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Inhibition of Immunoglobulin Production In Vitro by IgG and F(ab′)2 Fragments, but not by the Fc Portion

Antibody‐secreting B cells were measured as plaque‐forming cells (PFC) in a modified haemolysis‐in‐gel assay, using protein A coupled sheep erythrocytes as targets. Human lymphocytes from blood (PBL), bone marrow or spleen were stimulated in vitro by various polyclonal B‐cell activators and incubated with intravenous immunoglobulin (IVIG) or peptide fragments of IVIG. IgG and IgM production from PBL and bone marrow cells, measured as PFC, was inhibited more than 50% by IVIG 2.5mg/ml, compared to controls without IVIG. Inhibition of the IgG and IgM response of spleen cells by IVIG varied depending on the stimuli. Using Staphylococcus aureus protein A (SPA), inhibition was almost 90% (P<0.001). The inhibition of the IgG and IgM responses to lipopolysaccharide from Escherichia coli (LPS) were 70% (P<0.01) and 28% (P<0.05), respectively. IgG stimulation by pokeweed mitogen (PWM) was inhibited by 57% (P<0.01), but the IgM response was inhibited only by the higher IVIG concentration of 5.0mg/ml. In mixed lymphocyte cultures of spleen cells, IgG and IgM production were inhibited by more than 60% (P<0.05). The effect of IgG, IgG‐F(ab′)2 and IgG‐Fc on LPS or PWM‐stimulated spleen cells were compared, using equimolar concentrations of the various preparations. IgG‐ and IgM‐producing PFC were significantly (P<0.05) inhibited in a dose‐dependent fashion by IgG and F(ab′)2, but not by Fc. LPS‐induced IgG and IgM production was inhibited also when IgG and F(ab′)2 were added up to 48h after the stimulator. A comparison of IgG, F(ab′)2 and Fc products from different companies showed that all IgG and F(ab′)2 preparations significantly inhibited IgG and IgM production of LPS‐stimulated spleen cells. No significant inhibition was obtained with any of the purified Fc products.