Alveolar macrophages (AMs) are considered major effector cells in host defense against respiratory tract infections by virtue of their potent phagocytic properties. In addition, AMs may regulate the host inflammatory response to infection by production of cytokines and by their capacity to phagocytose apoptotic polymorphonuclear cells (PMNs). To elucidate the in vivo contribution of AM to host defense against pneumococcal pneumonia, we depleted mice of AMs via pulmonary application of liposomal dichloromethylene-bisphosphonate (AM- mice) before inoculation with Streptococcus pneumoniae; control mice received saline (AM+sal) or liposomal phosphate-buffered saline (AM+lip) before bacterial inoculation. AM- mice displayed a significantly higher mortality compared with AM+ control mice, whereas bacterial clearance did not differ. Poor outcome of AM- mice was accompanied by a pronounced increase of local proinflammatory cytokine production as well as strongly elevated and prolonged pulmonary PMN accumulation. Closer examination of infiltrating PMN in AM- mice disclosed high proportions of apoptotic and secondary necrotic cells, reflecting the lack of efficient clearance mechanisms in the absence of AMs. Furthermore, caspase-3 staining showed only slightly higher activity in AM- mice, arguing against accelerated apoptosis per se. These data suggest that AMs are indispensable in the host response to pneumococcal pneumonia by means of their capacity to modulate inflammation, possibly via elimination of apoptotic PMNs.

TLRs are important for the recognition of conserved motifs expressed by invading bacteria. TLR4 is the signaling receptor for LPS, the major proinflammatory component of the Gram-negative cell wall, whereas CD14 serves as the ligand-binding part of the LPS receptor complex. Triggering of TLR4 results in the activation of two distinct intracellular pathways, one that relies on the common TLR adaptor MyD88 and one that is mediated by Toll/IL-1R domain-containing adaptor-inducing IFN-β (TRIF). Nontypeable Haemophilus influenzae (NTHi) is a common Gram-negative respiratory pathogen that expresses both TLR4 (LPS and lipooligosaccharide) and TLR2 (lipoproteins) ligands. To determine the roles of CD14, TLR4, and TLR2 during NTHi pneumonia, the following studies were performed: 1) Alveolar macrophages from CD14 and TLR4 knockout (KO) mice were virtually unresponsive to NTHi in vitro, whereas TLR2 KO macrophages displayed a reduced NTHi responsiveness. 2) After intranasal infection with NTHi, CD14 and TLR4 KO mice showed an attenuated early inflammatory response in their lungs, which was associated with a strongly reduced clearance of NTHi from the respiratory tract; in contrast, in TLR2 KO mice, lung inflammation was unchanged, and the number of NTHi CFU was only modestly increased at the end of the 10-day observation period. 3) MyD88 KO, but not TRIF mutant mice showed an increased bacterial load in their lungs upon infection with NTHi. These data suggest that the MyD88-dependent pathway of TLR4 is important for an effective innate immune response to respiratory tract infection caused by NTHi.

Salmeterol exerts antiinflammatory effects in the pulmonary compartment of humans exposed to LPS.