Research
Type III Secretion.
Type III secretion systems (T3SS) is a specialized apparatus expressed by many Gram negative bacteria, including a number of human pathogens. The hallmark of the T3SS is contact-dependent delivery of bacterial effector proteins directly into the cytosol of target host cells. These T3SS effector proteins then mediate diverse effects on normal host cell processes, including inhibition of phagocytosis and inflammatory cytokine production. The T3SS is composed of a basal body that spans both the inner and outer membranes and a needle-like structure that extends ~60 nm from the outer membrane (see figure, right). Components of the T3SS tip complex are secreted by the T3SS and act concertedly to form a pore in the target host cell membrane. This tip complex then facilitates the directed translocation of effector proteins into the host cell cytosol.
Yersinia pseudotuberculosis.
Yersinia pseudotuberculosis and Y. enterocolitica are human gut pathogens that cause gastroenteritis and inflammation of mesenteric lymph nodes. These enteropathogenic Yersinia species most often cause a localized, self-limited infection in healthy individuals. However, immunocompromised patients and people with iron-overload disease such as hemochromatosis have a higher risk of developing much more serious disseminated infection.
Enteropathogenic Yersinia are among the dozens of bacterial pathogens that use a T3SS to cause disease. Yersinia express at least six T3SS effector proteins, YopHEMOJT. Most of these effector proteins are important for virulence, as they interfere with innate immune defense functions of target host cells.
Innate immune system.
The mammalian innate immune system must recognize previously unencountered microbes and immediately launch an appropriate response. Overreaction to beneficial microbes, which far outnumber harmful ones, are thought to contribute
to inflammatory disorders. Therefore, mechanisms must be in place to distinguish beneficial bacteria from pathogens.
Mammalian cells can sense the presence of potentially dangerous microorganisms through so-called “danger signals”,
such as the the presence of microbial products in the cytoplasm of mammalian cells. A large number of mammalian
innate immune receptors have been described that recognize different microbially-derived molecules. One group, called
the Toll-like receptors (TLRs), reside on host membranes and recognize molecules such as lipopolysaccharide from
Gram negative bacteria. Another group of receptors reside in the cytosol of host cells and recognize moieties such as
specific RNA structures, flagellin, and peptidoglycan.
Innate immune recognition of Yersinia type III secretion.
We have recently shown that mammalian cells are specifically able to recognize Y. pseudotuberculosis expressing a functional T3SS, allowing differentiation between virulent and avirulent bacteria. This recognition involves the T3SS-associated pore-forming proteins YopB and YopD, but does not require the six known effector Yops. Because mammalian cells can distinguish between T3SS-positive and T3SS-negative Y. pseudotuberculosis in the absence of TLR signaling, we hypothesize that cytosolic innate immune receptors play a role in sensing this extracellular bacterium.
Our current areas of investigation include exploring...
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• How does T3SS-mediated pore formation and translocation affect the cytosolic innate immune response to Yersinia?
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• What are the bacterial and host molecules involved in innate immune recognition of the Yersinia T3SS?
We are also using innate immune recognition of the T3SS as a tool to screen for novel, small molecule T3SS inhibitors.
(Link to small molecule screening facility)
Differential host response to Yersinia type III secretion shown using Affymetrix microarrays.
