The Interaction of Neisseria Gonorrhoeae with Human Monocytes

Abstract

The ability of human monocytes to phagocytize and kill nonpiliated opaque (T3) and transparent (T4) gonococci was investigated in a tumbling tube suspension assay. A serum-sensitive strain, F62, and a serum-resistant strain, FA19, were studied. Viable colony-forming units remaining after incubation with monocytes were used to assess the extent of killing. The data show that 50% of T3 and T4 gonococci of both strains were killed by monocytes over a 2 hour period. Serum was necessary for the phagocytic killing of transparent gonococci of both strains as well as for FA19 T3. Concentrations of serum ranging from 0.5% to 10% were equally effective, and heat-labile components were required. Killing of opaque F62 T3 organisms, however, occurred in the absence of serum. Increased ratios of bacteria-to-monocytes decreased the efficiency of monocyte phagocytic killing, while a 30 minute pre-opsonization of gonococci in serum enhanced the rate of killing. Monocytes were able to kill plate grown bacteria, but not log phase organisms. A limited duration of phagocytic capability was demonstrated. The absence of lysosomal enzyme release and the Cytochalasin B inhibition of colony reduction indicated that killing was the result of intracellular bactericidal activity. Disruption of the monocytes by sonication to release internalized bacteria did not alter the number of viable colony-forming units recovered indicating that significant numbers of internalized gonococci fail to survive. Additional experiments with adherent monocyte monolayers suggested that all internalized gonococci are indeed killed immediately on ingestion. Similar results obtained from several modifications of the tumbling tube assay supported these observations. Human monocytes responded to contact by opsonized gonococci with an enhanced oxidative metabolism as measured by chemiluminescence. Non-opsonized gonococci also stimulated chemiluminescence although the magnitude of the response was 10-fold less. Both strains and types of gonococci evoked similar chemiluminescent responses which were unrelated to the extent of bactericidal activity. Inhibition of monocyte chemiluminescence by sodium azide, catalase and aminotriazole had no effect on the ability of the cells to phagocytize and kill gonococci, whereas, incubation in iodoacetic acid and phenylbutazone inhibited phagocytic killing, probably as the result of impaired phagocytosis. These data indicate that freshly isolated and in vitro cultured adherent monocytes are capable of phagocytizing and killing N. gonorrhoeae, and that the mechanism of bactericidal activity is not solely dependent on oxidative metabolism.