Immune Modulation by Bacterial Autolysins

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The SecA2 auxiliary protein secretion system is required for secretion of virulence promoting proteins from a number of Gram-positive pathogens, including Bacillus anthracis (category A) and Listeria monocytogenes (category B). We identified two SecA2-dependent autolytic proteins that promote virulence of L. monocytogenes in infected animals, yet do not affect the growth of this bacterium in tissue culture cells. The virulence of engineered bacterial mutants lacking p60 was restored by expression of full-length p60, but not by expression of a truncated, catalytically inactive protein. The catalytic specificity of p60 predicts that it digests peptidoglycan (PGN) to generate or destroy immune modulating PGN fragments (muropeptides), including respectively muramyl di- and tri-peptides (MDP and MTP). MDP and MTP influence mammalian cell cytokine responses by acting on cytosolic proteins of the Nod family. We have found that p60- expressing bacteria and small molecules released from these bacteria enhance the induction of specific immune-regulatory cytokines by macrophages. In this grant proposal we investigate how p60 promotes virulence and affects host innate immune responses to infection. Our first Aim will identify features of p60 that are required for PGN digestion and for its effects on bacterial virulence and cytokine gene expression. Our second Aim investigates the structure and phylogenetic distribution of a p60-dependent biologically active muropeptide or small molecule and tests whether responses to this molecule require known muropeptide-responsive Nod family proteins. For our third Aim, we investigate a potential mechanism for p60's effects on bacterial virulence by determining how expression of p60 and cytokines induced by p60 affect macrophage responses to activating stimuli. Our studies will define the mechanisms by which this bacterial autolysin contributes to the virulence of a clinically important bacterial pathogen and begin to explore whether similar mechanisms promote virulence of other Gram-positive pathogens, including potential agents of bioterrorism. The mechanisms used by pathogenic bacteria to cause disease include strategies to subvert host immune responses. A subversive strategy that may be common to a number of deadly bacteria is studied in this grant. Our studies will define the molecular basis for this strategy of immune subversion and may thus reveal novel therapeutic avenues to modulate inflammation during bacterial infection, vaccination, and chronic inflammatory diseases.