Optimal innate immune response to
infection includes eradication of potential pathogens, resolution of associated
inflammation, and restitution of homeostasis. Phagocytosing human polymorphonuclear leukocytes (
hPMN) undergo accelerated apoptosis, a process referred to as phagocytosis-induced cell death (PICD) and an early step in their clearance from inflammatory sites. Among human pathogens that modulate
hPMN apoptosis, Neisseria gonorrhoeae delays PICD, which may contribute to the exuberant neutrophilic
inflammation that characterizes
gonorrhea. To elucidate the mechanisms underlying delayed PICD, we compared features of
hPMN cell death that followed phagocytosis of N. gonorrhoeae FA1090 wild-type (GC) or serum-opsonized
zymosan (OPZ), a prototypical stimulus of PICD.
Phosphatidylserine externalization required
NADPH oxidase activity after ingestion of GC or OPZ, and
annexin V staining and DNA fragmentation were less after phagocytosis of GC compared to OPZ.
Caspase 3/7 and
caspase 9 activities after phagocytosis of GC were less than that seen after ingestion of OPZ, but
caspase 8 activity was the same after ingestion of GC or OPZ. When
hPMN sequentially ingested GC followed by OPZ, both
caspase 3/7 and 9 activities were less than that seen after OPZ alone, and the inhibition was dose dependent for GC, suggesting that ingestion of GC actively inhibited PICD. Sequential phagocytosis did not block
caspase 8 activity, mitochondrial depolarization, or
annexin V/
propidium iodide staining compared to responses of
hPMN fed OPZ alone, despite inhibition of
caspases 3/7 and 9. Taken together, these data suggest that active inhibition of the intrinsic pathway of apoptosis contributes to the delay in PICD after
hPMN ingestion of N. gonorrhoeae.