Alveolar macrophages (AM) are critical cell determinants of the pulmonary innate immune system and they are the first professional phagocytes to internalize and typically degrade invading microbes. These cells are considered alternatively activated since they limit the pro-inflammatory response and exhibit high expression of pattern recognition receptors (PRRs) [e.g. mannose receptor (MR) and Toll-like receptors (TLRs)] which aid in the clearance of invaders. While TLR2 and TLR4 can initiate several pro-inflammatory signaling cascades following their activation, TLR negative regulators exist to limit the inflammatory response. It is of interest how certain components within the alveolus affect the activation state of AM. Surfactant protein-A (SP-A), a pulmonary surfactant protein, has been shown to regulate macrophage biology through its effects on the pro-inflammatory response and phagocytic PRR expression and activity (e.g. MR). Work in this thesis examined whether SP-A regulates the expression, activity, and intracellular trafficking of TLRs in human macrophages.
We examined basal and SP-A-induced transcription and surface protein expression of TLR2 and TLR4 on human monocytes and monocyte-derived macrophages (MDMs) and we observed that SP-A significantly increases TLR2, but not TLR4, surface expression on MDMs. While SP-A selectively increases TLR surface expression, it dampens the pro-inflammatory response by inhibiting NFκB activity and TNFα secretion in the presence of TLR ligands and by decreasing the phosphorylation of key TLR signaling molecules, including the major cell regulator, Akt.
We examined the relationship between the MR and TLR2 or TLR4 since SP-A regulates all three receptors and it has been suggested that cross-talk between these receptors exists. We observed that the MR and TLRs co-localize on the surface of MDMs, but these receptors are located in distinct intracellular vesicles, which suggest that these receptors may collaborate during the initial recognition of a pathogen but then diverge in terms of intracellular function. In addition, SP-A regulates the intracellular trafficking of TLR2 and TLR4. Our findings strongly support the role of SP-A as a major alternative activator of macrophages that regulates the expression, activity, and intracellular trafficking of major PRRs in the alveolar innate immune response.