Department of Microbiology, A Leader in Microbiology and Microbial Pathogenesis Research and Training.

Analysis of the host oxygen-dependent antimicrobial arsenal in bacterial pathogenesis. The NADPH phagocyte oxidase produces a wide array of reactive oxygen species that are essential to the host’s antimicrobial arsenal. Many pathogenic microorganisms avoid the deleterious actions derived from NADPH oxidase activity by for example detoxifying or scavenging oxyradicals or by repairing the damaged inflicted by these toxic compounds. Our laboratory has described a novel strategy used by S. typhimurium to avoid contact with incoming vesicles harboring functional NADPH oxidase that relies on the contact-dependent type III secretion system encoded by the Salmonella pathogenicity island 2 (SPI2). Effector proteins secreted by SPI2 avoid contact with NADPH oxidase by interrupting the TNF-initiated, acid sphingomyelin signaling pathway of the host phagocyte. The host on the other hand can counteract the actions associated with this complex Salmonella type III secretion system. Transcriptional profiling has revealed that nitric oxide produced by IFN-activated macrophages inhibit transcription of SPI2 regulated genes. Oxidation of the free cysteine of the DNA binding domain of SsrB, which is the response regulator that governs overall SPI2 expression, interferes with the recognition of cognate SPI2 promoters. Our current research is utilizing biochemical and molecular biology approaches to characterize in detail the NO-mediated regulation of signaling pathways that control global transcription in the intracellular pathogen Salmonella.

Dual role of mononuclear phagocytes for disseminating enteric pathogens and elicitating systemic immune responses. Transmigration of Salmonella from the gastrointestinal tract into deep tissues is initiated upon the invasion of M cells lining ileal Peyer’s patches in a process that is dependent on a type III secretion system encoded by the Salmonella pathogenicity island 1 (SPI1). Paradoxically, Salmonella strains deficient in this island disseminate extraintestinally and are able to cause systemic disease. We have demonstrated that phagocytes are a double-edged sword that can disseminate pathogens from the gastrointestinal mucosa to the systemic circulation. Phagocytes provide both an entryway and a site of bacterial replication, while also induce protective acquired immunity. We are investigating the anatomical location and molecular basis for the transmigration of Salmonella-containing phagocytes as well as the mechanisms by which gut phagocytes trigger systemic immunity against this enteropathogen. Our Salmonella vaccines capable of loading antigen into the cytosol of intestinal dendritic cells have elucidated that mucosal vaccines elicit strong effector memory but weak central memory T cell responses. A weak central memory response may eventually lead to T cell exhaustion, providing a mechanism for the short lived protection conferred by current clinical Salmonella oral vaccines. We are investigating the early dendritic cell signals that may strengthen T cell central memory responses to Salmonella. The goal of these studies is to shed light into the basic organization of the inductive sites of the complex gastrointestinal mucosa and ultimately expedite the development of the next generation of inexpensive, easily administered oral vaccines.