The Ligand and Function of the RegIII Family of Bactericidal C-Type Lectins
Beginning at birth, the intestines of humans and other mammals are colonized with a diverse society of resident bacteria that play a crucial role in host nutrient metabolism. To maintain this commensal relationship, resident microbes must be prevented from crossing the intestinal epithelium into host tissues where they can cause inflammation and sepsis. The innate immune system plays a crucial role in preventing bacterial incursions across gut epithelial surfaces. Mucosal epithelial cells produce a variety of secreted antimicrobial proteins that help to prevent bacterial attachment and encroachment at epithelial surfaces. Among these, Paneth cells are specialized small intestinal epithelial cells that have been shown to produce and secrete antimicrobial proteins and peptides. To gain new insights into the adaptation of mucosal surfaces to microbial challenges, the Hooper lab has used DNA microarrays to screen for Paneth cell genes whose expression is modulated by intestinal microbes. This screen revealed that expression of two C-type lectins, RegIIIbeta and RegIIIgamma , is strongly induced following intestinal colonization with resident microbes. Two features suggested that members of the RegIII family may have microbicidal functions. First, they are C-type lectin family members. Other C-type lectins, including the mannose binding lectin, have well-characterized innate immune functions and play critical roles in microbial killing by recruiting complement. Second, I have shown that the murine RegIII lectins localize to intestinal crypt cells, including Paneth cell secretory granules, and that they bind to luminal bacteria harvested from intestinal conditions. Based on these observations, we hypothesized that this family of proteins may perform an innate immune function, specifically antimicrobial defense. The studies reported in this thesis characterize a family of C-type lectins. Specifically, we determined that these proteins interact with peptidoglycan by binding with high affinity to its glycan structure, representing a unique blend of peptidoglycan recognition and lectin function. Additionally, we have demonstrated that this binding results in the specific disruption of the Gram positive bacterial cell wall, where peptidoglycan is exposed, which is the first example of a family of directly bactericidal C-type lectins. We also present evidence for the regulation of these bactericidal proteins by colonization with an intestinal microflora. Therefore, the research presented in this thesis elucidates the function of three members of the RegIII family, in both mice and humans.