Intracellular killing of micro-organisms by phagocytic cells: the effect of extracellular stimuli (Abstract)

Abstract

The rate of intracellular killing of micro-organisms by monocytes and granulocytes can be measured independently from the rate of ingestion. This method allowed us to study the effect of extracellular factors (serum proteins and lectins) on intracellular killing. The intracellular killing of microorganisms by human granulocytes and monocytes proved to be dependent on the presence of extracellular serum. In the absence of serum, killing of S. aureus by monocytes did not occur and was sub-optimal for granulocytes. In the presence of heat-inactivated serum the killing index lay between the values obtained in the absence of serum and in the presence of fresh serum. The killing in the presence of inactivated serum is very probably dependent on an interaction between the Fc part of IgG and the Fc receptor in the cell membrane, as indicated by the following findings: (1) IgG preparations were as active as inactivated serum: (2) p-Fc fragments of IgG stimulated intracellular killing, whereas (Fab’)₂ fragments were inactive in this respect; (3) only IgG subclasses IgG₁ and IgG₃ were stimulatory, and the cells only carry receptors for these subclasses. Complement component C3b also stimulated the intracellular killing of S. aureus ingested by monocytes and granulocytes. In case of the monocytes, C3b is generated via the alternative pathway, whereas for the granulocytes factors of the classical as well as the alternative pathway are involved in this process.