FIG 7.

Proposed model for differential trafficking of TR NTHI through macropinocytosis resulting in IBC formation. In the absence of EIPA (top), both transiently restricted (TR, left) and continuously exposed (CE, right) NTHI enter the cells through endolysosomal pathways: clathrin-mediated endocytosis (blue circles), lipid raft/caveola-mediated endocytosis (orange rectangles), and macropinocytosis (membrane ruffling by actin polymerization, red). The ability to enter the cell through these various pathways appears to be independent of prior heme-iron status. Once internalized, CE NTHI (yellow bacteria) traffics to the early endosomes (red circles) and finally to the lysosomes (purple circles), where the bacteria are degraded. TR NTHI (green bacteria) also enters the cells through endolysosomal pathways and traffics to the early endosomes and lysosomes. In contrast, the subpopulation of transiently restricted NTHI that enters through macropinocytosis either completely evades or escapes this pathway (indicated by “?”) to form intracellular bacterial communities in the cell cytoplasm. In the presence of the macropinocytosis inhibitor EIPA (bottom), trafficking of continuously exposed NTHI through the endolysosomal pathway remains unchanged. Transiently restricted NTHI, entering the cell through clathrin- or lipid raft/caveola-mediated endocytosis, now localizes to the early endosomes. This shift in trafficking targets TR NTHI for degradation by the endolysosomal pathway and significantly decreases intracellular survival of this population.