BACKGROUND: Scavenger receptors are important components of the innate immune 
system in the lung, allowing alveolar macrophages to bind and phagocytose 
numerous unopsonized targets. Mice with genetic deletions of scavenger 
receptors, such as SR-A and MARCO, are susceptible to infection or inflammation 
from inhaled pathogens or dusts. However, the signaling pathways required for 
scavenger receptor-mediated phagocytosis of unopsonized particles have not been 
characterized.
METHODS: We developed a scanning cytometry-based high-throughput assay of 
macrophage phagocytosis that quantitates bound and internalized unopsonized 
latex beads. This assay allowed the testing of a panel of signaling inhibitors 
which have previously been shown to target opsonin-dependent phagocytosis for 
their effect on unopsonized bead uptake by human in vitro-derived alveolar 
macrophage-like cells. The non-selective scavenger receptor inhibitor poly(I) 
and the actin destabilizer cytochalasin D were used to validate the assay and 
caused near complete abrogation of bead binding and internalization, 
respectively.
RESULTS: Microtubule destabilization using nocodazole dramatically inhibited 
bead internalization. Internalization was also significantly reduced by 
inhibitors of tyrosine kinases (genistein and herbimycin A), protein kinase C 
(staurosporine, chelerythrine chloride and Gö 6976), phosphoinositide-3 kinase 
(LY294002 and wortmannin), and the JNK and ERK pathways. In contrast, inhibition 
of phospholipase C by U-73122 had no effect.
CONCLUSION: These data indicate the utility of scanning cytometry for the 
analysis of phagocytosis and that phagocytosis of unopsonized particles has both 
shared and distinct features when compared to opsonin-mediated phagocytosis.