Intracellular pathogens are responsible for an enormous amount of worldwide morbidity and mortality and the development of vaccines and therapeutics to treat diseases caused by these pathogens continues to represent one of the biggest challenges facing the international biomedical community. By virtue of their intracellular niche, these pathogens avoid extracellular immune defense mechanisms, and consequently, vaccine strategies that target the production of antibodies have been largely ineffective. The Portnoy lab tackles a wide range of problems related to the pathogenesis and host response to intracellular pathogens with the goal of developing vaccines and therapeutics. Specifically, the lab works on Listeria monocytogenes, a facultative intracellular food-borne bacterial pathogen that is an outstanding model system with which to dissect basic aspects of host-pathogen interactions.

Specifically, the lab is focused on the interaction of the facultative intracellular bacterial pathogen Listeria monocytogenes and its mammalian host. This fascinating microorganism is able to enter cells, escape from a phagosome, circumvent autophagy, avoid cell death pathways, and grow rapidly in the cytosol. By exploiting a host system of actin-based motility, the bacteria move through the cytosol to the cell membrane and into pseudopod-like projections (listeriopods) that are ingested by neighboring cells. This mechanism allows pathogens to spread from one cell to another without ever leaving the host cytoplasm thereby avoiding the immune response. Current research covers many topics including basic microbiology, the cell biology of infection, innate immune responses, acquired immunity, and vaccine development to both infectious diseases and cancer.

Characterization of the essential virulence factor LLO.

The L. monocytogenes pore-forming cytolysin, Listeriolysin O (LLO) is an essential determinant of pathogenesis that mediates escape of the bacteria from host phagosomes allowing the bacteria to grow in the cytosol. Although LLO is related to dozens of other members of this family of cytolysins, LLO is the only one that is required by an intracellular pathogen. Accordingly, LLO has evolved numerous properties that allow it to specifically act within the host cell without causing cell death. We are currently studying the regulation of LLO synthesis and the cell biology of its trafficking in cells. We recently discovered that LLO synthesized by cytosolic bacteria forms pores in the cytoplasmic membrane but prevents killing the host cell by mediating endocytosis and targeting LLO for degradation. Mutants lacking this activity are 10,000-fold less virulent. Most recently, we have uncovered a mechanism that controls the translation of LLO so that it is synthesized and secreted during the starvation environment in a phagosome, but much less so during growth in the host cell cytosol.

Regulation of virulence gene expression.

L. monocytogenes lives a biphasic lifestyle that includes growth in the environment, including food, and infection of warm-blooded animals, including humans. Most determinants of pathogenesis are controlled by the master virulence transcriptional regulator. We recently found that intracellular bacteria detect the host environment by responding to the redox state of the host cell. We are currently using bacterial genetics to study how the bacteria sense and respond to alterations in redox. Most recently, we identified an 8-gene locus that encodes a flavin-based extracellular electron transport system that makes L. monocytogenes electrogenic. Importantly, we have also found these genes in 100s of other bacterial species including members of the intestinal microbiota. We are now studying how this system contributes to growth in the intestine and during infection.

Interaction of L. monocytogenes with the innate immune system.

We have three new projects that relate bacterial metabolism to host innate immunity.

(1) We discovered that the bacteria secrete c-di-AMP, a conserved signaling molecule that binds to and activates STING, a critical hub for the detection of microorganisms and tumors. We are currently studying the basic microbiology of c-di-AMP and its role during pathogenesis. Most recently, we have begun to explore how activation of STING can lead to trafficking from the intestine to distal sites. 

(2) Humans possess a particular type of T-cell (gamma delta T- cell) that is activated by an intermediate of bacterial isoprenoid biosynthesis. Some bacteria synthesize this intermediate while others use the same pathway as humans. L. monocytogenesis one of the very few bacteria that use both pathways. We are currently seeking to understand why L. monocytogeneshas both pathways and determine the role of each during infection.

(3) Humans and mice have T-cells called MAIT cells that are stimulated by bacterial metabolic intermediates of riboflavin biosynthesis. L. monocytogenes is a riboflavin auxotroph and is not predicted to stimulate MAIT cells. We are current examining if MAIT cells respond to L. monocytogenes infection and if these T cells play a role during L. monocytogenes infection and immunity.