Innate immune cells respond to invading microbes upon detection of pathogen-associated molecular patterns (PAMPS). PAMP-recognition machinery is evolutionarily conserved, allowing for characterization in model organisms. The model organism Dictyostelium discoideum can exist as single-celled amoebae, which phagocytize bacteria for nutrients. Although D. discoideum is used extensively to study phagocytosis, it has not been determined if D. discoideum detects bacterial PAMPs using pattern-recognition machinery. Here we show that D. discoideum mounts responses against the bacterial cell wall PAMP, lipopolysaccharide (LPS). Upon treatment with LPS or its active component Lipid A, D. discoideum cells more efficiently clear phagocytized bacteria. LPS-enhanced bactericidal activity appears dependent both on MAPK signaling pathways as well as on the D. discoideum toll/interleukin-1 receptor domain-containing protein, TirA. These findings indicate that pattern-recognition machinery required to detect and respond to bacterial PAMPs may be conserved in D. discoideum.

Innate immune cells use evolutionarily-conserved pattern-recognition machinery to detect pathogen-associated molecular patterns (PAMPs). The conservation of pattern-recognition machinery allows for its characterization in various model organisms. The social amoeba Dictyostelium discoideum is a unique model system that can exist as single-celled amoebae that phagocytize bacteria for nutritional uptake. Although D. discoideum has been used to study bacterial phagocytosis, it has not been determined whether D. discoideum uses pattern-recognition machinery to detect bacterial PAMPs. Here we show that D. discoideum mounts responses against the bacterial cell wall PAMP, lipopolysaccharide (LPS). LPS treatment of D. discoideum results in more efficient uptake of bacteria and more efficient clearance of phagocytized bacteria. In addition, as shown by qRT-PCR, D. discoideum treated with LPS upregulate expression of putative pattern-recognition and phago-lysosomal genes. Both LPS-enhanced bactericidal activity and gene expression are dependent on MAPK signaling. We currently are determining the mechanisms by which LPS stimulates D. discoideum by studying intracellular trafficking of phagocytized bacteria and by creating D. discoideum cells that overexpress or are deficient for pattern-recognition and phago-lysosomal genes. The characterization of pattern recognition in D. discoideum should provide insight into conserved molecular mechanisms underlying microbial detection.

Cells of the innate immune system detect pathogen-associated molecular patterns (PAMPs) through highly conserved pattern-recognition machinery. The conservation of this machinery allows for its characterization in model organisms. The unique model organism Dictyostelium discoideum can exist as single-celled amoebae that phagocytize bacteria for nutritional uptake. However, it has not been determined if D. discoideum uses pattern-recognition machinery to detect PAMPs. Here we show that D. discoideum mounts a response against the bacterial cell wall PAMP, lipopolysaccharide (LPS). LPS treatment results in an increased clearance of phagoctyzed bacteria by D. discoideum. LPS-induced bactericidal activity appears to be dependent on MAPK and Tir domain-containing protein A (TirA)-mediated pathways. Recent results suggest that increased bactericidal activity upon treatment with LPS may involve the induction and maturation of autophagosomes. We show here, by following the delivery of the autophagosomal marker Atg8 to degradative compartments, that Staphylococcus aureus induces autophagosomal maturation in D. discoideum only upon addition of LPS. We are currently characterizing the signaling pathways underlying bacterial induction of autophagy in D. discoideum. Characterization of the cellular events induced upon microbial pattern recognition in D. discoideum should provide valuable insight into conversed cellular mechanisms underlying microbial detection and killing.