Superinduction of neonatal IgA relies on the maternal microbiota vertically transferred during and after birth. (A) Shannon diversity of microbiota in different sites of the neonatal gastrointestinal tract at 16 d of age (n = 4). ST, stomach. (B) The relative abundance at the order level in 16-d-old pups (n = 4). (C) Principal coordinates analysis of the microbiota in 16-d-old pups (n = 4). (D) Breeding scheme to generate littermate dams. (E) Shannon diversity of the microbiota in the stomach of 16-d-old pups nursed by littermate dams (n ≥ 10). (F) Principal coordinates analysis of the microbiota in the stomach of 16-d-old pups nursed by littermate dams (n ≥ 10). (G) Serum IgA level of 16-d-old pups nursed by littermate dams (n ≥ 21). (H) Serum IgA level of 16-d-old pups nursed by antibiotic-treated dams (n ≥ 7). (I) Scheme showing microbiota transplantation experiments in GF C57BL/6 wild-type mice. The donor material was the stomach contents of 3-d-old immunocompetent pups nursed by SPF Rag1+/+ (WT) or Rag1−/− (KO) dams. (J) Serum IgA levels of 16-d-old pups from GF dams receiving PBS (no), WT, or KO microbiota transplantation (n ≥ 16). One-way ANOVA and two-tailed Student’s t tests were used as appropriate. Error bars show SEM. *P < 0.05, ***P < 0.001; n.s., not statistically significant.