Tissue culture in hollow-fibre systems: implications for downstream processing and stability analysis.

Murine hybridoma cells, not adapted to low-protein or serum-free culture media, were grown in a hollow-fibre bioreactor or a stirred-tank perfusion system. In both situations, the serum content of the medium was gradually decreased during the process. The harvest obtained from the hollow-fibre reactor contained a much higher concentration of monoclonal antibody (MAb) as compared to the supernatant harvested from the stirred-tank perfusion system. Moreover, the production rate of the cells grown in the hollow-fibre reactor system was not affected by decreasing the concentration of serum in the medium to as low as 0.5% while the same cell line required 2% of serum in the stirred-tank reaction system. Consequently, the ratio of the concentration of MAb over serum proteins in the tissue culture supernatant derived from the hollow-fibre system was much higher than the one present in the harvest derived from the stirred-tank perfusion system. To study the stability of the cell mass immobilized in the hollow-fibre perfusion system, culture supernatants were tested for the presence of immunoglobulin heavy chain isotype switch variants at various intervals after initiation of culture from the cell bank. The implications of the features of hollow-fibre bioreactor systems for the production of MAb for clinical applications are discussed.

A human hybrid hybridoma.

Hybrid hybridomas are obtained by fusion of two cells, each producing its own antibody. Several authors have reported the construction of murine hybrid hybridomas with the aim to obtain bispecific monoclonal antibodies. We have investigated, in a model system, the feasibility of constructing a human hybrid hybridoma. We fused two monoclonal cell lines: an ouabain-sensitive and azaserine/hypoxanthine-resistant Epstein-Barr virus-transformed human cell line that produces an IgG1 kappa antibody directed against tetanus toxoid and an azaserine/hypoxanthine-sensitive and ouabain-resistant human-mouse xenohybrid cell line that produces a human IgG1 lambda antibody directed against hepatitis-B surface antigen. Hybrid hybridoma cells were selected in culture medium containing azaserine/hypoxanthine and ouabain. The hybrid nature of the secreted antibodies was analyzed by means of two antigen-specific immunoassays. Our results show that it is possible, with the combined use of transformation and xenohybridization techniques, to construct human hybrid hybridomas that produce bispecific antibodies.

Requirements for growth of Epstein-Barr virus-transformed cells at low cell densities.

We investigated the requirements for growth of Epstein-Barr virus-transformed cells at low cell densities in a homotypic system: only Epstein-Barr virus-transformed cells or their products were used to supply feeder activity. The cloning efficiency was increased markedly under conditions favouring close cell-to-cell contact: culture in round-bottomed rather than in flat-bottomed wells or the addition of irradiated Epstein-Barr virus-transformed cells. Further enhancement was obtained by the addition of the tumour promoter phorbol-myristate acetate. Part of this effect was also induced by supernatants of Epstein-Barr virus-transformed cells, in particular when the latter had been stimulated with phorbol ester. Thus, under limiting dilution conditions, Epstein-Barr virus-transformed cells were found to be dependent upon autologous cell-mediated and soluble proliferation-stimulating signals. In such a homotypic feeder system, the cloning efficiency could be enhanced up to eight-fold; a 100% cloning efficiency could not be achieved. Evidence is presented that the residual deficit in cloning efficiency is inherent to these cell lines.

Xenohybridization of Epstein-Barr virus-transformed cells for the production of human monoclonal antibodies.

Transformation of human B lymphocytes, obtained from hyperimmune donors with Epstein-Barr virus, yields polyclonal cell populations in which a minority of cells produce IgG antibodies of predetermined specificity, whereas the majority of cells produce 'non-specific' immunoglobulin (mainly of the IgM class). Such lymphoblastoid cell lines can be easily propagated in high-density cultures. Because cloning at 1 cell per well is not possible, stabilization of lymphoblastoid cell lines by limiting dilution is not feasible and most newly established lines cease to produce specific antibody within a few weeks. Xenohybrids, resulting from fusion of Epstein-Barr virus-transformed cells with NS1 mouse plasmacytoma cells, can be cloned at 1 cell per well. Stable xenohybridoma subclones, producing antibody of the desired specificity, can be isolated after a series of limiting dilutions. In a model system, we have studied the efficiency of xenohybridization of human lymphoblastoid cells. Using this system, we have constructed IgG anti-tetanus-toxoid- and IgG anti-HBsAg-producing cell lines. Next, we investigated whether transformation with Epstein-Barr virus is essential in such a two-step procedure or whether a polyclonal stimulator, such as pokeweed mitogen, could also be used. It was found that antibody-producing xenohybrids can be obtained after stimulation with pokeweed mitogen. However, this latter system is subject to more variations and lacks the advantage of pre-selection of antibody-producing cells as compared to xenohybridization after transformation.

A human monoclonal IgG1 lambda anti-hepatitis B surface antibody. Production, properties, and applications.

Peripheral blood mononuclear cells from a donor with a high titre of anti-hepatitis B surface (HBs) antibodies were fused with a cell line that was positive for Epstein-Barr virus nuclear antigen and sensitive to hypoxanthine-aminopterine-thymidine. A cell line was established that produces a monoclonal IgG1 lambda anti-HBs antibody. Afterwards, it appeared that the anti-HBs antibody-producing cell line had arisen from Epstein-Barr virus transformation of the donor B cells. The cell line is capable of producing up to 60 micrograms/ml of the monoclonal antibody, which has a high avidity for HBs antigen (Ag) and recognizes both ad and ay subtypes. The antibody is useful as a reagent for the detection of HBsAg in human serum. Over 1000 patient sera have been tested with a conventional third-generation assay in parallel, and only a single discrepant serum was found.

Human lymphoblastoid cell line producing protective monoclonal IgG1, kappa anti-tetanus toxin.

Peripheral blood lymphocytes from an individual, recently boosted with tetanus toxoid (TT), were transformed with Epstein-Barr virus. No antigen-specific selection nor stimulation of B cells was performed prior to transformation. One stable cell line, designated CLB-Hu-TT-1, was established. This cell line has a doubling time of 24 h and yields 10 micrograms/ml of a monoclonal IgG1, kappa anti-TT antibody in bulk cultures. The antibody is biologically active in that it can protect mice against the effects of tetanus toxin. The cell line has been characterized with regard to some cytoplasmic and membrane markers.