Transfer of anti-D antibodies across the isolated perfused human placental lobule and inhibition by high-dose intravenous immunoglobulin: a possible mechanism of action.

Using an in vitro perfusion model, therapeutic intravenous immunoglobulin (IVIgG) and IgG anti-D have been shown to cross the placenta from the maternal circuit to the fetal circuit. The transfer of all IgG species was linear with respect to time, and the amount of IgG transferred was proportional to the concentration of IgG in the maternal circuit ([IgG]m), but reached saturation at upper limits. With total [IgG]m at 6.5 g/l, 11.1 g/l or 26.2 g/l the increase in the fetal concentration of total IgG was 4.6 mg/l/h. 8.9 mg/l/h and 9.9 mg/l/h respectively. The rate of transfer of specific anti-D antibody to the fetal circuit was 0.026 IU/ml/h at a concentration of 38 IU/ml in the maternal circuit ([anti-D]m). High-dose therapeutic IVIgG added to the maternal circuit (total [IgG]m 29.2 g/l) significantly inhibited (P < 0.001) anti-D transfer to 0.004 IU/ml/n. Addition of the same IVIgG at a lower concentration (total [IgG]m 11.1 g/l) also reduced anti-D transfer, but only to 0.015 IU/l/h. The inhibitory effect of IVIgG does not appear to be mediated by anti-idiotypic or non-specific complexing with the anti-D, but may be the result of competition with IgG anti-D for placental Fc gamma receptors involved in the endocytotic uptake of IgG. The efficacy of IVIgG in this model suggests that it may be clinically useful in preventing HDN and other immune cytopenias, provided a sufficiently high dose is given.