Research
Effector-triggered immunity and host defense
As accidental pathogens of humans, Legionella species are an excellent tool to investigate the mammalian innate immune response to bacterial pathogens. Effector translocation by the Dot/Icm secretion system is required for intracellular replication, but detection of effector activity within host cells can also trigger bacterial clearance through a process termed 'Effector-Triggered Immunity'. A major focus of our lab is to define how individual effectors enhance pathogen restriction by healthy hosts.
We discovered that the effector LegC4 is a virulence factor in amoebae but confers a fitness disadvantage on L. pneumophila within cytokine-activated macrophages and in the mouse lung. We subsequently found that LegC4 binds host proteasome activator (PA)28α a subunit of the PA28αβ proteasome regulator that clears oxidant-damaged proteins under inflammatory conditions. Our data support a model in which effector-mediated subversion of PA28αβ perturbs host proteostasis and drives phagolysosomal fusion with the Legionella-containing vacuole. This was the first demonstration that PA28αβ contributes to cell-autonomous antibacterial defense, and we are now working to define how induced proteasome regulators mediate innate immunity and whether this mechanism extends to other intracellular pathogens.
Molecular mechanisms of metaeffector function
Legionella pneumophila translocates over three hundred effector proteins into infected host cells. How these effectors are regulated after translocation is poorly understood. Canonically, effectors have been associated exclusively with subversion of host factors; however, Legionella encodes multiple metaeffectors that function to regulate other effectors.
We study the metaeffector MesI (Lpg2505), which is essential for intracellular replication only when its cognate effector, SidI, is present. SidI is a cytotoxic glycosyl hydrolase that potently blocks host mRNA translation. We found that MesI binds SidI with nanomolar affinity and suppresses SidI activity. Unexpectedly, we found that SidI is toxic to L. pneumophila when uncoupled from MesI, and that MesI translocation into host cells is not required for intracellular replication, suggesting that metaeffectors also function within the bacterium. These findings challenge the dogma that Legionella effectors act exclusively in host cells. We are currently defining the mechanism of intrabacterial SidI toxicity, identifying SidI's physiological substrate, and exploring how other metaeffectors regulate their cognate effectors during infection.
Antimicrobial action of sorghum polyphenols
Polyphenols derived from a variety of plants have demonstrated antimicrobial activity against diverse microbial pathogens. In collaboration with the U.S. Department of Agriculture and the Kansas State University Agricultural Research Center, we discovered that high-polyphenol extracts from Sorghum bicolor bran augment macrophage killing of L. pneumophila without impairing bacterial growth in rich media or macrophage viability. Restriction was accompanied by increased TNF and IL-6 secretion from infected macrophages, suggesting that sorghum polyphenols enhance pro-inflammatory responses rather than act directly as microbicides. We are working to identify the specific bioactive components and to determine whether sorghum polyphenols can augment macrophage restriction of a broader range of bacterial pathogens.