Neutrophil products inhibit LLO secretion and activity, and Listeria monocytogenes intracellular growth

Listeria monocytogenes is a facultative intracellular pathogen that infects a large variety of host cells, including macrophages and diverse non-phagocytic cells. To avoid the phagosome microbicidal environment, L. monocytogenes secretes a pore-forming toxin (listeriolysin O; LLO) that releases the bacterium into the cytoplasm. Once in the cytosol, L. monocytogenes proliferates and infects adjacent cells through cell-to-cell spreading. Innate immune cells like neutrophils play an important role in the control of infection, yet the interaction between neutrophils, other host cells, and L. monocytogenes is not well understood. Neutrophils produce a high concentration and variety of antimicrobial molecules, including defensins and proteases; thus it is likely that these cells enhance the anti-listerial response of other host cells. This dissertation addresses if: i) human defensins, which can be released into the extracellular milieu by neutrophils, enable macrophages to control intracellular replication of L. monocytogenes; ii) L. monocytogenes is able to replicate in human neutrophils in a LLO-dependent manner as observed in macrophages; and iii) human neutrophils cooperate with macrophages to prevent L. monocytogenes replication in human macrophages. Addressing i), we found that the α-defensin HNP-1 (one of the most abundant proteins in neutrophil primary granules) cooperates with macrophages to inhibit L. iii monocytogenes phagosomal escape and intracellular growth. Importantly, HNP-1 is acquired by macrophages and trafficked to the phagocytosed bacteria. Finally, HNP-1 inhibits LLO secretion from the bacteria and directly blocks LLO activity. In conclusion, neutrophil defensins inhibit LLO function through two mechanisms (secretion and activity), and inhibit L. monocytogenes escape from macrophage phagosomes. Addressing ii), we found that LLO enhances the phagocytic efficiency of neutrophils and does not protect L. monocytogenes from neutrophil intracellular killing. L. monocytogenes produces multiple virulence factors, including LLO, that induce rapid neutrophil degranulation, even before closure of the phagosome. Intriguingly, degranulation protects neutrophils from LLO-mediated membrane damage. Neutrophils degranulate matrix metalloproteases, which degrade LLO, irreversibly blocking its activity. In summary, upon interaction with L. monocytogenes, neutrophils rapidly release matrix metalloproteases that degrade LLO, likely maintaining the bacterium in a bactericidal phagosome from which it cannot escape. Addressing iii), we determined that co-incubating neutrophils with macrophages during L. monocytogenes infection does not significantly alter L. monocytogenes association with or phagocytosis by macrophages but does markedly reduce L. monocytogenes growth in macrophages. In conclusion, human neutrophils produce molecules that inhibit LLO activity and intracellular replication of L. monocytogenes. Furthermore, neutrophils help limit L. monocytogenes replication in macrophages. We propose that during infection, macrophage internalization of neutrophils and/or neutrophil granule components enables