ridium sp. parasitizes intestinal epithelium, resulting in enterocyte loss, villous atrophy, and malabsorptive diarrhea. We have shown that mucosal expression of inducible nitric oxide (NO) synthase (iNOS) is increased in infected piglets and that inhibition of iNOS in vitro has no short-term effect on barrier function. NO exerts inhibitory effects on a variety of pathogens; nevertheless, the specific sites of iNOS expression, pathways of iNOS induction, and mechanism of NO action in cryptosporidiosis remain unclear. Using an in vivo model of Cryptosporidium parvum infection, we have examined the location, mechanism of induction, specificity, and consequence of iNOS expression in neonatal piglets. In acute C. parvum infection, iNOS expression predominated in the villous epithelium, was NF-B dependent, and was not restricted to infected enterocytes. Ongoing treatment of infected piglets with a selective iNOS inhibitor resulted in significant increases in villous epithelial parasitism and oocyst excretion but was not detrimental to maintenance of mucosal barrier function. Intensified parasitism could not be attributed to attenuated fluid loss or changes in epithelial proliferation or replacement rate, inasmuch as iNOS inhibition did not alter severity of diarrhea, piglet hydration, Cl Ϫ secretion, or kinetics of bromodeoxyuridine-labeled enterocytes. These findings suggest that induction of iNOS represents a nonspecific response of the epithelium that mediates enterocyte defense against C. parvum infection. iNOS did not contribute to the pathogenic sequelae of C. parvum infection. nitric oxide; barrier function; diarrhea; cryptosporidiosis A NONINVASIVE Protozoan, Cryptosporidium, parasitizes the small intestinal epithelium. Infection results in accelerated loss of villous enterocytes, leading to severe villous atrophy and a malabsorptive and secretory diarrhea. Acute epithelial defense against infection requires the elimination of parasitized enterocytes with preservation of epithelial continuity, as provided by enterocytes generated in the crypt and migrating onto the villus. These mechanisms must keep pace with ongoing infection until specific immune mechanisms result in clearance of the organisms. Surprisingly, little is known regarding the mechanisms and mediators of epithelial defense that predominate at the time of acute epithelial infection and diarrhea in the naturally susceptible host. Although studies in mice with defined immunodeficiency have identified a number of mediators (e.g., IFN-␥, CD40, and the CD4-positive T cell) that are necessary for onset of acquired immunity to chronic Cryptosporidium parvum infection, these animals do not develop villous atrophy or diarrhea, which characterize the acute clinical infection. Similarly, epithelial cell culture models of Cryptosporidium infection are unable to recapitulate the mechanisms of epithelial loss and replacement that characterize in vivo infection.Synthesis of nitric oxide (NO) is consistently elevated in patients with infectious diarrhea of a vari...

In intestinal inflammation, mucosal injury is often exacerbated by the reaction of NO with neutrophilderived superoxide to form the potent oxidant peroxynitrite. Peroxynitrite also has antimicrobial properties that aid in the killing mechanism of macrophages and neutrophils. Cryptosporidium parvum parasitizes intestinal epithelium, resulting in loss of epithelial cells and mucosal inflammation. Synthesis of NO is significantly increased and arises from the induced expression of inducible nitric oxide synthase (iNOS) by the infected epithelium. Inhibition of iNOS results in intensified epithelial parasitism and oocyst excretion. We hypothesized that formation of peroxynitrite is restricted to sites of iNOS expression by the epithelium and contributes to host defense in C. parvum infection. Accordingly, the location and biological effects of peroxynitrite formation were examined in neonatal piglets infected with C. parvum.