Human Antibodies

The contribution to IgG effector function of exposed galactose residues on the oligosaccharide chains in IgG has been tested experimentally. We studied a human monoclonal antibody (BRAD-5) to the Rh D blood group antigen. The preparation used contained a very low percentage of agalactosyl IgG (3.6%). After digestion with β-galactosidase there was an increase in terminal GlcN Ac indicating an increase in % agalactosyl IgG to approximately 30%. Comparison was made of the Fc receptor-(FcγR)-mediated functional interactions of the two glycoforms of BRAD-5 in assays which measured the recognition and destruction of sensitised erythrocytes by various effector cells. After β-galactosidase treatment, there was a slight reduction in FcγRI-mediated adherence of erythrocytes to U937 cells and phagocytosis of erythrocytes by monocytes. FcγRII-mediated binding of erythrocytes to K562 cells was also reduced. However there was little difference in adherence of erythrocytes to either Daudi cells (via FcγRII) or NK cells (via FcγRIII). There was a consistent reduction in lysis of erythrocytes mediated through FcγRIII on K cells with the γ-galactosidase treated anti-D. Overall the results showed that reduced levels of galactose on IgG anti-D were associated with reduced biological activity in these assays, but the experiments failed to cast light on the physiological role of the agalactosyl glycoform of IgG.

Human plasma cell lines producing IgG were cloned by the limiting dilution method from polyclonally immortalized B-cell lines with the plasmid DNA (pSVTLbsr) containing simian virus 40 (SV40)-gene from primary peripheral blood mononuclear cells of healthy and nonimmune volunteers. The plasma cell lines fused well with conventional partner cells and became evenly JgG-producing hybridomas at high efficiency, especially with the partner cell from human origin. One of the difficulties in obtaining stable IgG-producing hybridoma using Epstein–Barr virus (EBV) transformation followed by back fusion with partner cells, might mainly be attributed to the inability to immortalize plasma cells on account of the very low density of CD21 (EBV receptor) on the cell surface. The present CD21-independent immortalization by plasmid DNA and by the infection with SV40 protein-coated plasmid DNA will be a potentially effective method to obtain IgG-producing human monoclonal antibodies.

To increase the avidity of single-chain antibodies (scFv) for their antigen, we have fused them to core-streptavidin. The chimeric protein, expressed by the vector pSTE (plasmid for streptavidin-tagged expression) from Escherichia coli, can form tetrameric complexes, binds its antigen and contains four biotin binding sites per tetrameric complex. An additional cysteine inserted near the carboxy terminus further stabillsed the complex. The scFv fusion protein tetramers could be enriched by affinity chromatography using the biotin analog 2-iminobiotin from periplasmic inclusion bodies after refolding. We have also shown that the scFv fusion protein could be used for direct detection of its antigen in ELISA when stained with biotinylated horseradish peroxidase. The affinity of the scFv-antibody complex was substantially increased by avidity effects due to the tetrameric structure. The biotin binding sites may be used for coupling other antibodies and molecules to form bispecific and bifunctional reagents.

In vitro immunization of human B-lymphocytes was performed with liposomes containing the monosialoganglioside GM3, with or without either complete tetanus toxoid or a synthetic T helper epitope derived from tetanus toxin (determinant 830–843). The immunized B-cells were Epstein–Barr virus transformed and the human anti-ganglioside antibody response was evaluated using an indirect ELISA against different mono- and disialogangliosides. Clones producing antigen-specific human antibodies of the IgM isotype against the ganglioside GM3 used as the immunogen were selected and one clone, IM-11, was further characterized. In addition, a method of positive selection using GM3-coated magnetic beads has been developed which allowed us to rescue unstable clones. The binding of the human antibody M-11 to a large panel of glycosphingolipids separated on thin-layer plates was studied. The human MAb IM-11 was found to bind strongly to NeuAcGM3, IV³NeuAcnLc₄ and sulfate containing glycosphingolipids and weakly to NeuGcGM3. Immunohistological staining of melanoma and breast cancer biopsy sections showed a selective reactivity of IM-11 with tumor cells which varied among different tumors.

The display of repertoires of antibody fragments on the surface of filamentous phage offers a new way to produce immunoreagents with defined specificities. Here we report the selection of antibody fragments against the cytoskeletal fraction of T24 bladder cancer cells. To focus selection to a specific antigen, we eluted bound phage with a mouse monoclonal antibody directed against cytokeratins. Our initial studies proved that such a selection procedure with a library, carrying the mouse antibody fragment repertoire, resulted in phage specificity for the antigen against cytokeratin 8, recognized by the mouse monoclonal antibody. To facilitate detection of reactive clones, monoclonal antibodies against phage epitopes were developed. A human synthetic library (> 10⁸ clones) was used for selection by competitive elution after binding to T24 cytoskeleton. About 50% of the phage reacted in ELISA with cytoskeletons of T24 cells, while with noncytokeratin containing cells no reaction was observed. Immunofluorescence studies and Western blotting with a number of these clones showed reactivity against cytokeratin. We conclude that the competitive elution method can be used as a rapid technique to obtain immunoreactive phages, and eventually human single chain antibodies directed against defined epitopes which were formerly characterized and validated by mouse monoclonal antibodies.