Distribution of annexin I during non-pathogen or pathogen phagocytosis by confocal imaging and immunogold electron microscopy.

Annexin I is an abundant protein in U937 cells differentiated towards a macrophagic phenotype. These cells become able to kill Escherichia coli, however, the intracellular pathogen Brucella suis, known to interfere with phagosome maturation, multiply in these differentiated cells. We have analysed by confocal and electron microscopy the cellular localization of annexin I during phagocytosis of yeast, non-pathogenic E. coli and the intracellular pathogen B. suis. Using immunocytochemical detections annexin I was found mainly as patches in the cytoplasm of uninfected cells. Upon phagocytosis of yeast or E. coli organisms, annexin I rapidly translocated and concentrated around phagosomes. On the other hand, annexin I was never detected around live B. suis-containing phagosomes. However, when dead brucellae were used, annexin I did translocate to the periphagosomal region. Our results suggest that annexin I could play a role in the molecular mechanism of phagosome maturation, which is impaired by some intracellular pathogens.