Gut complement system: Induction and protection against enteric infection - SUMMARY/ABSTRACT For decades, the complement system has been primarily considered to originate from hepatocyte synthesis and to function within the bloodstream. Our recent research has uncovered a new perspective on the complement system as an integral part of the innate immune system in the gut, functioning independently from systemic complement. Through the integration of gnotobiotic mouse models, microbiota analysis, single-cell RNA sequencing, and microscopic imaging, we have demonstrated that the central component of the complement cascade, C3, is synthesized locally by three crucial intestinal cell types (epithelial, myeloid, and stromal). Intestinal C3 is regulated by the composition of the gut microbiota, resulting in host- specific levels of C3 in the gut lumen. The broad applicability of these findings was confirmed by validation studies conducted on a cohort of healthy human subjects, emphasizing the conservation of the gut complement system across species. Of importance, our studies have shown a significant increase in luminal C3 levels during enteric infections, which is essential for protecting against various enteric pathogens. Furthermore, a notable correlation has been established between an individual's intestinal C3 levels and their ability to effectively resist enteric infections, highlighting the crucial significance of gut C3 in the host's defense mechanisms against infectious diarrhea. Infectious diarrhea represents a significant and serious health concern, particularly affecting children in low- and middle-income countries. Vaccines are pathogen-specific and pose challenges to deliver to a targeted population. Our findings on gut C3 have uncovered a potential new target for preventing and treating infectious diarrhea. We hypothesize that microbiota-derived molecules can regulate luminal C3 levels, and by modulating C3 levels could serve as a novel strategy for combating infectious diseases. We will examine this hypothesis in three aims. In Aim 1, we will determine the cellular source of C3 in the gut that is most critical for combating infection. In Aim 2, ) we will identify the proteins and pathways through which the gut complement system is activated and functions. In Aim 3, we will develop a synthetic molecule with a specific lipid A structure that exhibits low proinflammatory activity but high potency in inducing C3.
