Entry and intracellular survival of group B streptococci in J774 macrophages

The mouse macrophage-like cell line J774 was used to analyze opsonin-independent entry and survival of group B streptococci (GBS). Efficient entry of GBS in J774 cells occurred within 5 min postinfection, and streptococci persisted intracellularly without loss of viability for at least 8 h. At 24 h postinfection, 30% of the total intracellular GBS was recovered from macrophages. Inhibition studies using different biochemical modulators of cellular functions showed that bacterial entry seemed to involve nonglycosylated J774 surface structures different from known receptors such as fibronectin-binding integrins. Internalization of GBS by J774 cells occurred by a microfilament-dependent phagocytosis-like process also involving participation of receptor-mediated endocytosis. Prior opsonization of GBS with human serum containing anti-GBS antibodies did not affect bacterial entry but significantly reduced the intracellular survival of GBS. Transmission electron microscopic analysis confirmed these findings and demonstrated that both opsonized and nonopsonized bacteria were contained within phagosomes during the whole infection period. Transmission electron microscopy further revealed that decreased intracellular survival rates of opsonized GBS appeared to be due to increased lysosomal activities of the macrophages. These results suggest that in the absence of opsonins, GBS are able to enter and persist efficiently in macrophages by evading intracellular antibacterial activities commonly associated with opsonin-mediated uptake.