Significance An experimental system was developed in mice to study the long-term benefits of early exposure to secretory antibodies of the IgA class (SIgA) in breast milk. We found that breast milk-derived SIgA promoted intestinal epithelial barrier function in suckling neonates, preventing systemic infection by potential pathogens. Long-term benefits of early exposure to SIgA included maintenance of a healthy gut microbiota and regulation of gene expression in intestinal epithelial cells. These findings suggest that maternal antibodies provide benefits to the intestinal immune system of the breast-fed infant, which persist into adulthood.

Intestinal epithelial cells (IECs) form a physical and immunological barrier that separates the vast gut microbiota from host tissues. MyD88-dependent Toll-like receptor signaling is a key mediator of microbial–host cross-talk. We examined the role of epithelial MyD88 expression by generating mice with an IEC-targeted deletion of the Myd88 gene (MyD88ΔIEC). Loss of epithelial MyD88 signaling resulted in increased numbers of mucus-associated bacteria; translocation of bacteria, including the opportunistic pathogen Klebsiella pneumoniae, to mesenteric lymph nodes; reduced transmucosal electrical resistance; impaired mucus-associated antimicrobial activity; and downregulated expression of polymeric immunoglobulin receptor (the epithelial IgA transporter), mucin-2 (the major protein of intestinal mucus), and the antimicrobial peptides RegIIIγ and Defa-rs1. We further observed significant differences in the composition of the gut microbiota between MyD88ΔIEC mice and wild-type littermates. These physical, immunological, and microbial defects resulted in increased susceptibility of MyD88ΔIEC mice to experimental colitis. We conclude that MyD88 signaling in IECs is crucial for maintenance of gut homeostasis.

IgA Abs help to maintain homeostasis at mucosal surfaces by promoting defense mechanisms that protect against pathogens while suppressing inflammatory responses to commensal organisms and food Ags. The polymeric Ig receptor (pIgR) mediates transport of IgA across mucosal epithelial cells. We hypothesized that signaling through TLRs may up-regulate pIgR expression by intestinal epithelial cells and thus enhance IgA-mediated homeostasis. To test this hypothesis we treated the HT29 human intestinal epithelial cell line with dsRNA, a ligand for TLR3, or LPS, a ligand for TLR4. Both dsRNA and LPS up-regulated levels of pIgR mRNA and cell surface pIgR protein. By contrast, dsRNA but not LPS up-regulated expression of TLR3 and TLR4 mRNA. However, cell surface expression of both TLR3 and TLR4 was enhanced by treatment of HT29 cells with their respective ligands. Transfection of HT29 cells with wild-type and mutated promoter/enhancer plasmids suggested that TLR3 and TLR4 signal primarily through NF-κB to enhance transcription of pIgR mRNA. TLR3 signaling resulted in a more pronounced inflammatory response than did TLR4, as evidenced by up-regulation of the transcription factor IFN regulatory factor-1, chemokines IL-8 and RANTES, and the proinflammatory cytokine TNF. Signaling through LPS/TLR4 appears to up-regulate pIgR expression while minimizing proinflammatory responses, a mechanism that could promote IgA-mediated homeostasis in the presence of commensal Gram-negative bacteria.

In the yeast Saccharomyces cerevisiae, the major membrane phospholipid phosphatidylcholine is synthesized by the CDP-diacylglycerol and CDP-choline pathways. We examined the regulation of phosphatidylcholine synthesis by CTP. The cellular concentration of CTP was elevated (2.4-fold) by overexpressing CTP synthetase, the enzyme responsible for the synthesis of CTP. The overexpression of CTP synthetase resulted in a 2-fold increase in the utilization of the CDP-choline pathway for phosphatidylcholine synthesis. The increase in CDP-choline pathway usage was not due to an increase in the expression of any of the enzymes in this pathway. CDP-choline, the product of the phosphocholine cytidylyltransferase reaction, was the limiting intermediate in the CDP-choline pathway. The apparent Km of CTP (1.4 mM) for phosphocholine cytidylyltransferase was 2-fold higher than the cellular concentration of CTP (0.7 mM) in control cells. This provided an explanation of why the overexpression of CTP synthetase caused an increase in the cellular concentration of CDP-choline. Phosphatidylserine synthase activity was reduced in cells overexpressing CTP synthetase. This was not due to a transcriptional repression mechanism. Instead, the decrease in phosphatidylserine synthase activity was due, at least in part, to a direct inhibition of activity by CTP. These results show that CTP plays a role in the regulation of the pathways by which phosphatidylcholine is synthesized. This regulation includes the supple of CTP for the phosphocholine cytidylyltransferase reaction in the CDP-choline pathway and the inhibition of the phosphatidylserine synthase reaction in the CDP-diacylglycerol pathway.

The influence of four bacteriocins from lactic acid bacteria on the proton motive force (PMF) of sensitive organisms was investigated. Pediocin PA-1 (20 pg/ml) and leuconocin S (48.5 jLg/ml) mediated total or major PMF dissipation of energized Listeria monocytogenes Scott A cells in a concentration-dependent manner, as has been shown for nisin. Lactacin F (13.5 ,ug/mi) caused 87% PMF depletion of energized LactobaciUlus delbrueckii ATCC 4797 cells, also in a concentration-dependent fashion. The energy requirements for the activity of these four bacteriocins were determined by using the ionophores nigericin and valinomycin to carry out partial and specific deenergization of the target organisms. Pediocin PA-1, leuconocin S, and lactacin F acted in an energy-independent manner, whereas the activity of nisin was confirmed to be energy dependent. These results together with published reports on other bacteriocins suggest that the bacteriocins of lactic acid bacteria share a common mechanism, the depletion of PMF.