Maternal IgG stimulates B lineage cell development in the progeny

To examine the physiological role of maternal natural IgG antibodies on the development of B lineage cells of the progeny, we have bred homozygous mu MT/mu MT or heterozygous mu MT/+ females to mu MT/mu MT or mu MT/+ males, respectively. We could thus compare normal or B cell-deficient mice born from Ig-deprived (Ig(-)) or phenotypically normal mothers (Ig(+)). B cell-deficient progeny of heterozygous mothers contain no detectable serum IgA or IgM, but IgG concentrations that peak at 2 mg/ml by 7-21 days of age, decay after weaning with a half-life of 7 days, and remain detectable for 2 months after birth. At 7 days after birth, mu MT/+ progeny born of Ig(+) mothers contain two- to threefold higher numbers of bone marrow (BM) pre-B and B cells, and of splenic B cells, compared to mice of the same age born from Ig(-) mothers. In contrast, the former progeny exhibit two to four times lower numbers of Ig-secreting plasma cells in spleen and thymus, and contain sixfold lower serum IgM concentrations. A similar maternal IgG-dependent stimulation of BM B cell precursors is also observed in mu MT/mu MT progeny. No significant differences were detected between the groups on day 3 after birth, suggesting the requirement for a minimal IgG concentration in the serum.