Enteric infections are endemic and epidemic infectious diseases that still afflict major populations around the world, particularly in developing nations, and also pose risks for US travelers (including deployed military personnel). Vaccines are widely viewed as cost-effective interventions to prevent and control endemic and epidemic infections. In spite of the need to develop new or improved vaccines against enteric pathogens of great public health importance, their development has been impeded by fragmentary understanding of the immunological mechanisms operational systemically and in the complex environment of the Gl tract. Thus, the central theme of this proposed CCHI is to advance our knowledge of Human Mucosal Immunity. to this end we will address the overarching hypothesis that the delicate Homeostasis of effector and regulatory Immunological mechanisms elicited systemically and In the gut Mucosa, and the Interplay with the resident gut microbiota, play a critical role In protection from typhoid fever In humans. Some of the unique resources to be utilized are specimens already collected during ground-braking clinical trials involving the challenge of volunteers with wild-type (wt) S. typhi before or following immunization with attenuated S. typhi vaccines, including the FDA licensed Ty21 a typhoid vaccine. In addition to sophisticated mechanistic immunological studies, because evidence is rapidly accumulating that the normal microbiota is likely to play a major role in the induction of host's Immune responses, we propose to dramatically expand the pioneering studies conducted during the current CCHI grant on the interactions between the host Immune responses and the gut microbiota in humans. Equally important, we will utilize state-of-the-art sophisticated in vitro organotypic models of the Human Intestinal Mucosa and Mucosal biopsy explants to study the initial interactions between S. typhi and the host, including innate immunity and physiological consequences. Finally, we will perform studies in children, who are primarily affected by typhoid fever but for whom virtually no information is available on the protective immunity elicited by Ty21a immunization. Because of the complexity of this undertaking, we have assembled a multidisciplinary team consisting of renowned Investigators in the fields of innate and adaptive immunity, vaccinology, Mucosal biology and physiology, clinical gastroenterology (with extensive experience in performing endoscopies), molecular biology, biochemistry, microbiology, genomics, bioinformatics and biostatistics. We expect this CCHI to yield much needed information in an area of great importance to Human health and to advance the development of much needed improved oral vaccines.