction in toxin neutralization activities of Fab and F(ab')2 fragments of our MAbs seems to be influenced by their Fc regions. Steric hindrance of the Fc region on the receptor-binding site of the toxin may explain the stronger neutralization of the toxin by the intact MAbs in comparison to their fragments.

Epitope Mapping of Tetanus Toxin by Monoclonal Antibodies: Implication for Immunotherapy and Vaccine Design

Tetanus as a life-threatening disease is characterized by muscle spasm. The disease is caused by the neurotoxin of Clostridium tetani. Active form of tetanus neurotoxin is composed of the light chain (fragment A) and the heavy chain. Fragment A is a zinc metalloprotease, which cleaves the neuronal soluble N-ethylmaleimide-sensitive attachment receptor (SNARE) protein, leading to the blockade of inhibitory neurotransmitter release and subsequent generalized muscular spasm. Two functional domains of the heavy chain are fragment C, which is required for neuronal cell binding of the toxin and subsequent endocytosis into the vesicles, and fragment B, which is important for fragment A translocation across the vesicular membrane into the neuronal cytosol. Currently, polyclonal immunoglobulins against tetanus neurotoxin obtained from human plasma of hyper-immunized donors are utilized for passive immunotherapy of tetanus; however, these preparations have many disadvantages including high lot-to-lot heterogeneity, possibility of transmitting microbial agents, and the adverse reactions to the other proteins in the plasma. Neutralizing anti-tetanus neurotoxin monoclonal antibodies (MAbs) lack these drawbacks and could be considered as a suitable alternative for passive immunotherapy of tetanus. In this review, we provide an overview of the literature discussing epitope mapping of the published neutralizing MAbs against tetanus toxin.

Comparative in vitro and in vivo assessment of toxin neutralization by anti-tetanus toxin monoclonal antibodies

Tetanus is caused by the tetanus neurotoxin (TeNT), a 150 kDa single polypeptide molecule which is cleaved into an active two-chain molecule composed of a 50 kDa N-terminal light (L) and a 100 kDa C-terminal heavy (H) chains. Recently, extensive effort has focused on characterization of TeNT binding receptors and toxin neutralization by monoclonal antibodies (mAbs). Toxin binding inhibition and neutralization is routinely assessed either in vitro by the ganglioside GT1b binding inhibition assay or in vivo using an animal model. These two assay systems have never been compared. In the present study, we report characterization of eleven mAbs against different parts of TeNT. The toxin inhibitory and neutralization activity of the mAbs was assessed in vitro and in vivo respectively. Our data demonstrated that seven mAbs bind to fragment C of the heavy chain, two mAbs react with the light chain, one mAb recognizes both chains and one mAb reacts with neither light chain nor fragment C. Six fragment C specific mAbs were able to inhibit TeNT binding to GT1b ganglioside in vitro but three failed to neutralize the toxin in vivo. One in vitro inhibitory mAb (1F3E3) was found to synergize with the in vivo neutralizing mAbs to reduce toxin lethal activity in vivo. Sequencing of the immunoglobulin heavy and light chain variable region genes revealed that the three in vivo neutralizing mAbs were derived from a common origin. Altogether, our data suggests that fragment C specific mAbs contribute to toxin neutralization in both systems, though some of the GT1b binding inhibitory mAbs may not be able to neutralize TeNT in vivo.

Dendritic cells that endocytosed antigen-containing IgG-liposomes elicit effective antitumor immunity

Liposomes represent a promising vehicle to deliver exogenous antigens to dendritic cells (DCs) for tumor immunotherapy. Targeting exogenous antigens to Fcgamma receptors on DCs has been shown to result in efficient presentation of antigen-derived peptides on major histocompatibility complex (MHC) class I and class II molecules. In this study, it was investigated whether DCs that endocytosed physicochemically optimized antigen-containing liposomes conjugated with IgG efficiently present antigens on MHC class I and class II molecules, and consequently induce strong antitumor immune responses. IgG-conjugated liposomes that were 200 nm in diameter without attaching polyethylene glycol were most efficiently endocytosed by DCs. Human monocyte-derived DCs that endocytosed tetanus toxoid (TT)-containing IgG liposomes via CD32 stimulated CD4(+) T cells more strongly than DCs pulsed with TT-containing bare liposomes or with soluble TT. Immunization of mice with DCs that endocytosed ovalbumin (OVA)-containing IgG liposomes but not OVA-containing bare liposomes or soluble OVA completely prevented the growth of OVA-expressing lymphoma cells. Importantly, administration of DCs that endocytosed OVA-containing IgG liposomes to the mice with established OVA-expressing tumors strongly suppressed tumor growth. This study demonstrates an IgG liposome with physicochemical properties suitable for delivering antigens to DCs and paves the way to the application of IgG liposomes for tumor immunotherapy using DCs.

Production and purification of ovine anti-tetanus antibody

We used the ovine as bioreactor for the production and optimization of anti-tetanus toxin antibody. Four female sheep were immunized with human tetanus vaccine (TT-alum) every two weeks for 16 weeks, after which serum was collected and its titer was estimated by ELISA. The highest titer obtained was 39,000 IU ml-1. To optimize a purification protocol for ovine anti-tetanus toxin, we used four procedures; weak anion (DEAE-Sephadex), weak cation (CM-Sephadex), ammonium sulfate precipitation alone or in combination with caprylic acid. Fifty percent saturation with ammonium sulfate combined with caprylic acid gave us the highest yield of protein with specific activity and the purest Fab product.

Production of neutralizing human monoclonal antibody directed to tetanus toxin in CHO cell

By the fusion of lymphocytes from hyperimmunized people with heteromyeloma cells, 600 human hybridoma cell lines were generated. Even though seven cell lines produced antibodies against tetanus toxoid, only two antibodies from hybrid CH8 and CH5 only neutralized the tetanus toxin and completely protected the mice that had been challenged with the toxin even at the level of 90 mean lethal dose. The cDNA of light (L) chain and heavy (H) chain variable region was isolated, and then inserted into expression vectors containing human IgG constant regions. After transfection of the recombinant human IgG gene into Chinese Hamster Ovary (CHO) cells, transformants secreting the complete human antibody were selected. The recombinant human antibodies produced from CHO cells possessed neutralizing activity against tetanus toxin just like the original human antibodies produced from human hybridoma cell lines. Western blot analysis showed that rCH8 and rCH5 antibodies recognized the H chain of tetanus toxin and did not bind to its L chain. The neutralizing test showed that HmAb rCH5 had 4.55IU and HmAb rCH8 had 1.09IU/100 micro g of IgG, respectively. Mixing of the two HmAbs resulted in synergistic effects. On a weight basis (IU/100 micro g IgG), the highest potency values were obtained when the two HmAbs were combined in equal quantity. The neutralizing activity of rCH8 and rCH5 mixture was 6.94IU/100 micro g IgG.

Cell hybridization, hybridomas, and human hybridomas

Cell hybridization is one of the most basic cytotechnologies. The hemagglutinating virus of Japan was first used to cause cell fusion; however, polyethylene glycol is widely used now because of simplicity of procedure. This chapter first explains the principles of cell hybridization methods and then describes the practical protocols for preparing mouse hybridomas using polyethylene glycol. So far, lack of an excellent human fusion partner cell line that has high fusion efficiencies and does not produce immunoglobulin has hindered the spread of human-human hybridoma preparation methods. In the authors' laboratory NAT-30 and HO-323, human parent cell lines with high fusion efficiencies, have been established to prepare many hybridoma cell lines producing cancer-specific human monoclonal antibodies. Because NAT-30 and HO-323 cell lines are IgM producers, it is difficult to obtain IgG-producing hybridomas because the types of immunoglobulin produced by hybridomas are strongly affected by the characteristics of parent cells. Thus a nonimmunoglobulin-producing human parent cell line, A4H12, derived from human T lymphoma was established that can efficiently obtain IgG-producing human hybridomas. Another problem with preparing human hybridomas is that it is difficult to obtain B lymphocytes immunized with optional antigens for ethical reasons. To overcome this problem, in vitro immunization methods have been developed that allow exposure of a large number of B lymphocytes to cultured cancer cell or soluble antigens. The section on human hybridomas explains human fusion partners, in vitro immunization methods, and the preparation of human-human hybridomas using an electrofusion method. Finally, the application of human monoclonal antibodies to medical uses and the preparation of supranatural monoclonal antibodies are reviewed. These include multifunctional monoclonal antibodies and altered monoclonal antibodies having increased affinity and specificity by exchanging or modifying light chains.

Successful in vitro antigen-dependent activation of 24-hour-old peripheral blood lymphocytes

We describe a simple, rapid and reproducible in vitro culture system in which human peripheral blood lymphocytes (PBLs), donated 24 h prior to initiation of culture can be stimulated to produce antigen-specific antibodies. Peripheral blood lymphocytes purified by Ficoll-Hypaque centrifugation were passed over a G10 Sephadex column and then activated in vitro in the presence of 0.003% staphylococcus Cowan A, 2.8 x 10(-6) M indomethacin and appropriate concentrations of tetanus toxoid antigen. After the first 24 h in culture, a five-fold concentrated supernatant from an allogeneic mixed lymphocyte culture was added. The cell surface phenotypes of the PBLs were analyzed by flow cytometry at the initiation and termination of culture, in order to provide a comprehensive characterization of the cellular composition of a successful in vitro stimulation system. Our results clearly show that the majority of peripheral blood B cells can be induced to an activated stage (blast transformation) and interleukin 2 (IL-2) receptor expression, following very simple manipulations of the lymphoid population. Tetanus toxoid-specific antibody production can be readily generated in this cell population. In contrast, T cells were not activated to express IL-2 receptors and reach blast transformation, and did not show appreciable proliferation. Our system provides a population of B cells producing antibodies of desired specificity which could be utilized for the generation of human hybridomas or could serve as a donor population for antibody engineering via the combinatorial library approach. Careful light scattering and cell surface phenotypic analyses of the cells entering, proliferating and differentiating in these cultures enabled several novel observations to be made.

Polyclonal preparations of anti-tetanus toxoid antibodies derived from a combinatorial library confer protection

We have compared the in vivo therapeutic potential of anti-tetanus toxin (TT) human Fab antibodies derived from a combinatorial phage display library to established polyclonal and monoclonal reagents. The oligoclonality and fine specificity distribution of the synthetic anti-TT Fab preparations was comparable to the antibody spectrum present in the donor serum and the affinities determined for the synthetic phage-bound Fab (Phab) and soluble Fab were in the same range as their monoclonal and polyclonal counterparts. On a weight basis, the protective capacity of the new oligoclonal preparations in vivo (16.4 IU/100 micrograms Fab) was comparable to those of the best combinations of hybridoma derived human monoclonal antibodies, and far better than those exhibited by the polyclonal serum antibodies of the donor (0.29 IU/100 micrograms IgG) or by a standard commercial human tetanus immunoglobulin preparation. These data suggest that recombinant antibodies may become a safe and effective alternative to human plasma-derived immunoglobulins for passive immunization.

Similar binding properties for a neutralizing anti-tetanus toxoid human monoclonal antibody and its bacterially expressed Fab

A high-affinity anti-tenanus toxoid (TT) human monoclonal antibody showing neutralizing activity was isolated from a fusion between mouse myeloma and human splenic cells. Fab fragments from this antibody were obtained using a recombinant phage surface-display expression system. The parental antibody and the corresponding Fab had identical immunological activities, including specificity and affinity. These results confirm the feasibility of developing Escherichia coli expression of monoclonal human Fab from hybridoma cells.

Affinities of endotoxin-specific human monoclonal antibodies, their polyclonal counterparts and murine monoclonal antibodies

Affinity as a measurement of the strength of binding is a crucial factor in biological significance. In general, high-affinity antibodies are most effective in mediating immunological effector mechanisms. Here, we compare the affinity distributions of corresponding polyclonal and monoclonal human antibodies specific for lipopolysaccharide determinants of the nosocomial pathogen Pseudomonas aeruginosa. The affinities of the 14 human mAb analysed ranged from 8.3 x 10(5) to 7.5 x 10(8). The average affinities of their polyclonal counterparts, assessed by analysing chromatographically separated antibody populations, ranged from 1.7 x 10(6) to 6.3 x 10(7). Furthermore, the affinities of murine mAb of the same specificity ranged from 3.7 x 10(5) to 1.4 x 10(7). These results suggest that the generated human monoclonal anti-carbohydrate antibodies exhibit affinities comparable to or higher than those of their human polyclonal counterparts and those of murine mAb of the same specificity.

Enhanced production of human monoclonal antibodies by the use of fructose in serum-free hybridoma culture media

It was found that the production of human monoclonal antibodies (MoAbs) by human-human hybridomas can be significantly enhanced by replacing glucose with fructose in the dish culture medium. Optimization of initial concentrations of fructose and glutamine, another influencing factor for MoAb production, enabled an enhanced production of human MoAb 2.1 times higher than that obtained using the conventional culture media employing glucose. It was shown by kinetic analysis that enhanced MoAb production at the optimum fructose concentration can be attributed to the retention of high specific antibody production rates and diminished time lag during the course of culture. These dish culture results with fructose-containing medium were successfully applied to the continuous perfusion culture with a slight modification, where 2.9- and 1.9-fold enhancements in specific antibody production rate and MoAb concentration, respectively, were attained as compared with the conventional glucose-containing medium. An inverse relationship was observed between the secreted concentrations of lactic acid and MoAb when the hybridoma was cultured in the media containing varying concentrations of fructose, i.e., the lower the lactic acid concentration, the higher the MoAb production and vice versa, suggesting that fructose at appropriate concentrations in the medium can serve as an alternative sugar for the efficient production of human MoAbs, with reduced pH shifts, for the serum-free culture of human-human hybridomas.

Scale-up and optimization of culture conditions of a human heterohybridoma producing serotype-specific antibodies to Pseudomonas aeruginosa

Three different stirred bioreactors of 0.5 to 121 volume were used to scale up the production of a human monoclonal antibody. Inoculation density and stirrer speed were evaluated in batch cultures, whereas dilution rate and pH were optimized in chemostat cultures with respect to high specific antibody production rate and high antibody yield per time and reactor volume. The cell line used for the experiments was a heterohybridoma, producing immunoglobulin M (IgM) against lipopolysaccharide of Pseudomonas aeruginosa. Cells were cultured in spinner flasks of 500 ml liquid volume for adaptation to stirred culture conditions. Subsequently cells were transferred to the 1.5-l KLF 2000 bioreactor and to the 12-l NLF 22 bioreactor for pilot-scale cultures. Chemostat experiments were done in the 1.5-l KLF bioreactor. Cell density, viability, glucose and lactate and antibody concentration were measured during culture experiments. In batch cultures in all three stirred bioreactors, comparable maximal cell densities and specific growth rates were achieved. Chemostat experiments showed that at a pH of 6.9 and a dilution rate of 0.57 per day the specific antibody production rate was threefold higher than similar experiments done at pH 7.2 with a dilution rate of 0.36 per day. By optimizing pH and dilution rate in chemostat cultures the daily yield of human IgM increased nearly threefold from 6 to 16 mg/day and per litre of reactor volume. The yield per litre of medium increased twofold.

Characteristics of toxin-neutralization by anti-tetanus human monoclonal antibodies directed against the three functional domains [A], [B] and [C] of the tetanus toxin molecule and a reliable method for evaluating the protective effects of monoclonal antibodies

Five anti-tetanus human monoclonal antibodies (MAbs) produced by hybrid cell lines we established previously were characterized. Their abilities to neutralize tetanus toxin in vitro and to protect mice against challenge with toxin were studied by observing the changes in the progress of symptoms in mice. Immunostaining showed that MAbs MAb-G4 and G2 recognized the N-terminal domain, [A] and the C-terminal domain, [C] of the tetanus toxin molecule, respectively, while MAbs MAb-G1, G3 and G6 recognized its middle domain, [B]. Enzyme-linked immunosorbent assay showed that the binding affinity of MAb-G3 was 2.9 x 10(10) M-1 and those of the other MAbs were as high as approximately 10(11) M-1. In in vitro neutralization experiments, at sufficient doses all the MAbs as single reagents protected mice completely against the effect of tetanus toxin. However, at lower doses than those sufficient to rescue mice, the kinetic patterns of progress of symptoms with the individual MAbs differed with each other and, except for MAb-G4, were different from that of anti-tetanus human polyclonal antibody. They suppressed the development and/or slowed the rate of progress of symptoms for over 96 h and delayed death of the mice. We propose that the comparison of the minimum survival dose with that of human polyclonal antibody of known international units is a reliable method for estimating the actual protective activity of a MAb. Intravenous (IV) injection of doses of individual MAbs or their mixtures at over 0.03 IU per mouse protected mice from subsequent challenge with 20 MLD of tetanus toxin.(ABSTRACT TRUNCATED AT 250 WORDS)