Phagocytosis and signaling in the innate immune system
Gonzalez, Elizabeth Anne Cates
Wu, Louisa P
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The innate immune response provides broad spectrum defense through germline encoded components. Many aspects of innate immunity, such as the activation of NFκB transcription factors and phagocytosis, are highly conserved within the animal kingdom. The innate immune response of the cow, in particular, is important due to the cow's agricultural value. A major proportion of acute disease in domestic cattle is caused by Gram-negative bacteria, which produce the outer membrane component lipopolysaccharide (LPS). LPS binds to Toll-like receptor (TLR) 4 and activates multiple signaling pathways, which have been well-studied in humans, but not in ruminants. Human myeloid differentiation-factor 88 (MyD88) and TIR-domain containing adaptor protein (TIRAP) are critical proteins in the LPS-induced NFκB and apoptotic signaling pathways in humans. We demonstrated through the expression of dominant negative constructs in bovine endothelial cells that both MyD88 and TIRAP activate NFκB in the cow. Additionally, bovine TIRAP was also shown to transduce LPS–induced apoptosis, indicating that multiple aspects of the TLR4–dependent signaling pathways are conserved between cows and humans. The model organism <italic>Drosophila melanogaster</italic>, was subsequently utilized to investigate the role of another branch of the innate immune response: phagocytosis. The extracellular fluid surrounding phagocytic cells in <italic>Drosophila</italic> has a high concentration of the amino acid glutamate. While glutamate has been well-characterized as a neurotransmitter, its effect, if any, on immune cells is largely unknown. We identified that a putative glutamate transporter in <italic>D. melanogaster</italic>, <italic>polyphemus (polyph)</italic>, is critical to the fly's immune response. Flies with a disrupted <italic>polyph</italic> gene exhibit decreased phagocytosis of microbial-derived bioparticles but not of latex beads. Additionally, <italic>polyph<italic> flies show increased susceptibility to <italic>S.aureus</italic> infection, decreased induction of the antimicrobial peptide (AMP) Cecropin, increased melanization response, and increased ROS production. Glutamate transport has previously been shown to regulate the synthesis of the antioxidant glutathione. We demonstrate that a polyph–dependent redox system is necessary to maintain the immune cells' function against an infection. By utilizing two species, the cow and the fly, to study the innate immune system, we have gained unique and novel insights into NFκB activation and phagocytosis.