The S. typhimurium LD₅₀ is reduced in pIgR^−/− mice. Groups of five naive B6 (▪) or pIgR^−/− mice (◯) were orally infected with either 10⁶ (A), 10⁵ (B), or 10⁴ (C) CFUs S. typhimurium. For ethical reasons, mice were killed when they became moribund; these mice were deemed “nonsurvivors.”

Naive pIgR^−/− mice are susceptible to “natural” infection with virulent S. typhimurium. One B6 mouse (□, A) or one pIgR^−/− mouse (◯, B) was orally inoculated with S. typhimurium and cohoused with naive B6 mi ce (▪) and naive pIgR^−/− mice (•). The course of the infection in all mice (n = 3, orally inoculated mice; n = 6, naive mice) was followed by measuring the number of bacteria shed in feces. Mice were killed when they became moribund, and the survival was recorded (C and D). The experiment was performed with three cages for each strain of mice, and the data represent the mean ± 1 SD.

Innate IgA inhibits S. typhimurium invasion. (A) Naive mice were orally infected with 10⁷ CFUs S. typhimurium. Shown are the numbers of S. typhimurium detected in PP 6 or 12 h after oral infection, or in ileal mucosa 12 h after infection. Presented are the combined results of three independently performed experiments. *P = 0.011. (B) An in vitro invasion assay was used to determine the ability of serum from naive B6 (hatched columns) or pIgR^−/− mice (black columns) to inhibit S. typhimurium invasion. Presented is the mean invasion ± 1 SD of triplicate cultures of one representative out of three performed experiments. *P = 0.04; **, P < 0.001. (C) Goat anti–mouse IgG (hatched columns), goat anti–mouse IgA (black columns), or culture medium (white columns) was added to serum from pIgR^−/− mice before use in an in vitro invasion assay. As controls, B6 serum or goat antibodies added to the assay in the absence of serum was used. Presented is the mean invasion ± 1 SD of triplicate cultures. *, P = 0.4; **, P = 0.01. (D) Normal (white bars) and preabsorbed serum (hatched bars) from B6 or pIgR^−/− mice (pools of three) was assayed for its ability to block invasion of MDCK cells by S. typhimurium. Shown is the mean invasion ± 1 SD of quadruplicate cultures of one representative out of six independently performed experiments. *, P = 0.04; **, P = 0.01. (E) ELISA was used to determine the IgA titers specific for gut microflora in normal (white bars) and preabsorbed (hatched bars) serum from pIgR^−/− mice. Symbols represent serum pools formed from three or four mice (n = 10 in total). The dotted line indicates the detection limit of the assay.