InlA Promotes Dissemination of Listeria monocytogenes to the Mesenteric Lymph Nodes during Food Borne Infection of Mice

Abstract

Intestinal Listeria monocytogenes infection is not efficient in mice and this has been attributed to a low affinity interaction between the bacterial surface protein InlA and E-cadherin on murine intestinal epithelial cells. Previous studies using either transgenic mice expressing human E-cadherin or mouse-adapted L. monocytogenes expressing a modified InlA protein (InlA^m) with high affinity for murine E-cadherin showed increased efficiency of intragastric infection. However, the large inocula used in these studies disseminated to the spleen and liver rapidly, resulting in a lethal systemic infection that made it difficult to define the natural course of intestinal infection. We describe here a novel mouse model of oral listeriosis that closely mimics all phases of human disease: (1) ingestion of contaminated food, (2) a distinct period of time during which L. monocytogenes colonize only the intestines, (3) varying degrees of systemic spread in susceptible vs. resistant mice, and (4) late stage spread to the brain. Using this natural feeding model, we showed that the type of food, the time of day when feeding occurred, and mouse gender each affected susceptibility to L. monocytogenes infection. Co-infection studies using L. monocytogenes strains that expressed either a high affinity ligand for E-cadherin (InlA^m), a low affinity ligand (wild type InlA from Lm EGDe), or no InlA (ΔinlA) showed that InlA was not required to establish intestinal infection in mice. However, expression of InlA^m significantly increased bacterial persistence in the underlying lamina propria and greatly enhanced dissemination to the mesenteric lymph nodes. Thus, these studies revealed a previously uncharacterized role for InlA in facilitating systemic spread via the lymphatic system after invasion of the gut mucosa. Ingestion of Listeria monocytogenes-contaminated food can be life-threatening for immune compromised individuals, can cause severe gastroenteritis in otherwise healthy people, and is also thought to occur frequently with little consequence. The factors that determine susceptibility to this infection are unknown, due to the lack of an appropriate animal model that closely mimics this wide range of human disease. Mice are highly resistant to oral L. monocytogenes infection, and the prevailing view has been that a low affinity between the bacterial surface protein InlA and E-cadherin expressed on the gut mucosa was largely responsible for limited invasion of the murine intestines. We used a novel food borne model of listeriosis to show that a mouse-adapted InlA offers little advantage over wild type InlA for initial colonization of the gut, and indeed, even bacteria lacking InlA can establish intestinal infection in mice. Thus, other aspects of the murine gastrointestinal environment appear to be the key to innate resistance against oral transmission. Surprisingly, our study uncovered a novel function for InlA later in the infection, when the bacteria begin to spread systemically. The natural feeding model presented here using susceptible and resistant strains of mice should be very useful for future studies investigating both mechanisms of microbial pathogenesis and host responses to oral infection.