Human mAbs produced by primary in-vitro immunization

Three-dimensional cell culture approach for in vitro immunization and the production of monoclonal antibodies

The generation of monoclonal antibodies using an in vitro immunization approach is a promising alternative to conventional hybridoma technology. As recently published, the in vitro approach enables an antigen-specific activation of B lymphocytes within 10-12 days followed by immortalization and subsequent selection of hybridomas. This in vitro process can be further improved by using a three-dimensional surrounding to stabilize the complex microenvironment required for a successful immune reaction. In this study, the suitability of Geltrex as a material for the generation of monoclonal antigen-specific antibodies by in vitro immunization was analyzed. We could show that dendritic cells, B cells, and T cells were able to travel through and interact inside of the matrix, leading to the antigen-specific activation of T and B cells. For cell recovery and subsequent hybridoma technique the suitability of dispase and Corning cell recovery solution was compared. In our experiments, the use of dispase resulted in a severe alteration of cell surface receptor expression patterns and significantly higher cell death, while we could not detect an adverse effect of Corning cell recovery solution. Finally, an easy approach for high-density cell culture was established by printing an alginate ring inside a cell culture vessel. The ring was filled with Geltrex, cells, and medium to ensure a sufficient supply during cultivation. Using this approach, we were able to generate monoclonal hybridomas that produce antigen-specific antibodies against ovalbumin and the SARS-CoV-2 nucleocapsid protein.

Production of human antibodies by in vitro immunization using a fusion protein containing the transcriptional transactivator of HIV-1

Antigen-specific activation of human B cells represents a key step for the production of monoclonal antibodies. Several approaches have been developed over the last thirty years in order to improve the process of lymphocyte activation in vitro. In the present study, we investigated whether the transcriptional transactivator (Tat) of human immunodeficiency virus, which possesses numerous biological activities, is able to trigger antibody secretion when incubated with human peripheral blood mononuclear cells. No such effect was observed when using Tat as a free protein. However, we found a significant IgM antibody production when Tat was previously fused to a double domain, called ZZ, derived from protein A of Staphylococcus aureus. The effect was also observed when the fusion protein, called ZZTat101, was incubated with purified B cells, indicating that the phenomenon does not require T-cell help. Antibody secretion was observed in the absence of cytokines that are usually used during in vitro immunization experiments, indicating that ZZTat101 provides the signals required for the initiation of the immune response. Antibody secretion was observed using a ZZTat mutant, containing only the Tat residues 22 to 57, called ZZTat22-57, indicating that this region is sufficient to initiate the immune response. In contrast, the effect was not found with a ZZTat22-57 mutant devoid of the seven Tat cysteines located between residues 22 and 37, demonstrating that these residues play a crucial role in the phenomenon. Our results pave the way to the development of a new in vitro immunization method based on antigens associated with ZZTat.

Receptors and signaling mechanisms for B-lymphocyte activation, proliferation and differentiation--insights from both in vivo and in vitro approaches

During the last three decades, a number of B-lymphocyte specific surface antigens have been defined some of which may also show activation/differentiation specific expression. Here, we review the various signaling events and the receptor-ligand interactions for B-cell development, activation and differentiation. Our discussion and presentation include reviewing the in vivo and in vitro mechanisms. Focus is on the experiments that give us valuable insights into the B cell signaling mechanisms in vitro. Three significant pathways in B-cell development - c-Kit, FLT-3 and IL-7 signaling pathways are elucidated upon. Both antigen dependent and antigen independent mechanisms of B cell stimulation are also reviewed.

Human Trichuris-specific antibody responses in vaccinated hu-PBL-SCID mice

Trichuris trichiura is a highly prevalent intestinal helminth of humans with a well-characterized animal model, Trichuris muris in the mouse. Relating the murine work back to the human infection has been difficult, however, as many of the questions addressed in the mouse cannot be asked in humans. The ability to reconstitute a mouse with a human immune system could help bridge this gap, allowing a human immune response to be studied under a controllable laboratory environment. In this study, we demonstrate that severe combined immunodeficient mice engrafted with naïve human peripheral blood lymphocytes are capable of mounting a Trichuris specific human antibody response after vaccination with T. muris antigens. The phenotype of the response depended on the vaccinating antigen with excretory/secretory antigens eliciting a human immunoglobulin IgG2 response, and whole worm homogenate stimulating IgG1 and IgG2 responses. Vaccination with homogenate also enhanced a human IgG response against a 66-kDa component of T. muris homogenate in a donor-dependent manner. This work shows the potential of using the humanized mouse model for studying the immune responses of humans living in T. trichiura endemic areas.

The use of in vitro immunisation, as an adjunct to monoclonal antibody production, may result in the production of hybridomas secreting polyreactive antibodies

The possibility that immortalisation of in vitro immunised splenocytes may result in hybridomas secreting polyreactive antibodies was investigated. A panel of nine murine hybridomas, secreting IgM(kappa) anti-goat immunoglobulin G (anti-GIgG), was produced by immortalising splenocytes that had been immunised in vitro with GIgG. The ability of the corresponding monoclonal antibodies (Mabs) to bind multiple antigens was investigated using two techniques. First, the affinity constants characterising the interactions of each of the nine Mabs with each of a panel of six antigens were determined. Second, the specific anti-GIgG activities of each hybridoma supernatant and its corresponding affinity-purified IgM fraction were determined and compared. In total, these experiments indicated that eight of the nine hybridomas were polyreactive.

Human therapeutic antibodies

The development of genetic engineering technologies has today advanced to the point where the generation of high-affinity human antibodies against therapeutic targets is not a major hurdle. Rather, it is the selection of target molecules in, for example, cancer therapy that poses a challenge. Targets that are not merely passive acceptors but those that signal into the cell are preferred. Recent advances in the clinical use of antibody-based therapy--such as anti-CD20 (rituximab) for the treatment of non-Hodgkin's lymphoma and anti-tumour-necrosis-factor-alpha for Crohn's disease--as well as novel antibody designs and improved understanding of the mode of action of current antibodies lend great hope to the future of this therapeutic approach.

Induction of antigen-specific isotype switching by in vitro immunization of human naive B lymphocytes

The use of in vitro immunization technology for the generation of human antigen-specific antibodies has essentially resulted in low affinity IgM antibodies, resembling an in vivo primary immune response. We now describe a detailed reproducible protocol for a two-step in vitro immunization, which yields isotype switched, antigen-specific human antibodies. The immunizing antigen was a 30aa synthetic peptide, containing both a B (15aa V3 peptide of the HIV-1) and a T helper cell epitope (15aa peptide from tetanus toxin). The immunization protocol includes: (i) a selection procedure of donors with a memory T cell response against tetanus toxoid; (ii) immunization of mature naive peripheral B lymphocytes in two distinct phases, involving a primary and a secondary step. None of the donors which were examined after primary immunization showed at any time an IgG anti-V3 specific antibody response, while all the donors showed an IgM response. After the secondary immunization step, anti-V3 antibodies of both IgM and IgG isotypes were detected. The switch frequency event was high among the tested donors (5/8).

Potential therapeutic use of antibodies directed towards HuIFN-gamma

IFN-gamma is an important regulator of immune responses and inflammation. Studies in animal models of inflammation, autoimmunity, cancer, transplant rejection and delayed-type hypersensitivity have indicated that administration of antibodies against IFN-gamma can prevent the occurrence of diseases or alleviate disease manifestations. Therefore, it is speculated that such antibodies may have therapeutical efficacy in human diseases. Since animal-derived antibodies are immunogenic in patients several strategies are being developed in order to reduce or abolish this human anti-mouse antibody (HAMA) response. In our laboratory, we have constructed a single-chain variable fragment (scFv) derived from a mouse antibody with neutralizing potential for human IFN-gamma. A scFv consists of only variable domains tethered together by a flexible linker. The scFv was demonstrated to neutralize the antiviral activity of HuIFN-gamma in vitro and therefore might be considered as a candidate for human therapy.

A promising model of primary human immunization in human-scid mouse

The engraftment of human peripheral blood mononuclear cells (Hu-PBMC) from adult donors in scid mice has been published by MOSIER et al. in 1988. The possibility to obtain a secondary human immune response in human-scid mice has also been reported but attempts to induce a primary human immune response still remain difficult to achieve. In this work, an antigen (Canine albumin) or a hapten (DNP) was coupled with tetanus toxoid, an antigenic protein against which our human donors already had memory T cells through vaccination. In this way, hu-scid mice immunized with coupled DNP-tetanus toxoid (TT-DNP) or coupled Canine albumin-Tetanus toxoid (Calb-TT) mounted a specific human immune response anti-DNP or anti-Canine albumin (Calb) respectively. A secondary human immune response anti-tetanus toxoid was also detected in the sera of hu-scid mice immunized with product containing TT but not in the sera of those injected with PBS alone. The scid mice grafted with Hu-PBMC from a TT naive donor and challenged with Calb-TT or Calb alone failed to produce specific anti-Calb antibodies. These observations demonstrate that memory T cells can give a substantial help to naive B cells which interact with them for obvious B cell activation and differentiation into plasma cells. This model of immunization might be useful for other antigens of choice, allowing the production of human monoclonal antibodies, in combination with a suitable system of immortalization. Attempts to immunize human cells in scid mice against DNP coupled to LO-BM2 (a rat monoclonal antibody anti-human IgM) failed to induce a specific human response either anti-rat immunoglobulins (Igs), or anti-DNP and led to a decrease of human Ig production in hu-scid. We also immunized hu-scid mice against ovalbumin alone but, only in some cases, a low specific human immune response was observed, so this system seems to be unreliable.

Optimizing production of human monoclonal IgG antibodies by in vitro-primed human PBMC: influence of CD56+ NK cell depletion

Freshly isolated human peripheral blood mononuclear cells (PBMC) were immunomagnetically depleted of CD56+ cells. When these CD56- PBMC populations were cultured in the presence of autologous donor serum, polyclonal activation with IL-2 and pokeweed mitogen (PWM) generally resulted in exclusive production of IgG antibodies. Fusion with SP2/O-Ag14 mouse myeloma cells was highly efficient and yielded a great number of IgG-producing heterohybridomas. These conditions were used for in vitro immunization with viable human HT29 tumor cells. After fusion, an increase in hybridoma clones producing IgG monoclonal antibodies (MAb) with HT29 specificity showing a higher portion of MAb binding to the surface of viable HT29 cells was recorded. This immunizing efficiency was not observed with HT29 membrane protein fractions or HT29 proteins integrated into ISCOM particles. Investigations with human anti-alpha Gal antibodies showed that the IgG antibodies produced by the human/mouse heterohybridomas did not contain the mouse-specific Gal alpha 1-3Gal epitope.

A rat monoclonal anti-(human CD2) and L-leucine methyl ester impacts on human/SCID mouse graft and B lymphoproliferative syndrome

The transfer of human peripheral blood mononuclear cells (hu-PBMC) from adult Epstein-Barr-virus(EBV)-seropositive donors in SCID (severe combined immunodeficiency) mice frequently leads to the development of a human B lymphoproliferative syndrome (hu-BLPS). Therefore, as 90% of adult potential donors are EBV-seropositive, efforts have to be made to avoid the occurrence of this B lymphoproliferative disorder. McCune et al. [Science 241:1632 (1988)] used human fetal organs for a human SCID graft. This system does not give rise to hu-BLPS but human fetal organs are much less available than peripheral blood leucocytes. The experiments reported in this paper show how crucial is the presence of functional T lymphocytes for a graft to take and for development of hu-BLPS in hu-PBMC-reconstituted SCID mice, since inhibition of T lymphocyte by a rat anti-(human CD2) monoclonal antibody (LO-CD2a) during the first 10 days of the graft prevents successful engraftment of human normal lymphocytes as well as hu-BLPS in SCID mice. The transfer of B cells alone or B cells plus monocytes in SCID mice does not permit either long-term engraftment or development of hu-BLPS. We also demonstrate that hu-PBMC treated with L-leucine methyl ester are less susceptible to the development of hu-BLPS after engraftment in SCID mice than are untreated hu-PBMC. The mechanism of action of L-leucine methyl ester on these cells is discussed.

Mimicking the humoral immune response in vitro results in antigen-specific isotype switching supported by specific autologous T helper cells: generation of human HIV-1-neutralizing IgG monoclonal antibodies from naive donors

Molecular and cellular requirements for antigen-specific isotype switch of human B cells have been investigated by mimicking signaling occurring in germinal centers. Peripheral blood mononuclear cells from healthy seronegative blood donors were first primary immunized in vitro, using a synthetic immunogen containing both a T and B cell epitope, which generated specific IgM-secreting B cells. We used the apex of the V3 loop of gp120 as B cell epitope linked to a promiscuous T helper epitope from tetanus toxin. In parallel, CD4+ T helper cell clones specific for the T epitope of the immunogen were established. In a secondary in vitro stimulation period, we co-cultured the antigen-specific T and B cells on CD32-transfected fibroblasts, together with an anti-CD40 monoclonal antibody. This resulted in isotype switching and human antigen-specific, IgG-secreting B cells were detected. This response was strictly dependent upon the presence of autologous T helper cells and the immunogen. Antigen-specific human B cells derived from this primary and secondary in vitro immunization were subsequently subjected to electrofield-induced somatic cell hybridization and hybridomas secreting human anti-V3 IgG monoclonal antibodies were isolated. One human antibody was further characterized and shown to be specific for the immunizing antigen with an affinity constant of 24 nM. This antibody also effectively neutralized different isolates of HIV-1, achieving a 50% neutralization at 0.46 microgram/ml.

Primary in vitro immunization with multimeric synthetic peptides of HIV-1 envelope glycoproteins: generation of neutralizing human monoclonal antibodies

Peripheral blood lymphocytes from healthy HIV-1 seronegative donors were immunized in vitro with the following synthetic peptides: (i) an octameric poly-L-lysine conjugated peptide of the HIV-1MN V3 loop and (ii) a resin bound synthetic peptide aa642-665 of HIV-1 gp41. Lymphoblastoid cell lines (LCL) were obtained by immortalization with Epstein-Barr virus (EBV). We produced four LCL secreting human monoclonal antibodies (HuMoAbs) of the IgM isotype: three were directed against the V3 domain (FC10, FC81 and CF41) and one against aa642-665 (CA45C). Two of these HuMoAbs (FC81 and CA45C) reacted to viral surface antigen on HIV-1-infected cells. All the HuMoAbs inhibited 40-53% of cell fusion induced by HIV-1-infected H9 cells at 5 micrograms/ml. They also neutralized, at lower concentrations, cell-free infection with HIV-1MN, HIV-1IIIB and four primary clinical HIV-1 isolates. No enhancing activity of the HuMoAbs in the presence of complement was observed. The results presented here show the feasibility of generating neutralizing human monoclonal antibodies against HIV-1 by primary in vitro immunization with selected synthetic peptides of HIV-1 envelope glycoproteins. This approach has provided tools for further studies of synergistic neutralization assays, and generated potential immunoglobulin candidates for passive immunotherapy.

Radioimmunodetection of solid tumors. Future horizons and applications for radioimmunotherapy

Seventeen years after the development of hybridoma technology, the clinical utility of radioimmunodetection of solid tumors using monoclonal antibody-based imaging agents has been definitively established. As expected, these first immunoscintigraphy agents demonstrate certain limitations (most notably, suboptimal tumor-to-background radiolocalization ratios and immunogenicity), suggesting that the full potential of this technology has not been realized. This article reviews research strategies for optimizing the imaging performance of radiolabeled monoclonal antibodies. Promising approaches include the development of humanized tumor-targeting vehicles, improved chelator technology to link the antibody and the radioisotope, the use of smaller immunoreactive targeting agents, modifications of the tumor or host determinants of antibody biodistribution, regional delivery of immunoscintigraphic agents, use of antibody "cocktails," and advances in image acquisition technology. The successful application of these strategies should lead to improved agents for tumor radioimmunodetection. The results of these research efforts should be useful in developing radiolabeled monoclonal antibody-based agents for solid tumor therapy.

In vitro sensitization for human monoclonal antibody production

We have found that cell adhesion prior to in vitro antigenic stimulation enriched the B-cell population and diminished the CD8 lymphocyte subset. These changes in lymphocyte proportions were favourable for increasing the percentage of antibody-producing cells after culturing in vitro followed by Epstein-Barr virus (EBV) infection. The immunodepletion of leukocytes by methyl esters did not yield satisfying results under analogous culture conditions. In vitro primary antigenic stimulations with the addition of IL-2 (25 U/ml medium) and low amounts of interferon-gamma provided better recruitment of antibody-producing cells and higher binding activity of antibodies to sperm than secondary antigenic stimulation. IL-6 did not positively influence the EBV-transformed cell lines.

Antibody engineering using Escherichia coli as host

The expression of immunoglobulin fragments with antigen binding activities in E. coli is now routinely possible. Using such expression systems, Fv, Fab, and scFv fragments and single VH domains can be produced as secreted proteins in yields of the order of milligrams per liter. Moreover, expression systems are being rapidly developed for the production of antibody scFv or Fab fragments by repertoire cloning followed by selection. Diverse repertoires of genes encoding VH and VL domains can be isolated by the PCR and cloned for expression using these systems, which allow the selection of recombinants that produce fragments with the desired antigen binding specificities. This technology is rapidly evolving and, coupled with the development of systems for the random mutagenesis and selection of higher-affinity antibody fragments, could, in the longer term, provide an alternative rapid route to hybridoma technology.

A single human monoclonal antibody that confers total protection from tetanus

Protective human monoclonal antibodies (HuMAbs) are superior to hyperimmune sera and murine monoclonal antibodies as far as human immunotherapy is concerned. In this report, we describe the successful generation of triomas secreting HuMAbs to tetanus toxin (tt). Lymphoblastoid cell lines secreting anti-tt antibodies were stabilized by back-fusion with a mouse x human heterohybrid myeloma partner, SBC-H20. One of the antibodies so produced, confers total protection of mice from tetanus, unlike a few recent reports where only partial protection (delay in the onset of tetanus) was achieved with single HuMAbs. Experiments to localize the neutralizing epitope(s) of the toxin using the protective monoclonal antibodies revealed that the antibody recognizes a conformational determinant that is destroyed on SDS-treatment. Preliminary studies show that Fab preparations of the protective antibody are capable of neutralizing tetanus toxin, suggesting that it might be possible to clone and express the Fab in a stable vector for large scale production.

Specific amplification of rearranged immunoglobulin variable region genes from mouse hybridoma cells

In this article we show how the polymerase chain reaction (PCR) and primers designed for conserved sequences of leader (L), framework one (FR1) and constant (CONST) regions of immunoglobulin light and heavy chain genes can be used for the cloning and sequencing of rearranged antibody variable regions from mouse hybridoma cells. RNA was extracted from the mouse hybridoma cells secreting MAbs: IOR-T3a (anti-CD3), C6 (anti-P1 of N. meningitidis B385), IOR-T1 (anti-CD6), CB-CEA.1 (anti-carcinoembryonic antigen), and CB-Fib.1 (anti-human fibrin). First strand cDNA was synthesized and amplified using PCR. The newly designed primers are superior to others reported recently in the literature. Isolated PCR DNA fragments of C6 and IOR-T3a were sequenced after asymmetric amplification, or M13 cloning. The FR1/CONST primer combinations selectively amplified mouse lights chain of groups kappa II, V, and VI, and heavy chains of groups IIa and IIc. The L/CONST primers for light chains amplified light chains from all four hybridomas. These methods greatly facilitate structural and functional studies of antibodies by reducing the efforts required to clone and sequence their variable regions.

A high-affinity human monoclonal IgM antibody reacting with multiple strains of Mycoplasma hominis

Human monoclonal antibodies were produced against Mycoplasma hominis by in vitro immunization of peripheral blood lymphocytes from a healthy seropositive donor using low amounts of antigen (5 ng/ml). The immune B lymphocytes were subsequently immortalized by Epstein-Barr virus transformation followed by somatic cell fusion. Using this procedure, several specific anti-M. hominis monoclonal antibodies of mu isotype were obtained exhibiting apparent affinity constants as high as 3 x 10(9)M-1. The productivity of a number of different hybridomas was determined to be in the range 7-22 micrograms x (24 hr x 10(6) cells)-1. The specificity of one antibody, 129.1, was further characterized by enzyme-linked immunosorbent assay against several different species of Mycoplasma and by Western blot analysis. The antibody specifically bound to a protein of Mr 100,000 present in 10 of 12 different strains of M. hominis.

Strategy for the production of human monoclonal antibodies using in vitro activated B cells

An up-to-date strategy to optimally produce human monoclonal antibodies by primary in vitro immunization or by secondary in vivo stimulation of immunized donors is discussed in detail. The effect of a lysosomotropic amino acid dipeptide on the B cell suppression by lysosome-rich cytolytic cells and the subsequent Epstein-Barr virus transformation of immune B lymphocytes is explained. The described strategy allows a routine production of human hybridomas, derived from peripheral blood lymphocytes and exhibiting a productivity in the range of 20-50 micrograms Ig/24 h per 10(6) cells. Furthermore, the possibilities to modulate antibody isotype and affinity by molecular biological methods is reviewed.

A repertoire of monoclonal antibodies with human heavy chains from transgenic mice

The introduction of human immunoglobulin gene segments in their unrearranged configuration into the germ line of mice might allow the production of a repertoire of human antibodies. Such transgenic mice could be used for the production of human monoclonal antibodies against human antigens. To test the feasibility of this approach, mice were created that carry a human heavy-chain minilocus comprising unrearranged immunoglobulin variable, diversity, and joining elements linked to a human mu-chain gene. The gene segments of this minilocus are rearranged in a large proportion of cells in thymus and spleen but not in nonlymphoid tissue. Some 4% of the B lymphocytes synthesize human mu chains resulting in a serum titer of about 50 micrograms of transgenic IgM antibody per ml. Hybridomas were established from the transgenic mice that stably secreted several micrograms of antibodies containing human mu heavy chains per milliliter.

Rapid cloning of rearranged immunoglobulin genes from human hybridoma cells using mixed primers and the polymerase chain reaction

A general method to directly obtain the DNA sequence of the variable regions of any immunoglobulin chain using a mixture of oligomer primers and the polymerase chain reaction (PCR) is described. Mixed oligonucleotide primers corresponding to the 5' signal peptide and a conserved 3' constant region primer were used for enzymatic amplification of each of the heavy and light chain variable regions of a human hybridoma producing a monoclonal antibody recognizing an epitope of gp120 of the human immunodeficiency virus 1. The amplified DNA segments were cloned and the sequence was determined for the heavy chain variable region. This method will greatly facilitate structural and functional studies of immunoglobulins by reducing the effort to clone and sequence the members of the immunoglobulin as well as other multigene families.