Alveolar macrophages (AM) originate from monocytes that immigrate into the lung and have unique biological characteristics referred to as an alternative activation state. AM display a greater phagocytic capacity due to increased activity of pattern recognition receptors [e.g. the macrophage mannose receptor (MR)]; and altered oxidative responses to microbes. These attributes allow AM to effectively clear microbes within the alveolus while minimizing """"""""collateral"""""""" inflammatory damage, but we speculate may make them particularly good targets for respiratory-adapted intracellular pathogens. Surfactant protein A (SP-A) is important in innate immunity to lung pathogens. We have determined that SP-A signals human macrophages to increase MR function and decrease oxidative responses to stimuli. SP-A can also uniquely drive monocyte differentiation to up-regulate MR function and alter oxidative responses. Our central hypothesis is that interactions of SP-A with both resident AM and immigrating monocytes help dictate the unique functional properties of AM that are important in the handling of respiratory pathogens. In this regard, SP-A traffics through the endo-lysosomal pathway and co-localizes with E. coil within the macrophage phagolysosome. Since SP-A has direct anti-microbial activity against E. coil, it may uniquely enhance microbicidal activity within the phagolysosome of the AM. Respiratory-adapted intracellular pathogens such as Mycobacterium tuberculosis (Mtb) subvert several innate host defenses. Mtb uses the MR to enter the macrophage and its phagosome does not fuse with lysosomes, thus potentially limiting its interaction with SP-A in this locale.
Our Specific Aims are to: 1) determine the mechanisms underlying SP-A-induced (A) up-regulation of MR activity and (B) inhibition of oxidative responses in macrophages; 2) determine the role of two key macrophage biochemical mediators in this process: Protein Kinase C and Phosphoinositide-3-kinases; 3) determine the effects of SP-A on monocyte differentiation; and 4) compare the impact of SP-A and other surfactant components on survival of the extracellular pathogen E. coli and the intracellular pathogen Mtb in human macrophages. We will use microscopy techniques and biochemical assays to assess SP-A's effects on MR trafficking, oxidative responses and signaling in human monocytes and macrophages, and cell culture for microbe studies. Our overall goal will be to better understand how surfactant components regulate macrophage biology to maintain the health of the individual against respiratory pathogens and how respiratory-adapted intracellular pathogens subvert this process.
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