Interactions of Neisseria meningitidis with human monocytes

The roles of capsule, pill and Class 5 outer-membrane proteins (Opa and Opc) of Neisseria meningitidis (Nm) in bacterial interactions with human monocytes were investigated using several meningococcal isolates of different serogroups. The presence of either Class I or Class II pili in capsulate strains of several serogroups had no significant effect on adherence to and internalisation by monocytes. Using clonal variants derived from a non-piliated serogroup A strain, C751, it was observed that capsulate bacteria (cap⁺) failed to interact with human monocytes in significant numbers whether or not they expressed outer-membrane proteins. These bacteria were also resistant to phagocytic killing. For capsule-deficient bacteria, expression of the Opc protein or OpaB_c751 correlated with high levels of association, while the expression of OpaD_c751 or OpaA_c751 resulted in comparatively lower levels. Bacteria expressing undetectable levels of Opc or Opa proteins (Opc^-, Opa) failed to interact with monocytes. In phagocytic killing assays, Opc-expressing bacteria (Opc⁺) as well as Opa-expressing bacteria (Opa⁺) were killed more readily than Opc^-, Opa bacteria (30% decrease in viability of Opc⁺ bacteria; 18%, 10% and 8% decrease in viability of OpaB⁺, OpaD⁺ and OpaA⁺ bacteria). A study of intracellular survival showed a gradual decrease in viability of both capsulate and capsule-deficient bacteria. However, proportionately greater numbers of capsule-deficient bacteria were internalized and consequently larger numbers survived over a 4-h period. Prolonged bacterial survival within phagocytic cells may have implications in dissemination of bacteria by carriage within these cells.