Antimicrobial mechanisms in neutrophilic polymorphonuclear leukocytes

Microorganisms ingested by PMNs are exposed to a variety of antimicrobial systems. Together they comprise a formidable armamentarium, and few organisms survive. The predominant antimicrobial system would be expected to vary with the species, the availability of oxygen and the type of microorganism ingested. There is considerable evidence that the MPO-mediated antimicrobial system plays an important role in the destruction of certain microorganisms in most species; chicken heterophils, however, do not contain MPO,40 and some microorganisms are resistant to this system due to the nature of their cell wall material.146 Further, microbial catalase may offer some protection. The granulocytes of some species (e.g., rabbit, chicken) are rich in cationic proteins and these agents may play a particularly important role in these cells. Granular cationic proteins are less plentiful in human cells.111 Organisms vary in their susceptibility to lysozyme and this enzyme is absent from bovine leukocytes.113 It is probable that the total microbicidal potential of the leukocyte is in excess of its needs under most circumstances. This "overkill" capacity is a reflection of both the level of activity of individual systems and their variety. Particular organisms are susceptible to more than one antimicrobial system and thus may be effectively handled by back-up systems when one is absent. Thus, an organism normally killed by the peroxidase system may be handled less efficiently but adequately when MPO is absent by other oxygen-dependent antimicrobial systems. When a defect in oxidative metabolixm is present as in CGD, both MPO-catalyzed and nonenzymatic oxygen-dependent systems are absent. The ingested organism can, in some instances, supply the needed product of oxidative metabolism (i.e., H2O2); in other instances, oxygen-independent antimicrobial systems are adequate to prevent microbial growth. However, in yet other instances, the organisms survive and multiply and severe infection results.