We previously reported that inhibition of nitric oxide (NO) increases the rate of bacteremia and maternal mortality in pregnant rats with uterine infection by Escherichia coli expressing the Dr fimbria (Dr ؉ ). Epithelial binding and invasion by Dr ؉ E. coli has also been shown to be dependent upon the expression level of the cellular receptor decay-accelerating factor (DAF; CD55). Here, we hypothesize that NO-related severity of infection could be mediated by changes in DAF expression and in the rate of epithelial invasion. The cellular basis of NO effects on epithelial invasion with Dr ؉ E. coli was studied using Ishikawa endometrial carcinoma cells as an in vitro model of the human endometrial epithelium. Initially, we show that Ishikawa cells produce NO and express both NO synthase enzymes, NOS II and NOS III, and DAF protein. We next tested the abilities of both Dr ؉ E. coli and a Dr ؊ E. coli mutant to invade Ishikawa cells, and invasion was seen only with Dr ؉ E. coli. Invasion by Dr ؉ E. coli was decreased by elevated NO production and increased by NO inhibition. Elevated NO production significantly decreased DAF protein and mRNA expression in Ishikawa cells in a time-and dose-dependent manner. Here, we propose that in vitro invasion of an epithelial cell line is directly related to NO-regulated expression of DAF. The significance of NO-regulated receptor-ligand invasion is that it may represent a novel unrecognized phenomenon of epithelial defense against infection.Although urogenital microbial infection in pregnancy is an important cause of maternal and neonatal morbidity and mortality, the mechanisms of defense against gestational intrauterine infection are poorly understood (8,14,23). Evidence obtained in studies of both humans and rats suggests that the bacteriostatic actions of nitric oxide (NO) are an important component of defense against urogenital infection (16,30,33). Nitric oxide is synthesized in situ from an L-arginine substrate by one or more of three NO synthase isoforms (NOS I, NOS II, and NOS III), each of which has been identified in the mouse, rat, and human (1,27,40,41).Several lines of evidence have demonstrated the involvement of intracellular NO in the host defense mechanisms against bacterial infections (5, 27). Recently, NO production and three NOS isoforms were reported to be present in rat uterine tissues, and elevated NOS II expression was demonstrated to contribute to the increased NO production in the rat uterus and consequent uterine quiescence during gestation (3, 4, 40). However, the role of the NO system in uterine defense mechanisms is not well understood.Three separate lines of evidence from our laboratories have suggested that increased NO production by the urogenital tract in pregnancy protects against Escherichia coli infection. First, inhibition of NO synthesis in pregnant rats with an intrauterine infection increases maternal death (30). Second, the sensitivity of the female rat or mouse urinary tract to E. coli infection was increased with inhibition of NO (30,3...