Nitric Oxide Production by Human Intestinal Epithelial Cells and Competition for Arginine as Potential Determinants of Host Defense Against the Lumen-Dwelling Pathogen Giardia lamblia

Abstract

Giardia lamblia infection of the human small intestine is a common protozoan cause of diarrheal disease worldwide. Although infection is luminal and generally self-limiting, and secretory Abs are thought to be important in host defense, other defense mechanisms probably affect the duration of infection and the severity of symptoms. Because intestinal epithelial cells produce NO, and its stable end products, nitrite and nitrate, are detectable mainly on the apical side, we tested the hypothesis that NO production may constitute a host defense against G. lamblia. Several NO donors, but not their control compounds, inhibited giardial growth without affecting viability, suggesting that NO is cytostatic rather than cytotoxic for G. lamblia. NO donors also inhibited giardial differentiation induced by modeling crucial environmental factors, i.e., encystation induced by bile and alkaline pH, and excystation in response to gastric pH followed by alkaline pH and protease. Despite the potent antigiardial activity of NO, G. lamblia is not simply a passive target for host-produced NO, but has strategies to evade this potential host defense. Thus, in models of human intestinal epithelium, G. lamblia inhibited epithelial NO production by consuming arginine, the crucial substrate used by epithelial NO synthase to form NO. These studies define NO and arginine as central components in a novel cross-talk between a luminal pathogen and host intestinal epithelium.

Release of metabolic enzymes by Giardia in response to interaction with intestinal epithelial cells

“…Prior to inducing apoptosis, this depletion reduces the ability of IECs to produce NO, an anti-microbial innate defense molecule. Our previous studies implicate NO toxicity to Giardia because in vitro NO donors inhibit giardial growth but not viability [19]. NO also inhibits both encystation and excystation of Giardia and could thus interfere with parasite transmission [19].…”

“…Our previous studies implicate NO toxicity to Giardia because in vitro NO donors inhibit giardial growth but not viability [19]. NO also inhibits both encystation and excystation of Giardia and could thus interfere with parasite transmission [19]. Many intracellular pathogens are killed by NO, but the role of NO in controlling infections of extracellular pathogens is not well established [19].…”

“…NO also inhibits both encystation and excystation of Giardia and could thus interfere with parasite transmission [19]. Many intracellular pathogens are killed by NO, but the role of NO in controlling infections of extracellular pathogens is not well established [19]. Interestingly, the NO levels in intestinal epithelial cells have also been shown to be important in the regulation of adsorption/secretion of water [41], suggesting that it could be associated with symptoms of giardiasis.…”

“…This enzyme plays an important role in the intracellular arginine metabolism of Giardia [28]. We showed earlier that Giardia deplete arginine required by intestinal epithelial cells for nitric oxide (NO) production [19]. Secreting ADI, may enable the parasite to disarm the potentially detrimental effects of NO-dependent host defenses.…”

“…Earlier studies had indicated that parasite proteins are released into the growth medium during incubation of axenic cultures of Giardia trophozoites in serum-free medium but no specific proteins have been identified so far. However, the highly immuno-reactive variable surface proteins (VSPs) [18,19] and an unidentified 58 kDa protein causing intestinal fluid accumulation in mice [5,20] have been reported to be excretory-secretory factors.…”