Serum but not pIgG treatment of unaffected young AID^−/−µS^−/− mice corrects extra numbers of total, MZ, and B1 cells but does not correct GC B cells at adulthood. Three-week-old Thy1.2⁺ AID^−/−µS^−/− mice intravenously received either 800 μg of pIgG or mIgG weekly, or 200 μL of sera from WT mice every other day (sera). Four weeks after the first injection, these 3 groups of treated mice together with untreated age-matched AID^−/−µS^−/− and WT controls were analyzed. (A) FACS analysis of BM of 3-week-old WT and AID^−/−µS^−/− mice. The numbers in dot plots show the percentages of B220⁺CD43⁻ cells in fractions D (B220^dimIgM⁻), E (B220^dimIgM⁺), and F (B220^brightIgM⁺). (B) Total number of splenic CD19⁺IgM⁺ B cells of 3-week-old AID^−/−µS^−/− and age-matched WT controls. (C) Levels of serum IgG in mice treated with pIgG or mIgG were monitored by ELISA performed on blood samples withdrawn just before each of the 4 injections. The x axis indicates WT age-matched mice and for each group of treated mice, the time after the first injection, in weeks 1 to 4. (D) Levels of IgM and IgGs in mice treated with WT sera at the time of the analysis. (E) The total numbers of CD19⁺IgM⁺ B cells per spleen of untreated WT and AID^−/−µS^−/− mice or treated AID^−/−µS^−/− mice. (F) Absolute numbers of Imm, Mat, follicular (FO), MZ, CD21^loCD23^lo, and GC B cells per spleen of untreated and treated mice. (G) Representative dot plots showing the percentages of GC B cells among mature splenic B cells of untreated WT and AID^−/−µS^−/− mice, or treated AID^−/−µS^−/− mice. Each dot represents one mouse. Results are from 2 independent experiments; ns, not significant; *P ≤ .05; **P ≤ .01; ***P ≤ .005.