Bridging of neutrophils to target cells by opsonized zymosan enhances the cytotoxicity of neutrophil-produced H2O2.

Hydrogen peroxide (H2O2) is a well-established cytotoxic agent released by activated neutrophils into the extracellular environment. However, a maximum of only 5 microM H2O2 was detected in the medium when 10(6) neutrophils/ml were activated with opsonized zymosan (OZ), more than 50-fold lower than the concentration of exogenous H2O2 required to produce equivalent killing of a cell line. In addition PMA-activated neutrophils were noncytotoxic, despite the capacity of PMA to generate two- to fourfold as much H2O2 for five times longer. The basis for this discrepancy was explored. NaN3 increased cytotoxicity to >90% only when neutrophils were activated with OZ due in part to inhibition of myeloperoxidase-mediated hydrolysis of H2O2, while catalase completely prevented cytotoxicity of OZ-activated neutrophils. These results indicate that H2O2 was solely responsible for the observed cytotoxicity. OZ-mediated cytotoxicity was prevented by intermittent agitation of the cultures or by the addition of soluble complement receptor type 1, suggesting that a physical association between neutrophils and target cells mediated by OZ was required to generate a cytotoxic environment. Significant numbers of neutrophil-target cell aggregates were observed by microscopic examination only under low hydrodynamic shear conditions. We conclude that the cytotoxic potency of H2O2 produced by neutrophils activated with OZ was due to a localized high concentration of H2O2 to which the target cells were exposed as a result of their labile adherence to OZ. This phenomenon may reflect a mechanism that neutrophils have acquired for maximizing the antimicrobial power of extracellular oxidants toward microbes that escape phagocytotosis.